Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
640 PM EST Sat Mar 07 2020
Valid 12Z Wed Mar 11 2020 - 12Z Sun Mar 15 2020
...Overview and Guidance/Predictability Assessment...
Early in the period guidance agrees fairly well that mainland
upper troughing from the short range will continue into
northwestern Canada while upstream energy filtering into a Gulf of
Alaska trough will progress southeast and push away initial low
pressure near the Panhandle. At the same time cold high pressure
should prevail over much of the mainland during Wed-Thu. Farther
west the forecast is fairly consistent in principle for a strong
storm to track across Siberia and into the Arctic late this week.
Expect the storm's associated strong winds to cross the Bering Sea
and northwest mainland with some significant terrain-enhanced
precipitation possible.
The most notable adjustment in most guidance over the past two
days is a pronounced stronger trend for upper ridging now forecast
to build over the northeastern Pacific into the mainland Fri-Sun.
This trend has continued with the 12Z guidance including the 12Z
ECMWF/ECMWF mean runs (12Z ECMWF mean arriving after initial
guidance evaluation). While it reflects recent trends the 12Z CMC
could be overdone with this ridge. The stronger trend for the
upper ridge provides ongoing support for discounting the GEFS mean
whose recent runs have been on the eastern side of the spread for
the Siberia into Arctic storm, corresponding to mainland heights
aloft that are generally somewhat lower than most other guidance.
Later in the period the models and individual ensemble members
become increasingly divergent regarding the details of the overall
ridge and energy that may flow around it. This favors trending
the forecast increasingly more toward the ensemble means (00Z
ECMWF mean and 12Z NAEFS mean, to lean away from questionable GEFS
mean characteristics) until better agreement develops. Most D+8
multi-day means--which cover the latter part of the extended
period and beyond--are fairly similar in principle so resolving
differences on a specific valid day seems to be a low-confidence
endeavor.
The Wed into early Thu part of the forecast started with the 12Z
ECMWF/GFS/UKMET/CMC in order of greatest to least weight. One
localized reservation about the GFS exists south of the Panhandle
as it shows a northeastward-tracking wave versus consensus that
depicts a system generally developing in place and then
progressing southeastward. Strength of the CMC upper ridge led to
its removal from the blend after early Thu. Total weight of the
00Z ECMWF mean and 12Z NAEFS mean was 20 percent on day 6 Fri and
increased to 50 percent Sat and 70-80 percent by next Sun.
...Weather Highlights/Threats...
The southeastern coast and Panhandle will see a drier trend
mid-late week as initial low pressure nearby weakens/departs.
Trailing high pressure over the mainland will bring much below
normal temperatures through midweek, followed by a significant
warming trend that initially arrives over western-northern areas
and progresses eastward. Still expect the greatest above normal
temperatures from late this week into the weekend to be over the
northern mainland. The strong storm responsible for this
temperature rebound, tracking northeast through Siberia and into
the Arctic Thu-Fri, should also bring a period of strong winds
across the Bering Sea and into the northwestern mainland with
significant terrain-enhanced precipitation likely. Recent trends
with the upper ridge building over the northeastern Pacific into
the mainland suggest that the strongest winds/heaviest
precipitation may not extend as far southeastward as suggested in
some earlier projections. By next weekend confidence is quite low
regarding any possible features that could flow around the mean
ridge aloft, though the general pattern may support some
precipitation over northern/western parts of the 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
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