Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Thu May 02 2024

Valid 12Z Mon May 06 2024 - 12Z Fri May 10 2024

...Overview...

An upper low initially over the northeastern Pacific/Gulf of
Alaska (associated with an impactful system during the short range
period) should steadily weaken and open up as its energy lifts
northward with time, pushing into mean ridging over Canada into
the northern mainland.  This energy may produce some areas of
meaningful precipitation over Southcentral and vicinity. 
Meanwhile, the combination of a northern Bering Sea upper low and
a system emerging from the northwestern Pacific should ultimately
support an Aleutians system whose surface reflection with mean
upper low may settle over or near the Alaska Peninsula by late
next week.  This system will likely bring increasing precipitation
to the southern coast and Panhandle during the middle to late part
of next week.  With lower confidence, another system may have some
influence on portions of the Aleutians by next Friday.

...Guidance Evaluation and Preferences...

A blend approach looks good for resolving the weakening upper low
expected to drift north and the northeast from the Northeast
Pacific/Gulf of Alaska, with the initial surface reflection
weakening in place.  Regarding the Bering Sea through North
Pacific evolution, there is at least the common theme of overall
upper troughing with an embedded low by mid-late week, along with
best emphasis for surface low pressure near the Aleutians and into
the Alaska Peninsula.  However dynamical and machine learning (ML)
guidance has been showing a lot of detail differences as can be
expected with such a scenario involving uncertain interaction of
separate features.  Adding in the 12Z guidance there is a notable
trend that favors trimming away the western side of the envelope
for the Bering Sea upper low by around midweek.  This leaves the
12Z GFS as a modest western extreme given adjustments in the other
guidance including the new 12Z ECens mean.  Beyond this aspect of
the forecast, there is better than average agreement toward the
overall upper/surface low reaching near the Alaska Peninsula by
Friday (while weakening at that time) but individual dynamical and
ML solutions offer minimal agreement on finer details that will
take a while to get resolved.

Between the initial Northeast Pacific feature and Aleutians
system, guidance shows a very wide spread for the potential track
of a mid-latitude wave forecast to be well south of the Aleutians
as of early Monday.  The majority of solutions would keep the wave
near or south of 50N latitude and have no effect on Alaska.  On
the other hand the 12Z CMCens mean tracks it to Kodiak Island
while the past couple CMC runs have brought it into the northeast
Pacific.  Such a solution would yield an earlier increase of
moisture along the southern coast and/or Panhandle.  The ML models
do not add any clarity, as their envelope generally spans the
range of dynamical guidance.  At this time preference the
maintains continuity via the southern scenario, keeping the
CMC/CMCens out of the forecast.

Thus far there has been some spread and trending in the guidance
for a western Pacific system that may affect parts of the
Aleutians by late next week.  Ensemble means have been on the
slow/ill-defined side while the ML models show a faster/northward
depiction that would have more influence on the Aleutians.  The
12Z ECMWF is closest to the 00Z ML idea while the 12Z may stray a
little north and the GFS runs (like the 00Z ECMWF) track somewhat
south but have similar timing.  Latest signals favor tilting the
forecast as close as possible to the 12Z ECMWF for now.

Above guidance comparisons led to starting today's forecast with a
40/30/30 blend of the 12Z ECMWF/12Z GFS/12Z UKMET early in the
period.  By day 6 Wednesday the blend started to add a little 12Z
GEFS/00Z ECens input while splitting the GFS among 06Z/12Z runs to
help center the Aleutians system.  Days 7-8 incorporated only 30
percent total of the means as the 12Z GFS/ECMWF were fairly close
to them in principle near the mainland and given the ECMWF
preference near the western Aleutians for Friday.

...Weather/Hazards Highlights...

Portions of the southern and eastern mainland may see one or more
episodes of locally enhanced precipitation early-mid week as the
initial upper low offshore the southern coast/Panhandle gradually
opens up and lifts northward/northeastward. Currently the details
are too uncertain for evaluating how this activity may influence
any river flooding due to ice jams and snowmelt.  Lingering mostly
light precipitation is possible along the southern coast and
Panhandle early in the week.  The system tracking near the
Aleutians should bring that region mostly light precipitation
during the first half of the week.  As the surface low reaches the
eastern Aleutians and Alaska Panhandle, leading flow will likely
bring increasing moisture to the southern coast and eventually
Panhandle.  By next Wednesday-Thursday the forecast rainfall is
sufficient to merit a heavy rain area from portions of the Alaska
Peninsula through Kodiak Island and into some of the Southcentral
coast in the Days 3-7 Hazards Chart.  The extent to which rainfall
may reach hazardous criteria into the Panhandle will be evaluated
tomorrow.  The Aleutians may see another episode of rainfall by
next Friday with another system but confidence in specifics is
fairly low at this time.

Expect below normal high temperatures over most of the eastern and
southern parts of the state early in the week, with the eastern
Interior tending to have the best chance for some above normal
readings.  The forecast pattern should keep the southern coast and
Panhandle below normal for highs through the rest of the week
while other areas trend toward a mix of above/below normal highs. 
Morning lows will tend to be more above normal over most of the
state (highest anomalies over the North Slope), though still below
normal over the far west early in the week.  The Southcentral
coast and Panhandle may see some localized below normal lows as
well.

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