Small Craft Advisory 304, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
400 AM AKDT Tue Apr 11 2017

.ANALYSIS AND UPPER LEVELS...

A shift in the upper air pattern is underway this morning as a
deep 497 decameter, 500 hpa upper low is moving into the
Southwest Bering Sea. Shortwave ridging currently over the Eastern
Bering Sea will quickly amplify in response to the warm air
advection pattern associated with the aforementioned low. Weak
upper troughing over Southcentral will amalgamate into a North
Gulf low then interact with the building ridge out west and
another digging trough currently over the North Pacific. Weak
onshore flow this morning across Southcentral will quickly
transition to offshore flow and drying conditions/clearing skies
today.

&&

.MODEL DISCUSSION...
The numerical weather models are in excellent agreement in both
the short term forecast period all the way through Friday night
before the wheels quickly fall off after Saturday. For the short
term forecast period through day 3, the NAM and high resolution
models were favored to better depict the offshore wind pattern.
Over the Bering, the GFS was favored for the current deep low and
its slow eastward progression and gradual weakening. Forecast
confidence is very high.

&&

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

&&

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

The remnants of yesterdays shortwave are over the Kenai Peninsula
and Afognak Island this morning bringing only a few isolated
showers. It continues to dissipate as the pattern change is
beginning today with the warm upper level ridge building in from
the west. As this ridge builds in and a weak surface trough
develops along the north Gulf coast it will increase the pressure
gradient. This will increase the winds in the Prince William Sound
area and over the Matanuska valley Tuesday into Wednesday. The
ridge will also bring clearing skies over most of the area by
Tomorrow with just a few showers persisting over the Wrangell
Mountains before clearing out there by Wednesday. The only
question is whether there will be any fog that develops under this
ridge. If it was the middle of winter it would be almost a sure
thing for fog to develop. However with the much warmer
temperatures and pressure gradient it looks much less likely to
have the fog develop. However it is worth mentioning since the
potential is there.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A period of quiet weather is in store for the southwest mainland
through midweek as a high amplitude ridge builds over the west
coast of Alaska later today. This will yield clearing skies and
larger diurnal temperature swings as daytime highs trend upward
and overnight temperatures bottom out as a result of more optimal
radiational cooling processes. Copious dry air in the low levels
has thus far mitigated any fog development, and have thus cut
back on fog potential the next few nights as these conditions
persist. Coastal locations may start to see increased cloud cover
again by Wednesday evening as the Bering front makes slow
eastward progress, however any precipitation should remain
offshore as the front continues to weaken and eventually
dissipates.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The front traversing the central Bering will slow its eastward
progress and stall over the Pribilof Islands and Eastern Aleutians
today as it encounters the stout ridge to the east. This will
bring a relatively prolonged period of rain and gale force winds
to these locations through Wednesday afternoon before the front
finally shifts to the east and eventually dissipates. The western
half of the Bering will also see gusty winds and widespread
showers as the parent low crosses the western part of the chain
later today and opens up into a weak trough over the central
Bering by Wednesday afternoon. The next front will then move over
the Western Aleutians Wednesday night accompanied by another
round of heavier rain and gale force winds.

&&

.LONG TERM FORECAST (Days 3 through 7/Friday through Monday)...
There is high confidence with the presence of a high amplitude
ridge over mainland Alaska on Thursday. However, model spread
rapidly increases (and forecast confidence decreases) Friday and
Saturday as the ridge retreats northward to the Arctic while a
short-wave in vicinity of the Yukon Territory undercuts the ridge
(moving westward). This will affect the progression of a frontal
system approaching from the Bering Sea. It seems likely the front
will reach Southwest Alaska by Saturday, but much more questionable
whether it will reach Southcentral. In any case, expect increasing
chances of precipitation across the southern mainland of Alaska with
continued above normal temperatures. As is typical for this time of
year, the Bering Sea and Aleutians will remain active with large
deep cyclones transiting the region every 2 or 3 days.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 170 173-174 177-179 185 411-414.

&&

SYNOPSIS AND MODEL DISCUSSION...JRA
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...SB