Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
354 PM EDT Wed Oct 23 2019
Valid 12Z Sun Oct 27 2019 - 12Z Thu Oct 31 2019
...Heavy coastal rain and mountain snow for portions of western
and southern coastal areas next week...
A very amplified pattern to start the period on Sunday may only
slowly weaken by late next week. The ensembles were in good
agreement overall on a trough (upper low)/ridge couple along
170W/140W that will edge eastward Sun-Tue. Next Wed/Thu, another
reinforcing trough southwest of the Aleutians should again favor
ridge building into the Gulf.
The 12Z and 00Z ECMWF/Canadian clustered with the 00Z ensemble
means to start the period with the latitudinally-stretched system
astride the southwest coast of Alaska. This will set up a long
fetch of moisture from the sub-tropics northward into southwestern
and southern coastal areas where precipitation (coastal rain,
mountain/inland snow) may be heavy. Both precipitable water
anomalies and M-Climate QPF were at very high levels (some at the
highest for this time of year) which suggests a much higher than
average chance of an extreme precipitation event. Details will
change over the next few days which will affect impacts.
The atmospheric river setup will drift eastward next Tuesday as
the frontal system weakens toward the Panhandle. By then, the 00Z
ECMWF/Canadian best clustered with the ensembles back toward/south
of the Aleutians as the 12Z GFS took a surface low toward the Gulf
whereas the ensembles suggest building ridging there. Thus,
trended toward a majority ensemble weighting and took the next
system toward the AkPen next Wed/Thu.
Temperatures will rise to well above average levels west of ~150W
due to strong southerly flow with the largest anomalies perhaps
along the North Slope once the warmer air mixes down to the
surface. This may change initial snow ahead of the warm front to
rain until cooler air comes back around the main surface low
behind the cold front. Temperatures over the Panhandle will be
cooler than average to start with high pressure overhead.
Fracasso
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
Alaska Hazards...
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sat-Mon, Oct 26-Oct 28.
- Heavy rain across portions of mainland Alaska, Sat-Mon, Oct
26-Oct 28.
- Much above normal temperatures across portions of mainland
Alaska, Sun-Tue, Oct 27-Oct 29.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Sat, Oct 26.
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