Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Wed Apr 17 2024

Valid 12Z Sun Apr 21 2024 - 12Z Thu Apr 25 2024

...Overview...

For the start of the forecast early Sunday, today's models and
ensembles continue to show a fairly strong upper ridge extending
from western Canada through the mainland while an elongated upper
low prevails over/south of the Alaska Peninsula.  Meanwhile a
compact upper low should be dropping south over the western Bering
Sea at that time.  Latest guidance suggests the Alaska Peninsula
upper low could split with most surface low pressure emphasis
tracking well south of the mainland.  A more pronounced large
scale trend seems to be taking shape to the west, with solutions
gravitating toward the idea of a longer term mean upper low
becoming established over the Aleutians/Bering Sea.  This mean low
would be comprised of the initial western Bering Sea low plus
input from one or more emerging western Pacific features and
possibly some additional energy dropping south from Siberia. 
Consensus suggests that mean ridging aloft may persist over most
of the mainland, but with uncertainty over its shape/orientation
and possible weak embedded shortwave(s) through the course of the
period.

...Guidance Evaluation and Preferences...

The full array of guidance is not very well clustered regarding
exactly what will happen with the initial upper low over and south
of the Alaska Peninsula as of early Sunday.  The 12Z GFS/UKMET are
the most pronounced in taking what splits off from the northern
side of the feature and carrying it eastward across the mainland. 
Most other solutions, including a majority of 00Z
ECMWF-initialized machine learning models (MLs), either keep a
more consolidated low farther south or at least keep the northern
energy west of the GFS/UKMET.  The smaller scale details involved
with this evolution lead to low predictability for specifics.  An
intermediate latitude provides a reasonable starting point given
the north-south spread with the primary surface low, with
consensus showing an eastward track well south of the mainland
after Sunday.

Over the past day, most of the dynamical guidance has trended
strongly toward prior signals from the MLs for the evolution over
the Aleutians and vicinity. Thus the latest consensus advertises
the western Bering Sea upper low dropping just south of the
western Aleutians and developing fairly strong surface low
pressure already by late Sunday or early Monday.  This low may be
strongest around Tuesday, with the 12Z GFS/ECMWF reaching as low
as the upper 960s mb (though the new 18Z run has tempered it to
the mid 970s mb).  MLs are split between greatest depth in the
970s or 980s mb.  Toward Wednesday-Thursday the typical
divergences develop regarding details of the upper low (due to
different sources of Pacific and Siberia energy potentially
feeding into the circulation) along with one or more surface lows.
 Latest GEFS/CMCens/ECens means are remarkably well clustered for
the low at the surface and aloft by this time, favoring a decent
weight of their ideas late in the forecast.  Minus any effects
from weak shortwaves surrounding or embedded within the mainland
upper ridge, the approaching upper low may lead to somewhat more
of a north-south orientation of the mainland ridge late in the
period (latest GFS/ECMWF runs) or broader southwesterly flow
(GEFS/CMCens mean).  The new 12Z ECens mean essentially becomes a
hybrid between the early-period configuration and the later
GFS/ECMWF cluster.

Combining the majority scenario among dynamical guidance and MLs
plus locally available data, the first half of the forecast
started with a 50/30/20 blend of the 12Z ECMWF/00Z ECMWF/12Z CMC. 
Then the forecast transitioned toward 30-50 percent input from the
12Z GEFS/CMCens means with the operational model component
comprised of the two ECMWF runs and 12Z GFS replacing the CMC (as
some details of the latter strayed more from the majority).

...Weather/Hazards Highlights...

The circulation around low pressure to the south of the Alaska
Peninsula as of early Sunday should continue to produce some
precipitation over the Peninsula and Kodiak Island plus the Kenai
Peninsula--and perhaps a little farther inland over the
southwestern corner of the mainland--into early next week. 
However uncertainty over important but low-predictability details
will keep confidence low for specifics in terms of precise
magnitude and location.  Overall totals should trend lighter
relative to earlier days.  Meanwhile confidence is increasing with
respect to the potential for fairly deep low pressure to develop
near the western half of the Aleutians early next week, with
greatest strength most likely to be around Tuesday.  This system
may produce strong winds and there will be ongoing evaluation to
see if wind speeds may approach hazardous criteria as guidance
stabilizes for the storm's evolution.  Surface low details become
more uncertain by Wednesday-Thursday due to potential influence of
one or more bundles of energy feeding into the overall circulation
aloft.  Thus the initial system may persist or another surface low
may track into or develop over the region.  Either way it looks
like a multi-day stormy pattern with episodes of precipitation
extending from the Aleutians into the Alaska Peninsula with the
leading edge of moisture possibly extending into other parts of
the western mainland late in the period.

The majority of the state should see above normal temperatures
during the Sunday-Thursday period.  As for exceptions, some
erosion of the northern part of the initial east-west upper ridge
across the mainland still appears likely and should allow a front
to settle over the northern-central mainland, with the North Slope
possibly trending toward near or slightly below normal readings
next week.  Also, localized parts of Southcentral and the northern
Panhandle could see one or more days of temperatures close to
normal.

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