Small Craft Advisory 695, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKDT Mon Sep 10 2018

.ANALYSIS AND UPPER LEVELS...

A 3 sigma 500 hpa rex block is still dominating the regional
weather pattern as Southern Alaska remains on the eastern edge of
the upper high. What this means is northerly upper level flow is
promoting an offshore pressure gradient which is bringing largely
clear skies and warm/sunny weather to most of Southern Alaska.
Over the Bering Sea, the usual sea stratus and fog is covering
almost the entire area, and some of the fog/stratus has moved
inland along the Kuskokwim Delta coast as well. A weak shortwave
trough is moving over the top of the ridge near the North Slope,
and this will move south late this afternoon and evening bringing
some high clouds. Otherwise, the main story is amazingly warm and
dry weather for September, with coastal locations seeing more
offshore winds which are allowing for incredible warmth.

&&

.MODEL DISCUSSION...
The numerical weather models are in excellent agreement with the
general pattern with little to no discrepancy. They do differ,
very slightly, with respect to a northerly shortwave trough moving
over the upper ridge into the Yukon Wed and Wed night, but the
sensible weather differences are minor. The GFS and NAM were
favored.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Tonight through
Thursday)...
The above normal temperature streak will continue as
a large area of high pressure remains centered over the Bering
Sea with ridging persisting over Southcentral Alaska today. The
upper level flow remains northerly over the region. Yesterday, the
high in Valdez was 72F and the the high in Seward was 71F. So far
this afternoon, 72F was observed in Seward breaking the old
record which was set in 2016. Additionally, Talkeetna is being
actively monitored for a potential record where the latest
observation is 70F. The ensemble guidance suggests that the
temperatures aloft will be 3 standards deviations above normal.
More warm and dry conditions expected through Thursday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Tonight through Thursday)...
Temperatures look to warm over the next few days, as the
persistent ridge across the central Bering moves eastward.
Increasing heights aloft will allow 850mb temperatures to climb,
with offshore flow persisting. This in terms allows for better
adiabatic mixing to commence under clear skies, allowing
temperatures to increase during the afternoon, aided by down-slope
effects. During the early morning hours, fog looks to be a fairly
widespread concern given the increasing heights and winds
decoupling. Some areas in the Lower Kuskokwim Delta may see
visibilities less than one mile, as this region gets a boost in
moisture from the marine layer. This is mainly due to winds being
parallel to the coast, then gradually becoming offshore. Heading
into Thursday, a front will cross the AKPEN during the day,
accompanied by scattered showers.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Tonight through
Thursday)...
The weather will become more unsettled as we head through the
next few days. The persisted ridge which has dominated our weather
for several days, gets shoved eastward, as several disturbances
bump into it from both the north Pacific as they make inroads into
the area, and also from a pair of troughs to our northwest and
northeast near the Arctic coasts. The net affect is the ridge will
de-amplify significantly as it gets shunted south to southeast.

This then opens the door for perhaps multiple days of
precipitation potential. The first significant increase in this
will develop across the western half of the Aleutians as an 850mb
warm front crosses the area, with showers developing in an over-
running pattern. Then, the parent surface low and warm front rides
north on Wednesday, allowing the precipitation shield to spread
north and east, especially towards the late evening hours as the
front begins to occlude and accelerate. Yet another low will move
into the western portions of the chain on Thursday, with a moist
southwest flow continuing as both of these lows have a subtropical
connection, allowing some pockets of moderate to heavy rainfall
to develop. The ridge will be overhead of Southwest Alaska and
progressing eastward on Thursday. The low currently south of the
Aleutians will track up the west side of the ridge. The Canadian
is an outlier in trying to push this system directly into the
ridge, tracking it further eastward. Prefer the remainder of
solutions which take more of a northerly track. This may bring
clouds and rain to coastal Southwest Alaska on Thursday, most
likely to the Kuskokwim Delta.

Winds and waves look to remain in small craft criteria or less for
most locations, with the exception being just south of the central
Aleutians, where some marginal gale force sustained winds may
develop.

&&

.LONG TERM FORECAST (Thursday through Monday)...
The big question on everyone's mind (at least in Southwest Alaska
and Southcentral Alaska) is "How long will this sunny and warm
weather last"? Models tend to break down strong blocking ridges
too fast, so it's no surprise that the latest model runs have
slowed this process down, which means dry and sunny weather will
continue through about Thursday for Southwest Alaska and through
Friday or Saturday for Southcentral Alaska.

A storm system moving through the Bering on Friday is the one
which may finally break down the ridge as we heads to next
weekend. However, would not be surprised if the arrival in
Southcentral is delayed further (as has been the trend in the
models over the last 24 hours). In any case, expect much more
active weather over the Bering Sea and Aleutians as we head
through the week, likely pushing eastward across Southwest Alaska
and into Southcentral sometime next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 176.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...SEB