MAweatherboy1's Blog

Review of June 23 New England Severe Weather Event

By: MAweatherboy1, 1:09 AM GMT on June 27, 2015

Today’s blog entry will be on the severe weather episode that took place in New England on Tuesday, June 23. I will specifically be focusing on Southern New England. I’ll examine what caused the atmosphere to be ripe for storms that day, what potentially limited the event, and I’ll take a detailed look at the storm that spawned a brief, EF0 tornado in Wrentham, Massachusetts. Given that Wrentham is also my hometown, I’ll be sharing my own personal story from that storm as well.

As much as three days in advance of the event, forecasters and computer models began to signal the potential for a significant severe weather event on June 23 over New England. Predictions showed that the region would be in the warm sector of a strong, expansive frontal system that was at the time providing multiple rounds of severe weather including tornadoes to portions of the Midwest United States. The Storm Prediction Center Day 3 convective outlook indicated a “slight” risk of severe weather over western and northern New England, with a “marginal” risk east of this region. The outlook is shown below.


Figure 1: SPC Day 3 Convective Outlook, issued 0730Z on 6/21/15.

The first Day 2 convective outlook issued by the SPC for the June 23 was very similar to the Day 3 outlook posted above. Uncertainty existed with regard to the timing of the front, as well as the potential for early morning convection to keep the atmosphere stable into the afternoon, preventing additional convection. Model forecasts were consistent, however, displaying impressive shear to near 50kts, good helicity values, steep lapse rates of around 7C/KM, and CAPE on the order of 1500-2000J, impressive values for New England standards. In addition, these parameters were not reserved for only western and northern New England. They extended further east, and as a result, substantial changes were noted in the second Day 2 outlook issued by the SPC. An “enhanced” risk was added for portions of central and western New England, with adjacent areas included in the “slight” risk category. This outlook is shown below.


Figure 2: SPC Day 2 Convective Outlook, issued 1730z on 6/22/15.

Shear and instability parameters remained consistent on the models into the evening of the June 22, and the Taunton, MA NWS forecasts office had noted their concern in several forecast discussions leading up to the event. The potential for wind damage as a primary threat but also isolated tornadoes was well advertised. At least some supercellular storms were forecast to develop. An excerpt from the NWS Taunton office discussion on the morning of the 23rd is given below.

“THE HIGHEST RISK WILL BE NEAR AND NORTHWEST OF A BOSTON TO
PROVIDENCE CORRIDOR...BUT WE CAN/T RULE OUT ACTIVITY MAKING IT ALL
THE WAY TO THE CAPE COD CANAL GIVEN STRONG FORCING. IN FACT...A LOT
OF THE LATEST GUIDANCE SHOWS QPF ACROSS THE CAPE AND THE ISLANDS
BY EARLY THIS EVENING. MAIN THREATS FROM THESE STORMS WILL BE
STRONG DAMAGING WIND GUSTS...LARGE HAIL...HEAVY RAINFALL...AND AN
ISOLATED TORNADO OR TWO. ALSO CANNOT UNDERPLAY THE LIGHTNING
RISK. PWAT VALUES DO CLIMB UP TO 2 INCHES WITH K VALUES ABOVE
40...SO ALTHOUGH THESE STORMS WILL BE MOVING QUICKLY...THIS COULD
BE A SCENARIO WHERE LOCATIONS COULD GET A QUICK INCH OF RAIN
WITHIN 20 MINS.”

There was one primary limiting factor identified at least 48 hours in advance of the event. This was the potential for early morning convection or cloud debris to prevent destabilization of the atmosphere into the afternoon. Several mesoscale convective systems (MCS) had developed in the previous few days in association with the frontal system being discussed. Models indicated one of these MCS’s would develop over the Great Lakes region on the evening of the 22nd, and these projections showed leftover clouds and showers from this complex moving through New England early on the 23rd. Northern New England in particular was forecast to receive considerable cloud and shower activity from the warm front, and indeed this greatly limited severe weather potential in this region, although a couple severe thunderstorms did develop in this region in the afternoon. Southern New England was a larger question mark, however, with models differing on how much cloud and shower activity would impact the region on the morning of the 23rd, and how long such cloud debris would linger.

The initial SPC Day 1 convective outlook for June 23 is shown below. In comparison with the previous Day 2 outlook given above, note how the enhanced risk is removed from much of central New England due to the stabilizing morning convection. However, also note how it is shifted south and east closer to the coast in southern New England. This fits with the Taunton NWS indicating the greatest threat to be near and northwest of the I95 Boston-Providence corridor. In addition, very noticeable is the southward expansion of the enhanced risk through much of the Mid-Atlantic. A significant severe weather event took place in this region as well, but for the purposes of this report I will keep focused on New England.


Figure 3: SPC Day 1 Convective Outlook, issued 0600z on 6/23/15.

As June 23 dawned, all expected parameters continued to remain in place. As expected, the key on the morning of the 23rd was the remains of a once impressive MCS that had tracked through the Great Lakes region and into western NY/PA by the predawn hours of 6/23. The day began partly to mostly cloudy across southern New England. The lack of overcast may have suggested that the MCS had almost completely fallen apart before reaching the region, but in fact this was not the case. The MCS was actually a bit slower and stronger than most of the models had indicated. As late as around 9-10AM, severe thunderstorm warnings were being issued in central-eastern New York on what remained of the MCS. In addition, the SPC issued a severe thunderstorm watch just after 10AM for nearly all of southern New England (excluding Cape Cod) to account for the potential lingering threat of this MCS, as well as the potential for new severe storms to form. This watch was only valid until 4PM, however, and it was thought by most forecasters that additional watches would be required later in the day, which they in fact were.

As the MCS approached, mostly overcast conditions overspread most of southern New England. The primary impact from this MCS occurred between around 11AM-1PM from west to east. The MCS was weakening on approach to the area, and some places received no rain at all, but cloud cover was widespread. The rest of the blog will present numerous captioned radar images spanning the duration of the event.


Figure 4: Boston radar image at 13z (9:00 EDT) on 6/23. Notice the significant amount of non-severe shower and thunderstorm activity over central New England, which limited severe weather in that region. Also, over New York, the aforementioned Great Lakes MCS is beginning to come into view of the radar, still packing quite a punch.


Figure 5: Boston radar image at 14z (10:00 EDT) on 6/23. Notice the fast moving remnant MCS just starting to enter western MA and CT, with convection continuing to the north.


Figure 6: Boston radar image at 15z (11:00 EDT) on 6/23. Notice the continued rapid eastward progression of the gradually weakening remnant MCS.


Figure 7: Boston radar image at 16z (12:00 EDT) on 6/23. The remnant MCS began to weaken more quickly as it spread further east, such that it was only a cluster of showers as it passed Boston, with a heavier storm on the North Shore of MA. I received only a brief shower at my house in Wrentham, MA.


Figure 8: Boston radar image at 17z (1:00 EDT) on 6/23. At this point, the last of the remnant MCS is pushing offshore, with additional weak convection far southwest around New York City. A brief tornado warning was issued for one cell on the North Shore of MA, with no touchdown confirmed. Much of Southern New England is now left with breaking clouds.

As the last of the old MCS pushed offshore, mostly cloudy skies remained in place over Southern New England. Clouds steadily broke up, however, and most places around the region were seeing partly cloudy skies by 2:00 P.M., with a couple more hours of these conditions occurring from then on. The MCS had lingered quite late into the day, but it’s weakening over central and eastern MA meant that the atmosphere was not particularly “turned over” by this morning rainfall as it was in northern New England. Undoubtedly, these clouds did have an impact, and I will go into this a bit more later on. It was clear that the midday clouds and showers would prevent a “worst case scenario.” It had been mentioned by the Taunton NWS office that should the MCS fall apart completely before reaching our area, there would be a chance for a very high impact event, at least by New England standards, and “a significant, long tracked tornado could not be ruled out.”
Despite the clouds, southern New England was not to be spared from severe weather on June 23. High shear and a warm, moist atmosphere remained in place, with temperatures of 85-90F and dew points near 70F, and CAPE steadily built to around 1500J. The forecast scenario was essentially on track. The SPC made generally minor changes to their convective outlook in all updates. The 20z Day 1 outlook is given below, and can be compared against the initial Day 1 outlook in Figure 3.


Figure 9: SPC Day 1 Convective Outlook, issued on 2000z on 6/23/15.


Figure 10: WPC Surface Analysis valid 18z on 6/23. The storm system being discussed can easily be located, with southern New England firmly in the warm sector. The warm front had pushed through in the early morning hours, and the cold front would push through overnight.

By about 3:00 PM, new convective development began in eastern New York and far western New England. Some of these storms would soon go severe.


Figure 11: Boston radar image at 1930z (3:30 EDT) on 6/23. Note the strong convection forming over New York, a few spotty showers forming in western MA, and a couple renegade showers and storms in southern NH, with one perhaps being a “mini-supercell”, although little came of it.

What transpired from here can be looked at in two main events, although there is more to it. One is the wind damage event that took place in Connecticut. The other is the tornadic supercell that formed in northern CT, straddled the RI/MA border, and then moved through eastern MA. I’ll continue to use radar images to playback the storms.


Figure 12: Boston radar image at 2000z (4:00 EDT) on 6/23. Severe convection is seen in southwest CT, and this would continue to produce widespread wind damage as it tracked ENE across the state. Microbursts with winds up to 95mph would eventually be produced. Separately, note the developing storm over northeast CT. While still weak at this point, it was already starting to take on the look of a supercell out in front of the main pack of storms, which are frequently the most dangerous types of storms since they can be first to take advantage of the atmospheric energy available.


Figure 13: Boston radar image at 2030z (4:30 EDT) on 6/23. The damaging storms over CT are the most visually prominent feature, with additional convection over western MA. However, note the previously mentioned storm now over far northeast CT. While quite small, this storm was now a bona fide supercell, with a severe thunderstorm warning issued for it. It is likely that this is about the time when weak rotation began to develop in the mid-levels of the storm, which is common in supercells.

At this point, I am going to start focusing fully on the soon-to-be tornadic supercell now over far northeast CT. The marker on the radar images below represents the location of my house. The storm was generally steady-state from about 4:30-4:55 P.M. After this point, the transformation from “supercell” to “tornadic supercell” began to take place.


Figure 14: Boston radar image at 2058z (4:58 EDT) on 6/23. Note our target supercell beginning to develop a “tail” section. This tail is known as a hook echo, and tornadoes generally form within this rotating hook. At this stage the hook is just beginning to develop and is not yet a threat to produce a tornado. Indeed, most storms never get passed this point.


Figure 15: Boston radar image at 2108z (5:08 EDT) on 6/23. Note the much stronger, more defined hook echo at the back of the storm, indicative of considerable rotation. The storm was not tornado warned at this point, but was now garnering significant interest.

At 5:15 P.M., the National Weather Service in Taunton issued a tornado warning on this supercell, with the warning noting radar indicated rotation that signaled a tornado may be on the ground or could imminently form.


Figure 16: Boston radar image at 2114z (5:14 EDT) on 6/23. This is approximately one minute before the tornado warning was issued. The hook echo on this small but intense supercell was continuing to grow better defined.


Figure 17: Boston radar image at 2127z (5:27 EDT) on 6/23. This is approximately 8 minutes before the estimated time of the tornado touchdown. Note how the hook echo is now truly attaining a hook shape, wrapping around the back of the storm, indicating strong low-level rotation and a likely ongoing or imminent tornado.

In a public information statement confirming and detailing the event, the Taunton NWS office estimated the time of the brief tornado to be 5:35 PM EDT. Radar images were taken at about 5:32 PM and 5:38 PM, so it is quite likely the radar actually missed the peak strength of the storm, given the tornado was likely only on the ground for a couple of minutes.


Figure 18: Boston radar image at ~2138z (5:38 EDT) on 6/23. Note the well-defined hook echo, as the storm was it its most intense at right about this point. Close proximity to the radar slight may slightly distort the image. The tornado was just north of my house. It was likely lifting at the time of this image.

Velocity images can also be used to attempt to track the path of the tornado. Given its weak strength, however, no very clear velocity “couplet” ever developed. In other words, there was no point at which strong winds moving towards and away from the radar met at the same point. A pattern like that usually indicates a tornado, sometimes a significant one. Below, using a storm velocity image from 5:32 PM, I’ve attempted to recreate the approximate path of the tornado.


Figure 19: Boston radar image (velocity) at ~2132z (5:32 EDT) on 6/23. Overlaid in white is my approximate track of the tornado. Towards the left of the image you can see some convergence of the greens and reds on the image. The tornado likely touched down about two or three minutes after this image was taken, and lifted at about the time of the 5:38 EDT image posted above. It tracked on a heading of about 80-85 degrees, about a mile north of my house.

As I mentioned above, the tornado likely lifted around 5:38-5:40 PM EDT. It was probably only on the ground for 5-8 minutes. The tornado warnings for the storm never indicated visual confirmation of the tornado. Instead they continued to base the warnings on radar indicated rotation. Limited if any visual evidence of the funnel exists, but NWS survey teams determined the damage path was consistent with that of a tornado. They estimated it to be a high-end EF0 tornado, with maximum winds of 80mph. It appeared to be entirely confined to the town of Wrentham in Norfolk County, MA. No injuries were reported, and damage was confined to trees only, including a large 80ft maple tree on the town common. An excerpt from the public information statement on the tornado is below.

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE TAUNTON MA
230 PM EDT WED JUN 24 2015

...TORNADO CONFIRMED IN WRENTHAM IN NORFOLK COUNTY MA...

LOCATION...WRENTHAM IN NORFOLK COUNTY MA
DATE...JUNE 23 2015
ESTIMATED TIME...535 PM EDT
MAXIMUM EF-SCALE RATING...EF-0
ESTIMATED MAXIMUM WIND SPEED...80 MPH
MAXIMUM PATH WIDTH...200 YARDS
PATH LENGTH...0.5 MILE
BEGINNING LAT/LON...42.07N / 71.33W
ENDING LAT/LON...42.07N / 71.32W
* FATALITIES...0
* INJURIES...0

* THE INFORMATION IN THIS STATEMENT IS PRELIMINARY AND SUBJECT TO
CHANGE PENDING FINAL REVIEW OF THE EVENT(S) AND PUBLICATION IN
NWS STORM DATA.

...SUMMARY...
THE NATIONAL WEATHER SERVICE IN TAUNTON MA HAS CONFIRMED A
TORNADO IN WRENTHAM IN NORFOLK COUNTY MA ON JUNE 23 2015.

THE TORNADO TOUCHED DOWN JUST EAST OF LAKE ARCHER WHERE TREE TOP DAMAGE
WAS OBSERVED. THE TORNADO CONTINUED INTO THE MIDDLE OF DOWNTOWN
WRENTHAM...CROSSING THE TOWN COMMON WHERE AN 80-FOOT TALL MAPLE
TREE WAS UPROOTED. SEVERAL TREES WERE UPROOTED AND LARGE LIMBS
WERE DOWNED IN THE TOWN CEMETERY EAST OF THE COMMON. THE TORNADO
LIFTED JUST EAST OF THE CEMETERY.

ALL THE DAMAGE THAT WAS OBSERVED WAS ORIENTED TOWARD THE TRACK OF
THE TORNADO...WHICH INDICATES TORNADIC DAMAGE. THE DAMAGE PATH WAS
ONE HALF MILE LONG AND 200 YARDS AT ITS WIDEST POINT.

BASED UPON THE DAMAGE OBSERVED...MAXIMUM WIND SPEEDS WERE
APPROXIMATELY 80 MPH WHICH IS CLASSIFIED AS EF-0 ON THE ENHANCED
FUJITA SCALE.

NO INJURIES WERE REPORTED. MANY PEOPLE INTERVIEWED RECEIVED A
WIRELESS EMERGENCY ALERT ON THEIR CELL PHONES WHEN THE TORNADO
WARNING WAS ISSUED AND TOOK SHELTER.

APPRECIATION IS EXTENDED TO THE WRENTHAM FIRE DEPARTMENT FOR THEIR
ASSISTANCE WITH THIS DAMAGE SURVEY.

After the Wrentham tornado touched down and lifted, the supercell thunderstorm continued to track E/ENE, and remained tornado warned for the duration of its journey to the Atlantic Coast, which took about another hour after the tornado. The storm remained visually impressive on radar for at least another 10-20 minutes after the tornado, with a continued well defined hook echo and radar indicated rotation. It would not produce another tornado, however, and within half an hour the storm’s radar appearance began to degrade, with the hook becoming less defined and core echoes weakening. This may have been due to increasingly stable air as the storm approached the coast or interference from convection to the south of the storm. The supercell moved offshore around 6:30 PM EDT and gradually dissipated from there.

The end of this storm was not the end of the event. Additional convection, much of it severe, redeveloped to the west and northwest, with this convection more directly associated with the cold front. Most of these storms in western MA and CT posed a wind damage threat, and they weakened as they moved ESE and into the area affected by earlier storms, where the atmosphere had been stabilized. One semi-supercellular storm developed northeast of the main line, however. It was given a severe thunderstorm warning, and weak rotation was detected by the radar. While no tornado warning was ever issued, video evidence and a damage survey concluded that a brief EF0 tornado with winds up to 75mph did touch down near Westminster in Worcester Country, MA, causing some tree damage. This tornado was estimated to occur at 7:24 PM EDT.


Figure 20: Boston radar image at ~2325z (7:25 EDT) on 6/23. This is just about the time of the weak tornado touchdown in Worcester County. Note the discrete storm northeast of the main line of convection. A bit of a notch can be seen on the southeast side of this storm, indicative of modest rotation capable of a brief tornado like what was observed. Another discrete severe storm is seen over southern CT, and it would remain severe as it moved east towards Cape Cod, but it never became a tornado threat.

https://www.youtube.com/watch?v=9Y9JQ6VnNik
Link 1: This is a YouTube video of the Westminster tornado.

As the last of the cold frontal convection pushed eastward and weakened in cooler, more stable air, this represented the end of the severe weather threat for southern New England. Skies would begin to clear, and calm, pleasant weather would take hold for Wednesday, 6/24 behind the cold front. In a year that has so far been quiet for southern New England severe weather, this event is one to remember. Lingering questions do remain. For example, just how much of an impact did the morning convection have? I think it was quite large of an impact. We did not have anywhere near full sunlight on 6/23, and because of this the atmosphere simply did not destabilize enough for more widespread severe convection and additional supercells. Some capping may have also caused a lack of storm development near and north of Boston, with lackluster instability unable to overcome this inhibition. Had the morning convection been less of a factor, it is very possible that this could have been a much more serious event, with the potential for multiple tornadoes and perhaps a significant one, as the Taunton NWS had noted in their forecast discussions. All in all, while an event like this is commonplace in central and southern parts of the United States, it is not so here in the Northeast, and watching the evolution of severe weather on 6/23 was an exciting change from the usual summer norm here in southern New England.


Figure 21: Preliminary SPC 13z Day 1 convective outlook verification for 6/23. Note how the event was generally very well forecast. The Wrentham tornado report can be seen, although the Westminster report is not present.


Figure 22: A large tree downed in Wrentham Common by the tornado.

My Experience with the Wrentham Tornado

As I described above, Wrentham, MA is my hometown, so I had a very up-close encounter with the tornadic supercell that impacted the town. Tornadoes are not unheard of in Wrentham. A stronger F1 tornado hit the town in August of 2004. This tornado had a path length of around six miles with winds up to 90mph. I happened to be out of town when this tornado occurred, but given I was only eight years old at the time I don’t have much recollection of the event, although I remember seeing some of the damage a couple days later, which was mostly confined to trees. A report by the Taunton NWS on this storm is linked below.

Link

Tornadoes are definitely not an everyday occurrence up here, however, and the last real memory I have of a tornado threat before last Tuesday was only a few years ago on June 1, 2011. This was a much more dangerous, widespread event, which included the EF3 Springfield, MA tornado that killed three and caused over $100 million in damage. I was still not very experienced with storm tracking at the time, but I remember seeing the classic hook/debris ball radar signature. It was the first time my family ever went to the basement for a tornado threat, though the tornado lifted a good distance to my west and did not touch down again. It is certainly a day I’ll never forget, however.

Like all weather enthusiasts in southern New England, I had been eagerly watching the model forecasts for 6/23, with high-end severe potential apparent a good two-three days in advance of the event. I wanted a storm- it’s been a very quiet severe weather season here so far, although it’s still pretty early in the season. For this reason, I made sure to keep my enthusiasm tempered, for fear of more disappointment. I may have even been overly pessimistic. As the clouds and light showers filled the sky for most of the morning, part of me figured our whole event had been cancelled. The forecaster in me knew otherwise, however, and while I was sure we had averted a worst-case scenario, I knew at least some severe weather threat remained, and I hoped I would be lucky enough to experience a strong storm.

The temperature at my house soared to 88F in the mid-afternoon of 6/23, with a dew point hovering around 70F. It was a pretty oppressive air mass, but one perfect for fueling storms. I wasn’t exactly sure where on the radar to be looking for my best storm chance. At first I thought it would be with the widespread convection developing over western MA and CT. My attention soon turned, however, to what would ultimately be my tornadic storm. It appears this storm first developed as a cluster of light showers over northern CT at around 3:40 PM EDT. It developed rather quickly, and by 4:00 PM it was garnering most of my attention due to its rapid development and heading that appeared to be taking it towards my area. By 4:30 PM, it was the only thing I was watching. While not as impressive as the convection to its west, I knew from its supercellular structure and location out ahead of the main batch of storms that it was likely the day’s best candidate to produce a tornado. In addition, despite its small size by supercell standards, I was quite confident it would directly impact me. The heading of the storm was generally just north of due east, about 85 degrees. As it became severe-warned, my excitement grew, and I watched every new radar frame come in like a hawk. Days like this are few and far between. I don’t mind geeking out and staying glued to the radar for a couple hours straight in a situation like this. Around 5PM, I went under a severe thunderstorm warning, my first of the year. As seen in all those radar frames above, 5:00 PM was just about the time when the tail feature began to develop on the storm. As soon as I saw that develop, I knew it had a chance to go tornadic. As the hook became better defined, that chance began to turn into a likelihood. Shortly after 5:00 is when I first began to feel some genuine nervousness to go along with my excitement. Seeing that hook echo develop reminded me of plenty of situations where a storm can quickly wrap up and drop a rather significant tornado.

The storm’s radar signature continued to improve, and at 5:15 PM, the alert sounded on my phone: tornado warning, until 6:00 PM. I was directly in the middle of the warning box. My mom and two sisters were home at the time, with my dad away on business. We did not go to the basement right away. I had enough knowledge to know we had at least a solid 10 minutes before we would be in the danger zone. Rain turned from steady to torrential and winds began to gust as the storm moved in. I was worried now. The radar velocities had not become overly impressive, but the reflectivity images, particularly the 5:27 PM image posted above, were quite intimidating. Around 5:25 PM, we headed to the basement, where we turned on the news on a downstairs TV, with all stations devoting full coverage to the tornado warning. The rain was prodigious, the lightning near continuous, and the thunder deafening. The wind wasn’t overly bad, although we could hear the gusts. I’d estimate it gusted to around 40mph at my house. There was no hail at my location, although some spotty reports of it were received. It’s possible I may have missed it, as my basement has only a coupled small windows, with bushes mostly obscuring the outside view. The heaviest echoes of the storm went just about right over top of me, but the clouds simply didn’t reach high enough for large hail formation.

We sat and waited out the storm. My concern peaked around 5:30, then began to ease. While I knew a weak tornado was still distinctly possible, it was clear that the storm would not produce a significant tornado by the time it passed me. Just after 5:40, we left the basement. While the warning was still in effect, the storm had clearly passed, and no danger remained. A quick survey of our yard revealed no damage, save for a couple small braches. I was curious as to our rainfall total- my weather station, which I find accurate, recorded 1.01” of rain in the duration of the storm, about 30 minutes time. I had actually expected a bit more, but this was still pretty impressive. It was not until the next day that we learned of the more significant damage at the town common, and at a noontime press conference on the common the NWS team confirmed the tornado touchdown. It made me glad that we had heeded the warnings. The tornado in fact tracked within about a mile of my house, with most of the damage just to my north and west. It could’ve been much worse. And as I thought it over, another idea occurred to me. Had the tornado been stronger, it’s very possible the storm would’ve “right-turned” just enough, as tornado producing supercells often do, to put me right in the most significant damage path. It was a close call, but in the end just a fun story to tell.

The Taunton NWS office did a commendable job with their warning efforts on this storm. Wrentham residents received a full 20 minutes of warning before the tornado touched down. Granted, this same office also failed to issue a tornado warning on the other tornado producing storm mentioned. Still, I think this early warning is a validation of the efforts of the meteorologists at our local office. It also goes to show how effective mobile communication of severe weather alerts has become, with many Wrentham residents noting that they received early warning of the storm via mobile devices, including myself.

Just a couple weeks ago, many of us in the WU community had some fun on the main blog by ranking our five most memorable weather events. I joked that given my young age- I’m only 19- I really didn’t have five events good enough to make a list. I gave a top 4 though, with June 1, 2011 as #1, a 2013 blizzard at #2, Irene at #3, and Sandy at #4, and Sandy is really stretching it. I feel confident I can now add this event to the list. Where exactly does it slot? I’m not quite sure yet. It’s too fresh in my mind to assign an objective ranking at this stage. I’m tempted to bring it straight to #1, but that would be a rash decision. Perhaps in a couple months I’ll be able to look back and see where in compares against the other events.

This blog has been a tremendous pleasure to write. Being in the middle of summer break, I have much more time than I normally would during the school year. The funny thing is, I wrote all this just to describe one little EF0 tornado, as if it’s some kind of remarkable event. It’s not of course, the US is hit by hundreds of tornadoes every year, many of them much stronger and more destructive than the one that hit my town. But that goes to prove how many opportunities there are to conduct research on individual events. If I can write all this on an EF0, what could be said about an EF2+? I just picked this storm because it was so relevant to my life. It was an experience I’ll certainly remember for a very long time. I had been asking for a storm, but I think next time I’ll ask for a storm with the condition that no tornadoes occur in my vicinity.

That just about wraps it up for today. Thank you as always for reading. Feel free to leave comments, questions, etc. below, as well as any mistakes/typos you may notice. I hope to continue writing entries on interesting events off and on for the next couple months before school starts back up. If you’re interested, I also have a new Twitter account, and you can check it out here.

References/Links

Link
The Taunton NWS forecast discussion can be found at this link, as well as an archive of the last 50 discussions, which I used to gather some information.

SPC product archive, used to gather images from past SPC convective outlooks.

Radar images used in this entry were ordered from NOAA’s NCDC.

Link
This is a link to another blog called Woods Hill Weather, a small blog community with entries written by an experienced meteorologist, focusing on southern New England. I post here under the handle “WxWatcher”. If you live in the area, I highly recommend checking it out. The link I provided is to the entry for the morning of 6/23, and I made a few remarks in the comments section.

-MAweatherboy1

Commentary on Blanca

By: MAweatherboy1, 1:54 PM GMT on June 04, 2015

Good morning. Today will be another “case study” blog, this time of the tropical variety. I’m going to take an in-depth look at hurricane Blanca, and its intensity fluctuations over the past couple of days. Before the storm even formed, Blanca had high expectations, with impressive model projections and seemingly perfect atmospheric conditions. Indeed, the disturbance 92E that became Blanca developed quickly, and the cyclone intensified steadily into a hurricane, then very rapidly into a major hurricane, strengthening 55 kts in 24 hours from the 15z advisory Tuesday to the 15z advisory Wednesday, reaching 115kts at that time and strengthening further to 120kts at the next advisory. With nearly zero shear and a moist environment, combined with a spectacular storm structure consisting of excellent poleward and equatorial outflow channels, as well as a well-developed inner core, forecasts showed continued intensification to a Category 5 storm by this afternoon.


Figure 1: RGB loop of Blanca during the final stages of yesterday morning's rapid intensification, approaching peak intensity. Note the clearing eye, large and symmetrical CDO, and spectacular outflow pattern. Thanks to TropicalAnalystwx13 for the saved loop.

Yesterday afternoon, however, Blanca hit a seemingly invisible obstacle. Central convection began to warm steadily, and the eye, which had popped out and warmed significantly, began to cool. The most intense hurricanes have very cold central convection indicative of intense, high-topped thunderstorms, and warm, calm eyes. So why did Blanca weaken under perfect upper level conditions? An eyewall replacement cycle was one possibility, as most intense hurricanes will “replace” their eyewalls one or more times in their life cycle, with a smaller, “old” eye contracting, dissipating, and getting “replaced” by a newer, larger eye. During this process, the storms’ winds can weaken temporarily, only to reintensify and grow in diameter once the cycle completes. This did not happen to Blanca, however, as a microwave image pass mid-late yesterday afternoon indicated only one eye which was still well developed, not a “concentric” eyewall structure which can indicate a pending or ongoing EWRC. It was also theorized that the warming convection could be a diurnal cycle, which has been proven to impact some cyclones. Generally, diurnal fluctuations are more of an issue for weak/developing cyclones, with convection tending to cool in the overnight period and warm during the day. It is possible that a diurnal fluctuation played a role, but I do not believe it to be the primary nor even a significant contributor to what ailed Blanca yesterday and into today.
With EWRCs and diurnal cycles eliminated, the only remaining possibility to explain Blanca’s unexpected weakening is ocean upwelling. Upwelling is a process by which water deeper in the ocean is transported to or near the surface. Strong tropical cyclones tend to cause upwelling due to the turbulent seas they produce, jumbling up the ocean so to speak. In most strong cyclones, this will result in a cold “wake” forming in trail of the cyclone as it moves, cooling the waters it traverses. This usually does not affect the cyclone producing the wake. In some cases, however, it can. If a cyclone is stationary or nearly so, cold, upwelled waters can choke off the storm, weakening it temporarily, or even permanently if it remains stuck for several days, which is rare. Blanca was certainly a stationary storm for much of yesterday, with weak steering currents allowing little movement. Upwelling was not an immediate problem. Initial SSTs in the region were around 30C, which is very warm, and warm waters extended to a depth of around 100m, which is quite deep. This allowed Blanca to rapidly intensify into a Category 4 storm with no difficulty. Eventually, however, time ran out. I believe the cyclone peaked, at least temporarily, around 16-17z yesterday. After that time, the convection began to warm and the eye began to cool. Tropical cyclones generally need SSTs of around 26C to develop, although they can be lower in some cases. For a major hurricane, however, higher SSTs are required to maintain intensity, with 28C being a good estimate for that requirement. Even though it had deep heat reserves under it, Blanca simply stayed in the same place too long for its own good.


Figure 2: Blanca at 1915z yesterday. If you're familiar with these images, you'll notice a lack of very cold convection (dark reds or grays), as well as an eye that is not as warm as would be expected in a storm like this. The effects of upwelling were well underway at this stage.

What happened with Blanca yesterday and into today is unique. Storms weakening due to cool waters is not unusual. It’s quite common in the East Pacific actually, with many storms out there weakening and dissipating due to cool waters and much drier air found further west in that basin. Andres, for example, a once powerful hurricane, is now in the final stage of its life cycle as it completes its decay over cool waters and dry air, as well as some shear. Blanca did not have dry air, however, with relative humidity values in its vicinity in the 80-85% range. And shear was also non-existent, generally 5kts or less. Blanca provided us a very rare opportunity to observe what happens when a virtually perfect cyclone (structure-wise) in perfect atmospheric conditions, is exposed to the single limiting factor of cooler SSTs. In other words, what we saw with Blanca yesterday was the impact of only one variable, since everything else was as close ideal as nature offers.
This morning, Blanca’s convective and core structure continues to deteriorate, although it may be stabilizing now. The eye became completely cloud filled and obscured, and the CDO shrunk and lost symmetry. Through an analysis of several recent microwave passes, I think it is possible Blanca is now beginning an EWRC, but that does not appear to be the primary reason for the collapse of the eye. Blanca continues to have an excellent outflow pattern, which shows that upper level conditions remain very favorable for strengthening. The next 48 hours will be very interesting, in terms of how well, if at all, the cyclone is able to recover from its ailments.


Figure 3: Blanca now. Notice the dramatic degradation of the CDO and eye, indicating a greatly decayed cyclone. However, also note the continued strong outflow pattern, proof of continued excellent upper level conditions.

A final point of interest with regards to Blanca is model intensity forecasts of the storm and how those forecasts relate to SSTs. Significant discussion has taken place in regards to the advantages and disadvantages of “coupled” vs. “non-coupled” forecasts. By this, it is meant that the major global models such as the GFS and ECMWF do not take changing SSTs into account in their forecasts, which for a storm like Blanca leads to a bogus forecasts, since SSTs under the storm are dramatically lower than what these models are simulating. This is leading to erroneously high intensity forecasts, particularly by the GFS. On the other side, you have hurricane specific models, the GFDL and HWRF, which are “coupled”, meaning they do take changing SSTs into account. This has led to those models correctly forecasting Blanca to weaken. However, these models are not without significant shortfalls. Neither model predicted the rapid intensification of Blanca yesterday, presumably due to their SST parameters dropping too much, too fast, for what really happened. The HWRF showed SSTs under Blanca tumbling from 30C to near 15C, which is unrealistic, leading the model (and the GFDL) to weaken Blanca too much. So in other words, no lone model did a great job with Blanca’s full strengthening and weakening cycle. Rather, a compromise of solutions would have provided the best forecast. It will be very interesting to observe just how low the SSTs get once Blanca finally moves, to try to determine just how accurate the HWRF and GFDL SST grids are for a major upwelling event like this.


Figure 4: 6z HWRF SST grid. Notice the extreme, likely well overdone, area of cooling under Blanca.

That’s it for today, thank you as always for reading. Cases like this are very interesting to me, and there are many avenues of research that could be conducted on this storm. I look forward to seeing additional, professional analysis of Blanca’s life cycle once that cycle is complete. Have a great rest of your week and weekend!
-MAweatherboy1


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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About MAweatherboy1

Average 20 year old weather nerd. Plymouth State University Meteorology, Class of 2018. NOAA Hollings scholar. Summer 2016 intern at NWS Boston.