Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
646 PM EST Thu Dec 3 2020
Valid 12Z Mon Dec 7 2020 - 12Z Fri Dec 11 2020
***Major Gulf of Alaska storm early next week brings high winds
and heavy precipitation***
...Guidance and Uncertainty Assessment...
The deterministic and ensemble guidance is in relatively good
agreement on the strong Gulf low on Monday in terms of magnitude,
with the 12Z ECMWF on the northern side of the guidance. More
noteworthy differences appear by Tuesday with the 12Z CMC
depicting another strong low lifting northeastward towards the
Gulf region. However, this solution has very little support from
the ensemble means and was not used in the forecast process.
There is better model support for a low pressure system near the
Aleutians by next Thursday, but the ECMWF is considerably farther
to the northwest than the GFS. Given the uncertainty among the
deterministic guidance for this time range, the forecast was based
primarily from a GEFS/ECENS compromise for the Thursday to Friday
time period. Some of the 12Z ECMWF and less of the GFS was used
for early in the week along with the ensemble means.
...Weather/Hazards Highlights...
An intense low pressure system over the northern Gulf of Alaska on
Monday is forecast to gradually weaken by the middle of the week.
This storm is expected to produce strong and gusty winds across
the Aleutians and the southern mainland coast, along with
additional heavy rain and mountain snow across areas that have
already been hammered by relentless precipitation over recent
days. This may result in additional episodes of flooding and
shoreline erosion. Farther inland across the Interior and the
North slope region, a generally benign weather pattern is
expected, except for more modest early next week spillover
moisture/snows into the Interior as the aforementioned deep low
lifts northward and weakens over the Gulf toward southern Alaska.
Additional surface lows upstream with lower predictability are
subsequently slated from the northern Pacific towards the
Aleutians and vicinity by the middle to end of the week.
Hamrick
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sun-Mon, Dec
6-Dec 7.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Sun-Mon, Dec 6-Dec 7.
- High significant wave heights for coastal portions of the Alaska
Panhandle and mainland Alaska, Sun-Mon, Dec 6-Dec 7.
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