Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Wed Nov 06 2019
Valid 12Z Sun Nov 10 2019 - 12Z Thu Nov 14 2019
Overall there is still considerable uncertainty over important
forecast details even though there are some common themes in the
guidance regarding large scale flow.
There is some agreement that a weakness will develop within the
initial mean ridge that extends from the British Columbia coast
through the mainland--with some energy possibly crossing the
northeastern Pacific/Gulf of Alaska and cyclonic flow developing
to the southwest of an Arctic/Canadian Archipelago upper low,
while an upper high closes off northwest of the mainland. Path
and strength of northeastern Pacific/Gulf energy will be key for
associated surface low pressure, with guidance showing a broad
envelope for low tracks between Kodiak Island and the Alaska
Panhandle. This part of the forecast is very difficult as recent
ECMWF/CMC/UKMET runs are fairly aggressive to bring energy into
the downstream ridge leading to low tracks toward the Panhandle,
while the ensemble means are much more subdued with the energy
(seemingly more plausible given occasional tendencies for guidance
to break down ridges too quickly) with a farther west low track.
Latest GFS runs are more compatible with the GEFS/ECMWF mean
surface low but even the GEFS mean suggests the GFS could become
to slow with the energy aloft. A compromise may ultimately be the
best option. At higher latitudes there are persistent differences
with the southwestward amplitude of cyclonic flow with an
intermediate solution appearing best. Fortunately surface
solutions are fairly similar with a cold front pushing into the
mainland from the north while high pressure becomes established
over the Arctic.
Farther west there is plenty of spread as well. Early in the
period ECMWF/CMC runs have been closing off a concentrated system
over the southern Bering Sea along an initial wavy front while
most other guidance is downplaying such a feature. Then there is
a decent signal for a significant North Pacific into southern
Bering Sea storm which could have some input from Super Typhoon
Halong, possibly combined with a mid-latitude Pacific wave that
catches up from the west. GFS runs have generally been more
consistent for track longitude than the ECMWF over recent days
(western half of the spread versus every 1-2 runs switching
between eastern and western tracks). A westward adjustment in the
ECMWF mean compared to yesterday now brings it into agreement with
the GEFS mean so there is a little more confidence for leaning
away from the eastern part of the envelope. The 00Z/12Z GFS runs
track the storm farther north than consensus though, and other
guidance even keeps the system south of the Aleutians for most of
its existence. The means provide a reasonable intermediate path.
Finally, toward the end of the period the ensemble means have been
fairly stable in advertising another system entering the domain
from the west, possibly brushing the Aleutians by day 8 Thu.
With fairly low confidence given the spread in the guidance, the
forecast blend emphasized the 06Z GEFS/00Z ECMWF means with some
GFS input where similarities existed--06Z-12Z runs early and only
the 06Z run late. The manual forecast adjusted this blend a
little stronger for mass fields and corresponding winds. While
day-to-day details are uncertain the overall pattern should favor
highest precipitation totals along the southern coast and
Panhandle with meaningful totals extending farther west as well.
The potentially deep northern Pacific/Aleutians storm may produce
areas of strong winds.
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
- Much above normal temperatures across portions of northern
Alaska, Sun-Tue, Nov 10-Nov 12.
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