Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
658 PM EST Sun Feb 14 2021
Valid 12Z Thu Feb 18 2021 - 12Z Mon Feb 22 2021
...Overview and Guidance/Uncertainty Assessment...
Latest model and ensemble guidance shows an area of upper
troughing with potential for multiple embedded closed lows within
an area extending from the Canadian Archipelago through the North
Slope and into western Mainland Alaska. Within the Alaska part of
the trough axis the ensemble means have generally trended deeper
over the past day or so, providing some incremental increase of
confidence in the general pattern. Meanwhile lower-predictability
progressive flow will prevail across the North Pacific. This flow
will likely carry multiple waves along the Aleutians and into the
Gulf of Alaska. The pattern aloft over the mainland will favor
persistence of mean low pressure over the Gulf with corresponding
precipitation focus along the southeastern coast and Panhandle.
For most forecast specifics, guidance has been sufficiently
diverse and variable to recommend a 12Z operational model blend
for about the first half of the period followed by a trend to
50-60 percent means as individual solutions increasingly diverge.
Already late this week there are a couple notable detail
discrepancies. Over the Northeast Pacific-Gulf, the 12Z GFS/UKMET
are much deeper and somewhat northward of the ECMWF/CMC for
possible low pressure development in response to vigorous
small-scale evolution (hence lower predictability) of energy on
the leading side of a larger overall upper trough. GEFS/CMC
ensemble means support the GFS cluster in principle though of
course in much weaker fashion. The initial blend with a little
manual enhancement yielded a system like the GFS in concept but
with intermediate depth. The new 12Z ECMWF mean has adjusted
toward the other means for general evolution, further suggesting
the operational 12Z ECMWF may be too far southeast. Meanwhile
over the northern mainland and vicinity the debate is whether a
single upper low will drop into the region and persist (GFS) or an
initial feature ejects, to be replaced by another bundle of energy
arriving from the northeast during the weekend (ECMWF/UKMET/CMC).
The GFS could have the advantage of aligning better with the mean
trough and recent deepening trends in the ensembles, but the model
run required tempering over other areas. Also the new 18Z run
appears to have hedged a bit northeastward after Thu as
northeastern energy approaches, followed by merging the two
features.
The guidance blend represented a fair degree of consensus for the
rapid progression of systems across the Aleutians and vicinity
then into the Gulf late week into the weekend. The fast nature of
the mean flow aloft will likely temper predictability for some
details though. By days 7-8 Sun-Mon guidance becomes more out of
phase for significant features over the western-central Pacific in
particular, favoring a conservative approach that incorporates
half or more weight of the ensemble means. There is a general
signal for a continued east-west axis of lower surface pressures
near the Aleutians but confidence is very low for resolving the
character of any embedded systems.
...Weather/Hazards Highlights...
The overall pattern will favor precipitation over the southeastern
coast and Panhandle during the period, with the best potential for
heavier activity to be late this week followed by a general
lighter trend during the weekend into early next week. Some
moisture may extend into other parts of the southern mainland late
week into the weekend. A developing northeastern Pacific system
that may lift into the Gulf Thu-Fri could provide some added
moisture focus, and depending on its strength a band of strong
winds just off the Panhandle. Upstream systems should spread at
least a couple areas of precipitation across the Aleutians and
Bering Sea before settling into the Gulf.
Most of the Panhandle should see below normal temperatures during
the period. Across the mainland, expect the start of the period
(Thu) to feature below normal readings over the North Slope and
far west, while remaining areas will likely be above normal. With
time expect a colder trend that will steadily expand the coverage
of below normal anomalies, reaching nearly the entire state by
Sun-Mon. This will be particularly the case for max temperatures
while some areas of above normal mins could still persist.
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