Winter Storm in Alaska, March 20 to 21, 2019

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 AM AKDT Wed Mar 20 2019

.ANALYSIS AND UPPER LEVELS...
At the surface, there is an area of high pressure over the
interior and a cluster of surface lows south of the chain. There
is an inverted trough extending into the Prince William Sound.
Yesterday afternoon, numerous locations were in the 40s in the
Anchorage Bowl and Mat-Su valley, while Klawock reached 70F.
While there are several systems looming near or over the southern
tier of the state, without a doubt the above normal temperatures
cannot be ignored. Essentially, spring is coming early to portions
of Southcentral Alaska. Southwest Alaska the temperatures have
been running much colder and that region has a storm closing in on
them today. Aloft, the tail end of one jet is over eastern AK and
there is another jet core is moving into the Gulf Of Alaska.
Looking at a mosaic of ASCAT/advanced scatterometer data, gale
force winds were detected this early morning south of the
Central/Eastern Aleutians and near Kamchatka.

&&

.MODEL DISCUSSION...
The global models are in good agreement with the synoptic scale
features through 00z Friday. Again, the tight clustering of the
GFS ensemble members is impressive with the high over Siberia, the
low near Hokkaido, Japan and the main low south of the Eastern
Aleutians/AKPEN. There are some differences with the timing of the
precipitation shield and with the thermal profiles in the boundary
layer especially over the southern tier of Alaska. Past 48 hrs,
the NAM and the GFS really start to diverge and the Canadian and
the ECMWF are most similar. And at this point the Canadian and the
ECMWF are the most plausible.

&&

.AVIATION...
PANC...VFR conditions will persist. However, the winds out of
Turnagain Arm will pick up later today and gusty winds will
materialize at PANC.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days today through
Thursday night)...

A return to warm, locally windy, and wet conditions is expected
for Southcentral as a series of weather fronts move north across
the Gulf over the next couple of days.

The first of the fronts, currently over the southern Gulf, heads
north to along the North Gulf Coast this afternoon then weakens
as it heads inland tonight. The brunt of the precipitation,
generally in the form of rain below 2000 ft, will fall along the
coastal areas as strong east-southeast cross-barrier flow limits
moisture intrusion to the lee of the coastal mountains. Gale force
winds are expected in advance of this front along with strong gap
flows. The strongest of these gap winds will likely be from
Portage Valley to along Turnagain Arm. The Matanuska, Knik Valley,
and Copper River winds are also expected, but should not be as
strong.

The next front to follow will be stronger and should produce gales
to storm force winds over northern Gulf marine zones as it heads
north Thursday. Strong gap flows are still expected, except for
along the Copper River which should see diminishing winds
throughout the day. The precipitation pattern and type should be
about the same, as well as, the unseasonably warm atmosphere.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Brisk northeasterly, offshore winds will continue to increase
today as the pressure gradient between a northward retreating
ridge and a surface low moving northwest from the western Gulf
tightens. Attached to this developing low is a westward rotating
frontal wave that will pinwheel over the AKPEN and across Bristol
Bay this morning. This feature will then lift northwest over the
Kuskokwim Valley later this afternoon and over the Kuskokwim Delta
this evening before stalling along the coast for Thursday.

Snow will develop out ahead of the front this morning and lift
north across Southwest Alaska today. This, in combination with the
strong northeasterly winds, will produce areas of blowing from
west of the Nushagak River to the Kuskokwim Delta. As the front
edges north, blowing snow will become less of a concern as
temperatures rise. For areas south of the Kuskokwim Delta, snow is
expected to change to a rain/snow mix for areas around the
Nushagak Hills and Kilbuck Mountains. Farther south, and along
the coast, a quick changeover to rain is expected with
temperatures climbing above freezing. Snow and blowing snow will
persist into Thursday for areas east of Bethel as the front stalls
and the colder air hangs on.

A second developing low will then lift north late Thursday into
Friday on the heels of the first with another frontal system
sweeping over the region from southeast to northwest. With little
if any cold air remaining at the surface behind the first system,
and this next frontal wave advecting even warmer air aloft, this
event will be primarily a rain event. The exception to this will
be the KilBuck Mountains and perhaps the immediate Kuskokwim Delta
coast where temperatures will remain a tad colder. Much like the
previous frontal wave, strong northeasterly wind will kick up
across the southwest mainland.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

Cold, northerly flow will continue today as an upper-level trough
persists across the western and central Bering. A vertically
stacked low south of the eastern Aleutians will remain stationary
through the first half of today before it becomes absorbed into a
developing North Pacific Low as it lifts northwest from the
western Gulf. As this system lifts north, strong northerly gales
will develop across the central Bering along the back side of the
system Thursday and Friday. Widespread snow showers are also
likely, with periods of blowing snow for the Pribilofs late
Thursday and Friday. Farther west, a Kamchatka Low will send a
weak frontal system to the Western Aleutians where it will stall
and then gradually shear apart through Thursday.

&&

.MARINE (Days 3 through 5/Friday through Sunday)...

We are still expecting a wide swath of northerly gale force winds
in the Bering through Saturday. Winds should taper on Sunday and
switch to a more southerly direction. Meanwhile in the Gulf of
Alaska, southeast winds will continue as they circulate around a
low in southwest of Kodiak. Winds should start out as gale force
on Friday and taper off Sunday. Confidence is high in forecasts
for both areas as models seems to be doing a good job of picking
up this pattern.

&&

.LONG TERM FORECAST (Days 3 through 7/Friday through Tuesday)...
Another round of warm and wet weather is in store for southcentral
AK to start the long term. This will be due to another low
pressure system tracking north into the Gulf of Alaska, with the
continued high pressure ridge to the east over Canada sending low
pressure in that direction. A brief break will be in store on
Saturday with another low moving into the Gulf again on Sunday.
The Sunday low should finally be the last of this pattern, as it
appears high pressure moves into the Gulf of Alaska early next
week. Model agreement is good enough in the long term that we are
confident in the overall pattern. However, as with most weather
systems as of late, we will have to wait until the short term to
be able to figure out your local forecast.

The Bering and Aleutians will start the long term with continued
northerly flow, keeping temperatures on the cool side with some
snow showers. Another large low pressure system looks to move
into the western Aleutians/Bering early next week, transitioning
the region to a more southwesterly pattern. Model confidence is
pretty good in the long term, so there is a high level of
confidence this forecast will generally hold true.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155 161.
MARINE...Gale 174 176 179 180 181 185 139 138 137 136 132 131 130
125 120 414 411 412 119 351 352. Heavy Freezing Spray 180 181
185.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...BB