Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
648 PM EDT Mon Jul 20 2020
Valid 12Z Fri Jul 24 2020 - 12Z Tue Jul 28 2020
...Overview and Guidance Evaluation...
Today's guidance generally agrees that upper troughing to the
south of a slowly retrograding Arctic upper low will promote a
cool and unsettled pattern over much of the state during the
period. A couple surface systems that may track near the southern
coast of the mainland could also enhance precipitation over some
areas between the southern mainland and Panhandle.
An ensemble mean average has been the most stable solution for the
Arctic upper low over the past couple days, showing the feature
eventually wobbling to a position a little northwest of the
mainland. Stability of the means favors a blend/ensemble mean
approach for that feature. The 12Z CMC depicts the most extreme
westward path while the 12Z ECMWF eventually strays to the east of
consensus.
The system initially over the Bering Sea should extend a front
eastward with a modest embedded wave most likely tracking along
the southern coast of the mainland/extreme northern Gulf by Sat.
Most guidance advertises ejection of the upper low corresponding
to the Bering Sea system but with considerable timing and latitude
differences among current and recent consecutive runs. Uncertain
influence from northern stream flow leads to some of this spread.
Therefore confidence is fairly low for specifics of surface low
pressure that could develop in response to this energy. The deep
00Z GFS/ECMWF surface systems may have pushed the envelope for
plausibility based on sea level pressure records along the
southern coast/Panhandle at this time of year. However the upper
dynamics appear strong enough to offer potential for a stronger
system than what the ensemble means show--though an average of
their timing seems reasonable relative to the model spread.
Overall favor an intermediate approach that ends up being closest
to the 12Z GFS in principle though perhaps a bit slower after day
6 Sun. This is slower than the 12Z ECMWF but faster than the
prior 00Z run.
Models are all over the place regarding the ultimate evolution and
location of initial Sea of Okhotsk/western Pacific energy, leading
to individual solutions showing a wide variety of surface patterns
over/south of the Aleutians--ranging between low and high
pressure. The extreme spread recommends a blend/ensemble approach
that keeps the pattern fairly ill-defined until better agreement
emerges.
Based on aforementioned preferences the forecast started mainly
with a blend of the last two ECMWF runs and 12Z GFS/UKMET early in
the period, followed by a steady trend toward 50-60 percent total
weight of the 12Z GEFS/00Z ECMWF ensembles.
...Weather Highlights/Threats...
The best potential for highest five-day precipitation totals
should be over the southern mainland and Panhandle, with an assist
from the one or more surface systems expected to track near the
southern coast of the mainland and/or over the Gulf. There is
still considerable uncertainty over precip coverage/intensity
especially during the latter half of the period with the potential
second wave. Locations farther north over the mainland may also
see one or more episodes of significant precipitation due to the
proximity of a deep Arctic upper low and to its south a wavy
frontal boundary that should persist over the northern mainland.
Clouds and precipitation should lead to particularly noticeable
cool anomalies for daytime highs, with some areas likely to see
readings 10F or more below normal for one or more days. Anomalies
for min temps should be more mixed but still with a tendency for
somewhat greater coverage of negative anomalies.
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: No significant hazards are expected over Alaska during
this forecast period.
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