Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
735 PM EDT Sat Jul 13 2024

Valid 12Z Wed Jul 17 2024 - 12Z Sun Jul 21 2024

...Wet pattern for the Alaska Panhandle continuing into late
week...

...Overview...

From Wednesday through next Sunday, two primary systems will
produce areas of active weather during the period.  The first will
be an upper low forecast to track from the southwestern mainland
to south of the Panhandle, leading to Northeast Pacific surface
development late this week and another episode of potentially
significant rainfall over the Panhandle with some moisture likely
extending back to the Southcentral coast.  Meanwhile, expect an
upper low tracking from near southern Kamchatka into the western
Aleutians/southern Bering Sea to support an associated surface
system (with leading wavy front) and broad area of organized
rainfall.  The upper pattern over the Interior should gradually
transition from troughing to ridging as the initial upper low
drops southeastward and an upper ridge over the Bering Sea builds
inland.  Most guidance shows the North Slope being on the
southeast fringe of high latitude cyclonic flow aloft, so one or
more fronts could get close to the northern coast.

...Guidance Evaluation/Preferences...

There have been some differences and variability for some aspects
of the forecast among the dynamical and machine learning (ML)
guidance, but overall agreement is better than average at this
time.  The 12Z operational model runs have improved clustering for
surface details over the Northeast Pacific/Gulf of Alaska during
the latter half of the week.  Accounting for the majority themes
for individual features, today's forecast started with a blend of
the 12Z GFS/ECMWF/CMC for the first half of the period and then
incorporated up to 30 percent total of the 12Z GEFS/00Z ECens
means thereafter.

Regarding the evolution over the northeastern Pacific/Gulf of
Alaska, a model/mean average looks reasonable for depicting the
overall idea of Wednesday's upper low over the southwestern
mainland slowly tracking southeastward and most likely reaching a
position south of the Panhandle by next Sunday. Like the various
operational model runs, the 00Z ECMWF-initialized ML models
diverge somewhat regarding precise upper low track and shape over
the course of the period with the ML models not favoring a
particular side of the spread.  There had been more spread with
important surface details due to sensitivities to the upper low
itself and separate supporting energy initially to the south.  12Z
model runs have come in with a more agreeable Wednesday-Friday
evolution with respect to low pressure lifting north and then
northwest into or close to the Gulf of Alaska--as supported in
principle by the ML models.  Consensus shows the surface system
weakening and dropping south by next weekend.

Most guidance displays well above average agreement and continuity
for the system tracking into the Aleutians and Bering Sea late
this week through the weekend.  ML models are close to the
dynamical model/ensemble mean majority, providing some added
confidence to the forecast for the time being.  The 12Z UKMET and
00Z ECMWF differed from the majority in being somewhat
slower/southwest with the system.  Thus the forecast did not
incorporate any of those two particular runs.

Likewise, an overwhelming majority of guidance agreed in principle
with the initial Bering Sea upper ridge building eastward into the
mainland to replace the leading trough, with typical detail
differences.  The past couple CMC runs have provided the most
notable discrepancy for the ridge by the weekend, straying farther
north than other dynamical/ML solutions.  The blend made sure to
keep CMC weight low enough to downplay its details for the upper
ridge.

...Weather/Hazards Highlights...

The Panhandle will likely see another episode of enhanced
precipitation during the latter half of the week as surface low
pressure most likely lifts northward and then northwestward over
the Northeast Pacific into or near the Gulf of Alaska.  Best
potential for highest totals is over the southern two-thirds of
the Panhandle with lesser amounts extending as far back as the
Southcentral coast.  The upper low over the southwestern mainland
on Wednesday may also support some areas of midweek rain over
southern areas before drifting southeastward and pulling in the
Pacific surface low.  Panhandle/southern coast rainfall should
trend lighter and less organized by the weekend.  Meanwhile a
leading wavy frontal system and then a larger-scale system
tracking into the Aleutians and Bering Sea will spread areas of
rainfall gradually eastward across the Aleutians during the
period.  The leading edge of this moisture could reach into the
southwestern mainland by next weekend.  Winds with the
Aleutians/Bering Sea system may become brisk but will likely stay
well below hazardous criteria.

Initial upper troughing will support below normal highs over much
of the state mid-late week, though with gradual moderation as the
upper trough weakens.  Some areas of above normal readings may
develop especially over the Interior by the weekend as an upper
ridge builds over the mainland.  Negative anomalies for min
temperatures should be more moderate than those for highs early in
the period, followed by increasing coverage of modestly above
normal lows by next weekend.

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