Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
707 PM EST Tue Nov 14 2023
Valid 12Z Sun Nov 19 2023 - 12Z Thu Nov 23 2023
...Heavy precipitation threat over portions of the Panhandle late
this week and possibly again on Monday...
...High winds likely over the Aleutians on Sunday...
...Overview...
A deep upper trough will be atop much of Alaska over the weekend
and bring a period of below normal temperatures to most areas
aside from the North Slope. Low pressure and frontal systems at
the surface could also direct moisture into Southeast Alaska for
some enhanced precipitation Friday and continuing into Saturday.
By early next week, a pattern change is likely, with upper ridging
first extending into the Bering Sea and into the southwestern
Mainland by Monday, traversing into the Interior on
Tuesday-Wednesday. But lows at the surface and aloft are likely to
be on the periphery of this ridge, including a deep surface low
over the Kamchatka Peninsula that should erode well west of the
state, while a low farther south takes over and tracks eastward
likely across the northern Pacific south of the Aleutians.
Additionally, after a flip in the guidance yesterday (Monday), the
signal has remained consistent for upper and surface low pressure
farther east in the northeast Pacific. While the details are
uncertain, this could lead to another round of enhanced
precipitation for at least the southern part of the Panhandle
around Monday.
...Guidance Evaluation and Preferences...
Model guidance is in reasonably good agreement with the initial
trough pattern across the state with a couple of embedded
shortwaves. The details of the trough axis do show some minor
spread though, which affects the exact surface lows/fronts pattern
and the precipitation and temperature forecasts somewhat. A blend
of the deterministic models seemed to work well to produce a
middle ground solution.
There is also a good signal in the guidance for defined mid-upper
ridging to stretch to the northeast from the Pacific and make its
way across the Aleutians and into the southwest Mainland early in
the week. The way it evolves over the Interior is a bit more
questionable into Tuesday-Wednesday, including how the ridge/high
may join up with western Canada ridging. This is especially true
by next Wednesday as the 12Z ECMWF and CMC bring some Arctic
troughing into the North Slope midweek and splits the ridge.
Actually this shortwave stems from the energy that helped form the
Kamchatka Peninsula low, so not willing to discount the
possibility of that shortwave, though GFS runs indicate a
continuous ridge.
Meanwhile guidance also is quite consistent in showing a strong
surface low (perhaps <960 mb) over the Kamchatka Peninsula early
in the week, with good agreement on the low position and then it
also weakening quickly as its energy shears eastward. Recent
guidance seem to show that other energy coming in from the west
and north is what tracks near/south of the Aleutians next week.
Farther east, low pressure and energy aloft tracking northeastward
toward the Gulf of Alaska next week still looks likely after
models keyed in on that about a day ago.
Overall, with no particular outliers in the model guidance, just
some typical spread, utilized a multi-model deterministic blend of
the 12Z guidance and then increased the proportion of ensemble
means in the model blend to about half by Days 7-8.
...Weather/Hazards Highlights...
The southern periphery of the mainland/Gulf upper trough and
associated surface wave(s) will likely promote moderate to locally
heavy snow and coastal/southern Panhandle rain late this week.
Some moisture may extend somewhat northwest of the Panhandle,
ahead of separate Gulf low pressure. Confidence is still lower
than desired for precise magnitude of highest totals and northward
extent of heaviest precipitation. Relative confidence is highest
for the southern Panhandle to see the most precipitation. Expect a
much lighter trend by Sunday, but by Monday into Tuesday another
heavy precipitation event may take place there. There is some
spread with the greatest moisture axis though, and there is a
possibility for the enhanced precipitation to stay south into
British Columbia. Some precipitation could make its way into the
southern coast by Tuesday-Wednesday.
Calmer conditions are possible across the Aleutians Saturday as a
narrow ridge crosses the region from west to east. Then a front
approaching/crossing the Aleutians and adjoining waters should
spread an area of high winds and precipitation across the region
around Sunday. Another potential storm coming into the picture by
next Tuesday could help to maintain brisk winds across the
Aleutians and vicinity early next week.
The deep upper trough crossing the mainland this weekend will
bring below normal temperatures to most of the southern two-thirds
of the state, with coldest departures from normal likely to be
over southern areas through the weekend and into the Panhandle by
early next week. A warming trend should progress from west to east
during Monday-Wednesday. The North Slope will be one area that
should remain above normal through the period, with anomalies
increasing as part of the general warming trend over the mainland
early next week.
Tate/Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Fri, Nov 17 and Mon, Nov 20.
- High winds across portions of the Aleutians, Sun, Nov 19.
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