034
FXAK02 KWNH 052354
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
754 PM EDT Sun May 05 2024

Valid 12Z Thu May 09 2024 - 12Z Mon May 13 2024

...Wet/snowy pattern along the southern coast and Panhandle late
this week through the weekend...

...Overview...

Models/ensembles have maintained the general theme of a mean upper
low over or near the Alaska Peninsula during the period, likely
consisting of one departing low and arrival of a second from the
west.  This pattern should favor potentially significant
precipitation focus along the southern coast and Panhandle.  With
typical minor adjustments, consensus still shows a leading
vigorous system tracking through the Gulf and into the mainland
Thursday into Friday, bringing a brief episode of heavy
rain/higher elevation snow to areas from the Kenai Peninsula
through the Panhandle.  Then latest guidance trends are increasing
the probability that a trailing system tracking near the Aleutians
and over or south of the Alaska Peninsula late week through
Sunday-Monday will spread organized rainfall across the Aleutians
and then rain/mountain snow from the Alaska Peninsula (which may
see a period of enhanced easterly wind/moisture focus) through the
Panhandle.  Meanwhile the majority of guidance still shows an
upper trough axis near the western mainland, possibly
transitioning to a col region at times.  Also, western Canada
upper ridging could periodically extend back into parts of the
mainland.

...Guidance Evaluation and Preferences...

Over the past day the operational models have remained fairly
consistent with the overall forecast of the wave tracking
northward through the Gulf of Alaska and into the mainland
Thursday-Friday.  There is still the typical spread and run-to-run
variability for exact strength/track details, with the latest GFS
runs and 12Z UKMET a bit on the western side of the envelope along
with the 12Z CMCens mean while the 12Z ECMWF/ECens mean are
similar to the 00Z run.  All but the CMC show a depth under 1000
mb.  00Z ECMWF-initialized machine learning (ML) models continue
to be on the weaker side.  While the supporting shortwave energy
is on the eastern side of the overall upper low circulation near
the Alaska Peninsula, there is lingering uncertainty over what
influence that upper low may have on this system.  The new 18Z GFS
even goes beyond this point, dropping the upper low farther south
which not only pulls the Gulf system farther west but then creates
a separate trailing Gulf wave by Friday as the remaining upper
shortwave lifts northeastward.

The other primary storm of interest should be over/near the
western Aleutians as of early Thursday.  24 hours ago there was
wide guidance spread for the latitude of this system as it
continues eastward (with the CMC/CMCens even holding it well
westward for a time), but now the 00Z/12Z guidance runs have
gravitated much closer together and in the direction of the
ECMWF/ECens which have generally been on the northern side of the
envelope near the Aleutians.  The 12Z ECMWF did nudge a tad south
of its 00Z run though, while the 12Z GFS/CMC are a bit on the
southern side of the narrowing envelope.  The ML models have been
favoring a track fairly close to the Aleutians.  By the weekend
into early next week the full array of dynamical and ML guidance
diverges for upper/surface low position, ranging between the
southeastern Bering Sea and the Gulf of Alaska/Northeast Pacific.
Currently the best intermediate track by the latter half of the
period would have the upper low over or just south of the Alaska
Peninsula with the surface low eventually tracking close to Kodiak
Island.  The ensemble means cluster well in principle at the
surface and aloft, though the GEFS mean is suspiciously weak
compared to the ECens/CMCens means.

Otherwise, details over the rest of the mainland are somewhat
ill-defined or variable aside from a continued general signal
toward a weak mean trough most likely aligned over the west, with
a col region possibly evolving at some point.  There is also
continued potential for periodic extension of western Canada upper
ridging into parts of the mainland.

The first half of the forecast started with 40 percent of the 12Z
ECMWF with the remaining weight split evenly among the
GFS/UKMET/CMC.  This yielded a reasonable intermediate solution
for the Thursday Gulf system.  While recent trends could argue for
even more ECMWF weight for the Aleutians storm, the blend already
provided a meaningfully northward adjustment from continuity while
waiting for guidance to stabilize regarding specific details.  The
CMC strayed south of remaining guidance especially around Sunday
so it was removed from the forecast at that time.  Days 7-8
Sunday-Monday shifted to 40 percent ensemble input (only the 12Z
CMCens/00Z ECens due to the GEFS being weak) while the relative
GFS/ECMWF weights adjusted to a more even balance than on earlier
days.

...Weather/Hazards Highlights...

Continue to expect a compact but vigorous storm to track northward
through the Gulf of Alaska Thursday and then weaken/broaden as it
reaches the Interior by Friday.  This system will likely bring a
period of heavy rain and high elevation snow to areas along and
near the Southcentral coast and Panhandle, with 12Z GFS/ECMWF runs
now agreeing on precipitable water (PWAT) values surging to 2-3.5
standard deviations above normal within the moisture shield.
Currently expect the highest precipitation totals over the
northern Panhandle while the western extent of enhanced
precipitation (near the Kenai Peninsula) will depend on exact
track of the surface low.  This system may produce strong winds
over open waters as well, while strong gap winds will be possible
near the Alaska Range as the low tracks northward through the
southern mainland.  Areas of lighter precipitation may extend
northward over parts of the mainland, still with uncertain
influence on any river flooding due to ice jams and snowmelt.
Behind this system, latest guidance trends are helping to increase
confidence that another storm may track along or just south of the
Aleutians and Alaska Peninsula late this week into the weekend,
bringing organized precipitation and areas of brisk to strong
winds depending on exact track.  As this solution is looking more
likely, there is increased potential for a period of easterly low
level flow to focus precipitation along the Alaska Peninsula--but
with lesser PWAT anomalies than forecast with Thursday`s Gulf
system.  Moisture should continue eastward along the southern
coast and into the Panhandle during the weekend with some
persistence over at least the eastern areas into Monday.

Southern areas will likely see below normal daytime high
temperatures through the period due to the unsettled pattern
across that region, while the North Slope should see below normal
highs as well given a tendency for the surface gradient to produce
a cool northerly low level flow.  This leaves the Interior as the
main region with the potential to see some areas of warmer than
average highs.  Expect greater coverage of above normal morning
lows across the state with mostly localized pockets of below
normal readings.

Rausch


Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html

$$