Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
453 am akdt Tuesday Jul 29 2014
Analysis and upper levels...
Water vapor imagery this morning depicts a shortwave enhanced
upper low over the lower Kuskokwim valley and Bristol Bay area.
This upper low has begun to phase with another upper low located
in the Gulf of Alaska. This ultimately created a deformation band
over the Cook Inlet area overnight and early this morning...which
has allowed precipitation to linger a tad longer than originally
expected. The phasing of these two lows will have a lasting impact
on the longwave pattern as troughing along the eastern periphery
of an Omega block over the Bering Sea becomes more amplified.
Another upper low located between the Kamchatka peninsula and the
western Aleutians continues to slowly push eastward as it ejects
multiple shortwaves over the western Aleutians. An unusually
strong ridge resides across the central and eastern Bering
Sea...with the 00z and 12z radiosonde observations at St Paul island reporting 500
mb heights between 583 and 584 decameters. The combination of the
low west of the Aleutians islands...the low in the Gulf of Alaska
and the strong ridge over the Bering Sea are in textbook Omega
block configuration. Water vapor imagery is also depicting an
easterly wave presently located along the alcan border rotating
around the aforementioned low in the Gulf of Alaska.
Models are very good synoptic agreement this morning. The 00z NAM
was a distinct outlier with the upper low over southwest Alaska...as
it tried to place this feature further east into the Susitna Valley.
The 06z run of the NAM corrected this initialization defect and
placed the upper low in in the correct location compared to
current satellite observations. Numerical guidance has come into
better agreement with an easterly wave currently located along the
alcan border. There are still small differences amongst the
guidance...but should have very minor impacts on the evolution of
precipitation today as the waves pushes into the Copper River
basin. The European model (ecmwf) was preferred this morning for forecast updates
given the best run-to-run consistency and best initialization.
All numerical guidance suggests that the Omega block over the
Bering Sea and southwest Alaska is here to stay through at least
the end of the work week.
Short term forecast...
Southcentral Alaska...precipitation will continue to taper off
from northeast to southwest this morning as an upper level low
over southwest Alaska pushes south toward Kodiak Island and
eventually into the North Pacific. Some breaks in the clouds will
be observed over the western Copper River basin and into the mat-
su valley and Anchorage bowl this morning through this afternoon.
Patchy fog will be an issue across the mat-su valley and Anchorage
area early this morning as residual moisture from overnight
rainfall becomes trapped near the surface. The patchy fog should
be quick to burn off shortly after sunrise. An easterly wave
presently located over the alcan border will push westward into
the Copper River basin during the day today. This will bring
light to moderate rainfall to the eastern half of the Copper River
basin...and scattered showers and the potential for an isolated
thunderstorm or two for the remainder of the basin. Models are
hinting at a decent chance for convective showers and a few
thunderstorms along the Talkeetna Mountains in the mat-su valley
as energy associated with the easterly wave pivots through the
area late this afternoon and evening. There will also be a slight
chance for convective showers and thunderstorms in the Anchorage
area this evening...but confidence with this is low and opted to
neglect it from the forecast for the time being. High pressure
will begin to build across the area on Wednesday...which will
allow for skies to clear across much of southcentral Alaska
through the end of the week with near normal to above normal
Southwest Alaska...scattered showers will continue along the
Aleutian Range and the Alaska Peninsula through this morning with
an upper level low pushing out of southwest Alaska and into the
Gulf of Alaska. A ridge of high pressure over the Bering Sea will
build eastward over southwest Alaska during the day today. This
will allow for skies to continue to clear in the wake of the upper
level low that pushed through the area on Monday. Above normal
temperatures will be observed through the end of the week across
much of southwest Alaska.
Bering Sea/Aleutians...the eastern half of the Bering Sea will
remain dry with a potent longwave ridge thoroughly entrenched
across the area. Low stratus and patchy/areas of fog are likely to
persist through the end of the week. An upper level low west of
the Aleutian chain will slowly drift eastward and eject a series
of shortwaves across the western Bering Sea. Periods of rain can
be expected for the western Bering Sea...with no appreciable change
in the weather pattern expected through the end of the week.
Long term forecast...
Omega blocking centered over the northern Bering Sea will
continue to drive the sensible weather across most of Alaska
region on Friday. Much of southern Alaska will remain under the
influence of weak shortwave ridging...which will keep most of the
area slightly warmer than average...with showers relegated to the
mountains. Southwest Alaska will remain along the periphery of the
blocking high over the Bering and troughing across the North
Slope...with rain and shower threats along the quasi-stationary
front. Ridging will remain persistent across the Bering through
the weekend...but uncertainty arises across southern Alaska by
Sunday. The European model (ecmwf) continues to hint at northern stream energy
diving into the Gulf and interacting with the current closed
low...while the Canadian gdps and GFS build the Bering ridge into
southern Alaska. The current forecast will take a middle ground
approach...highlighting the potential for increasing precipitation
chances by Sunday and Monday...and near climatological temperatures.
Mclay/ja Jul 14