Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
651 PM EDT Sun May 17 2020
Valid 12Z Thu May 21 2020 - 12Z Mon May 25 2020
...Overview...
Today's models and ensemble means have not changed much from
yesterday for the large scale pattern which itself is likely to be
slow to change. Continue to expect gradual weakening of an
initially strong southern Bering Sea storm (and associated upper
low) that should drift into the southeastern Bering by the weekend
and possibly wobble back to the west a little around next Sun-Mon.
Leading strong and moist deep-layer flow and one or more frontal
waves will focus significant precipitation along portions of the
southern coast and possibly parts of the northeastern Alaska
Peninsula/southwestern mainland. Some moisture may reach the
Panhandle as well. A downstream upper ridge crossing the mainland
and Gulf of Alaska on Thu should settle into position from western
Canada across or near the northeastern mainland and into the
Arctic by Sat-Mon. An embedded high center may become apparent
near or just north of the northern coast of the mainland late in
the period.
...Guidance/Predictability Assessment...
Emphasizing a 12Z model consensus early-mid period followed by a
gradual increase of 12Z GEFS/00Z ECMWF ensembles late (reaching up
to 40 percent total) provided good continuity for the overall
pattern with only minor adjustments for some details. Operational
input by Sat-Mon consisted mostly of the 12Z GFS and 12Z/00Z ECMWF
runs.
Continuity and clustering for the southern Bering low were good
during Thu-Fri. There has not yet been a pronounced signal for
the upper low to reach as far east as depicted in the 12Z ECMWF by
around Sat, favoring the aforementioned split for the operational
ECMWF component of the forecast. The new 12Z ECMWF mean, while
remaining a little east of the GEFS/CMC means, also recommends
keeping the upper low somewhat west of the operational run on Sat.
The Gulf of Alaska continues to be an area of considerable
uncertainty for frontal waves, given typically low predictability
of such small features beyond the short-range time frame. There
is fair clustering for one defined wave to reach southeast of
Kodiak Island around early day 5 Fri. Thus far there is less
support for the 12Z CMC idea of a trailing strong wave tracking to
Kodiak Island by early Sat so the CMC was removed from the blend
after Fri. Latest GFS runs and 00Z ECMWF showed another wave
south of the Panhandle by day 7 Sun. Confidence in this wave's
specifics are low but such a wave is quite plausible given the
general frontal progression leading up to that time.
Guidance trends have become mixed for the strength of the
downstream upper ridge, with the latest model/mean consensus a bit
weaker in contrast to the moderately stronger trend from 48-24
hours ago. ECMWF mean runs have tended to be a little weaker than
GEFS/CMC means. The blend approach helped to maintain forecast
stability within moderate spread/variability for the precise shape
and location of the overall ridge.
...Weather/Hazard Highlights...
The forecast is fairly consistent in showing a focus for heaviest
precipitation late this weekend and possibly into the weekend over
locations along and close to the eastern Kenai Peninsula as well
as the northeastern Alaska Peninsula. Kodiak Island and parts of
the southwestern mainland are within the envelope for meaningful
precip as well. By the weekend some moisture may extend across
the southeastern coast and Panhandle. Low-confidence frontal wave
details will play a significant role in determining the timing and
intensity of precip in this eastern portion of the moisture
shield, with current model/ensemble guidance showing a wide spread
for potential precip totals.
The surface pattern across the mainland will be stable through the
period, highlighted by a persistent east-west trough over central
areas. Highs will tend to be above normal over the central
mainland and somewhat cooler than normal over the southern
mainland and parts of the North Slope region. Some of the
southern mainland below normal readings may extend into parts of
the Panhandle. Most areas should see above normal morning lows
with the greatest anomalies over parts of the northern half of the
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
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