Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
533 PM EST Wed Dec 04 2019
Valid 12Z Sun Dec 08 2019 - 12Z Thu Dec 12 2019
...Heavy precipitation expected across southern mainland Alaska...
Models and ensembles showed general consensus at the large scale
across Alaska during the extended period. Upper-level ridging
should persist/expand from western Canada into eastern mainland
Alaska. Meanwhile, an active and relatively progressive flow
pattern across the North Pacific will help to reinforce and
amplify a mean trough/upper low near/south of the Aleutians.
Despite relatively good large scale agreement, the details of
specific systems become highly uncertainty past days 4-5
(Sun-Mon), with a relatively high degree of run-to-run variability
and inconsistency among deterministic solutions. Sun-Mon, models
showed some consensus that a low pressure system should approach
the Alaska Peninsula with a surface front crossing the Gulf. Deep
southerly flow associated with this system east of the upper
trough axis will transport significant amounts of Pacific moisture
northward, resulting in heavy precipitation potential for portions
of southern mainland Alaska. The GFS suggest the potential for
another deep wave of low pressure to develop along the surface
front across the Gulf of Alaska Mon night, moving north toward
southern mainland Alaska on Tue. Confidence in this scenario is
low to to high run-to-run variability, but this scenario would
further enhance heavy precipitation potential across southern
mainland Alaska if it were to occur. Ensembles show consensus that
the next deep surface low should move east near/south of the
Aleutians Mon-Tue, approaching the Alaska Peninsula by Wed-Thu.
Deterministic solutions diverge significantly on the potential
placement, timing, and intensity of any potential triple point
lows across the Gulf during this time frame. While development of
such a wave of low pressure seems likely given the scenario, the
poor consensus makes for very low confidence on the specifics at
this time.
Given these considerations, the WPC forecast was initially based
on a multi-model deterministic blend, including the 12Z
GFS/ECMWF/CMC during days 4-5. By days 6-8, low forecast
confidence warranted a shift heavily toward ensemble means
(ECENS/NAEFS) until some of the forecast specifics become clearer.
Ryan
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sat-Mon, Dec 7-Dec 9.
- High significant wave heights for coastal portions of mainland
Alaska, Tue, Dec 10.
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