Small Craft Advisory 356, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 AM AKDT Tue May 1 2018

.ANALYSIS AND UPPER LEVELS...

A broad trough associated with a low over Kotzebue Sound extends
down across the majority of the Bering. Meanwhile, an upper level
ridge over the panhandle is bringing southwest flow across
Southcentral with an upper level jet maxing out at near 140 knots.
With ample moisture present, rain showers are spreading in across
the Kenai Peninsula and extending north to the Matanuska Valley.
The heaviest precipitation can be found along the northern Gulf
coast. As the trough progresses eastward, westerly flow will
develop ahead of another low approaching the Western Aleutians.

&&

.MODEL DISCUSSION...

As usual, the numerical models are in excellent agreement with the
large scale synoptic pattern, and they don't really diverge until
late in the week with a deepening low moving into the Gulf of
Alaska. In the immediate short term, it seems the Canadian model
performed best with isentropic ascent up Cook Inlet which has
resulted in widespread light rain from the Kenai Peninsula north
to the Matanuska Valley. That model was favored early for
precipitation, with the NAM Nest being favored for winds.

&&

.AVIATION...
PANC...MVFR conditions this morning will improve back to VFR as
rain comes to an end...likely by late morning. Thereafter, dry
and VFR conditions will prevail through the remainder of the TAF
period. Southeast winds will also diminish and become W to SW up
Cook Inlet...and may briefly become gusty this afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weather front over Southcentral this morning will slide east
during the day, exiting into Canada this afternoon. Weakly
cyclonic flow aloft will allow showers to continue into the
evening and tonight, diminishing from west to east. Weak high
pressure aloft will build over the area later tonight into
Wednesday. This will allow a brief dry period over much of the
area, with just a few showers inland over the higher elevations.
Westerly winds will develop over the northern Gulf and coastal
locations today. The next system will approach from the southwest
Wednesday afternoon and Wednesday night. Winds will switch to
easterly as a front works its way towards the Gulf coast early
Thursday morning. Precipitation will increase across the area,
especially for coastal locations. Inland locations near northern
Cook Inlet will only have a slight chance of precipitation with
the downslope flow off the mountains.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The upper level trough currently stretching across western Alaska
will continue east through the day bringing cool temperatures, an
unstable air mass and scattered to numerous showers along the
mountains. Flat ridging will move to the coast this evening
tapering off showers and bringing the potential for patchy valley
fog to much of Southwest Alaska as cloud cover diminishes. The
break in between systems will be short lived as the next frontal
system tracks east across the Alaska Peninsula Wednesday. After
swinging inland into the Greater Bristol Bay area Wednesday
evening the front lifts north further into Southwest Alaska
Wednesday night and Thursday. Thursday Night, a rapidly
strengthening frontal system will track north towards the Alaska
Peninsula.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A weak surface low currently over the western Aleutians will track
east across the Bering today and tonight as it gradually
strengthens to be centered in the vicinity of the Pribilof
Islands by Wednesday morning. The surface low will continue to
deepen and drift a little further east to remain in the eastern
Bering through Thursday. Northwesterly flow and cold advection will
strengthen across the central Bering to west of the low,
particularly as it begins to interact with the strong low
tracking up towards the Alaska Peninsula Thursday.

&&

.LONG TERM FORECAST (Days 3 through 7: Wednesday night through
Sunday)...
The long term forecast will begin with a continued active pattern
as much of the area remains under the influence of a series of
developing lows in the Bering Sea and North Pacific. The first of
these will be a triple point low that will develop off the AKpen
late Wednesday and move into the Gulf on Thursday, bringing
another rain and mountain snow to the Gulf Coast. A much deeper
low will then develop immediately behind this low late Thursday
into Friday just south of the Aleutians and ride north into the
Southwest Mainland. This is where most of the concern in the long
term forecast exists, as this system could potentially bring
another round of strong winds to much of Southcentral as well as
rain and high elevation snow to the coast. The models had diverged
significantly in both the track of intensity of this low. The GFS
has consistently been the most aggressive with bringing a much
deeper low up the Southwest Mainland, while the EC had previously
brought a much weaker low further east into the Gulf. With the 12z
runs this morning, the EC has come more in line by bringing a
also bringing a deeper low to the west of the Alaska Range. This
system will continue to be monitored in the coming days, but the
general trends and consensus will be for continued active weather
for the South Mainland late this week into the weekend.

The one piece of good news in the long term forecast is that there
is very good model agreement that widespread high pressure will
return to the Gulf and Mainland early next week. This would bring
the sunny skies and warmer temperatures typical for this time of
year back into the region.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 155.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...KO/JRA
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...DEK