Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 221257
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKDT Tue May 22 2018

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough centered over the state extends from
the eastern Bering and reaches as far south as the Northern
Pacific. An area of low pressure in the Gulf has weakened
slightly over the last 12 hours and a stationary front remains in
place over much of Southcentral this morning. Behind this trough,
a weak ridge is attempting to build over the central Bering.
However, a fast-moving front with a well-defined jet streak is
suppressing the area of high pressure, forcing it to the south and
causing it to dissipate.

Current observations this morning show continued overcast across
Southcentral and southwest Alaska. Rain showers are spreading in
along the northern Gulf Coast, across the Kenai Peninsula, and as
far west as the Alaska Peninsula. Out in the Aleutians, light
rain and fog are occurring in the wake of the eastward moving
front. Gusty southeast winds persisted overnight in Anchorage and
the Matanuska Valley - a response to the interaction between a
coastal surface ridge in place over the Copper River Basin and the
aforementioned low in the Gulf. Areas of patchy fog and stratus are
likely along the southwest coast this morning as calm conditions
allow for little mixing.

&&

.MODEL DISCUSSION...
Models remain in solid agreement through the short-term portion of
the forecast (48hrs). There are some minor discrepancies in how
they handle a weak triple point low that tries to form along the
decaying front over the Western Gulf Thu into Fri. Either way, the
sensible weather from any of the model`s solutions would be very
similar to the others. The only other item of interest is the
timing of southeast winds across Southcentral today. Hi- res
models and hand edits were once again utilized to handle local-
effect winds and potential thunderstorms.

&&

.AVIATION...
PANC...VFR conditions will persist. Winds should be very close to
dying down for the morning hours. But they will once again become
gusty this afternoon. Timing on this increase could be off an
hour or two and the current TAF is on the low-end for gust
potential.

&&

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

Broad upper level cyclonic flow continues through Wednesday night.
That said, the resident upper low over the gulf does begin to lift
north on Wednesday as an long wave approaches from the west.
At the surface, weak low pressure centers continue over the gulf
with accompanying troughs rotating north and west. The coastal
ridge overall will remain in place and combined with the interior
thermal trough will continue gusty afternoon and evening winds
along the Copper River, out of the Knik Valley, and over south
and west Anchorage. Winds over south and west Anchorage will most
likely linger through the late night hours tonight. Showers or
rain will be more widespread along the coasts. Inland areas will
generally see showers as weak upper waves propagate across the
mainland. Conditions over the Copper River Basin are expected to
be a little more stable than Monday with cooling 850 mb temps, so
a repeat of Monday`s thunderstorms are not anticipated. The
inland areas of the western Kenai Peninsula may be an area to
watch for thunderstorm development this afternoon as stability
indices are becoming more favorable. It will depend heavily on if
there is enough clearing for surface heating.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)... The
continued presence of the thermal trough will bring another round
of isolated to widely scattered thunderstorms as well scattered to
numerous showers to portions of Southwest Alaska this afternoon
and evening. As compared to yesterday the axis of the greatest
concentration of showers and thunderstorms will likely be shifted
further to the east, limiting them to the middle Kuskokwim Valley
and northern portions of the Greater Bristol Bay area. A frontal
system approaching the coast on Wednesday but stalling before
arriving will help stabilize conditions somewhat and limit
precipitation to isolated to scattered showers Wednesday and
Thursday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
The large vertically stacked low located to the west of the
western Aleutians early this morning will track east to cross over
Shemya this afternoon and then continue east-northeast across the
southern Bering and towards the south central Bering tonight
through Wednesday. The low will then drop slowly south Wednesday
night and Thursday before turning more east-southeasterly as it
crosses the central and eastern Aleutians Thursday night. The
associated frontal system currently pushing into the central
Bering will continue east to stall and shear apart over the
eastern Bering and Alaska Peninsula Wednesday and Wednesday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast begins Thursday with a weak surface low
rotating in the Southern Gulf spreading clouds and rain showers across
the Gulf Coast and most of Southcentral. This system will slowly
fill in and die out, though persistent onshore flow will lead to
clouds and rain, especially along coastal areas on Friday. Another
low will rapidly move in from the west on Saturday providing no
real break in the weather for the Gulf Coast and no prolonged
periods of sun and warm weather for the inland areas. This low
will remain in the Gulf through most of the long holiday weekend
as a stubborn long wave ridge over the Western US will continue
to direct systems north into the Alaskan mainland. The pattern
doesn`t look to be a total washout though, especially for more
inland areas, with breaks in between individual waves bringing
periodic dry weather, though coastal areas look to remain rather
cloudy and wet. Models do show this system slowly moving off to
the east on Memorial Day (Monday) providing a better chance of
decent weather for the holiday. High pressure looks to build in
over Western AK for the later half of the weekend, though
afternoon showers/thunderstorms should develop over the higher
terrain and may drift over more populated areas.

Out West, a long wave ridge will build in over the Western Bering
by the weekend bringing drier northerly flow to the western two-
thirds of the Bering/Aleutians. There is some disagreement between
models with the next system coming into the Western Aleutians but
it appears the ridge looks to hold through most of the weekend.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 177 178.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KO/MSO
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
LONG TERM...KVP



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