Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
610 PM EST Thu Jan 30 2020
Valid 12Z Mon Feb 03 2020 - 12Z Fri Feb 07 2020
The majority of guidance agrees fairly well for the large scale
pattern evolution during the days 4-8 Mon-Fri period. Arctic
upper trough/low energy should gradually settle toward a position
just west of the northern mainland, while the combination of a
couple features aloft will support deep low pressure over the
Aleutians/Bering Sea followed by steady weakening as the system
eventually heads inland. Fast northern Pacific flow will carry
along multiple features that have less consensus in the guidance
though there is a decent signal for a better defined western
Pacific system coming into the picture toward the western
Aleutians late next week.
Over recent runs there have been some notable differences/trends
in specifics of Arctic flow. ECMWF mean runs through 00Z had been
trending toward fairly quick movement of an initial upper low to a
position just west of the mainland--with the operational 00Z
ECMWF, 00Z/12Z UKMET, and 12Z CMC mean generally supporting that
idea. On the other hand GFS/GEFS runs have been holding the
initial upper low farther east while using flow to its west to dig
the trough/embedded low into the consensus late-week position.
12Z ECMWF adjustment delayed the timing of the evolution,
ultimately favoring a model blend (00Z/12Z ECMWF and 12Z
GFS/UKMET) followed by a model/mean blend (00Z ECMWF mean/12Z GEFS
for the ensembles) for a single deterministic forecast. The 12Z
ECMWF mean trended close to this blend approach.
The aforementioned model blend also provides the best starting
point for resolving differences with the strong Aleutians/Bering
Sea system. There are multiple influences on system specifics,
including at least two individual upper features as well as
details of flow to the north. Consensus surface low track is
farther north than continuity and the past couple ECMWF mean runs
through the 00Z cycle, with the 12Z ECMWF mean adjusting toward
latest preference.
Across the North Pacific domain there is a hint of a leading
system reaching into the Gulf of Alaska by around early Tue,
followed by a front extending south from the Bering Sea storm. An
upstream wave may brush the Aleutians around Wed-Thu, though after
Wed models/ensembles diverge enough to recommend only a modest
reflection until better agreement develops. Another trailing
feature could reach the western Aleutians as well. Models and
means show a larger scale upper trough reaching the western
Pacific late in the week so there is a somewhat better signal for
low pressure reaching near the western Aleutians by day 8 Fri.
The overall pattern will likely support highest precipitation
totals from just east of the Kenai Peninsula through the
Panhandle, with lesser amounts extending west through the
Aleutians. The strong system initially over the Aleutians/Bering
Sea and then weakening as it eventually heads into the mainland,
plus interaction with a leading stationary front, should produce
meaningful precipitation over portions of the southern and western
mainland.
Rausch
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 mainland Alaska, Sun, Feb
2.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Thu, Feb 3-Feb 6.
- Much below normal temperatures across portions of mainland
Alaska, Sun, Feb 2.
- Much below normal temperatures across portions of mainland
Alaska, Sun-Mon, Feb 2-Feb 3.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Sun-Mon, Feb 2-Feb 3.
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