Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
646 PM EST Tue Dec 10 2019
Valid 12Z Sat Dec 14 2019 - 12Z Wed Dec 18 2019
Today's guidance offers better than average agreement/continuity
in principle for two strong North Pacific/Aleutians systems during
the period. The first should be over the Aleutians as of early
day 4 Sat and then continue in a general eastward direction,
averaging out some wobbles along the way, before dissipating after
Mon. Small scale shortwaves spinning around the upper low
continue to produce some uncertainty for precise details/track.
The second system should follow behind by about two to three days
and take a fairly similar track. GFS/ECMWF runs are signaling
some degree of wave development along the leading front, and while
the ensemble means are more subtle, their surface low evolutions
suggest that a leading wave may ultimately become better defined
than the parent low by day 8 Wed. Latest GFS/ECMWF/CMC runs
maintain the parent low for a little longer than the means while
the GFS is a little more concentrated than other solutions with
the leading frontal wave.
Farther east, most operational guidance maintains the general
signal from yesterday for a northward-moving wave tracking fairly
close to the Panhandle by around day 7 Tue. This continuity in
the models would favor at least a defined system in the
deterministic forecast even if not yet with the strength that some
individual runs show. Ensemble members are sufficiently scattered
to yield a mean that does not reflect this feature. Meanwhile
various guidance shows opposite ideas for the upper ridge that
should be just north of the mainland as of early Sat. Recent
GFS/ECMWF runs have trended toward southeastward progression while
the 12Z CMC/UKMET keep the ridge in the Arctic as per the ensemble
means. The means have adjusted southeastward for forecasts valid
Sat but still keep the ridge north of the mainland after Sat.
Prefer an intermediate approach that accounts for some of the
GFS/ECMWF (and day 4 mean) trends early-mid period but then
reflects some persistence of the ridge over the Arctic late.
Based on forecast considerations the initial blend consisted of
the 00Z/12Z ECMWF and 12Z GFS/CMC early in the period followed by
moderate inclusion of the 12Z GEFS/00Z ECMWF means--30 percent on
day 6 Mon and 40 percent on days 7-8 Tue-Wed. One or more periods
of brisk easterly low level flow may enhance precipitation over
windward terrain across the northeastern Alaska Peninsula as well
as Kodiak Island and Kenai Peninsula but with fairly high
uncertainty for intensity at any particular time. The wave
forecast to track off the Panhandle around next Tue may increase
precipitation over that region.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
No significant hazards are expected over Alaska during this
forecast period.
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