Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
754 PM EDT Wed Apr 14 2021
Valid 12Z Sun Apr 18 2021 - 12Z Thu Apr 22 2021
...Overview...
Latest guidance continues to depict a blocky regime during the
period. This pattern will be highlighted by upper ridging that
covers the mainland and western Canada (possibly drifting into the
northeastern Pacific for a brief time) and eventually joined by
another ridge that builds over the western Bering Sea/eastern
Siberia, while a consolidating Pacific mean low aloft should
settle into a position well south of the eastern Aleutians and
Alaska Peninsula. Shortwave energy within progressive flow around
the north side of the upper ridge may push a cold front into the
northern/eastern mainland by Tuesday-Thursday. Many locations
should be fairly dry for most of next week with the best potential
for precipitation existing over the Aleutians and Peninsula in the
gradient between high pressure to the north and Pacific low
pressure to the south.
...Guidance/Uncertainty Assessment...
The most pronounced trend seen in the guidance over the past day
has been toward a stronger ridge aloft from western Canada through
the mainland from the start of the period early Sunday onward. To
some degree this trend is a nod to yesterday's 12Z GFS which held
the strongest ridging over the mainland into late period. By the
latter half of the forecast, guidance diverges over details of the
ridge that builds into the western Bering/eastern Siberia, with
the more progressive GEFS mean merging it into the mainland ridge
while the ECMWF/CMC means hold it farther west (near continuity)
while the ECMWF and to a lesser extent CMC have it give way,
allowing lower heights to extend into the Aleutians/Bering. The
shape of ridging over the northeastern Pacific/Alaska Panhandle by
late in the period will also have some influence on this evolution.
The primary forecast consideration was to lean away from the ECMWF
scenario after Monday as its farther north extent of lower heights
aloft and surface low pressure from the North Pacific into the
Aleutians/southern Bering during the mid-late part of the period
conflicted with other operational models, the latest
GEFS/ECMWF/CMC means, and recent trends for stronger upper ridging
to the north. As the preferred operational model blend
transitioned to a model/mean mix later in the forecast, the
ensemble component tilted a bit more in the GEFS direction since
trends supported its stronger mainland ridge (regardless of the
specifics of how it gets there). The GFS/CMC provided some
lingering model input by day 8 Thursday. Also on day 8, trends
for the upper ridge seemed to favor maintaining a drier forecast
over the southeast mainland and Panhandle (per operational models)
versus the ensemble-based PoP guidance. Finally the stronger
upper ridge early in the period deflects Arctic fronts a little
northward versus continuity but there is a continued signal for
the front reaching the mainland by Tuesday-Thursday--albeit with
typical variability in timing.
...Weather/Hazards Highlights...
Most precipitation should be confined to the Aleutians/Alaska
Peninsula and vicinity, with a fairly persistent flow of moisture
carried in the gradient between one or more Pacific lows and high
pressure well to the north. Light to moderate amounts are most
likely but some locally higher totals could be possible depending
on exact strength and track of Pacific systems. Also expect a
broad field of brisk winds around these lows.
Most of the state should see above normal temperatures at the
start of the period Sunday, with the warmest anomalies over the
North Slope and for morning lows. A gradual cooling trend with
time will tone down the warm anomalies somewhat while possibly
bringing at least high temperatures moderately below normal over
parts of the southeastern quadrant of the mainland and the
Panhandle.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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
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