Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
649 PM EST Mon Feb 03 2020
Valid 12Z Fri Feb 07 2020 - 12Z Tue Feb 11 2020
...Overview...
During the course of the period guidance shows a mean ridge
building over the northeastern Pacific into the mainland of Alaska
and northwestern Canada, with varying degrees of troughing to the
west. One or more shortwaves will likely pass through/around the
mean ridge to provide a brief interruption in the trend toward
higher heights within the overall ridge. As has been the case in
recent days, models and ensembles display a fair amount of spread
for multiple individual features in spite of the better agreement
for large scale evolution. Among today's solutions the best
guidance clustering varies depending on the feature. This
required using a multi-model blend (12Z/00Z ECMWF and 12Z
GFS/UKMET/CMC) to emphasize the most common specifics during the
first half of the period, followed by a trend to a model/ensemble
mean blend (the means represented by 12Z GEFS/00Z ECMWF mean) as
details become increasingly uncertain.
...Guidance Evaluation...
The forecast of the upper low and surrounding trough expected to
be near the northwest corner of the mainland continues to be
problematic with a multitude of ideas for precise
evolution/position depending on the day. The full range of
guidance ranges between keeping the upper low/trough over or very
close to the northwestern mainland into day 6 Sun, and
retrogression or shearing/reforming of the upper low into Siberia.
Then the question becomes how this feature ejects eastward around
the building ridge downstream. At the very least the latest GFS
runs may be somewhat too deep/amplified early next week. General
preference to trend the forecast somewhat more toward the means by
the latter part of the period across the full domain ultimately
leads to reflecting the retrogression scenario, but with low
confidence. Behavior of guidance thus far is suggesting it will
take into the short range time frame for this aspect of the
forecast to be resolved.
The preferred forecast of North Pacific into Bering Sea/western
mainland low pressure Fri-Sun stays close to continuity, with the
00Z ECMWF and 12Z CMC/UKMET/GEFS mean providing the majority
cluster. This would have a triple point wave tracking from the
North Pacific into the Bering, and then across the western
mainland. During Fri-Sat the GFS differs from most other guidance
in rapidly weakening the upper low associated with the western
parent low and forming a deep upper low to the east--but still
behind the leading triple point system depicted in other guidance
including the 12Z GEFS mean. Track/evolution of this system will
depend on the very uncertain specifics of the upper low/trough
initially near the northwestern mainland.
During the latter half of the period the continued
northward/northeastward progress of western mainland low pressure
should bring a cold front across a large portion of the mainland.
Meanwhile North Pacific/Aleutians shortwave energy may briefly
generate a surface wave over or near the Gulf of Alaska around day
7 Mon. Current model/ensemble mean consensus indicates that such
a wave should be somewhat weaker and northward/faster than the
past couple ECMWF runs--though the 12Z run at least trended weaker
and faster than the 00Z run.
Farther west there has been a decent signal for organized low
pressure from the western Pacific into western Bering Sea during
the latter half of the period. Thus far the ensemble means have
provided the most consistent solutions. GFS runs through the 00Z
cycle had been the most consistent operational guidance for
general strength/track/timing but 06Z and 12Z runs have strayed in
various ways. On the other hand the last two ECMWF runs have come
into better agreement with the means. The 12Z CMC is near the
favored cluster into early Mon before becoming a fast northeast
extreme.
...Weather Highlights...
A weakening system near the southwest corner of the mainland as of
early Sat should produce light-moderate precipitation along the
southern coast late in the week. The stronger trailing system
forecast to track into the Bering Sea/western mainland should
bring an episode of brisk to strong winds, as well as enhanced
precipitation from the eastern Aleutians/Peninsula into parts of
the mainland. The moisture should extend eastward across the
southern coast as well. This system will support a northward
surge of warmer air and lead to well above normal temperatures
over much of the mainland after northern/western areas see well
below normal readings Fri-Sat. Passage of a trailing cold front
may provide a modest cooling trend late weekend and early next
week but with anomalies staying on the warm side of normal. Low
pressure expected to track from the western Pacific into the
western Bering Sea around Sun-Tue may spread a broad area of
fairly strong winds across the Aleutians and Bering Sea.
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:
- High winds across portions of mainland Alaska and the Aleutians,
Fri-Mon, Feb 7-Feb 10.
- Much below normal temperatures across portions of mainland
Alaska, Thu-Sat, Feb 6-Feb 8.
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