Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
751 PM EDT Wed Jul 06 2022
Valid 12Z Sun Jul 10 2022 - 12Z Thu Jul 14 2022
...Heavy Precipitation Threat for the Alaskan Southern Tier...
...Overview...
Recent guidance has maintained some general themes for the general
pattern evolution. These include a trend toward broadly cyclonic
Arctic flow while initial mainland upper ridging weakens after
early next week, a northeastern Pacific/southern Gulf upper low
that should ultimately track across the Panhandle and into western
Canada, and farther west a slow-moving system to the south of the
western Aleutians. However there has been a lot of variability
and spread for amplifying energy reaching the Bering Sea and
vicinity. At the moment, consensus is pointing to an upper low
that should be dropping into the northern Bering from Siberia as
of early Sunday and then exiting across the southwestern
mainland/Alaska Peninsula around midweek or so. The forecast
pattern evolution will favor areas of potentially significant
precipitation over parts of the southern mainland and southern
Panhandle, while a front setting up over the northern mainland
next week could provide some precipitation focus as well. Expect
very warm high temperatures over the interior Sunday-Monday to
trend cooler thereafter as the upper ridge weakens.
...Guidance/Predictability Assessment...
Recent guidance spread and variability temper confidence, but for
now there is decent agreement into the first part of next week for
the upper low dropping into the Bering Sea from the north.
Ensemble means are steadily trending toward the operational model
cluster. With time there are still widening differences over how
quickly the upper low departs into the northeastern Pacific. The
12Z CMC is the fastest, though the 12Z ECMWF trended somewhat
faster than the prior 00Z version. At the same time, GFS runs in
particular have been inconsistent with a separate system to the
south of the western Aleutians. ECMWF/CMC runs and their ensemble
means have consistently kept this system to the south of the
Aleutians through the period. Earlier GFS runs did as well, but
the 00Z run brought it into the Bering after ejecting the initial
upper low while the 12Z run phased it with a more persistent
Bering low. The 06Z GFS leaned more to the majority and the new
18Z run has reverted back to look more like consensus. By the end
of the period the 12Z GEFS mean showed more phasing between the
Bering/North Pacific systems versus the ECens/CMCens. Across
northern areas, predictability is low for the shortwave specifics
within the broadly cyclonic mean flow that is forecast to become
established to the south of an upper low near the North Pole.
This favors a conservative approach for specifics of Siberia into
northern mainland surface waves whose general existence becomes
more likely late in the period.
Based on guidance comparisons, the early part of the forecast
started with a blend of the 12Z ECMWF/GFS/CMC. The forecast
excluded the UKMET which was farther east than the otherwise
agreeable model/mean consensus for the initial northeastern
Pacific upper low. Various divergences in the guidance led to
some transitions after Monday. First was to phase out CMC input
due to being fastest with departure of the Bering Sea upper low.
Then the forecast split ECMWF input between the past two runs
given developing timing differences for the Bering low as well as
for high latitude details. Also the GFS input changed from the
12Z run to the 06Z version since the latter matched better with
the majority evolution over the Bering/North Pacific. While
employing these adjustments with the operational guidance,
ensemble mean weight gradually increased toward about half total
and leaned more to the 00Z ECMWF mean relative to the 12Z GEFS
mean since the GEFS depicted the less likely Bering/North Pacific
phasing.
...Weather/Hazard Highlights...
Moisture flowing around the northeastern Pacific/southern Gulf of
Alaska upper low (likely to track over the Panhandle by around
midweek) will contribute to a multi-day threat for moderate to
locally heavy/terrain enhancing precipitation over parts of the
southern mainland. A more focused band of moisture on the
southeast side of this feature may produce heavy precipitation
over the southern Panhandle. Meanwhile, the upper low/surface
system tracking south/southeast through the Bering Sea and
eventually into the northeastern Pacific should bring some
moisture across the Bering, eastern Aleutians, and Alaska
Peninsula/southwest corner of the mainland. The 12Z GFS scenario
would have this system phase with a separate North Pacific system
and bring more moisture northeastward across the mainland. This
potential requires monitoring, but the full array of guidance
suggests this currently has fairly low probability. Farther
north, leading shortwave energy should bring a cold front into the
northern part of the mainland early next week, with a wavy
stationary front likely to persist through Thursday. The front
and embedded waves could focus light to moderate precipitation
across parts of the North Slope. Locally heavier pockets of
precipitation are a possibility but with very low confidence at
this time.
A northeast to southwest band of well above normal high
temperatures over the mainland during Sunday-Monday, corresponding
with the initial upper ridge, will moderate thereafter as the
ridge weakens. Clouds/precipitation will tend to keep
Southcentral and the Panhandle below normal for highs, while the
North Slope will trend to below normal readings after cold frontal
passage. Above normal lows will tend to be more widespread,
especially Sunday-Monday. As with the highs, North Slope lows
will trend below normal from west to east. Some locations over
the southeastern Coast and Panhandle could see slightly below
normal lows.
Rausch
Hazards:
- Heavy rain across portions of mainland Alaska, Sat, Jul 9.
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Sun-Tue, Jul 10-Jul 12.
- Much above normal temperatures across portions of mainland
Alaska, Sat-Mon, Jul 9-Jul 11.
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