Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
636 PM EST Sun Nov 24 2019
Valid 12Z Thu Nov 28 2019 - 12Z Mon Dec 02 2019
Viewed through the perspective of the ensemble means, the large
scale pattern evolution during the Thu-Mon period has fairly good
agreement and continuity. Shortwave energy initially over the
eastern Aleutians/Bering Sea into the North Pacific should
progress eastward and weaken while a mid-latitude system to the
south remains separated from most of this energy and eventually
drops southeastward. Toward the end of the week and weekend the
pattern will become more dominated by a deep upper low moving into
or just inland from the extreme western/northern Bering. The
overall trough containing the upper low should extend south into
the mid-latitude Pacific, with the trough axis most likely
reaching the eastern Bering around next Mon. Expect a downstream
ridge aloft to cross the mainland around Fri-Sat and continue into
western Canada. Not surprisingly operational model runs and
individual ensemble members display enough spread and run-to-run
variability to temper confidence in some important details.
During Thu-Fri the most significant model discrepancy involves the
handling of shortwave energy over and just south of the eastern
Bering/mainland, possible interaction with the system well to the
south, and a surface wave/boundary that may promote a concentrated
focus for heavy precipitation along a portion of the southern
coast. The majority cluster has adjusted a little west from
yesterday for the surface feature but there is still good
agreement that it should remain somewhat east of recent GFS
runs--with a little less stream interaction aloft. Thus
confidence is fairly low in the heavy GFS precip depicted over the
Alaska Peninsula and southwestern mainland early in the period.
The better signal for highest totals is over and/or just east of
the Kenai Peninsula.
Meanwhile the forecast is fairly well on track for the storm over
or near the extreme western Bering Sea. There is still potential
for one frontal wave to wrap into the main area of low pressure
during the first half of the period, with enough uncertainty not
to depict explicitly in the manual forecast. A consensus
adjustment from yesterday is for faster progression of the
associated front across the Bering after early Fri with
introduction of a southern coast wave by Sun after the occluded
front dissipates. Then there is wide model/ensemble spread
(similar to yesterday) for another wave that may track northeast
from the central Pacific Sat onward, with various depth and
potential tracks ranging from the southeastern Bering/western
mainland (GFS) to the Gulf of Alaska (ECMWF). Continuity and
increasing emphasis on the ensemble means by that time lead to the
forecast reflecting the northwestern part of the spread for the
wave of interest, but again with low confidence. A broad area of
precipitation will accompany the front and wave(s) with precise
coverage and intensity determined by wave specifics.
During the first half of the period the forecast started with
components of the 00Z/12Z ECMWF and 12Z GFS/CMC with 30 percent
total input from the 12Z GEFS/00Z ECMWF means. The blend provided
better than average continuity for this sensitive evolution aloft
and reflected the modest consensus adjustment for the wave near
the southern coast. In light of increasingly divergent model
details, the latter half of the forecast increased ensemble
weight, reaching 70 percent by day 8 Mon. The 12Z ECMWF mean
reinforces the ensemble consensus that suggests the 00Z ECMWF may
have become too fast with the trough crossing the Bering while the
12Z run may be too slow--but at the same time fast to bring strong
energy into the Gulf of Alaska by day 8 Mon.
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
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