Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Wed Jul 07 2021
Valid 12Z Sun Jul 11 2021 - 12Z Thu Jul 15 2021
...Heavy rain threat for the Panhandle and coastal Southcentral
this weekend...
...Overview...
An upper low with associated surface system will drift over the
Gulf of Alaska/Northeast Pacific during the period, with the
greatest moisture likely to extend into the Panhandle and coastal
parts of Southcentral through the weekend before tapering down
somewhat with time. Behind this system, a Bering/Siberia upper
ridge will build over portions of the mainland while a trailing
weak Bering upper low/trough exhibits some combination of slow
northward and/or eastward motion. Guidance shows very fast and
low-amplitude flow across the Arctic during the period. A mean
frontal boundary on the southern side of the upper jet should
meander near the northern coast from the weekend into the first
part of next week and then possibly drop a little farther south by
midweek or so. This front and any waves may produce periods of
precipitation.
...Guidance/Predictability Evaluation...
Among today's guidance the ECMWF along with the 12Z CMC/UKMET
generally clustered the best among each other and the ensemble
means early in the period. Then by days 7-8 Wednesday-Thursday a
transition toward the past two ECMWF runs, 12Z CMC, and 12Z
GEFS/00Z ECMWF means represented the most common themes of
guidance while downplaying details that have lower predictability.
The 12Z GFS differed from the majority for the shape/evolution of
the Bering into mainland upper ridge and this eventually caused
the low south of the mainland to become more progressive. The
UKMET strayed a bit north/northeast with the upper low near the
end of its run so its weight was reduced at that time. As for
trends, over the past couple days there has been somewhat of an
eastward tendency in forecasts valid early in the period but then
latest models and means offer more of a mix between slower and
faster trends--supporting the blended/compromise approach.
Meanwhile the fast and low-amplitude nature of Arctic flow lends
itself to low predictability for specifics over northern areas.
The GFS has been particularly erratic lately, with the 12Z run
bringing lower heights into the northern mainland versus other
models/means while the 06Z run had ridging that deflected flow a
lot farther northward. The last couple ECMWF runs and 12Z CMC
compared fairly well to the ensemble means in principle.
...Weather/Threats Highlights...
Expect the Gulf system to bring the highest precipitation totals
into the Panhandle during Sunday-Sunday night, with 1-2 inches
possible during that time. Locally higher totals would not be out
of the question depending on exact track and strength of the
system. Some meaningful precipitation may extend into the
southeastern corner of Southcentral as well. Precipitation
intensity should decrease to light-moderate from Monday onward but
could persist over at least the Panhandle for one or more days.
High temperatures will be below to well below normal over the
Panhandle and parts of Southcentral during the period. Snow
levels will likewise be lower than average for the middle of
meteorological summer, especially during the weekend. Northern
areas may see one or more episodes of precipitation with the mean
frontal boundary that should be near the northern coast early in
the week and possibly drop southward by Wednesday-Thursday. Thus
expect northern areas to see some above normal readings early in
the week followed by a cooling trend that may bring highs down to
below normal levels. Interior areas are likely to see near to
below normal highs trending to mostly above normal by Tuesday as
an upper ridge builds in, followed by slight moderation as the
ridge weakens a bit.
Rausch
Hazards:
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Sat-Sun, Jul 10-Jul 11.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards 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