Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Tue Jul 27 2021

Valid 12Z Sat Jul 31 2021 - 12Z Wed Aug 04 2021

...Heavy rainfall possible for parts of western and northern
Alaska...

...Overview and Guidance/Predictability Evaluation...

The majority of guidance continues to show strong
amplification/deepening of an upper trough with embedded closed
low between Siberia and the Aleutians while upper ridging builds
downstream over portions of the mainland and into northwestern
Canada.  This upper ridge could keep an upper low meandering off
the Panhandle/Haida Gwaii for most of the period.  The developing
pattern over and west of the mainland should be favorable for a
significant amount of moisture (possibly a combination of western
Pacific tropical moisture and another source from west-southwest
of Kamchatka) to reach parts of the western and northern mainland.
 However individual model runs show a disconcerting degree of
spread for important large scale aspects of this evolution--which
historically tend to have better predictability than what the
models currently suggest in this case.  Therefore confidence is
currently lower than desired for the location, duration, and
magnitude of what could be a significant rainfall event over some
areas.

General preference for the overall forecast evolution is closest
to the ensemble means in principle as they have been the most
agreeable and stable for the potentially strong Bering Sea storm
system along with its supporting dynamics, plus the downstream
ridge.  Note that the means have generally been showing a trend
toward a stronger ridge over the past couple days.  Recent ECMWF
runs have been closest to the means in a broad sense but have also
been on the extreme side of guidance for the duration and
magnitude of high precipitable water values over the western
mainland--up to a few standard deviations above climatology.  GFS
runs have tended to be southward and become more negatively tilted
with the upper low/trough versus the means and other guidance
while pulling downstream ridging farther northwestward than other
guidance.  The 06Z/12Z GFS evolution tends to favor southwestern
areas.  Early in the period the 06Z run did compare better to
consensus than the 12Z run.  The 18Z GFS has adjusted closer to
the majority cluster.  Meanwhile the latest CMC runs have tended
to shift the overall pattern east of consensus, leading that model
to spread a broader and eastward area of heavy rainfall over the
mainland.

Aside from the 12Z ECMWF's faster trend, the more common trend has
been slower for the upper low off the southern Panhandle and Haida
Gwaii.  This corresponds to the recent stronger trend for the
ridge that builds to the north, with the average of ensemble means
suggesting the low could meander over the northeastern Pacific
through the end of the period.

Based on guidance comparisons, today's forecast started with a 70
percent model/30 percent mean blend for days 4-5 Saturday-Sunday. 
Models consisted of 00Z/12Z ECMWF runs along with the 06Z GFS and
12Z UKMET while the 12Z GEFS/00Z ECens provided the mean input. 
The blend quickly transitioned to 70 percent means and 30 percent
total 00Z/12Z ECMWF by days 7-8 Tuesday-Wednesday.


...Weather/Threats Highlights...

Expect one or more episodes of potentially heavy rainfall over
parts of the western and northern mainland.  The best overlap of
guidance in terms of enhanced moisture is around Sunday-Monday
while favored terrain over the Seward Peninsula and Brooks Range
currently has the best probability for highest totals during the
period.  Potential exists for other areas over the west/north to
see significant rain as well as before and/or after the most
favored days.  A leading shortwave and surface system may produce
one period of focused precipitation over the northwest early in
the weekend while the combination of a stalled front and possibly
some western Pacific tropical moisture included within southerly
flow ahead of a Bering Sea storm could provide the added
enhancement thereafter.  Deceleration of the front extending from
the Bering storm may support persistence of rain late in the
period.  It is still within the realm of possibility that this
event could produce some rainfall amounts approaching
climatological extremes.  The Panhandle may see mostly
light/scattered rain at times ahead of the lingering northeastern
Pacific upper low.

The upper ridge building over the eastern mainland/northwestern
Canada should promote a warmer trend for highs over
eastern/interior areas.  The northern coast of the mainland may
also see above normal highs.  Below normal highs should prevail
elsewhere, though the southern Panhandle could see pockets of
warmer readings.  Min temperatures will tend to be above normal
over most of the state with only a few localized areas below
normal.

Rausch


Hazards:
- Heavy rain across portions of mainland Alaska, Sat-Mon, Jul
31-Aug 2.

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