Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
552 PM EST Wed Nov 25 2020
Valid 12Z Sun Nov 29 2020 - 12Z Thu Dec 03 2020
...Stormy pattern to continue through the first half of next week
favoring heaviest precipitation over the Panhandle/southern
coast...
...Overview and Guidance/Predictability Assessment...
Today's guidance is consistent and agreeable with the general
evolution aloft for most of the period, with an amplified pattern
featuring a trough over the far western mainland and ridge over
western Canada. This trough-ridge configuration that should
promote heavy precipitation focused over the Panhandle and
southern coast of the mainland may begin to relax a bit toward the
end of the period. Also late, upstream progressive shortwave
energy with lower predictability will likely bring a rapid
increase of surface detail uncertainty across the Pacific.
Leading low pressure reaching the Gulf of Alaska/southern coast by
the start of the period early day 4 Sun should weaken in place
into early Mon. Consensus is taking a little longer to weaken
this low compared to yesterday due to a modest slower/southward
trend for the upstream deep low forecast to track south of the
Aleutians Sun into Mon. The trend for the second low ultimately
leads to a somewhat later arrival into the southern mainland. The
12Z ECMWF has adjusted deeper with the western mainland upper
trough and a little faster/northward with the surface low versus
the 00Z run by Tue, in a partial nod to GFS runs. The ECMWF trend
reversal along with GEFS/ECMWF/CMC ensemble mean support recommend
a solution among the GFS/ECMWF and their means for track/timing
while GFS/ECMWF strength looks reasonable (leaning closer to the
GFS where differences exist). In contrast the 12Z UKMET is slower
from the start of the forecast and CMC becomes slower after early
Mon.
As this storm weakens mid-late week, expect confidence in upstream
specifics to decline rapidly due to progressive Pacific flow with
multiple difficult-to-resolve embedded shortwaves. Forecast
difficulties are highlighted by a general signal for one or more
surface low developments over the central/eastern Pacific but poor
clustering and considerable run-to-run variability for any
individual systems. Meanwhile an elongated area of energy aloft
may support one or more surface lows over or near the Aleutians
but again with fairly low confidence in specifics.
Forecast considerations led to starting with a blend of the 12Z
ECMWF/GFS/CMC days 4-5 Sun-Mon followed by the 12Z-00Z ECMWF/12Z
GFS and modest (20 percent total) 12Z GEFS/00Z ECMWF mean input
for days 6-7 Tue-Wed to reflect the best consensus and eventually
account for modest spread that develops among that cluster,
followed by manual enhancement where appropriate. The forecast
quickly transitioned close to an even model/ensemble mean blend by
day 8 Thu given the increasing detail uncertainty at that time.
...Weather/Threats Highlights...
Expect the leading storm that weakens over the Gulf of
Alaska/southern coast Sun into Mon to produce some focused
precipitation along the southern coast and Panhandle. The next
storm should bring a period of strong winds and the northern part
of its precipitation shield to the Aleutians Sun into Mon,
followed by eastward progression of winds/precip into the Gulf of
Alaska and southern mainland/Panhandle. Recent trends that have
shifted the surface low a bit to the south over the North Pacific
and then southeast on its approach to the mainland would suggest
somewhat lesser impacts over the Alaska Peninsula compared to some
previous forecasts. Agreement is still good regarding heavy
precipitation potential along the southern coast and Panhandle
with Tue being the most agreeable day for having the greatest
moisture flow. However there are still meaningful differences
among models and their means with respect to the magnitude and
duration highest precipitable water anomalies. By Wed-Thu the
precipitation should begin to taper down somewhat as the upper
pattern relaxes but amounts could still be significant over some
locations.
A majority of the state should see above to well above normal
temperatures during the period. The main exception may be over
the far western mainland where a persistent upper trough could
keep highs in particular somewhat below normal on one or more
days. Below normal readings may extend into the
Peninsula/Aleutians while some pockets of below normal temps may
exist over the Panhandle as well.
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, Sat-Tue, Nov 28-Dec 1.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Sat, Nov 28 and Tue, Dec 1.
- High winds across portions of mainland Alaska and the Aleutians,
Sat-Mon, Nov 28-Nov 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