Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
509 am akdt Tuesday may 5 2015
Analysis and upper levels...
An amplifying upper level trough extending from the Kamchatka
peninsula to southwest Alaska continues to absorb the cut off
upper low over the Alaska Peninsula that has continued to bring
rain showers to southwest Alaska. The jet streak responsible for
the amplification of the upper level trough is well depicted on
water vapor satellite imagery this morning and appears to be
entering the base of the trough. As the jet stream rounds the base
of the trough...the amplification of the trough will temporarily
come to an end before another jet streak enters the trough later
in the week. As this trough digs further south...it has allowed an
upper level ridge to amplify and nose northward toward southwest
Alaska. An upper level and surface based ridge is set up across
the northern Bering Sea which is also well depicted on water vapor
imagery this morning. The ridge is limiting the northward
propagation of the upper level energy associated with the cut off
low over southwest Alaska as it phases with the large upper low
over the Bering Sea and North Pacific.
Southcentral Alaska tells quite a doldrum story (when compared to
that of the Bering sea) with high pressure continuing to nose
northward into the Copper River basin and Susitna Valley
overnight. Some middle to high level moisture ahead of the warm
occlusion attached to the low over the western Aleutians has begun
to push across southcentral Alaska overnight. The subsidence
inversion has prevented much in terms of rain showers thus far
over Kodiak Island. However...the ridge is shifting ever so
slightly eastward which will likely bring in some precipitation to
Kodiak Island later in the day.
Models remain in good agreement synoptically through Tuesday
afternoon before models begin to struggle with a few upper level
features. A second jet streak is poised to enter the upper level
trough late Wednesday and into Thursday which should help re-
amplify the upper level trough to some extent while reinvigorating
an area of low pressure in the North Pacific. The timing of the
jet streak is not handled very well at this time with all of the
guidance showing a slightly different story. This slight
difference in timing will effect the track and intensity as the
low curves into the Gulf of Alaska on Friday with a potent front
pushing along the northern Gulf Coast. Based on the 00z and 06z
model packages...the NAM was heavily utilized through Thursday due
to the detail with the warm occlusion working into the western
Gulf of Alaska. By late Thursday into Friday...a blend of the GFS
and European model (ecmwf) was utilized with better agreement on the jet streak
location and intensity.
Short term forecast southcentral Alaska (days 1 and 2)...
the remnants of the upper low over Bristol Bay this past weekend
will exit across the Copper River basin today with just enough
instability to produce a few showers. More importantly the upper
ridge which had been over the area has been pushed off to the
east. This is allowing moisture exiting the deep trough over the
Aleutians/North Pacific to stream northward toward southcentral.
Initially this will just bring increasing middle to high level clouds.
Deeper moisture along a weakening occluded front is pushing into
the western Gulf/Kodiak Island bringing light rain. The front
will be reinforced by a short-wave exiting the trough late tonight
through Wednesday...resulting in much stronger winds and heavier
rain for much of the Gulf/Kodiak Island on up to the southern
Kenai Peninsula. As everything lifts northward Wednesday night
rain will make its way into the Prince William Sound region...but
easterly flow will keep areas in the Lee of the Kenai and Chugach
Mountains mostly dry.
Taking a look at fire weather conditions...despite the increase in
middle to upper level clouds today inland areas will still see some
sun and low levels will remain quite dry. As the front strengthens
over the Gulf Wednesday a ridge will build back over southcentral.
The shift in low level flow will keep low clouds confined to the
coastal zone with plenty of sunshine and continued dry conditions
over interior southcentral. With a surface ridge along the north
Gulf Coast there will be some weak gap winds (turnagain arm/Knik
Arm/copper river)...but these winds should remain well below red
Short term forecast southwest Alaska (days 1 and 2)...
the once closed upper level low over the southwest Mainland
continues to weaken into an open trough and elongate while slowly
tracking northward tonight. Showery precipitation associated with
this feature will begin to transition to more stratiform rain...as
the weakening front associated with the Aleutian low pushes
inland today. East to southeast winds will also continue to
increase through Wednesday night...with gusty winds developing
along the gaps of the Aleutian Range as the gradient over the
areas tightens further. The pattern will be slow to change
tonight into Wednesday as the front drapes itself over the area
and slowly falls apart. A new piece of energy will move quickly
north from the North Pacific on Wednesday. This will
re-invigorate the front with another round of rain...mainly for
the southern portion of Bristol Bay.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the broad upper-level low over the central Aleutians continues to
keep much of the Aleutians in a showery regime today...with some
lingering gusty winds over the western/eastern Aleutians and the
Bering Sea. This feature will be slow to move over the next
several days as a closed high sits just to its north in the Bering
Strait. These two features will work together creating a quasi-
blocking pattern for the area. This will keep much of the eastern
Aleutians in warmer weakening southerly flow with the western
Aleutians in slightly cooler...more showering northwesterly flow.
High pressure starts to build into the western Bering late
Wednesday which should bring even calmer winds and a decent
potential for some fog.
Long term forecast (thursday through monday)...
Low pressure system moves north to the Gulf Coast Thursday into
the weekend bringing rain to the Gulf Coast and Prince William
Sound as well as Kodiak...with isolated to scattered rain showers
inland. Things become more uncertain toward early next week as the
pattern seems to become a bit more progressive so this will bear
close watching...but a generally showery regime is likely to
continue into early next week for most areas.
Rainy weather will continue Thursday into the weekend over
southwest Alaska and the Alaska Peninsula on the north and west
side of broad low pressure that will set up over the western Gulf
and Bristol Bay. A progressive low will move quickly across the
Aleutians Friday into the weekend bringing another chance of
rain/showers...with another low approaching the western Aleutians
late this weekend into early next week bringing another chance of
rain to the western and Central Island chain.
Synopsis and model discussion...mc
southwest Alaska/Bering Sea/Aleutians...tp