Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
645 PM EDT Wed Sep 23 2020
Valid 12Z Sun Sep 27 2020 - 12Z Thu Oct 01 2020
...Overview and Guidance/Predictability Assessment...
Model/ensemble guidance is agreeable and consistent in showing a
transition to a very amplified upper pattern, featuring a strong
ridge over western Canada and a mean trough (with one or more
embedded lows) over and extending well south from the western
mainland. This configuration should lead to a significant
increase of precipitation over the Panhandle and portions of the
southern coast. Guidance behavior thus far suggests well above
average confidence in the mean pattern but models have had
considerable difficulty with the specifics for individual systems.
Forecast preferences essentially fall into three segments with
fairly quick transitions between each. The first, centered on day
4 Sun, focuses on low pressure that may track toward the
Panhandle. The starting blend emphasizes the 12Z CMC/00Z ECMWF
(with 20 percent total 12Z GEFS/00Z ECMWF means) as those two
model solutions are the most consolidated and closest to the
means. GFS runs are somewhat fast, the 12Z UKMET quite
deep/westward, and the 12Z ECMWF more fragmented. Then day 5 Mon
is a transition to days 6-7 Tue-Wed which incorporate 70 percent
total 12Z ECMWF/GFS (30 percent means) for the low pressure
expected to track northward toward Kodiak Island. The GFS may be
a tad fast given the ensemble means, thus favoring somewhat more
ECMWF input, but the ECMWF does represent a faster trend from the
00Z run. The operational model emphasis helps to maintain a
reasonable amount of gradient for what could be a windy system in
light of the amplified pattern. After this system arrives the
surface details near the mainland rapidly become more ambiguous.
An elongated area of Northwest Pacific low pressure (extratropical
evolution of T.S. Dolphin) dropping into the mid-latitude Pacific
should become the dominant system within the forecast domain by
day 8 Thu. At that time detail uncertainties lead to majority
weight of the ensemble means in the forecast.
...Weather/Threats Highlights...
The general pattern evolution will favor increasing potential for
heavy precipitation along the Panhandle and portions of the
southern coast next week. The details of timing, magnitude, and
coverage are far from resolved though. The most common theme in
the guidance is for a leading system tracking into the Panhandle
on Sun to focus the most precip over or south of the southern
Panhandle, followed by a Mon-Wed system tracking northward toward
Kodiak Island--pulling significant moisture into the Gulf of
Alaska and the Panhandle/southern coast. The trailing front could
contain one or more waves that would produce localized enhancement
beyond midweek. Elsewhere, expect periods of mostly light
precipitation over the Aleutians/Alaska Peninsula with some
scattered activity over the mainland away from the main focus in
the south.
Below normal high temperatures will prevail over the North Slope
(though gradually moderating with time) and southern
mainland/Panhandle. Some pockets of warmer than average highs
could exist over the south though. Other locations should see
above normal highs during the period. Expect widespread coverage
of above normal lows with greatest anomalies tending to be over
some central/northern areas. Any modestly below normal lows
should be localized and limited to the extreme
western/southwestern mainland.
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Wed, Sep 28-Sep 30.
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