Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
437 am akdt Thursday Jul 31 2014
Analysis and upper levels...
An upper level disturbance that originated over the far eastern
coast of Russia has pushed over Saint Lawrence Island is now
impacting Norton Sound early this morning. Light to moderate rain
associated with this disturbance are the only impacts observed
this morning. The rainfall currently extends from the northern
coast of the Kuskokwim Delta through Norton Sound and along the
Bering Strait coast. This upper level disturbance has slightly
shifted the axis of the ridge of high pressure over the central
and eastern Bering Sea slightly westward. This will allow the
shortwave energy to traverse southwest Alaska and the far eastern
Bering Sea through the remainder of the week. Even with this weak
disturbance pushing through the area...the Omega block pattern
remains supreme. A 587 decameter ridge remains firmly entrenched
over the eastern two-thirds of the Bering Sea and show no signs
of weakening at this time. The western anchor of this blocking
pattern is an upper level low located to the west of Shemya/Attu.
A quasi-stationary front attached to the surface reflection of
this upper low is currently located over the western Aleutians...with
steady rainfall in vicinity of the frontal boundary. The eastern anchor of
the blocking pattern is a complex upper low over the southern Gulf
of Alaska/North Pacific. The only impacts associated with this
upper low are clouds and rain showers across the southern Gulf of
The 00z runs of numerical guidance were struggling with the
initialization of the upper level disturbance presently located
near Norton Sound. The European model (ecmwf) was by far the best at initializing
this feature when comparing water vapor imagery to 400 mb relative
humidity and vorticity fields. The GFS/NAM/Gem were all nearly 6
hours off when comparing the quantitative precipitation forecast fields to the Nome (paec) and
Bethel (pabv) reflectivities depicted by the 88-d's. Thus...utilized
the European model (ecmwf) for forecast updates this morning.
Overall though...numerical guidance is in above average agreement.
The Omega block over the Bering Sea and southwest Alaska continues
to show no sign of dissipating in the short-term...but extended
guidance is showing the potential for The Block to begin falling
apart early next week. However...it it much to soon to even think
about this. The models are also struggling slightly with the upper
low currently located in the southern Gulf of Alaska/North Pacific.
The complex upper low will continue to slowly drift south through
Friday before beginning to drift north over the weekend. The models
are struggling with the placement of this low and a series of weak
upper level disturbances that will get ejected toward Kodiak Island.
Short term forecast...
Southcentral Alaska...high pressure will continue to dominate the
area through the remainder of the week. Most of the region will
remain clear and on the dry side...but isolated to scattered
shower activity will likely be seen near mountainous areas today
and again on Friday. However...a strong subsidence inversion (as
seen on the Wednesday afternoon/evening soundings) will likely put
a damper on most of the convection. The remnants of a frontal
boundary will approach the Susitna Valley from the northwest late
Friday morning/early Friday afternoon. It appears that the
majority of the precipitation will upslope along the windward
(western) side of the Alaska Range and very little will actually
make it into southcentral Alaska. Above normal temperatures will
continue to persist into the weekend.
Southwest Alaska...an upper level disturbance will push over the
Kuskokwim Delta during the day today...bringing periods of light
to moderate rain and some gusty winds to the coast. Rainfall will
begin to overspread the lower Kuskokwim valley and the western
convective available potential energy of the Bristol Bay area tonight and on Friday. A weak
deformation band extending from the Alaska Peninsula to the
Kuskokwim Delta should develop Friday and night and persist into
the weekend. This will keep rain chances across much of southwest
Alaska through the weekend.
Bering Sea/Aleutians...a ridge of high pressure will continue to
be the dominant factor for the eastern two-thirds of the Bering
Sea through the weekend. The upper level ridge will get shunted
slightly westward on Friday as an upper level disturbances pushes
off the southwest coast of Alaska and bring some rain chances to
the eastern Aleutians and Pribilof Islands. A quasi-stationary
front over the western Aleutians will slowly meander eastward
during the remainder of the week...keeping in rain chances across
the western and central Aleutians. With the anomalously strong
ridge over the Bering Sea...a low stratus deck with a few breaks
and patchy/areas of fog will continue to be seen through the weekend.
Long term forecast...
The Omega blocking over the Bering Sea will continue with stratus
and fog with light winds through Sunday. By Monday an upper level
weak shortwave trough will drop down from the upper Kuskokwim
valley into the southwest Alaska region. This will bring increased
clouds and showers throughout the area. By Sunday the the weak
upper level ridge in southcentral Alaska will be exiting the
region...followed by an upper level trough pushing into the Gulf
of Alaska bringing northeasterly winds and increased cloud cover
with rain along the coastline before spreading inland.
Mclay Jul 14