Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
759 PM EDT Wed Aug 04 2021
Valid 12Z Sun Aug 08 2021 - 12Z Thu Aug 12 2021
...Heavy rainfall threat over parts of southern Mainland Alaska
into the Panhandle during the first half of next week...
...Overview and Guidance/Predictability Evaluation...
Some of the details remain uncertain but there is a decent signal
in the guidance that flow ahead of a Bering Sea into northern
Pacific mean trough aloft may promote a period of significant
rainfall over portions of the southern mainland and the Panhandle
from Sunday into the next work week. Precipitation intensity
should decline around midweek or so as the upper trough fills
somewhat but the pattern will remain unsettled. Meanwhile the
northern part of the mainland should be under a col region in the
upper flow into early next week and then transition to flat or
broadly cyclonic flow as an Arctic upper low drops down to 80N
latitude or so.
The most prominent issue early-mid period is what becomes of a
Pacific wave (which may be related to current T.D. 15W) that comes
into the picture by day 5 Monday. On one side of the spectrum,
the 12Z UKMET/ECMWF suggest the upper energy initially over/south
of the western Aleutians will promote significant development and
a track into the Bering Sea/Bristol Bay, ultimately replacing the
system over the Bering Sea as of Sunday. On the other hand the
CMC is much weaker/eastward. The 12Z GFS becomes quite fast and
somewhat east of the middle of the envelope while the 18Z GFS has
adjusted slower. Ultimately preferred a 12Z model blend that led
to more reflection of such a wave than seen so far in the ensemble
means but with a track that hedged more to the means and
ECMWF--providing a more moderate continuity adjustment than would
be the case with other solutions. Details of any trailing frontal
wave are even more uncertain.
Farther upstream, by day 8 Thursday 12Z guidance as a whole is
starting to gravitate to a variation of recent GFS runs which have
been showing some combination of emerging western Pacific energy
and flow on the west side of the Bering/Pacific mean trough
separating into an upper low, with potentially significant surface
development. Latest GEFS means hint at a system and the new 12Z
ECMWF mean has also lowered its surface pressures compared to the
prior run. This system may have some initial connection to T.D.
14W.
At higher latitudes the primary extreme solution of note is the
12Z CMC which brings its Arctic upper low down to near the
northern coast of the mainland instead of 77-80N latitude as
indicated by most remaining models/means.
The aforementioned 12Z model blend early in the period trended
toward a model/mean mix later in the forecast, with a little more
than half total input from the 12Z GEFS/00Z ECMWF means and the
rest coming from the past two ECMWF runs plus only a small
minority 12Z GFS component.
...Weather/Threats Highlights...
Currently expect the most significant precipitation to be during
Sunday-Tuesday across southern parts of the mainland and into the
Panhandle. A leading Bering Sea low and rapidly weakening frontal
system will likely bring one surge of moisture around Sunday while
a trailing Pacific wave and its associated fronts bring another
area of enhanced moisture by Monday-Tuesday. These moisture
surges could bring precipitable water values up to at least 2-3
standard deviations above climatology. Some brisk to strong winds
could also be possible if the latter wave develops on the more
aggressive side of the spectrum. Unfortunately confidence is
still lower than desired for the strength and track of this
trailing wave. Ensemble guidance has been fairly consistent in
highlighting areas from the eastern side of the Kenai Peninsula
into the Panhandle, and possibly favored terrain in the Alaska
Range, for receiving the greatest five-day rainfall totals but
other areas may see significant totals as well. Some locations
could see several inches of rain during the period. Precipitation
may remain fairly widespread into Wednesday-Thursday but with
lesser intensity. Central Pacific low pressure may bring an
increase of rain/winds to the Aleutians after midweek.
The cloudy/wet pattern over many locations as well as the trend to
cyclonic flow aloft over northern areas will support below normal
high temperatures across much of the state through the period.
Min temperatures will be more moderate with various areas of
somewhat above or below normal anomalies.
Rausch
Hazards:
- Heavy rain across portions of the Alaska Panhandle and
Southcentral mainland Alaska, Sun-Tue, Aug 8-Aug 10.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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