Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
653 PM EDT Sun Apr 05 2020
Valid 12Z Thu Apr 09 2020 - 12Z Mon Apr 13 2020
...Overview...
In the wake of a short-range midweek system reaching the Gulf of
Alaska and late week Arctic upper low with associated trough
brushing the northern mainland, the best guidance cluster suggests
that mean ridging aloft will build into the Northeast Pacific.
Meanwhile there will be a persistent area of low heights with one
or more upper lows/surface systems likely near the western
Aleutians. Within this general evolution the guidance provides
various ideas for some details. These include how shortwave
energy crossing the Aleutians and initial mid-latitude
trough/upper low energy may filter into the mean ridge and then
with timing of dynamics/low pressure reaching the Aleutians/Bering
Sea during the latter half of the period. Therefore confidence is
lower for some of the forecast specifics.
...Guidance/Predictability Assessment...
One of the persistent uncertainties during recent days has been
with the combination of energy aloft crossing the Aleutians/North
Pacific and initial trough/upper low energy over the east-central
Pacific. Solutions still range between fairly aggressive
northward progress of associated surface low pressure into the
northeastern Pacific (12Z ECMWF in particular) and a suppressed
surface pattern (12Z CMC most extreme in that regard). Ensembles
available through arrival of the 12Z GEFS/CMC means have been
downplaying the strength of energy aloft and northward progress of
surface low pressure. Taken as a whole the recent ECMWF/GFS runs
suggest at least some potential for shortwave energy to approach
the upper ridge with mean flow aloft in most guidance as a whole
providing some opportunity for a wave/warm front to lift
northeastward eventually. Trying to gauge how the energy evolves
as it passes into/through the mean ridge is a very low-confidence
endeavor, so current preference is to nudge the forecast toward
more northward progress of a weak feature but not to the extent of
the 12Z ECMWF day 6 Sat onward. The 12Z ECMWF mean has developed
more of a reflection, essentially a weaker version of a 12Z/00Z
ECMWF compromise through next week.
Farther north, latest GFS runs have backed away from earlier runs
that had brought Bering Sea low pressure all the way into the
northern mainland by next weekend. At the same time other
guidance has evolved toward a better defined wavy front that
reaches the northern mainland late this week into the weekend.
By next weekend/early next week the guidance diverges for the
degree of eastward progress of low pressure reaching the
Aleutians. CMC runs have tended to be on the eastern side of the
spread but the new 12Z ECMWF has also shifted to this more eastern
scenario. Continuity thus far recommends holding this system back
toward the ensemble means, GFS runs, and ECMWF runs prior to the
12Z cycle.
The initial mass field forecast reflected the potential for
differing ideas of the operational models and means each to have
some validity. Days 4-5 started with a 75/25 proportion of models
to means. The model component evenly weighted the 12Z GFS/12Z
ECMWF/00Z ECMWF while the ensemble part used more 00Z ECMWF mean
relative to the 12Z GEFS. Preference to lean away from 12Z ECMWF
specifics mid-late period and increase weight of the means as
detail uncertainty increases led to trending the blend toward 60
percent means/40 percent total 12Z GFS-00Z ECMWF by days 7-8
Sun-Mon.
...Weather/Threats Highlights...
Locations along the Panhandle and southeast corner of the mainland
will trend drier late this week with the departure of the
short-range midweek Gulf system. Around the late week/early
weekend time frame a wavy front should bring a period of light to
moderate precipitation to parts of the western and
northern/central mainland, with some remaining uncertainty over
exact latitude. Western Aleutians systems will bring episodes of
moisture and possibly brisk/strong winds depending on system
strength. There is considerable uncertainty over how much
moisture may reach areas from the eastern Aleutians to Gulf of
Alaska. From late this week into the weekend expect the North
Slope region to see above normal temperatures with more modest
positive anomalies over the extreme western part of the mainland.
Chilly high pressure settling over the southern-southeastern
mainland will support below normal temperatures elsewhere.
Pattern evolution aloft by late weekend and early next week should
favor a warmer trend over most of the state even if high pressure
may persist over the southeast.
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
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