Scientific Forecaster Discussion
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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
443 am akdt Thu may 25 2017
Analysis and upper levels...
an upper level trough begins to push into the southern Mainland
this morning from the eastern Bering Sea, with ridging over
Southeast Alaska continuing to progress further eastward. At the
surface, a weakening front dissipating over the northern Gulf
Coast continues to bring some showery and gusty conditions to the
area as seen with radar imagery and observations. Another
weakening front is tracking into the southwest Mainland from the
eastern Bering, keeping onshore flow and another round of showery
conditions over the area. Further west, a weak surface low over
the western Aleutians has pushed further eastward into the central
Aleutians, bringing some rain and gusty winds to the area.
model guidance overall is in fairly good agreement with the timing
of synoptic features through the remainder of the week. However,
models begin to show some differences going into the weekend with
regards to a low south of the western Aleutians and another low
south of the Alaska Peninsula. These discrepancies have caused
some uncertainty with mainly the timing and amounts of
precipitation moving over the western/central Aleutians and over
the Gulf Coast/southern Mainland over the weekend. With that said,
a more blended approach was used for the forecast this morning to
product a more consolidated solution.
panc...gusty southeast winds will diminish slightly this morning
across the Airport complex, then begin to pick back up in the
afternoon and evening hours. VFR conditions are expected today,
except for a slight chance of some MVFR conditions in showers this
afternoon and evening.
Short term forecast southcentral Alaska (days 1 and 2)...
everything looks on track for heavy showers to develop across
portions of southcentral today as a sharp trough approaches and
the atmosphere quickly destabilizes due to impressive cooling
temperatures aloft. The most likely areas for convective bands
to set up include along the west side of the Kenai and Chugach
Mountains, as synoptic low level southwest flow coming up Cook
Inlet converges with shallow southeast flow crossing the mountains.
Further north, heavier bands could set up just about anywhere
across the Susitna Valley and Copper River basin. Maintained the
slight chance of thunderstorms over all of the mentioned areas
due to the steep mid-level lapse rates, dynamic lift, and upslope
along the mountains.
Meanwhile, as colder low level air moves in snow levels will
continue to fall. Generally expect snow levels to fall to 2000
feet or so this afternoon, though the heavier precip could drag
these locally lower. Issued a Special Weather Statement to
highlight both the heavy showers and potential for snow. Showers
will taper off from south to north tonight as the trough exits to
the north. For areas where showers linger, such as up along the
Alaska Range, there could be some light snow accumulation as
temperatures and snow levels continue to drop. The presence of
some wind will keep temps above freezing for most of the valleys
across the region. However, for wind sheltered areas temperatures
could bottom out near freezing.
A weaker short-wave trough moving up from the south on Friday
combined with continued instability with the cold air aloft will
keep some showers in the forecast for Friday. A low pressure
system tracking south of the Alaska Peninsula and Kodiak Island
will then spread steady rain across Kodiak Island and the Gulf
Friday night and Saturday. With flow shifting to southeast the
bulk of rain will hang up along the coast, with drier conditions
over inland areas of southcentral.
Short term forecast southwest Alaska (days 1 and 2)...
the cold upper level trough swinging onshore this morning will
track from the Kuskokwim Delta coast, inland across the Kuskokwim
valley this afternoon. Instability associated with this trough
will bring isolated to scattered thunderstorms this afternoon and
evening. With very low snow levels, small hail is likely out of
the thunderstorms as well as from some of the stronger showers.
Snow will also mix in with the showers at times this morning and
Although a broad upper level trough will remain over the area,
showers will diminish to a more scattered coverage late tonight
through Friday. A surface low south of the Alaska Peninsula and
accompanying trough will increase shower coverage along the Alaska
Peninsula and around the greater Bristol Bay area a bit more
Friday afternoon and evening. Easterly flow will increase through
the weekend as an upper level low moves into the southwestern
Gulf. In addition to bring increasing easterly gap/downslope winds
to the Lee/west side of the Alaska and Aleutian Range, easterly
waves rotating around low will bring chances for rain at times
through the weekend.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
zonal flow will continue across the Bering and Aleutians with the
next frontal system tracking quickly to the east along the
Aleutian chain today. The surface low will arrive in the eastern
Aleutians this evening and then continue east to be south of the
Alaska Peninsula Friday. The low will then sink slowly to the
south Friday night and Saturday before lifting back north to the
southwest Gulf on Sunday. Another frontal system will push into
the western Aleutians late Friday night and Saturday morning and
continue east to the central Aleutians Saturday night before
weakening and dropping south Sunday.
Long term forecast (days 3 through 7)...
the weekend will begin with the elongated cold upper level trough
over western and southwest Mainland Alaska. This will keep below
normal temperatures over most of the forecast area...especially in
southwest Mainland Alaska. Southcentral will be in due south flow
aloft which is favorable for precipitation. As the weekend
progresses a cut off low should develop near Kodiak off the main
upper level low allowing the main low to retreat into the Arctic.
After this the main pattern remains a bit unclear as models
continue to struggle with the synoptic pattern. Overall the
deterministic runs are all over the place with the GFS ensemble
means a little bit better. Best bet is that early next week
remains cloudy with some showers, but temperatures should be
increasing and back to near normal for much of the week. This
unsettled pattern is producing below normal confidence in the long