Gale Warning 102, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 PM AKST Sat Feb 2 2019

.ANALYSIS AND UPPER LEVELS...
A broad long wave trough over the Bering extends from a
vertically stacked low in the northern Bering. The jet is taking
on a meridional pattern with a 160 kt jet streak at 30,000 ft. The
jet streak over the central and eastern Aleutians is
strengthening the gradient within the flow around the trough in
this area. The weakest part of the jet is a northerly oriented
segment over Southcentral. With enhanced upper level support,
there is strong south-southwesterly flow aloft from the eastern
Bering through the Gulf of Alaska. This southwesterly flow is
advecting warm and moist air aloft. In satellite imagery, this
active area can be identified by a long swath of cloud cover
branching from the western mainland down to the North Pacific.
This is reflected as an occluded front at the surface.

The occluded front is slowly tracking eastward into the western
Gulf bringing increased precipitation and southwesterly winds
along with it. Over the western mainland and AK-Pen, snow has
turned over to rain. Ahead of the front, a ridge over the Gulf
brought clear skies, northerly flow at the surface and cool
temperatures to much of Southcentral over the last few days. As
this front continues to migrate eastward, it is slowly eating away
at this ridge. Additionally, a weak meso- low spinning in the
northern Gulf is bringing light precipitation to the area which
shows in radar.

&&

.MODEL DISCUSSION...
Models are struggling with the overall pattern throughout the
state. While they are consistent with a front moving into
Southcentral, the precipitation type, intensity and location
varies between models and between runs. In the west, guidance is
in decent agreement but again struggling with intensity regarding
the North Pacific low moving over the central Aleutians on Sunday.
As it tracks into the central Bering on Monday afternoon, the EC
moves further west into the western Bering while the NAM, GFS and
Canadian keep the low more centralized. The strength of this low
differs between the models. The GFS, which was once the strongest,
has started to nudge closer to the other solutions.

&&

.AVIATION...
PANC...VFR conditions will persist through tonight. Timing the
onset of the snow is still somewhat uncertain. The wind forecast
will be challenging late in the period. While it looks like north
winds will remain over the terminal, we can't rule out the
possibility of a brief period of Turnagain Arm southeasterlies.
Some light very low-level wind shear will also be possible at this
time, however, it would be marginal at best and was not included.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Monday)...
A big pattern change is in the works across much of Southcentral
ALaska tonight as a front currently moving in from the west and
ending today's sunshine essentially stalls over the area. The
front will act as a conveyor belt or train track for a series of
impulses of moisture to ride into Southcentral from the southwest
and right up Cook Inlet. Thus, with this pattern, the upslope
areas will be the Talkeetna Mountains, with Hatcher Pass a big
bullseye as far as expected snow is concerned. Hatcher Pass is
expected to pick up 2 feet of new snow through Sunday night. A
Winter Weather Advisory is also in effect for the Susitna Valley
with broad new snowfall amounts around a foot expected through
Sunday night.

Further south towards Anchorage and the Matanuska Valley, the
forecast is quite a bit more uncertain due to the expectation of
gusty southeast winds increasing across the Chugach and down
Turnagain Arm and the Knik River Valley through Sunday night. The
models are struggling with whether to bring the Turnagain Arm
wind into South Anchorage or keep it out over Chickaloon Bay.
Since there appears to be nothing that will draw the Turnagain Arm
Wind north into Anchorage, it seems most likely it stays south. A
similar scenario plays out with the Knik River Valley with the
wind likely being weak compared with the Matanuska Valley wind,
which too will take a large bite out of the expected snowfall
totals through that area.

Thus, with the Turnagain Arm wind staying south of Anchorage,
snowfall totals have been increased with a storm total through
Sunday night expected to be in the 2 to 4 inch range. The highest
amounts are expected on the far upper Hillside and over West
Anchorage. Timing-wise, the snow looks to start up late tonight in
Anchorage, and diminishes Sunday afternoon. On the one hand, East
Anchorage may see very little if anything with this first round.
On the other, some areas may see the snow diminish to flurries but
never fully stop. Steadier snow returns with the second system
Sunday night, and ends again Monday morning. All areas should get
an inch or two with the second system as all the flow should be
southwesterly by then with minimal downsloping. Then a third round
starts up Monday night and continues through the day Tuesday. An
important note: Each successive round of precipitation will bring
increasingly warmer air along with it, so the likelihood of rain
increases with each one with the day Tuesday more likely to be
rain than snow in Anchorage.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The southwest to northeast running frontal system currently
stretching from the Alaska Peninsula across southwest Alaska will
continue a bit further east this evening before stalling over the
greater Bristol Bay area and lower Kuskokwim Valley. With the
warm, moist air aloft this front is carrying north, precipitation
has changed over to rain along the coast, however it continues as
snow further inland where cold low level air is better entrenched
in the Kuskokwim Valley. With the front stalled over the area
tonight and a reinforcing frontal wave lifting north along it
early Sunday morning, QPF amounts are expected to be high with
around an inch of rain along the coast in the next 24 hours, 6 to
10 inches of snow inland and a wet mix of rain, snow and possible
light freezing rain in between. Once the frontal wave lifts north
precipitation will taper off from west to east midday Sunday. The
next frontal wave will cross the Alaska Peninsula Sunday afternoon
and continuing northeast across Southwest Alaska Sunday night. A
very brief ridge will build in behind it on Monday morning,
quickly followed by another fairly warm and moist frontal system
Monday afternoon through Monday night. Colder air and an upper
level trough behind this front will push onshore and over
southwest Alaska Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Currently, an elongated upper level low stretches from west of
the western Aleutians, north to eastern Russia with strong
southwesterly flow aloft and an active storm track persisting
across the Aleutians and Bering. A small surface low will swing
past, just to the west of the western Aleutians tonight while a
couple small shortwave troughs embedded in the cold southwesterly
flow aloft track to the northeast across the Bering. A larger
developing surface low in the North Pacific will track north
towards the central Aleutians tonight and Sunday to cross into the
southern Bering Sunday night. The strong low will then become
vertically stacked and continue north into the central Bering
Monday and northern Bering Tuesday. Another North Pacific frontal
system will approach the western Aleutians Monday night and swing
into the Bering Tuesday.

&&

.MARINE (Days 3 through 5)...
The Gales expected in the Eastern Aleutians and eastern Bering Sea
Monday look to be on track and the strongest winds should remain
just below storm force. A sizable area of 20+ ft seas should
accompany the strongest winds in the Bering Sea Monday but then
settle below 20 ft by Monday night.

A second low should also bring some Gales and near-Gale force
winds from the Central Aleutians through Alaska Peninsula and
Bristol Bay Tuesday night into Wednesday. While winds will not be
quite as high with this second low, the persistent southwesterly
fetch will likely bring in 20 to 30 ft seas along the Pacific side
of a large potion of the Aleutian Islands Tuesday into Wednesday.

Confidence in the winds and waves around this second low is above
average as models are in good agreement with the development and
movement of the low that is causing these Gales.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There is not a lot of overall change in thinking for the long
term forecast from Tuesday through next Saturday. The storm track
remains over the Bering Sea to the western Gulf of Alaska which
will keep a warming southwest flow over southwest and
Southcentral Alaska. One notable change in the forecast is that
there is an indication that there will be warmer air moving into
Southcentral Alaska on Tuesday which will push the rain-snow line
farther north at that time before a slow and slight cooling trend
begins Wednesday through Saturday for Southcentral. At this time
the rain-snow looks to be over Anchorage and the Matanuska valley
with the Kenai Peninsula mainly rain and the Susitna Valley
remaining snow. This system will likely bring another another
round of significant snow to the Susitna Valley Tuesday and
Tuesday night. By Wednesday, Southcentral temperatures for areas
not along the immediate coast should remain at or below freezing
and keep precipitation type all snow after that.

Southwest Alaska will see warmer air than Southcentral with
widespread above freezing temperatures Wednesday through Friday
with Thursday likely being the warmest day.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory (Snow) 145 152.
MARINE...Gale Warning 172>176 178 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/MSO
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
MARINE/LONG TERM...EZ