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FXAK02 KWNH 040005
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
805 PM EDT Fri May 03 2024

Valid 12Z Wed May 08 2024 - 12Z Sun May 12 2024

...Overview...

The leading upper low over the northeastern Pacific/Gulf of Alaska
(associated with an impactful system during the short range
period) is expected to weaken and open as its energy lifts
northward with time, pushing into mean ridging over Canada into
the northern mainland. This feature will be accompanied with
meaningful precipitation to the south-central and eastern parts of
the state. 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...

Followed a similar model blend approach as the previous forecast
to provide some continuity with the average to below average
confidence pattern evolution. Initial weighting was 40 ECWMF/30
UKMET/30 GFS since the overall handling of the leading low
weakening/lifting northward was decent, although did include two
runs of the GFS to help fine tune the location of the system near
the Aleutians as it progressed. The EC ensemble means and GEFS
means will added in by Day 5 and increased weighting to 30% for
days 7 and 8. This helped reduce some of the noise, particularly
the Aleutians/western Gulf region.

Overall, there was decent clustering with the weakening upper low
as it drifts north and the northeast from the Northeast
Pacific/Gulf of Alaska, with the initial surface reflection
weakening in place. By midweek, there is a general consensus for
upper trough with an embedded low  along with best emphasis for
surface low pressure near the Aleutians and into the Alaska
Peninsula; however there continues to be a fair amount of spread
with any specifics for this feature. There is better than average
agreement toward the overall upper/surface low reaching near the
Alaska Peninsula by Friday. 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.  There has been trends from the CMC
ensemble mean to track 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. At this time, use of the CMC/CMC
ensemble means were not utilized.

...Weather/Hazards Highlights...

Episodes of locally heavy rainfall will be possible across parts
of the eastern and southern Mainland initially and into midweek as
the initial upper low offshore the southern coast/Panhandle
gradually opens up and lifts northward/northeastward. PW values
will be in the 0.25 to 0.50 range which will be 1 to 2 standard
deviations above average for early May. Currently the details are
too uncertain for evaluating how this activity may influence any
river flooding due to ice jams and snowmelt. Light precipitation
may hang around 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. A burst of PW values
0.5 to 0.75 inch (about 1.5 standard deviations above average)
will help bolster rainfall amounts and intensity for portions of
the southern Coast and the Panhandle. A heavy rain area spanning
from portions of the Alaska Peninsula through Kodiak Island, along
the southern coast and covering a majority of the Panhandle in the
Days 3-7 Hazards Chart. The Aleutians may see another episode of
rainfall by next Friday with another system but confidence in
specifics is fairly low at this time.

Given the increased cloud cover and potential for rain early in
the week for southern and eastern portions of the state the daily
maximum temperatures are expected to be below normal for much the
extended period; with the eastern Interior tending to have the
best chance for some above normal readings. Elsewhere, the weather
pattern would support mix of both above and below seasonal average
temperatures over the far western reaches of the state. 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.

Campbell/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

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