Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
434 PM akdt Tuesday Sep 1 2015
Analysis and upper levels...
the northerly flow that has persisted over southcentral over the
last few days is beginning to slowly weaken as low pressure over
the eastern Gulf dissipates and moves inland. Behind this
system...a transitory ridge currently over the western Mainland is
pressing eastward towards southcentral. This ridge provided enough
stability to produce patchy fog/stratus over the West Coast early
this morning...and will bring cooler overnight low temperatures to
southcentral as it moves into the area tonight. Over the Bering
Sea...a frontal system associated with a low centered just east of
the Kamchatka peninsula stretches from the north central Bering
into the akpen. Behind this front...dry air entraining aloft has
allowed for isolated thunderstorms to develop over the central
Bering to the west of the Pribilof Islands for much of the
morning. Gusty southeasterly winds and rain mainly accompany this
front...which will continue as it moves towards the southwest
coast for tomorrow morning.
the models continue to be in decent synoptic agreement with the
progression of the frontal system over the Bering as it moves
towards the southwest coast tonight. There continues to be some
relatively minor timing differences with the arrival of this
precipitation and how quickly inland it will move...however these should
not have a significant impact in forecast confidence. Agreement
then breaks down on Wednesday evening as the models begin to
struggle as a secondary or triple point low develops near Kodiak
Island. At this point the GFS is the strongest outlier and slowest
solution with the precipitation. It also has a much broader surface low
near Kodiak Island. The NAM on the other hand has flip flopped
between the fastest and slowest solutions in the last 2
runs...while the ec and Canadian have remained the most
consistent. As as result the ec/Canadian were both used for this
system...however due to the timing differences there remains some
Fire weather discussion...
as the pressure gradients over the Gulf continue to fall and weak
ridging moves in...winds throughout the area will continue to
diminish this evening. Skies will remain clear under this
ridge...allowing for cold temperatures to develop in most Interior
Valley and good overnight relative humidity recoveries. Ridging will persist over
the area on Wednesday and allow for another mostly sunny day...but
with diminishing winds and higher relative humidity values. Over Kodiak
Island...the system will begin to approach the area with
increasing cloud cover and moist onshore flow. This system will
then spread into the Gulf...bringing a widespread rain and moist
conditions throughout southcentral for Thursday and Friday.
Short term forecast southcentral Alaska (days 1 and 2)...
With the persistent troughing which has been in place over
southcentral since late last week finally moving east...northerly
offshore flow will gradually weaken tonight and tomorrow.
Moreover...with clear skies and cold air in place...along with the
weakening gradient...rather cold temperatures are expected to
develop overnight. This means more widespread freezing
temperatures will develop around populated areas from Anchorage to
the Matanuska Valley. Shortwave ridging aloft will build over
southcentral tomorrow before rapidly moving east as a broad
southwest flow aloft moves into the region from the Bering Sea.
Clouds and precipitation will move from Kodiak Island during the
late morning and afternoon then into southcentral Thursday and
Thursday night. With an expected weak low over the northern
Gulf...the best rainfall is expected from Kodiak Island to the
northern Gulf and Prince William Sound...with lighter amounts
inland. There is enough discrepancy with the strength and track of
low that precipitation amounts north of Anchorage are less certain...with
the possibility that climatologically dry locations downwind of
the chugach and Talkeetna Mountains such as the Matanuska and
Susitna valleys remain much drier.
Short term forecast southwest Alaska (days 1 and 2)...
a frontal system will begin to track into the southwest coast
tonight...increasing cloud cover and spreading precipitation
onshore from west to east overnight tonight into Wednesday
morning. The frontal system will stall slightly over the Kuskokwim
Delta and Bristol Bay areas Wednesday and Wednesday night...as a
frontal wave lifts northeast from the western tip of the Alaska
Peninsula across western Bristol Bay. On Thursday a trough from
the west will quickly catch up to the stalled front and drag the
front further inland into southwest Alaska...where it will remain
as a deformation band through Friday.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the broad upper level low centered to the east of the Kamchatka
peninsula will remain a dominant feature drifting east into the
western Bering Wednesday and Thursday as the upper level ridge
shifts east and inland over Mainland Alaska. The surface low over
the Bering will continue to lift northward tonight...with windy
and rainy conditions over the eastern Bering/Aleutians tracking
eastward overnight tonight and inland by Wednesday morning. A
trough will swing across the western and central Aleutians/Bering
Wednesday through Thursday out ahead of the large upper low coming
in from the west...keeping showery conditions and southwesterly
flow over the area through Thursday evening.
Long term forecast (days 3 through 7)...
There is still a lot of discrepancy amongst the guidance today
starting as early as Thursday evening as a vorticity maximum rotating
around the upper low in the Bering Sea either pushes into
southwest Alaska or skirts the Alaska Peninsula and moves into
the Gulf Friday afternoon. The models have depicted the
progression of rainfall along the front moving across southern
Alaska pretty well. Models are starting to trend a tad drier
across southcentral Alaska late Thursday and Friday as they try to
create a surface low that transits across the northern Gulf
rapidly. This would allow southwest flow to usher in moisture
before the low shifts south of Prince William Sound and the
moisture then becomes confined to the northern Gulf Coast.
Nonetheless...Thursday afternoon through Friday morning should be
pretty wet across southcentral Alaska. Chances for rainfall will
persist through the extended for the Alaska Mainland as models are
more aggressive with the upper level ridge being more shortwave
induced vs a longwave ridge which should break down the ridge
quicker than the models hinted at in past days. Temperatures will
remain near normal to slightly below normal through the middle of
public...freeze warning 101 111.
Marine...gale 180 181.
Synopsis and model discussion...dek?/Amd
southwest Alaska/Bering Sea/Aleutians...tp