Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Fri Mar 26 2021
Valid 12Z Tue Mar 30 2021 - 12Z Sat Apr 03 2021
...Guidance/Uncertainty Assessment...
Latest models and ensemble means continue to show a mean trough
aloft settling over the mainland and northeastern Pacific mid-late
week. The northern part of the shortwave will initially support
weakening mainland low pressure from the deep storm forecast to
track over the Bering Sea in the short term, while energy farther
south should bring deepening low pressure into the Gulf of Alaska
by early Wednesday. Some northern stream energy digging into the
overall trough may reinforce the mean trough and associated
surface low over the Gulf later in the week. Farther west, most
guidance suggests that a couple shortwaves and associated surface
systems will track around the northern periphery of a deep layer
ridge that builds over the northern Pacific/Aleutians/Bering Sea
mid-late week. However agreement and continuity are poor with
respect to the details of these shortwaves and surface systems.
The broad consensus of guidance does show the overall pattern
flattening some by early next weekend, which would at least
promote a trend toward more shallow/broad mean trough over and
southeast of the mainland.
Similar to yesterday, the favored forecast blend emphasized the
12Z GFS/ECMWF/UKMET for about the first half of the period and
then quickly transitioned toward about half models (12Z/00Z ECMWF
and 12Z GFS) and half means (12Z GEFS/00Z ECens) for days 7-8
Friday-Saturday. The CMC continues to be slower than other
solutions to bring developing low pressure into the Gulf.
Otherwise there is fairly good continuity/clustering with the
systems over the mainland and Gulf, though the latter has trended
somewhat deeper over the past day. Lingering detail differences
are fairly small in scale and thus have low predictability. The
progressive shortwaves around the upstream ridge yield low
confidence for specifics of individual waves expected to track
across the Bering Sea. Best potential for one of these waves to
reach the western mainland will be toward early next weekend.
Unfortunately the current envelope at that time is vary
wide-ranging, from another deep and impactful northern Bering
storm (00Z ECMWF and 12Z CMC) to a weak and suppressed wave (12Z
GFS). The 06Z GFS and 12Z ECMWF are between the two extremes.
Ensemble means signal a most likely track reaching the western
mainland around 60-65N latitude.
...Weather/Hazards Highlights...
Expect the coverage and intensity of precipitation over most of
the mainland to decrease with time from Tuesday onward as mainland
low pressure weakens. Meanwhile a developing frontal wave should
track into the Gulf of Alaska by midweek and linger for at least a
couple days, bringing a period of heavy precipitation to areas
from the southeastern coast through the Panhandle. Highest totals
are likely around midweek followed by a steady decrease as low
pressure weakens and upper troughing slowly broadens. Waves along
a mean frontal boundary around the northern periphery of a
deep-layer ridge that extends from the Pacific into the Bering Sea
may produce precipitation areas that extend from the Bering Sea to
locations anywhere between the eastern Aleutians and western
mainland. Any moisture reaching the mainland should take until
Friday-Saturday to arrive. At that time the wide spread of
guidance for possible track/strength of low pressure keeps
confidence low for resolving the magnitude and coverage of
precipitation and other sensible weather effects.
On Tuesday, morning lows will still be above to well above normal
over at least the western half of the mainland but highs over the
state will be more moderate with near to somewhat below normal
readings. From midweek onward the arrival of upper troughing will
spread below normal highs over much of the state with morning lows
tending to be less extreme including some pockets of modestly
above normal. Possible flattening of the mean flow by next
Saturday would start a gradual warming trend.
Rausch
Hazards:
- Heavy precipitation across the Alaska Panhandle into the
southern coastal ranges of mainland Alaska, Tue-Wed, Mar 30-Mar 31.
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