Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
600 PM EST Mon Nov 09 2020
Valid 12Z Fri Nov 13 2020 - 12Z Tue Nov 17 2020
...Guidance/Predictability Assessment...
From late this week into the weekend there is good agreement that
the upper low initially over the western coast of the mainland
will drop southeast into a Northeast Pacific mean trough. Behind
this evolution a progressive upper ridge entering the western
Bering Sea early in the weekend may decelerate as it eventually
reaches the eastern Bering Sea and possibly parts of the mainland.
Forecast details become increasingly uncertain with time as
recent guidance diverges with how shortwave energy emerging from
eastern Asia by the start of the period ultimately evolves. Some
of the significant differences become apparent over the Bering Sea
already during the weekend (with a leading compact wave) and then
increase in scale and eastward effect going into next week.
The primary debate in the guidance is whether enough of the
southern half of the emerging Asia upper trough energy will
separate from the westerlies to allow for incorporation of a
lower-latitude Pacific wave. This wave would develop into a deep
storm that reaches the vicinity of the western Aleutians/Bering
around the start of the week followed by added uncertainty over
strength/track as it heads into the mean ridge aloft downstream.
Another scenario that some guidance has been advertising is for
greater northern stream emphasis, leading to a farther northwest
primary low and a mere front crossing much of the
Aleutians/Bering, while the southern wave is more suppressed with
no effect on the Aleutians.
Over the past couple days or so operational GFS/ECMWF runs have
varied significantly between each other and from run to run within
the broad envelope of possibilities for the western evolution,
favoring a measured approach to the forecast. From Fri into Sat a
12Z operational model blend (though for ECMWF input splitting
between the last two runs due to questions over some details)
provides a reasonable depiction of consensus. Aside from the
UKMET whose western evolution was like the old 00Z ECMWF, there is
currently more of a signal for flow separation with the 12Z GEFS
mean hinting at this idea more than the 00Z ECMWF mean. Thus
after Sat the forecast trends to a model/mean blend with more GEFS
influence than ECMWF mean into day 7 Mon with a system reaching
the western Aleutians/Bering at that time (12Z ECMWF mean adjusted
at least as far south as the GEFS mean). However with operational
runs diverging even more after Mon, prefer to trend further toward
the more stable means by day 8 Tue. This keeps low pressure over
the western Bering with progressive leading front. Note that
ensemble means converge by day 8 Tue so GEFS/ECens relative
weights return to even then. Modest input of individual models
helps to sharpen the mean pattern a bit. In addition the blend
makes sure to downplay a leading Bering wave in the 12Z ECMWF/CMC
around Sun in light of consensus suggesting a fairly strong ridge
aloft over the region. Meanwhile there is better consensus toward
surface high pressure building over more of the mainland from late
weekend into early next week.
...Weather/Threats Highlights...
A fairly broad area of scattered precipitation should gradually
decrease in coverage with time as the upper low initially near the
western coast of the mainland drops southeast into a Northeast
Pacific trough. One or more surface lows over the Gulf of
Alaska/northeastern Pacific may provide some focus at times along
the Panhandle and possibly the southeastern coast. Broad cyclonic
flow late this week into the weekend will favor unsettled
conditions over the Aleutians/Alaska Peninsula. Low pressure and
associated frontal system may bring an episode of enhanced
precipitation and strong winds across the Aleutians and Bering Sea
around early next week but so far guidance has displayed poor
agreement/continuity for the details.
The cooling trend forecast to be in progress late this week will
continue into the weekend, leading to below normal temperatures
over a majority of the mainland and Panhandle by the weekend and
early next week. The main exception will be over the North Slope
where above normal readings should persist. Some locations near
the western coast of the mainland may see modestly above normal
temperatures on one or more days.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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