Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
514 am akdt Monday Jul 25 2016
Analysis and upper levels...
The main feature dominating nearly all of southern Alaska weather
this morning is a vertically stacked low centered over Bristol
Bay. For southcentral, the low is drawing wave after wave
northeastward up Cook Inlet and Prince William Sound. In many
cases, however, the precipitation gradients have been large,
resulting in some areas picking up substantial rain while nearby
areas get relatively little. The low is in the process of both
cutting off from the jet and drifting further south away from
southcentral, resulting in future waves not having nearly the
energy to cause such large amounts of rain as far north.
A distinguishing factor of the air mass allowing for such large
amounts of rain is the large depth the atmosphere that is nearly
saturated. Yesterday afternoon's 00z Anchorage sounding showed
nearly total saturation through the entire depth of the
troposphere, through 400 mb. This only happens a couple times each
year. More rain is filling the radar maps over the southern Kenai,
Over southwest Alaska, the upper level low being positioned nearly
overhead today resulted in a relatively dry day as most organized
precipitation ended yesterday morning. Just a few nuisance showers
remain on radar on King Salmon radar this morning.
The upper level pattern remains in very close agreement over the
next two days on both the global and high resolution models. The
synoptic picture shows the upper low drifting southward south of
the Alaska Peninsula over the next 24 hours, with areas of
ridging over both the far western Bering and the Panhandle. Some
weak ridging is depicted on most of the models over southcentral
as well. Since most of the disturbances affecting the Mainland
over the next couple days look to be mesoscale in nature, the high
resolution models, particularly the NAM, were preferred for their
deference to the details of these features. Forecast confidence is
average, with high certainty that there will be occasional showers
threatening most areas of southern Alaska, but low confidence on
Short term forecast southcentral Alaska (days 1 and 2)...
An unsettled weather pattern continues across southcentral. With
a broad upper level low centered over Bristol Bay, a moist
southwesterly flow is in place across the region. This morning an
upper level wave is providing ample support for widespread showers
across much of southcentral, with greatest amounts across Prince
William Sound. In addition, the surface coastal ridge is
supporting a coastal jet through Prince William Sound and gusts
through Turnagain Arm. The jet will weaken tonight as the wave
slowly pushes inland. However, by early Tuesday the pressure
gradient will enhance once more as a new upper level wave
approaches the coast. The low level jet will help to partially
limit precipitation in typically downsloped areas of the Anchorage
bowl, especially this evening and tonight.
Short term forecast southwest Alaska (days 1 and 2)...
Numerous showers with isolated afternoon and evening thunderstorms
are expected through Tue across the region.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
Light west to northwest wind will persist over the area through
Tue. Showers will gradually diminish through this evening, but
redevelop over the eastern half of the Bering and Aleutians
beginning Tue night. Fog will slowly decrease in coverage during
the same period.
Long term forecast (days 3 through 7)...
the upper levels have a closed low over the Shumagin Islands with
support from a 90 knot westerly jet streak located south of the
eastern Aleutians late Tuesday. The jet stream will transition to
a southwesterly flow streaking into the southern portion of the
southeast Panhandle by late Wednesday night. By Thursday the upper
level low will be camped out in the northern Gulf before exiting
the region into the Yukon territory. This will bring a wetter
pattern with shortwaves tracking through the region with the
southwesterly flow. Meanwhile, the western/central Bering is
influenced by the high pressure west of the Shemya accompanied by
widespread marine layer stratus/fog heading into early Friday.
The southwest Alaska region will see a showery weather regime as
an inverted trough associated with the closed low south of the
Shumagin Islands as this feature tracks into the Gulf. The Bristol
Bay and the lower Kuskokwim valley will be affected by this
feature through Thursday. By Thursday morning a trough swings
through from the north over the y-k Delta moving eastward. By
Friday morning this trough swings through the southcentral region
with more numerous showers heading into the weekend. Upstream,
westerly flow will reside over the Bering, and the
southwest/southcentral regions through the weekend. There will be
multiple shortwaves with this flow bringing a showery weather
regime through the weekend.
Synopsis and model discussion...jw
southwest Alaska/Bering Sea/Aleutians...ds