Small Craft Advisory 452, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 AM AKDT Fri Jun 8 2018

.ANALYSIS AND UPPER LEVELS...
Mainland Alaska remains locked in a weakly cyclonic upper level
flow pattern this morning on the northern periphery of a broad
vertically stacked low centered over the northeast Pacific. This
is primarily yielding spotty diurnal convection over inland
portions of Southcentral and Southwest AK, with that trend
poised to continue today. A weak disturbance dropping south along
the Gulf coast is spreading rain over parts of the Copper River
Basin and eastern Prince William Sound. Out west, low stratus and
some patchy fog has persisted overnight across the Kuskokwim
Delta and has also started to encroach on coastal portions of
Bristol Bay in the presence of weak onshore flow. Extensive
stratus and fog also prevail across the Bering Sea, with a weak
transient ridge in place over the Alaska Peninsula an anemic upper
level wave dissipating north of Adak.

&&

.MODEL DISCUSSION...
Guidance remains in good short term agreement, yielding generally
minor NAM based changes for the morning forecast package. The
more significant differences continue to revolve around the arctic
trough descending upon mainland Alaska on Sunday, with the GFS
remaining somewhat of a west outlier which would signal larger
sensible weather impacts for Anchorage and surrounding areas.

&&

.AVIATION...
PANC...VFR conditions will persist through the TAF period. A light
seabreeze should develop around 18z before giving way to modest
southeast winds coming out of Turnagain arm by early evening. The
window for southeast winds should range from roughly 02-09z
before the local pressure gradient relaxes and light/variable
winds return to the terminal.

&&

.FIRE WEATHER...

From previous discussion issued 437 PM AKDT Thu Jun 7 2018.

Convective activity will be the main concern for the next few
days, but the weather to watch will be the pattern change on
Sunday. The trajectory of the trough from the north could mean
downslope winds off the Alaska range, producing a period of drier
and windy weather as the through pushes through late Sunday.
Depending on how much cloud cover moves in ahead of the trough
itself, solar heating in the day could add to instability, or the
event could be a cool and cloudy change for the area. It is
unclear which outcome is most likely at this point, but either
way, expect a pattern change for the late weekend.

&&

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

Weak upper waves drift south across Southcentral today as high
pressure noses in some from the west. A shortwave drops south
along the Alaska Range late Saturday night. The thermal trough
continues from the Yukon Territory to along and north of the
Alaska Range with a wedge over the Copper River Basin. Expect some
light rain over the Copper River Basin to over the North Gulf
Coast this morning with the passage of an upper wave. Otherwise,
generally unstable conditions and the lack of a well defined
forcing mechanism will favor airmass convection during the
afternoon and evening hours through Saturday across Southcentral.
The most favorable locations for wet thunderstorms will be
generally across the Copper River Basin, Susitna Valley, and along
the Talkeetna Mountains. Winds overall will be on the light side
though afternoon seabreezes could be fairly brisk for Kachemak Bay
and Resurrection Bay. A weak Turnagain Arm jet will bring some
locally gusty southeast winds into west and South Anchorage this
afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days through 3)...

Thunderstorm coverage may increase somewhat today for locations
near and north of the lower Kuskokwim River, and near the Alaska
Range further east. Instability will be somewhat higher compared
to yesterday as increased heating ramps up MUCAPEs to about 1000
J/kg with lifted indices down to -4, and mid-level lapse rates
around 7.3 deg. C/km. Freezing levels will be in the 6000 to 6500
ft range which given the instability parameters would make some
small hail a possibility, but, 0-6km bulk shear is only around 13
knots which should help mitigate storm organization and how
widespread this concern would be. We were tempted to add this to
the grids if all things considered above were just a touch higher.

Otherwise, as an area of mid-level low pressure near the Kuskokwim
coast retrogrades to the northwest, and the persistent Gulf low
remains in place, the COL (area co-located between both two areas
of low pressure and two areas of high pressure, causing
convergence or divergence in the isotherms) which has resided over
the region the last 24 hours will start to dissipate over the
next 24 to 36 hours, allowing for northerly flow to increase
while pushing any convective potential south towards the Bristol
Bay region/AKPEN. This will begin to limit overall precipitation
coverage towards the end of the forecast period.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days through 3)...

Relatively calm seas and winds with dry conditions look to
prevail over the next few days, as high pressure over eastern
Siberia tries to build south through the western Bering then
eastward towards Saint Paul Island. The only storm system of
concern will be an area of low pressure tracking west to east,
generally near 50N latitude. This system will get close enough to
bring some precipitation to locations from the west central
Aleutians to the east central portions of the chain. In this same
general area south of the chain, mariners can expect small craft
criteria winds and seas, with some gusts to gale force likely.

&&

.LONG TERM FORECAST (Days 4 through 7)...
A fair amount of uncertainty has creeped into the long term
forecast today as models are showing a significant pattern change
this weekend and into next week. An upper level low currently
over in Northern Europe will transit across the North Pole and
approach the north slope of Alaska this weekend, bringing with it
unseasonably cold air and generally unsettled weather. The global
models are all in decent agreement resolving the strength and
location of this system through Sunday with the upper low just
north of the North Slope. Models really begin to diverge as we
head into next week regarding how far south and west energy
associated with the low digs, which will have a significant impact
to the sensible weather over Southcentral. The GFS is the most
aggressive solution bringing an upper level shortwave across all
of Southcentral, eventually showing a closed low becoming cut-off
over the Prince William Sound by mid-week. The EC keeps most of
the southward diving energy further east over the Copper River
Basin and across the ALCAN border, keeping the western two-thirds
of the state under the influence of high pressure to the west.
Due to recent model inconsistency and the fact that models
struggle greatly with lows transiting across the poles, kept with
the ensembles today which leans more towards the further east
solution, i.e., the drier and warmer solution for southcentral.

Further west, there is more confidence in the forecast as a large
area of high pressure will dominate the Bering and Western Alaska,
leading to fog and stratus over the Bering Sea with warm and dry
conditions over the western mainland.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...CB
FIRE WEATHER...LF
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...KVP