Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
658 PM EST Mon Nov 21 2022
Valid 12Z Fri Nov 25 2022 - 12Z Tue Nov 29 2022
...Overview...
Most guidance shows a fairly progressive pattern aloft with a
series of troughs and ridges, with a general tendency for these
features to become more amplified with time. This regime will
support a succession of surface lows/fronts across the Bering
Sea/Aleutians into the Gulf of Alaska/Northeast Pacific, with
primary precipitation emphasis from the Aleutians through parts of
western and southern Mainland Alaska as well as the Panhandle.
Guidance agrees on this general pattern but displays meaningful
differences for some system details. Meanwhile, high pressure
building into the northern portion of the state during the weekend
and shifting into western Canada early next week should favor
driest conditions over northern/northeastern areas.
...Guidance/Predictability Assessment and Preferences...
Already to start the period early Friday, there are important
differences regarding the evolution of a system expected to be in
the vicinity of the Alaska Peninsula. This feature originates
from a fairly strong mid-latitude central Pacific system on
Thursday, with some elongation/weakening expected in the majority
of guidance going into Friday based on potential separation of
streams aloft. Over the past couple of runs there has been one
solution (00Z UKMET, 12Z CMC) to keep the upper system phased to
yield a strong surface low into Friday. On the other extreme,
latest GFS/ECMWF runs have been the weakest and most suppressed.
Ensemble means as a whole hint at a most likely surface low track
across the Alaska Peninsula/Bristol Bay before the system
consolidates over the Gulf of Alaska (with better clustering on
day 5 Saturday versus the day 4 forecast). By Saturday the
primary guidance difference of note is that the 12Z ECMWF brings
the system more to the southern Panhandle than the other models
and means. Prefer an operational model blend that yields a
solution closest to the ensemble means for the initial Alaska
Peninsula system early Friday and reflects better detail than the
means for the Gulf low by Saturday.
The next forecast challenge involves the system forecast to reach
the Bering Sea by Sunday, with differences early that day widening
as the low and/or front possibly reach the mainland before the
dynamics aloft support Gulf low pressure, whether from the
progression of a Bering Sea low or development along a front. As
with the evolution a couple days earlier, there is somewhat better
agreement with Gulf low pressure by early Monday (though with the
typically greater scatter for a day 7 forecast versus day 5) after
the greater uncertainty in how it gets there. The Bering into
Gulf details hinge upon the strength and amplitude of the upper
shortwave pressing into/around the somewhat larger scale upper
ridge, with predictability for a feature of that scale not great
for several days out. Currently the 12Z ECMWF/CMC and new 18Z GFS
show the best potential for a significant low to track into the
southwestern mainland/Alaska Peninsula. The prior model cycle
generally favored a farther northwest parent low track and only a
front reaching the western mainland followed by subsequent Gulf
development. The favored blend reflected a conservative approach
following the latter scenario while waiting for better agreement.
Finally, guidance generally agrees with the next upper trough
coming into the picture supporting a western Bering Sea/Siberia
system with a trailing front pushing through the Bering
Sea/Aleutians during the first part of next week. There are
typical differences in how close the front may get to the mainland
by day 8 next Tuesday, with a model/mean blend providing a
reasonable starting point.
Guidance preferences for systems of interest led to the early part
of the forecast starting with an operational model blend
consisting of the 12Z/00Z ECMWF, 12Z GFS/UKMET, and the least
weight on the 12Z CMC. After early day 5 Saturday the forecast
incorporated a little 12Z GEFS/00Z ECens weight to replace the
phased-out UKMET (due to straying fast with its Gulf system) and
by day 8 Tuesday reached a 40 percent ensemble weight relative to
the remainder coming mostly from the ECMWF runs/12Z GFS. The
decent weight of operational guidance maintained through the end
of the period helped to reflect the general model theme of the
upper features being more amplified than depicted in the means at
that time.
...Weather/Hazards Highlights...
Parts of the southwest corner of the mainland may see some
precipitation on Friday with a system tracking in the vicinity
before heading into the Gulf. Late this week into the weekend the
Panhandle should see some heavy coastal rain and inland/mountain
snow, from a combination of lingering moisture from a weakening
short-range storm and the upstream low expected to be offshore by
Saturday. Passage of a cold front progressing southward over the
mainland behind the Friday-Saturday system may lead to strong gap
winds over some areas in the south. The next system in the series
will likely spread a broad area of precipitation across the Bering
Sea/Aleutians during the weekend, with some of this moisture
likely to reach the southwestern half of the mainland by Sunday.
Confidence is low for details of precipitation coverage and
intensity at this time due to uncertainty over whether a
well-defined surface low or a mere front will serve as the
precipitation focus. The surface low scenario could lead to some
brisk winds. How much moisture reaches the Panhandle on Monday
will be sensitive to the exact track of low pressure emerging over
the Gulf/northeastern Pacific. Yet another frontal system may
spread moisture over the Aleutians/Bering Sea early next week and
some of the precipitation could reach the far western mainland.
The gradient between the front and high pressure departing into
northwestern Canada may lead to some brisk winds, but again with
low confidence in specifics at this time.
System/frontal progression as well as high pressure building
across the North Slope during the weekend should lead to a colder
trend over most of the state from Friday through Sunday, with
central/southern areas most likely to see the greatest below
normal anomalies. Then the eastward shift and amplification of
upper features should bring a pronounced warming trend to above
normal temperatures over western areas early next week.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Thu-Fri, Nov 24-Nov 25.
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