Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Wed May 04 2022
Valid 12Z Sun May 08 2022 - 12Z Thu May 12 2022
...Overview...
As next week begins, an upper trough that is looking stronger
today is forecast to shift into the western mainland and then
across the state for early-mid next week along with some moisture
and a low pressure/frontal system, supporting some showers across
much of the state. This trough may be strong enough to shunt an
eastern Pacific ridge well south of Alaska. Meanwhile an upper low
should stagnate well south of the Aleutians with less potential
for phasing with the flow farther north in today's forecast, so
this feature and an associated surface low may not directly impact
Alaska but could provide moist flow into the state for increasing
precipitation chances along the southern coast.
...Guidance/Predictability Assessment...
As the medium range period begins early next week, most guidance
is in fairly good agreement with the overall pattern, with
considerably stronger energy (compared to yesterday's guidance)
diving into the western mainland. The main exception was the 12Z
CMC, which is on its own with an earlier shortwave that is
stronger than other models divingÂsouthward, whereas consensus
shows that initial shortwave as less impactful and the second
shortwave is what creates the larger scale troughing. But the past
few runs of the ECMWF, GFS, and UKMET show better than typical
overall agreement with this troughing, so the WPC forecast
utilized those models early in the forecast as the troughing
deepens and shifts eastward. By midweek the trough still shows
reasonable agreement the operational GFS and ECMWF and their
ensemble means in terms of timing and pushing the eastern Pacific
ridge south, as well as the track of a surface low and frontal
system that are somewhat stronger and faster moving than
yesterday's forecast.
An upper low centered well south of the Aleutians in the Pacific
(around 40N and the International Date Line) has shown a trend
toward slow movement along with less potential to phase with the
flow farther north. The 12Z UKMET did show a faster track with the
Pacific low which led to some phasing of those features, so it is
not out of the question, but preferred the better clustered GFS/EC
and their means that keep the Pacific low detached. With the
fairly good ECMWF and GFS agreement, was able to keep the
deterministic proportion of the forecast blend pretty high through
much of the period, just over half (with EC/GEFS means the other
half) by day 8.
...Weather and Hazard Highlights...
Increasing precipitation chances are likely across the western
mainland shifting into and interior Alaska Sunday into Tuesday as
troughing and a surface front/trough pass across the area, with
perhaps some moderate amounts. The deeper northern trough and
sufficient moisture streaming in from the Pacific could lead to
light to moderate precipitation spreading from the Alaska
Peninsula to Southcentral Alaska possibly becoming moderate to
heavy for the Panhandle by around midweek. Chances for
particularly heavy amounts have generally gone down over the past
couple of days though given the trend toward the Pacific upper low
and its associated surface low staying well south. Another round
of light precipitation is possible for the interior Wednesday and
beyond next week.
Another concern during the period will be for river flooding as
above freezing temperatures under a significant snow pack will
lead to possible flooding in the Tanana basin between the Yukon
and Alaska Range of the eastern mainland and westward along the
Yukon River until Koyukuk Island. Flooding is also possible due to
potential ice jams in the Lower Kuskokwim Valley in southwestern
parts of mainland Alaska.
Temperatures should be fairly stable and not terribly anomalous
during the period. The Panhandle to far southeastern mainland can
expect cooler than normal conditions for both lows and highs. Much
of the mainland should see around normal to above normal lows,
with more of a mix in terms of high temperatures, with the
southwestern mainland particularly cool.
Tate
Hazards:
- Flooding possible across portions of mainland Alaska, Sat-Wed,
May 7-May 11.
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