Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
418 am akdt Wednesday Jul 30 2014
Analysis and upper levels...
The Omega blocking pattern centered over the central Bering Sea
continues to be the dominant factor over southern Alaska and the
Bering Sea. An upper level low located between the Kamchatka
peninsula and the western Aleutians (the western anchor of the
Omega block) continues to eject weak shortwaves across the western
Aleutians. This has continued the rainy period across much of the
western Aleutians as weak frontal boundaries associated with a
surface reflection of the upper low push through the area toward
the central Bering Sea. Another...and more complex...upper level
low is located across the southern Gulf of Alaska/northern Pacific
(the eastern anchor of the Omega block). This upper level low has
begun to slowly drift southward and amplify the longwave troughing
on the eastern periphery of an anomalously strong ridge of high
pressure centered over the Bering Sea. A weakening easterly wave
will continue to rotate along the periphery of the upper level
low and continue to bring rain to the northern Gulf Coast and
southern portions of the Copper River basin. The aforementioned
ridge over the Bering Sea (the final component of the Omega block)
has begun to build eastward over southwest Alaska and parts of
southcentral Alaska. Until a strong upper level system pushes
through the Bering Sea or North Pacific...this blocking pattern
should remain firmly entrenched.
Numerical guidance is in excellent synoptic agreement through the
short-term. A few discrepancies are observed with the timing/strength
of a weak wave dropping south along the eastern periphery of the
Bering Sea ridge late this week. Regardless...precipitation will
return to parts of southwest Alaska late this week and parts of
south central Alaska this weekend. Another minor discrepancy is
noted with the upper level low over the southern Gulf/North Pacific
heading toward the weekend. The GFS is trying to keep this upper
low much further north than all other guidance and seems to go
against what one would generally think in this type of pattern.
Overall...went with an European model (ecmwf)/NAM blend for forecast updates this
Short term forecast...
Southcentral Alaska...an easterly wave rotating around an upper
low in the southern Gulf of Alaska will continue to bring
precipitation to parts of the northern Gulf Coast and southern
portions of the Copper River basin through late this morning.
Precipitation will begin to taper off late this morning as the
upper low slowly drifts into the North Pacific and the easterly
wave moves offshore. The ridge of high pressure located over the
Bering Sea and southwest Alaska will build across southcentral
Alaska during the day today and persist through the remainder of
the week. This should bring mostly clear skies to the region...with
high temperatures 5 to 10 degrees above normal for most locations.
There will only be a slight chance for convective showers near the
mountains in the afternoons due in large part to strong subsidence
associated with the strengthening ridge.
Southwest Alaska...high pressure will dominate the overall weather
pattern through most of Thursday. Mostly clear skies with a few
cumulus clouds developing near the mountainous regions will be the
norm for today and Thursday. Temperatures across the region will
be 5 to 10 degrees above normal today and again on Thursday. By
Thursday evening/early Friday morning...a short wave will drop out
of the Arctic and bring rain chances back to the Kuskokwim Delta
and lower Kuskokwim valley...and linger through at least the first
half of the weekend.
Bering Sea/Aleutians...high pressure will continue to dominate the
eastern two-thirds of the Bering Sea through the short-term. Low
stratus and patchy fog/areas of fog will continue to be seen
across the Bering Sea and Aleutian chain. Some collision
coalescence should take place with the established stratus
deck...leading to areas of light drizzle along the Aleutian chain
and over the Bering Sea. An upper level low and its surface
reflection will continue to push upper level short waves and
frontal boundaries across the western Aleutians and western Bering
Sea...leading to yet another wet period for the western Bering Sea.
Long term forecast...
The Omega blocking over the central to northern Bering Sea will
continue through this upcoming weekend. The predominate feature
will be stratus and fog with light winds for the Bering Sea. The
southcentral Alaska will be under the influence of weak shortwave
ridging keeping most of the region slightly warmer than average
with showers along the mountains through the weekend.
Mclay Jul 14