Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
337 PM EDT Tue Jul 23 2019
Valid 12Z Sat Jul 27 2019 - 12Z Wed Jul 31 2019
The most stable and agreeable aspects of the extended Alaska
forecast are the shortwave expected to track from near the Alaska
Peninsula early day 4 Sat across the Gulf/Panhandle through the
weekend and an Arctic ridge that drifts westward with time.
Elsewhere guidance over the past couple days has shown a lot of
spread and run-to-run variability. Since yesterday model/ensemble
clustering has strongly gravitated toward the idea of keeping the
short range northern Bering/eastern Siberia upper low on the
western side of prior spread--versus a modest majority that had
brought it southeastward. Even early in the period there are
still significant differences for the initial upper low but loose
agreement exists for an elongated trough (with one or more
embedded upper lows) over portions of northeastern Siberia and
extending into the western mainland. In addition there is spread
regarding details of flow to the south of this axis of lowest
heights aloft, plus northern Pacific/Aleutians flow along with
what interaction may occur between the two. Among the most
pronounced differences, 06-12Z GFS runs become slower than other
guidance with flow near the Aleutians during the weekend while the
past four ECMWF runs through the 00Z cycle have been alternating
between high and low pressure at the surface over the Bering in
the Mon-Tue time frame.
A 70 percent weight toward the 00Z operational models relative to
the 06Z GEFS/00Z ECMWF means reflects the most common ideas/detail
of guidance while downplaying other less confident specifics for
days 4-5 Sat-Sun. With the ensemble means providing the most
stable forecast late in the period the initial blend adjusts to an
even model/mean blend for day 5 Mon and then goes 90-100 percent
to the means thereafter. This approach maintains the forecast of
a North Pacific warm front lifting toward/to the western Aleutians
while a modest surface ridge covers the Bering. Meanwhile there
is a hint of a weak trough aloft settling over the Gulf of Alaska
with modest low pressure over the northern Gulf.
The Peninsula through Gulf/Panhandle shortwave and corresponding
surface low/frontal system this weekend will provide a brief
period of enhanced precipitation to areas along the southern
coast/Panhandle. Otherwise a fairly broad area will see some
precipitation potential for one or more days during the period but
with low confidence in location/timing specifics due to the
uncertainty in flow details aloft and corresponding surface
evolution.
Rausch
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
No significant hazards are expected over Alaska during this
forecast period.
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