Small Craft Advisory 139, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
616 AM AKST Tue Feb 14 2017

.ANALYSIS AND UPPER LEVELS...

The upper level pattern remains very amplified with high
pressure/riding to the east over the panhandle, and a deep trough
over the Bering Sea with a nearly stationary upper low parked
south of the Alaska Peninsula. A vigorous shortwave that actually
developed its own mid-level circulation has raced northwestward
through the Gulf overnight, and is currently northeast of Kodiak
Island. Heavy rain continues to fall all along the Gulf Coast and
Kodiak Island. Interior areas are mostly - but not completely -
downsloped as the deep moisture available in the atmosphere with
this storm is leading to light precipitation even in these
typically dry areas. With temperatures remaining slightly below
freezing, this has led to persistent light freezing rain around
parts of Anchorage, where a freezing rain advisory is currently in
effect. Meanwhile, temperatures are likely cold enough to
support several more inches of snowfall over the western Susitna
Valley through mid morning.

West of the Alaska Range, blowing snow has come to an end in most
places as the first weather front has largely moved through, but
another round of winter weather is just beginning to move into
eastern Bristol Bay zone and will be spreading westward throughout
the day. Way out west, windy and cold northerly flow continue
across the entire area. The worst conditions by far are at the
Pribilof Islands where steady snow and cold temperatures are
combining with the wind to produce a long-lived blizzard in the
area.

&&

.MODEL DISCUSSION...

The models are in pretty good agreement for the next few days. The
challenge, as always, is in the smaller-scale details. The next
area of remaining model variability comes Wednesday afternoon as
another shortwave, weaker than the previous shortwaves, moves
northwestward through Prince William Sound and generally over top
of Anchorage. There should be a band of heavy wet snow that
accompanies this feature, but pinpointing the timing and speed of
snowfall is still a bit up in the air. For now, have gone with a
middle-ground solution based on the NAM that brings a few inches
of snow to the Anchorage area by Wednesday night, but the
potential is there for a bit more if things work out just right.
Future model runs will be monitored closely.

&&

.AVIATION...
PANC...VFR conditions are expected to continue throughout the day
as southeast winds keep the worst of the weather on the other side
of the mountains. Wind shear in the area should diminish by mid to
late morning as the pressure gradients re-orient and bring gusty
southeast winds all the way into the airport. The gustiness to the
winds will diminish this evening and eventually become variable
overnight before snow and MVFR conditions move in early Wednesday
morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

The complex storm system affecting Southcentral continues its
onslaught today. A surface low and attendant upper level shortwave
will swing northwestward this morning, bringing another round of
moisture to much of the area.

A Freezing Rain Advisory is in effect for Anchorage through the
morning. As copious moisture moves over the mountains this
morning, freezing rain is falling at higher elevations and
sheltered valleys. Warming temperatures later this morning will
end the icing threat. For the Cook Inlet region, the models
continue to emphasize downsloping as a primary limiter to
precipitation for lower elevations of Anchorage and the Matanuska
Valley today. Any precipitation that reaches the ground is
expected to be intermittent and light. The first point of
uncertainty remains as to what type of precipitation will fall.
With temperatures in the lower atmosphere fairly uniform right
near the freezing mark, it stands to reason that any precipitation
type will be possible. In fact, depending on the intensity of the
precipitation, a single cell could run the gambit of types.
Fortunately, regardless of precipitation type, well above
freezing temperatures at the surface late this morning and
afternoon should keep impacts minimal on well-traveled roads and
sidewalks. However, such may not be the case for less traveled and
untreated side streets, where any precipitation type could cause
the packed down snow and ice to become very slippery. Travelers
should exercise caution.

In the upper levels, this morning's shortwave is unanimously
expected to eject the jet streak fueling the heavier precipitation
across Prince William Sound well off to the east, which will end
the feed of warm air moving into Southcentral. Initially behind
the wave this afternoon, cold air will not be in any rush to
return to Southcentral. Thus temperatures both aloft and near the
ground will be slow to cool, making for a lovely Valentine's Day
afternoon with periods of sunshine along with temperatures
eclipsing 40 degrees nearly area-wide.

Tonight, cold air will slowly return to Southcentral as the upper
level low near Bristol Bay representing the core of the coldest
air moves northward in its latest looping track. The cooling of
all of the levels of the atmosphere will end any threat of
precipitation falling in any form other than snow. With winds in
the entire column becoming more variable in direction, the warming
and drying effect of downsloping should end tonight, making for
increasing chances for snow, especially after midnight.

Then on Wednesday, things get very interesting again. A surge of
cold air associated with another upper level wave and southwest
winds throughout the lower atmosphere will make for widespread
snowfall across Southcentral. While there are differences between
the models as to when the heaviest precipitation will occur, they
all agree that for the event as a whole, roughly a quarter inch of
liquid equivalent will fall. Using a 10:1 ratio, that equates to
2.5 inches of snow, though falling temperatures may increase those
ratios further.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The frontal system associated with the surface low in the western
Gulf is crossing the Aleutian Range early this morning and
spreading moisture into Southwest Alaska. With temperatures above
freezing in areas close to the mountains such as Iliamna, Port
Alsworth and Igiugig, where warming downsloping easterly winds had
been blowing earlier, precipitation is starting as rain but should
begin mixing with or over to snow shortly as increasing
precipitation intensities cools the air column. Elsewhere with
temperatures in the teens to single digits snow will predominate.
The surface low will track west across the Barren Islands this
morning, pushing into Southwest Alaska by midday. With reasonably
high QPF amounts associated with this system expecting 5 to 8
inches of snow for the Bristol Bay area. As the frontal system
lifts northwest over the Kuskokwim Delta this afternoon and
evening snowfall totals will be a little less but with
northeasterly winds continuing across the delta expect blowing
snow to be an issue.

The surface low will track over Bristol Bay in the afternoon with
the cold upper level low south of the Alaska Peninsula following
close behind and pushing into Southwest Alaska tonight. The
surface low and the upper level low will continuing to rotate
around each other with the center of this motion slowly
progressing east up the Alaska Peninsula Wednesday and Thursday to
become vertically stacked over Southwest Alaska. Throughout this
progression, frontal bands rotating through and scattered showers
will bring periods of snow to Southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Blizzard conditions continue over the Pribilof Islands as a
frontal system brings strong north winds and continuing snow. As
the trough associated with this system drops southeast following
the rotation and eastward track of the upper level low south of
the Alaska Peninsula, snowfall intensities will diminish, allowing
conditions to improve tonight. Cold northerly flow will keep snow
showers persisting over the Bering and Aleutians, while a surface
low rotating across the western Alaska Peninsula tonight brings
heavier snow to that location.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Beginning the long term forecast Wednesday evening a closed arctic
low will be dominating the regional weather pattern. Continued
cold northerly flow will be over the Bering Sea with that cold air
wrapping all the way around the cyclone into Southcentral.
Shortwave disturbances wrapping around the low will keep snow
threats across all Southern Alaska locations including the coast.
However, the noise in the pattern is high enough where there is
low confidence on the placement of the stationary front or weak
troughs rotating toward the coast. This pattern will persist
through Monday before the trough finally weakens and moves
eastward.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 195
Freezing Rain Advisory 101
Winter Weather Advisory 141 155 161
MARINE...Storms 119
Gales 120 125 129 130 131 173 174 179 181 185 351 352 412 414
Heavy Freezing Spray 179 185

&&

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...JA