Scientific Forecaster Discussion
fxak68 pafc 220049 aaa
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
429 PM akdt Fri Jul 21 2017
Analysis and upper levels...
The most significant feature on the weather map today is a
decently strong ridge of high pressure over the Bering Sea that is
slowly spreading eastward into Mainland Alaska. This pattern is
producing substantial amounts of sunshine over much of the
Mainland and substantial amounts of low clouds and fog over much
of the Bering Sea and Aleutians. A weak shortwave trough is
digging down the frontside of the ridge today and will be crossing
over the Bristol Bay and Cook Inlet areas this afternoon, and may
help to boost some shower development over the mountains.
An isolated thunderstorm or two may even develop in the Bristol
Bay area, with any storms that do develop expected to move from
the north to the south.
The models are in very good overall agreement and none were
excluded from use today. The higher resolution NAM best captures
the intricate flow through the complex terrain surround Cook Inlet
and the Prince William Sound area, so the NAM was primarily
utilized in the Cook Inlet and Prince William Sound areas.
Panc...VFR conditions will persist throughout the period. A sea
breeze will develop this afternoon and diminish by late in the
evening. A sea breeze is expected to develop again on Saturday,
but will probably be a little later to develop (mid afternoon)
than the sea breeze today (friday).
Short term forecast southcentral Alaska (days 1 and 2)...
The drying trend that has marked the end of the work week will
continue through Sunday afternoon. The area remains under a col
with an upper-level ridge nosing in from the west. As the ridge
builds east through Sunday, enough subsidence should be present to
suppress most diurnal convection. Offshore flow will support a
warming trend through Saturday, with highs around 70 expected in
coastal areas and towards 80 inland.
The break in rain will be short-lived however, as the progressive
pattern continues to move disturbances through the state. The next
feature of importance will be an Arctic trough moving through the
northern Mainland Sunday night. An associated front will bring
chances of rain back into southcentral Sunday night into Monday,
however most of the rain should be confined to the northern and
western Susitna Valley along with the northern Copper River
Short term forecast southwest Alaska...
high pressure continues to build and the ridge axis will nose its
way directly into southwest Alaska. Kept slight chance pops to
account for lingering pockets of instability and moisture which
could fuel isolated convection over interior locations. With
robust ridging in place this forecast period...expect the warming
trend to continue and be most notable for interior locations.
Along the coast the marine layer will continue to be a forecasting
challenge. The models continue to spin up a low over the Bering
and bring an organized front into western Alaska late in the
Short term forecast Bering Sea/Aleutians...
high pressure dominates the Bering and Aleutians bringing stratus
and fog to the area. The latest satellite image has stratus from
Cold Bay to Adak and north of St Paul island. As the forecast
period unfolds, late on Saturday a front will push through the
Bering from the west. The precipitation shield will reach the
Pribilof Islands and Nunivak Island on Sunday and will then
continue tracking to the eastward with the tail end of the front
draped across the western and central Aleutians.
Long term forecast (days 3 through 7)...
The long range forecast will be in a progressive pattern as an upper
low over the Arctic sends several shortwaves to the southern
Mainland. Under this regime, occasional showers are expected Monday
night through Friday across the southern Mainland and the Gulf of
Alaska. To the west, a ridge takes residence along the western half
of the Aleutians keeping much of the Bering dry, however low clouds
and fog will persist under the stable air mass.
The central and eastern Bering will mainly be influenced by the
upper low. Models are in relatively good agreement with the
synoptic pattern through the extended timeframe, therefore a blend
of the operational models and their ensembles were used to update
the latest package.