Gale Warning 180, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 PM AKST Wed Feb 20 2019

.ANALYSIS AND UPPER LEVELS...
Beautiful satellite imagery this morning showing a textbook low
pressure system moving northward through the state. The low center
is well stacked from the surface through the upper levels and is
located just to the south of St Lawrence Island and drifting
north. Over the mainland the main jet is shifting eastward into
the ridge and separating the baroclinic leaf from the occluded
front moving through western Alaska. A wave developing along the
jet is currently moving toward the AK Pen. This wave, along with
the area of colder cloud tops in the southern Bering indicates
instability moving into the base of the trough, which should
continue development of a secondary low in the Gulf.

Close to the surface, the precipitation has tapered off along the
Kuskokwim Delta, but winds are still gusty along the coast, which
is closest to the retreating low. In Bristol Bay, precipitation
continues as the area of instability moves overhead, bringing
showers. In southcentral, radar is showing evidence of
precipitation moving up the inlet, however at this point it seem
to be mostly virga, since observations at the surface are not
reflecting any falling precipitation.

&&

.MODEL DISCUSSION...
Models are in good agreement today, although solutions have
changed slightly from previous runs. In the east, the movement of
the low into Prince William Sound has slowed slightly, and the QPF
amounts have backed off, leading to a slightly delayed and drier
forecast than yesterday. West of the Alaska Range, observations
show an increase in winds from what models were indicating,
meaning they might be slightly underdoing the strength of the low
in the Bering. That being said, overall they are tracking well.

&&

.AVIATION...

PANC...Radar is showing returns over the Anchorage bowl as of 21Z
so it looks like we're on track for the snow onset. Heaviest snow
amounts are still expected to be overnight, but it should snow
pretty continuously through tomorrow morning. As with any snow, it
won't take much to lower cigs and vis and even when it's not
snowing heavily MVFR vis is the best we'll expect. Conditions
should clear rapidly tomorrow and be clear for the rest of the TAF
period, with the risk of fog as we move into the weekend.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
(Tonight through Friday night)

Quite the interesting forecast continues to play out, with many
challenges ahead. The first obvious one is with regard to snowfall
totals. Snow has been slower to increase in coverage today, and
this is mainly due to a few things. First, the original strong
tropical feed that we we're expecting to encompass the region by
now was "cut off" by a tropical low west northwest of Hawaii.
Secondly, the latest model trends continued to show the mid-level
disturbance talked about the last few discussions, showing a
further south track (about 50 miles further south). Third, SE
winds off the Turnagain Arm area have proven stronger than
earlier anticipated and taken a bite out of precipitation amounts.

That said, a secondary surge of moisture associated with another
low to the north of the Hawaiian low has developed, and the latest
CIMMS MIMIC Layer Precipitable Water is showing a renewed surge
northward, which has been marching north through the day. While
the brunt of this moisture will be shunted eastward, the western
flank of said surge will encompass the Kenai and Anchorage Bowl
for a few hours if this trend continues, with the heaviest
snowfall likely to occur between 10 PM and 3 AM overnight.

After that, the main story looks to be strong winds through
Thompson Pass and Keystone Canyon, as a surface low develops in
Prince William Sound tomorrow before heading into the eastern
Gulf. A very strong pressure gradient will lead to strong winds
developing for the Copper River Basin. Given a very good dendrite
growth layer and high snow to liquid ratios, this "fluffy" snow
should have no problem being transportable for blowing snow and
whiteout conditions to develop here. As such, a Winter Storm
Watch has been issued for potential blizzard conditions.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...A shortwave
approaching Bristol Bay will bring snow to the Alaska Range
tonight, with generally up to an inch in lower lying areas of
Bristol Bay and along the AKPEN. That said, due to the convective
nature of the precipitation, locally higher amounts are possible.
A weakening low approaching St. Lawrence island will continue to
maintain south to southwest flow over the Kuskokwim Delta tonight,
and cold air wrapping around the south side of the low will
continue to support showers over the Kilbuck and Ahklun
mountains. A brief ridge will build over the central Bering
Thursday and precipitation will diminish.

A storm force low developing southwest of the western Aleutians
will become the next focus in the short term. A secondary low will
develop over the northern Pacific, supporting a long moisture
fetch from the south. Precipitation is possible over the Kuskokwim
Delta/ lower Kuskokwim Valley Friday, with precipitation types
initially snow. The Kuskokwim Delta coast could see blowing snow
early Friday. Persistent south flow will bring temperatures above
freezing by Saturday but precipitation will have diminished.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...A
weakening gale force low is shifting north towards St. Lawrence
Island, leaving much of the Bering in broad west flow with areas
of gales. Cold air originating from an Arctic low over eastern
Siberia will continue wrapping around the south side of the low,
supporting showers over the Bering including the Pribilofs.

A storm force low located roughly 400 miles southwest of Attu will
be the next focus in the short term. The low will push northward
tonight, supporting widespread gales with areas of storm force
winds in the central and western Bering and Aleutians into
Thursday. The low will continue shifting northward Thursday into
Friday, and gales will spread eastward toward the Pribilofs
Thursday night. As the low shifts northward Thursday, a secondary
low will develop along the associated cold front in the north
Pacific. Ample warm air advection ahead of the front front will
maintain precipitation over the western and central
Aleutians/Bering with southerly gale or near gale-force winds
expected through Friday.

&&

.MARINE (Days 3 through 5)...
Thursday through Saturday continue to look windy across the
Bering Sea as a strong low moves up the Kamchatka Peninsula. Winds
ahead of the low in the central Bering are still expected to be
around gale force, but the forecast winds around the low center
have increased in strength and storm force winds are expected for
the Bering Sea and just north of the western Aleutians through
Friday. Saturday appears to be a bit of a break as most of the
energy will go into a low moving up into the central Aleutians,
but gales can be expected to persist across the central Bering.

In the Gulf of Alaska, a low moving into the Prince William Sound
and sliding off to the east on Thursday will increase
northwesterly winds through the gaps. These are expected to be
gale force through the Barren Islands and into the Gulf of Alaska
but higher gusts are possible especially along the coast in the
gap areas.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast continues to be on track from yesterday,
with a ridge of high pressure building in over mainland Alaska,
pushing the main storm track westward over the Bering Sea. Models
are trending towards a stronger high pressure system, which means
that it looks less and less likely for precip to make its way
inland from any lingering fronts over the weekend. This high is an
unusually warm high pressure system for this time of year,
looking more like a spring time high pressure system than the cold
winter highs we see in late January. Solar heating is still
marginal, so spring is not here yet...but temperature forecasts
for the weekend and early next week across the mainland look to be
a bit of a challenge.

As for the Bering Sea, the usual active storm track of the winter
will continue. The storms moving through aren't expected to be
unusually strong, but the amount of warm air aloft will bring rain
much further north than we normally expect at this time of year.
Warm and wet conditions are expected to continue for the eastern
half of the Bering through early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 131
		 Winter Weather Advisory 101 111 141 121
MARINE...Storm Warning 177 178
         Gale Warning 411-414, 150 170 173-176 179 181 185 119-125 127
		              129-131, 351-352

&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...SA

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1032 PM AKST WED FEB 20 2019

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0900 PM     SNOW             GIRDWOOD                60.94N 149.11W
02/20/2019  M6.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            SNOW STILL FALLING.

&&