Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
339 PM EDT Thu Oct 24 2019
Valid 12Z Mon Oct 28 2019 - 12Z Fri Nov 01 2019
...Heavy coastal rain and mountain snow for southeastern coastal
areas and much above normal temperatures for the central and
northern mainland early next week...
A highly amplified upper pattern will be in place at the start of
the medium range period, with a strong closed low in the eastern
Bering Sea and a well-defined ridge downstream from the panhandle
northward through the eastern part of the mainland. Aside from
relatively minor differences handling individual short waves and
associated surface lows rotating through the trough, the models
are in good agreement showing broad southerly flow encompassing
much of the mainland through Day 4. The amplified nature of the
pattern will allow moisture from the subtropics to be transported
northward, and where it impinges on the southeastern coast, there
will be very heavy rain and high elevations snows. In addition to
the precipitation, the southerly flow will allow a much warmer
than normal air mass to overspread the state (except for the
panhandle). Readings across the Brooks Range and North Slope may
exceed 20 degrees above normal from Monday through Wednesday of
next week.
After Day 4, the models struggle to handle the interaction of the
strong Bering Sea closed low and a system in the eastern Siberian
Sea. The 12Z ECMWF shows the two systems phasing Day 5, while the
GFS and CMC keep them well separated. The latest ensemble
guidance doesn't really support this ECMWF solution, so we
generally favored a blend of the GFS/CMC and the 12Z GEFS (and 00Z
ECMWF ensembles, which also don't support the 12Z deterministic
ECMWF run). The atmospheric river impacting the southeastern
coast will move slowly eastward into the northern part of the
panhandle Day 5 and gradually weaken as it runs into the
downstream upper ridge.
During the middle to latter part of next week, a short wave is
expected to rotate around the leftover Bering Sea upper trough
toward the Aleutians or Gulf of Alaska. There are timing and
track differences among the guidance, with the 12Z GFS most
progressive and 12Z ECMWF slowest. The Canadian is initially a
decent compromise on Wednesday, but then quickly lifts
northeastward as the system gets swept up by an amplifying trough
in the arctic stream. The GFS tends to have a fast bias with
mid-latitude features, so we are favoring a slower solution; at
the moment this seems to be best encapsulated by the 00Z ECMWF
ensemble mean. This system may bring a renewed outbreak of
widespread precipitation for the southwestern and south-central
coastal sections Thursday into Friday, but considering the model
spread, expect the details surrounding the associated sensible
weather to change with future model runs.
Klein
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, Sun-Wed, Oct 27-Oct 30.
- Heavy rain across portions of mainland Alaska, Sun-Mon, Oct
27-Oct 28.
- Much above normal temperatures across portions of mainland
Alaska, Sun-Thu, Oct 27-Oct 31.
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