Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Sun Sep 19 2021
Valid 12Z Thu Sep 23 2021 - 12Z Mon Sep 27 2021
...Stormy North Pacific into Gulf of Alaska Pattern Likely to
Bring Heavy Precipitation to the Southeast Coast and Panhandle...
...Guidance and Predictability Assessment...
Today's models and most means continue the series of vigorous
storm systems tracking along or south of the Aleutians and into
the Gulf of Alaska, bringing significant multi-day precipitation
totals to the southeastern coast and Panhandle along with episodes
of strong winds across the North Pacific. Following a short-range
system early this week expect another storm to reach the Gulf
later in the week, and with slowly improving confidence yet
another one by the weekend/early next week. These systems will
track around the southern side of a large scale upper trough
extending from the mainland southwestward/southward. The tilt of
the mainland upper trough may change gradually with time as an
Arctic upper low initially near 80N latitude wobbles
southwestward.
The forecast of the late week Gulf system has been consistent in
principle but some fine-scale detail uncertainties persist, with
low predictability multiple days out in time. Best consensus
appears to show a surface low track slightly northwest of
yesterday's forecast due in part to stronger upper dynamics
sliding through the southwestern mainland. What solutions have
been on the slower side of the spread seem to be thinning out
somewhat. On the other hand the 12Z CMC becomes questionably fast
by Friday.
Some guidance is still shuffling around for details of the next
system which should track along/south of the Aleutians late this
week and then into the northeastern Pacific/Gulf, but the overall
signal is gradually improving after starting to gain prominence
24-36 hours ago. Resolving how separate bundles of energy east of
Kamchatka and dropping south from Siberia may evolve/interact may
still lead to forecast challenges for a time. Over the past day
the ECMWF has been the most consistent in principle and ECMWF/CMC
ensemble means have been steadily deepening the surface low in
successive runs. GFS runs have been erratic over the North
Pacific (06Z and new 18Z runs closer to the majority cluster than
the suppressed 00Z/12Z runs) before converging toward the Gulf.
12Z CMC details also differ before reaching the Gulf. Beyond the
track, important strength differences exist as well and will be
sensitive to specifics of upper support. Most GEFS mean runs so
far have been struggling to define this system before reaching the
Gulf.
The main consideration for the Arctic upper low is to lean away
from the 12Z and new 18Z GFS runs that bring the feature farther
south/southeast than most other guidance by Sunday-Monday.
Based on the above guidance comparisons, the early part of the
forecast incorporated the 12Z ECMWF, 06Z GFS/12Z UKMET, and 12Z
CMC in order from more to less weight. Then the blend
transitioned toward a mix of models (mostly 00Z/12Z ECMWF and 06Z
GFS) and means (00Z ECens and 12Z CMCens). The lack of North
Pacific system definition in the GEFS mean favored use of the
CMCens in its place today. The main adjustment from continuity
for the second system is a faster trend along with moderate
deepening which reflects gradual improvement in the guidance
signal. Depth is still somewhat conservative relative to the deep
side of the envelope though.
...Sensible Weather...
The combination of storms reaching the Gulf of Alaska during the
period should bring significant totals of rain and higher
elevation snow to areas from the far eastern Kenai Peninsula to
the Panhandle. Most guidance is suggesting general five-day
totals of 5-10 inches liquid with some locally higher totals.
Both storms should also produce a broad area of brisk to strong
winds. There is still a moderate degree of uncertainty for the
precise track and strength of the second system as it tracks along
or south of the Aleutians toward the end of the week. Thus
potential effects on the Aleutians could range from moderate/heavy
rain and strong winds to much less extreme conditions. Currently
the most likely scenario leans more than halfway toward the
heavier rain/strong winds. Exact track will also determine the
degree of rain/winds across the Alaska Peninsula and vicinity.
The potential for heavy precipitation farther east appears less
sensitive to the storm's exact track/evolution before reaching the
Gulf. The first system reaching the Gulf late this week may
spread some moisture across parts of the central/eastern mainland.
Otherwise expect a majority of the mainland to see lighter and
more scattered precipitation. The upper trough persisting over
the mainland will keep temperatures below normal over much of the
state. Coldest anomalies should be over Southcentral and near the
Brooks Range.
Rausch
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