Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
745 PM EDT Wed May 26 2021
Valid 12Z Sun May 30 2021 - 12Z Thu Jun 03 2021
...Periods of heavy rain likely from the Peninsula region to the
Panhandle, as well as in parts of the southern Alaska mainland...
...Overview and Guidance Evaluation...
There is general agreement on the broad pattern but some important
discrepancies exist for the details. From Sunday into Monday the
guidance consensus shows two separate upper lows, one retrograding
over the southeastern half of the Bering Sea and the other to the
south of the eastern Aleutians/Alaska Peninsula. Then solutions
diverge regarding their relative strength, track, and how they may
interact or merge. By Tuesday-Thursday progressive North Pacific
flow may carry a couple waves that could also interact with the
overall mean low aloft. Meanwhile most solutions suggest that
some degree of upper troughing will extend from the Canadian
Archipelago into the far northern/northwestern mainland and Bering
Strait (perhaps digging into parts of the Bering Sea by mid-late
next week) while mean ridging extends from western Canada into the
mainland. The general pattern will support a period of wet
weather from the Alaska Peninsula eastward to the Panhandle with
some moisture eventually making its way northward as well.
After reasonable clustering into early Monday, operational models
diverge for specifics of the two upper lows with no common theme.
Through arrival of the 12Z ECMWF, that model was on its own in
holding the Bering Sea upper low nearly in place through the end
of the period. At the same time the 06Z/00Z GFS runs became quick
to drop the Bering low southward and merge it into the North
Pacific Low, and in contrast the UKMET weakened the North Pacific
low in deference to the Bering low. These differences ultimately
led to significant spread for details of individual surface lows
and fronts from the Bering/North Pacific into the Gulf. Given the
low predictability of this evolution, preference followed a 12Z
model blend early (splitting the ECMWF contribution between the
12Z and prior 00Z runs) and then transitioned to 70 percent total
ensemble weight (12Z GEFS/00Z ECens) by day 8 Thursday. This
provided an intermediate approach into Tuesday and then reflected
the most common theme of the large scale pattern, along with a
conservative reflection of North Pacific waves that may interact
with flow to the north. The new 12Z ECMWF mean has hedged
westward some with the Bering Sea low but not to the point of the
operational run, and at least into Tuesday the 18Z GFS has
adjusted favorably toward preference with this low.
...Weather/Threats Highlights...
The details remain uncertain but the overall pattern continues to
favor southerly upper level flow that should bring a series of
fronts into the Gulf of Alaska along with periods of enhanced rain
and wind from the eastern Aleutians or Alaska Peninsula to the
Panhandle. Models and ensembles generally show the most anomalous
moisture remaining along the southern coast into the early part of
next week. Then the axis of greatest moisture should lift
northward with some disagreement over magnitude. Thus expect the
best potential for heavy rain along the southern coast during
Sunday-Monday. Totals should be more moderate Tuesday-Thursday
but some localized heavier rainfall may still be possible
depending on exactly how features at the surface and aloft evolve.
Northward progression of moisture after Monday will likely
increase coverage and intensity of rain/mountain snow over more of
the mainland with time.
The forecast pattern should yield below normal high temperatures
over a majority of the state, including the Panhandle. The North
Slope and Panhandle/Southcentral should see the coolest anomalies
relative to normal. Far western parts of the mainland have the
best potential to see above normal highs during the period. For
low temperatures expect below normal readings to prevail over the
northeastern half to third of the mainland while mostly positive
anomalies extend over remaining areas.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Sat-Sun, May 29-May 30.
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