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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2333Z Mar 18, 2024)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
733 PM EDT Mon Mar 18 2024

Valid 12Z Fri Mar 22 2024 - 12Z Tue Mar 26 2024

...Much warmer than normal temperatures expected especially for
northern and western parts of the Mainland...
...Active pattern with precipitation and winds for the
Aleutians/Alaska Peninsula to the west coast of the Mainland...

...Overview...

By late week into the weekend, an amplified pattern will be well
established across Alaska, as a strong northeastern Pacific to
Mainland ridge is overhead. Warmer than normal temperatures are
expected across the Mainland, with the highest anomalies in
northern and western areas. To the west of the ridge axis, active
weather is expected with mean southerly flow in the North
Pacific/Aleutians/Bering Sea. Bouts of moderate to heavy
precipitation and windy conditions are likely across the Aleutians
to Alaska Peninsula as well as farther north across the western
coastal areas. Meanwhile the northerly flow on the east side of
the ridge axis should produce a cool and dry pattern for Southeast
Alaska. The ridge is forecast to gradually weaken and shift east
early next week, but maintaining the general pattern.


...Guidance Evaluation and Preferences...

Model guidance has consistently shown persistent upper ridging
through late this week into the weekend, with an omega block
possible as troughing comes into the southwestern edge of the
ridge. Confidence is relatively high on this larger scale. The
main uncertainties during the period include details of the flow
to the west of the ridge axis, so from the Aleutians to Bering Sea
to West Coast, as shortwaves and embedded upper lows within the
general troughing pattern show some spread. Models are still
indicating relatively good agreement in indicating a shortwave
over Bristol Bay/Alaska Peninsula and vicinity early Friday and
tracking north across the West Coast toward the North Slope by
Saturday, providing a little more confidence in one of the
smaller-scale features impactful to sensible weather. Farther
west, the details of energy are still murky even for late week, as
a couple of surface lows over the northern Pacific that look
distinct on Friday potentially interact, with models showing
various solutions for whether they merge by Saturday and their
placement. The EC and CMC show both lows maintaining some depth
through Saturday, while GFS runs merge the two quickly. Ensemble
means are not much help since they average the low positions
together regardless of whether the members have multiple lows.
Either scenario seems possible but at least on the large scale, a
large low pressure system (with possibly multiple centers)
crossing the Aleutians and setting up in the Bering Sea over the
weekend looks likely.

By early next week, increasing spread is seen with this surface
low track. Questionable evolutions of incoming energy from the
Arctic and Pacific do not help matters. There is a general signal
that the relentless bouts of energy into the ridge should
gradually push the ridge axis east and perhaps reorient it
southeast-northwest while weakening.

Considering these factors, the WPC forecast utilized a multi-model
blend of deterministic guidance early on. Gradually reduced the
amount of deterministic guidance as the period progressed in favor
of the ensemble means, reaching over half the blend by Days 7-8
amid the increasing spread.


...Weather/Hazards Highlights...

Expect the most active area for precipitation to be across the
Aleutians and Alaska Peninsula and the western coast of the
Mainland, where rounds of low pressure and frontal systems will
help focus moist Pacific southerly flow through late week into
early next week. These features could also be accompanied by some
enhanced winds. During late week, a shortwave and onshore flow
should produce some enhanced precipitation for western areas of
the Mainland in general, and particularly for favored southern
parts of the Seward Peninsula and southwestern Alaska. Both rain
and snow are possible depending on the area and elevation,
considering the warming temperatures. While multiple rounds of
precipitation are likely for the Alaska Peninsula, currently the
amounts look to be maximized there Friday into Saturday with a
favorable surface low placement to the west. The details of the
flow pattern make the precipitation totals and placement more
uncertain into next week, but generally there is some signal that
energy should press a bit east and bring precipitation to parts of
the Kenai Peninsula and Southcentral. Meanwhile, the northerly
flow east of the upper ridge axis should keep Southeast Alaska
mostly dry, though with perhaps some increasing chances for light
amounts by Monday-Tuesday.

Much warmer than average temperatures will likely last through the
entire period for much of the Mainland due to the ridge aloft and
warm mean southerly flow underneath/on the western side. Parts of
the North Slope to Brooks Range can expect temperatures over 30
degrees above normal with some above freezing readings becoming
more likely especially on Friday. Temperatures look to gradually
moderate after that but remain well above normal. Temperatures
that are above average but to a lesser extent are likely across
the rest of the Mainland too, with highs reaching the 40s on
average. On the other hand, Southeast Alaska can expect near to
slightly below normal temperatures initially due to northerly flow
aloft to the east of the upper ridge axis.


Tate


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