Scientific Forecaster Discussion
fxak68 pafc 191434
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
534 am akst Tue Nov 19 2019
Analysis and upper levels...
A low that is rapidly developing right now over the eastern
Bering will become the cause of widespread winter weather across
much of southern Alaska. Currently the radar is quickly filling
in along the Bristol Bay coast. Snow is being reported near
Dillingham, with King Salmon and points south at plain rain. So
far most of the radar echoes over southcentral are not being
recognized as precipitation at the ground. Thus, the snow aloft
is evaporating as it hits drier air underneath it, which is also
known as virga. Temperatures are expectedly rapidly rising where
winds have turned out of the east, namely from Iliamna south
through King Salmon and Port Heiden, the latter of which is
sitting at 50 degrees this morning. The winds are still northerly
around Dillingham through New Stuyahok, but even there
temperatures are already rapidly rising through the 20s.
The upper level pattern is highly amplified, but simple to
interpret. Ridging is building over southcentral and eastern
Alaska, while an upper level low is near Atka. Between these
features is a highly energetic upper level jet, which is strongly
supporting the rapid deepening of the surface low over the
eastern Bering. Warm, moisture-laden air associated with the
ridging is streaming northward from off the Gulf, while the next
polar outbreak is developing southward across the northern Bering.
There are a tremendous number of factors playing into this
evolving weather scenario that are adding complexity to the
forecast. Those factors include: the amount of moisture the low
is bringing north from off the Pacific, the potentially heavy
precipitation's effect on precipitation type, strong winds
including expected blizzard conditions in the Kuskokwim Delta,
cold air advection near the surface over southwest Alaska
initially, warm advection in the mid-levels of the atmosphere over
southwest Alaska, the marginally cold (near freezing)
temperatures over much of southcentral that makes the
precipitation type forecast for Anchorage, the western Kenai
Peninsula, and the Matanuska Valley very difficult, the potential
for Turnagain Arm winds to warm temperatures up near southcentral,
a fetch of moisture that will bring heavy snow to portions of the
Susitna Valley, Copper River basin, and Thompson Pass, potential
southeasterly winds through Glennallen that may rapidly raise
temperatures in that area, and large disagreement in the models
about many of those above factors, despite this event unfolding
from now through the next 24 hours.
The large model disagreement therefore is making forecast
confidence in smaller scale details such as precipitation-type
extremely low. However, where there is some agreement, confidence
is high enough to have issued a plethora of warnings and
advisories across the large majority of the southern Alaska area.
Based on developments across southcentral from the 12z soundings,
the namnest seems to have the best handle on current conditions as
compared with the NAM/GFS.
panc...moisture associated with the next round of precipitation is
moving in from the Gulf. For the next few hours, any precipitation
will evaporate before reaching the ground. All precipitation today
will be light, and only MVFR ceilings and visibilities are
expected at any time today. Precipitation type is the biggest
challenge, as a warming atmosphere will cause precipitation to
start out as snow early this morning, before transitioning to a
brief period of freezing rain and then plain rain by this
afternoon. The taf may be overly pessimistic on how long any
freezing rain will last, with temperatures starting out this
morning so close to freezing already. Winds will also be a concern
through the taf period, as the Turnagain Arm wind is likely to
make an appearance late this morning in the form of wind shear,
then reaching the surface by this afternoon. This will send
temperatures into the 40s. Scattered off-and-on rain shower
activity will continue through the night with the Turnagain Arm
wind and associated very warm, above freezing temperatures
persisting through Wednesday.
Short term forecast southcentral Alaska (days 1 and 2/today
through Wednesday night)...
a potent storm system will affect southcentral Alaska over the
next 24 hours or so, leading to widespread impacts to travel.
Advisories and warnings are in effect for much of the region.
The key features include a 150 kt upper level jet and atmospheric
river which will be pointed directly at southcentral Alaska this
afternoon through tonight. At the surface, a warm front will lift
northward across southcentral this morning, followed by an
occluded front tonight.
The air mass in place this morning is cold enough for all snow,
but much warmer air will advect northward into southcentral
throughout the day. Models continue to vary widely on the amount
of warming and northward extent of warming. The current forecast
represents a middle ground solution. For interior areas, including
the northern Susitna Valley and northern Copper River basin,
expect precipitation to remain all snow. For areas farther south,
from the western Kenai to Anchorage, the Matanuska Valley, and
the southern Susitna Valley, expect a transition from snow to
freezing rain and then to plain rain throughout the day. As the
front passes by Anchorage and the Matanuska Valley late this
afternoon into evening, gusty southeasterly winds will cause
temperatures to jump to well above freezing for most of the area.
Strong southerly low level flow will enhance what will already
be impressive precipitation rates along south facing mountains.
Thus, expect heavy snow focused over the northern Susitna Valley,
along the Alaska Range, and for Thompson Pass and vicinity. Heavy
rain can be expected along the Gulf Coast. Southeasterly flow
across the Kenai and Chugach Mountains will keep precipitation
lighter, though it doesn't take much freezing rain to cause
impacts on area roads.
One of the biggest areas of uncertainty is the wind and temperature
forecast for the Glennallen area. If gusty southeast winds
develop, temperatures will likely rise above freezing and any
precipitation which falls will be primarily in the form of rain.
If the cold air holds in and keeps the winds at Bay, then
precipitation will fall as snow.
The upper jet and front will exit eastward tonight. However, some
models indicate a surface wave riding up the front and pushing it
back westward on Wednesday. This would bring more heavy
precipitation to portions of the Copper River basin as well as the
Valdez and Cordova area. Meanwhile, a series of upper waves will
move in from the Bering Sea, maintaining gusty south to southeast
winds and an overall wet pattern as we head through Wednesday
Short term forecast southwest Alaska (days 1 through 3:tuesday
Low pressure is currently moving through Bristol Bay and entering
Mainland southwest Alaska. All of our weather hazard products have
gone active, so we are watching closely to see what happens.
Currently, bands of precipitation are moving ashore along the
northern Bristol Bay coast. Temperatures are also warming quickly
with the southerly push of warm air. Dillingham has risen 18
degrees overnight and is rapidly rising (at 28f as of 4am) and
will be above freezing soon. Precipitation is moving rapidly and
we expect the main push of weather pass the coast by 9am.
Further inland and farther north, we are still waiting for the
weather to arrive. Snow will reach into the interior areas later
this morning, as well as the y-k Delta. Winds are picking up
around the y-k Delta and are gusting 30 mph right now, so we are
on track for blizzard criteria when the snow does arrive. We are
also still anticipating heavy snow for interior areas. Later today
warming temperatures will also reach both regions, making a
rain/snow mix for some locations. The weather setup with this
system does not look favorable for freezing rain inland. This is
because this low is not overrunning cold air -its pushing it out-
meaning the setup needed for freezing rain won't be there.
After this low moves north, winds will die down tomorrow and
showery precipitation will remain in southwest Alaska. Expect a mix of
rain and snow showers, with snow be the more predominant
precipitation type in southwest Alaska.
Short term forecast Bering Sea/Aleutians (days 1 through
3:tuesday through thursday)...
Broad low pressure is moving through the Bering Sea as expected.
Gale force winds are continuing around this low. We are still
expecting gale force winds to continue through tonight and then
taper tomorrow. Wind direction will shift to a northerly direction
behind this low pressure system Thursday, which will cool
temperatures. With low pressure in the area and northerly flow
behind it, a chance of showers will remain in the forecast, with
precipitation becoming snow for much of the Bering with cool air
entering later in the week.
Marine (days 3 through 5)...
Beginning the period early Thursday morning, a weakening storm
force low in the eastern Bering will still be responsible for
gales in the northern Bering as well as coastal waters of the
Alaska Peninsula. There will be isolated areas of storm force
winds as a strengthening frontal wave tracks into the
southwestern Gulf early Thursday morning with widespread gales
across the Gulf of Alaska and coastal waters of southern Alaska.
Expecting storm force winds to impact Prince William Sound and
offshore waters of the southwestern Gulf Thursday afternoon
through Friday morning. Friday evening expect northerly gales
from the Bering Strait into the southern Bering as cold air pushes
south from Siberia. An area of gales will move across the
southern Gulf through Saturday evening while high pressure builds
into the central Bering. The next storm appears to be developing
off the Kamchatka peninsula Saturday evening and looks to be on
track to arrive in the western Aleutians sometime Sunday morning.
Long term forecast (days 3 through 7)...
By Thursday, a deep upper level trough containing numerous
smaller troughs and surface lows will stretch from the Arctic
Ocean, south along to West Coast of Mainland Alaska, across the
Bering and Aleutians. A very strong west southwest to east
northeasterly oriented jet stream will stretch from south of the
Aleutians into the southern Gulf will provide good upper level
support for developing lows along its north side.
A vertically stacked low centered over Bristol Bay Thursday
morning will swing inland Thursday afternoon and evening with the
portion of the associated frontal system over southwest Alaska
lifting north. The portion of the front stretching across the
Kenai Peninsula and into the western Gulf will swing northeast to
reach the northern Gulf Coast by Thursday afternoon and evening
while a second frontal wave develops into a closed surface low
east of Kodiak Island. Another frontal system stretching from a
surface low tracking to the east through the central Aleutians to
a second low south of the eastern Aleutians will continue east to
merge into a single low near the eastern Aleutians Thursday
evening with an upper level trough and frontal system stretching
east then south of the western Alaska Peninsula.
The very active storm track will continue Friday through the
weekend. The surface low developing off Kodiak Island Thursday
afternoon will continue to strengthen as it tracks northeast just
offshore of the Kenai Peninsula coast Thursday night to reach
Prince William Sound Friday morning. The next incoming frontal
system will track to the east just south of the Alaska Peninsula
late Thursday night through Friday morning then move quickly
northeast across the Gulf through the day Friday. Another strong
frontal wave slightly further to the south will follow Friday
through Saturday. Colder upper level troughing will dig in over
the eastern Bering and Bristol Bay area Friday and Saturday.
Upper level ridging building over the western Bering and Aleutians
on Saturday will shift over the eastern Bering on Sunday as a
strong frontal system pushes into the western Aleutians. The
ridge will push east to reach the eastern Bering on Monday with
the front swinging into central Bering. The front will reach the
central Aleutians Monday morning and eastern Aleutians Monday
afternoon and evening. The cold upper level trough will become
well establish over Mainland Alaska Sunday through Monday with the
Bering frontal system slowing Monday night as it approaches the
southwest Alaska coast then shearing apart with the bulk of the
remaining energy sliding south across the southern Gulf in the
form of another developing low.
public...blizzard warning: 155.
Ice storm warning: 161.
Winter storm warning: 131 141 145.
Winter weather advisory: 101 111 121 152.
Gales: 119 120 125 130-132 138 139 150-170 173-177 180
185 351 352 411-414.
Heavy freezing spray: 181.