Scientific Forecaster Discussion
fxus61 kphi 072334
Area forecast discussion
National Weather Service Mount Holly New Jersey
634 PM EST Sat Dec 7 2019
high pressure will continue settling into the region tonight then
moves offshore on Sunday. A warm front will lift north across the
area on Monday ahead of the next low pressure system impacting the
region through mid-week. A cold front will pass through late Tuesday
then strong high pressure will build into the northeastern US
through Thursday. Low pressure will return into next weekend.
Near term /until 6 am Sunday morning/...
630 PM update: temperatures are a challenge early this evening,
as the urban heat island is keeping things warmer than forecast
in the philly Metro while the Rural/Valley locations are tanking
quickly. Suspect there will be a few hours in which temperatures
are much more variable than the grids depict, but I did make an
effort to simulate this variability a bit more.
High pressure will be directly across the area tonight. Clear
skies and cold temperatures are expected. Lows will be close to
10 across the far north and in the teens and low 20s elsewhere.
Reduced the temps across the north sharply from earlier with the
snow on the ground and near ideal radiational conditions
expected. Readings for Metro Philadelphia will be in the mid
20s. Winds will be light and variable.
Short term /6 am Sunday morning through 6 PM Sunday/...
the center of the high will be offshore by morning but it will
remain in control of the weather thru the day. Sunny or mostly
sunny skies are expected through the day. After a very cold
morning, temperatures will rebound back to near normal readings
by afternoon. Highs will range from the upper 30s/low 40s north/west to
near 50 S/east and Metro Philadelphia. Winds will increase from
the southeast to S at 5 to 10 mph by early afternoon then switch around
to SW late.
Long term /Sunday night through Saturday/...
a busy start to the extended forecast period as a strong surface low
moves out of the plains and into the Great Lakes region. The first
affects of this system will be the arrival of a warm front Sunday
night from south to north. An elongated vorticity impulse along the
front will likely drive an area of moderate rain northward along the
coast Sunday night. Several vorticity impulses will move from
southeast to northeast through the region Monday through early
Wednesday, which will translate into several periods of enhanced
lift and heavier rounds of rainfall. Forecast soundings suggest
precip could begin as a mix of rain and snow across the higher
elevations of the southern Poconos and across Sussex County, New Jersey-
maybe even into the Lehigh Valley as well into early Monday morning.
No accumulations are expected though, and rising temperatures into
the 50s Monday will lead to quickened melting of any snow.
Heading into Tuesday, a cold front is expected to move across the
area into the evening. Rainfall rates will likely increase as the
precipitation shield expands across central PA along the front. Precipitable water
values will likely exceed 1.00 inches and approach 1.25-1.50 inches
Monday-Tuesday, so there could be period of moderate to heavy
rainfall at times. With the first round of rainfall Monday-Monday
night, we could get around 1.00-1.25 inches, and with the second
round of rainfall, we could get 0.50-0.75 inches. There is a chance
for some poor drainage/nuisance type flooding in areas of heavier
rainfall during Monday-Tuesday. The longer duration of the rain
should preclude any major flood concerns beyond this, however. Storm
total rainfall generally ranges from 1-2 inches.
The front may stall to an extent as it moves offshore. The ec is
more centralized in this Camp, forecasting one additional impulse to
ride along the front Wednesday morning, which would linger rainfall.
The GFS still under-rides the cold air advection behind the front, suggesting
a more substantial sneaker snow event may unfold early Wednesday
morning. Am not currently buying this completely, contrary to
many of our wishes. Climatologically, it is tough to keep the
moisture around behind the fropa in these types of systems. This
may very well follow a similar pattern to the storm system
before Thanksgiving with a quick hour of snow showers with
little to now accumulation. There is also the chance a few
isolated lake-effect streamers may make their way into the
Poconos in the afternoon, but chances of this are not very high
attm. Highs will be much cooler in the 30s and 40s Wednesday
compared to the 50s and 60s on Monday and Tuesday.
On Thursday, strong 1040-1045 mb high pressure will be moving from
the Great Lakes region toward the northeastern Continental U.S. With strong
zonal flow aloft. Cold air advection will be underway early, then
the north to northwesterly winds will decrease throughout the day.
High temps will struggle to climb into the 30s area wide. Otherwise
mainly clear and dry conditions are expected with surface dewpoints
likely in the single digits. Temps will fall quickly Thursday
evening, but lows will depend on the proximity of the center of the
high across New England. If it's farther away, winds will stay more
elevated and temps would not be as cold. Nevertheless, temps will be
on the order of 10 degrees below normal.
On Friday, the cold and dry airmass will be recovering with cold
advection shut off, but temps will likely top out a few degrees
below normal. The GFS is aggressive in amplifying a southern stream
wave ejecting from the southwestern Continental U.S. Arriving along the East
Coast Friday, however it certainly appears to be an outlier
compared to other deterministic medium range guidance; I have
discounted its validity and left sub-mentionable pops for this
period. Clouds will be on the increase throughout the day though
with longwave upper troughing developing across the central US.
The next big rain-making system will impact the region starting
early Saturday as the aforementioned upper troughing propagates
closer to the East Coast. Depending on how persistent the cold
surface air remains entrenched, could certainly a period of wintry
mix at the precip onset early Saturday, but confidence on this is
low. Medium range guidance differs quite a bit on the exact timing
of the precipitation, but precipitation does appear likely.
Aviation /00z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Tonight...VFR with light/variable winds. High confidence.
Sunday...VFR with light/variable winds in the morning becoming
southerly up to 10 kt during the afternoon. High confidence.
Sunday night...conditions may lower to MVFR later in the overnight
as a warm front lifts through the region. South-southeasterly winds
from 5 to 10 knots. Moderate confidence.
Monday-Tuesday...an extended period of MVFR and IFR conditions
likely as periods of rain move across the area. Southeast winds
early Monday become southwest once a warm front lifts across the
area Monday. Winds from 5 to 10 knots shift to the northwest behind
a cold front later Tuesday into Tuesday night with a few gusts to
15 knots possible as this occurs. Moderate confidence.
Tuesday night-Wednesday...rain or snow remains possible with MVFR or
IFR conditions possible. IFR conditions to improve to VFR Wednesday
afternoon. West to northwest winds become gusty to 20-25 knots on
Wednesday. Westerly winds should drop below 15 knots Wednesday
night. Moderate confidence.
high pressure across the waters this evening will move to the east
overnight. A return flow will develop behind the high on Sunday.
Sub-Small Craft Advisory conditions are expected. Fair weather. Seas 2 to 3 ft
tonight and Sunday. On Delaware Bay waves 1 to 2 ft at most.
Small Craft Advisory conditions will likely be developing
throughout the day Monday with seas building up to 9 feet Monday
night and winds gusting up to 30 kts on ocean waters. These
conditions will persist through Tuesday before gradually
diminishing Tuesday night.
On Wednesday, offshore winds will be in the 20-25 kts range with
seas 4-5 feet. Small Craft Advisory conditions probable.