Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Tue May 03 2022
Valid 12Z Sat May 07 2022 - 12Z Wed May 11 2022
...Overview...
Over the weekend, a mean trough in both the surface and upper
levels is forecast to drift into the western mainland and inland
during the early part of next week along with some moisture,
supporting some showers over those areas. An initial upper low
over the Gulf of Alaska Saturday should weaken and get shunted
into western Canada as next week begins, but a mid-latitude upper
low in the Pacific slowly drifting northeastward for the first
half of next week should be the more impactful feature. There is
uncertainty in how this feature will interact and possibly phase
with the troughing farther north, which affects its timing and
track of an associated surface low, and therefore precipitation
for the southern coast of Alaska as well. In general a wetter
pattern is expected there next week, but with uncertainty in rain
amounts and timing.
...Guidance/Predictability Assessment...
Model agreement as the medium range period begins Saturday is
fairly good with the larger scale features: the upper low over the
Gulf of Alaska, another centered well south of the Aleutians in
the Pacific (around 40N and the International Date Line), and
troughing just west of Alaska stemming from an Arctic upper low,
with perhaps weak ridging eroding in the southwestern Interior.
Troughing coming into western Alaska on Sunday still shows some
detail differences among models, like the GFS runs showing one
deeper 500mb trough 12Z Sunday while the ECMWF/CMC show two
shortwaves. With these differences, utilized a blend of the 12Z
deterministic guidance to get to a middle ground solution.
However, the evolution of this northern energy through the first
half of the workweek is uncertain along with the upper low farther
south. There has been considerable model waffling with how phased
the northern and southern features will get. The GFS runs remain
more connected with the features with ECMWF ones more separate.
However, the 12Z and 18Z GFS show slightly more separation than
older runs and show a slower upper low, while the 12Z ECMWF has
sped up a bit with the upper low, leading to somewhat better model
consensus in both the upper and surface low positions. The general
trend was for slower upper/surface lows than the previous WPC
forecast, so today's forecast leaned that direction. The WPC 500mb
heights show a closed low to the south but still with some
indication of a connection at times to the Arctic low/trough, for
the purpose of compromising between the more separated ECMWF/EC
mean and the less separated GFS/CMC/GEFS mean. The 00Z EC mean
seemed particularly slow compared to the consensus of other
guidance and the 12Z EC mean did come in a little faster. Overall
troughing should be persistent over much of Alaska by early-mid
next week, though with potentially some separation in features
allowing for more zonal flow in between around the AK Peninsula,
and this will continue to be refined. The troughing may get hung
up west of a strong eastern Pacific ridge. The latter part of the
WPC forecast used a blend of the latest available GFS, ECMWF, CMC,
and GEFS and EC mean guidance, with the blend tempering individual
differences.
...Weather and Hazard Highlights...
Chances for mostly light precipitation across the southeastern
mainland in conjunction with the initial upper low should become
more scattered and then diminish into early next week, but
increasing chances are likely across the western mainland shifting
into interior Alaska Sunday into Monday as troughing and a surface
front/trough pass across the area. Precipitation chances across
southern Alaska next week are less certain as they depend on upper
level flow interactions and the resultant surface low position. A
more phased solution would bring deeper moisture into the Alaska
Peninsula to Southcentral and the Panhandle, allowing for heavier
precipitation, like GFS runs show, while EC runs keep the area
drier given more separation in the upper level features. At this
time, the most probable outcome still seems to be for some
moisture to spread across areas from the Alaska Peninsula through
the southern coast and Panhandle early to mid next week, with some
locations reaching at least moderate intensity for a time.
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. Sufficiently
warm temperatures combined with the remaining snow pack could
present flooding concerns in the Tanana basin between the Yukon
and Alaska Range of the eastern mainland.
Tate
Hazards:
- Flooding possible across portions of eastern interior Alaska,
Fri-Tue, May 6-May 10.
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