Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
746 PM EDT Sun May 10 2020
Valid 12Z Thu May 14 2020 - 12Z Mon May 18 2020
Pattern Overview
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There has been relatively good continuity regarding the synoptic
scale pattern evolution across the Alaska domain and adjacent
maritime regions over the past day. One of the main features will
be an anomalously strong Arctic high that will extend to the
Brooks range, and over time this ridge is forecast to retrograde
to the west. The westward progression of this high will introduce
height falls from northwest Canada to support a trend toward
cooler mainland temperatures for much of the Interior and Arctic
Coast region. There will also be two noteworthy oceanic lows of
interest, with the first tracking across the Gulf of Alaska during
the first half of the forecast period but likely remaining well
south of the mainland. The second storm should pass south of the
Aleutians by next weekend.
Guidance/Predictability Assessment
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The 12Z model suite is in relatively good agreement on the
synoptic scale at the beginning of the forecast period Thursday,
with the main difference being the CMC farther north with the low
tracking across the Gulf of Alaska through the end of the week.
One of the main forecast challenges involves the height falls with
the retreating Arctic ridge and the building upper level trough
from western Canada, with the 12Z GFS much more pronounced and the
ECMWF indicating a considerably more subtle change. In regards to
the second low that tracks south of the Aleutians, the models
appear to handle the downstream ridge well, but differ on the
timing and intensity of the low. Both the GFS and ECMWF are
suggesting a triple point low that develops and becomes the
dominant feature, whereas the CMC is stronger with the initial low
over the western Aleutians and does not have as much signal for
secondary low development. By the end of the forecast period, the
12Z GEFS mean is strongest with the trough across central and
northern Alaska, whereas the CMC mean is more broad with more of
an upper ridge axis across west-central Canada.
For the Thursday and Friday time period, a blend of mainly the
ECMWF/GFS/UKMET was used as a baseline, along with a little of the
CMC on Thursday. By Saturday, there was a greater weighting of
the ECMWF and its ensemble mean given the differences with the GFS
across northern Alaska, and greater percentages of the means by
Sunday and next Monday. About 10 percent of the previous WPC
forecast was maintained through day 7 since there were no major
changes in continuity.
Weather/Hazard Highlights
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There is reasonable confidence in the general trend toward cooler
temperatures over a majority of the mainland over the upcoming
week. Above normal readings should decrease in coverage and
become more confined to southern/western areas of the state while
the pattern will gradually become more favorable for below normal
temperatures over northern and eastern areas. Expect significant
precipitation to be fairly limited in coverage during the period
owing to the southern extent of the oceanic lows and the surface
high governing the overall weather pattern for central and
northern sections. The front settling across the southern
mainland may focus some activity at times while light
precipitation may reach northeastern areas next weekend with the
arrival of upper troughing. As the Gulf of Alaska system moves
eastward, expect its track to remain far enough south for at most
a little moisture to reach the southeast Alaska Panhandle. The
system tracking over the western Pacific may bring some moisture
into the Aleutians late in the weekend but with limited confidence
on placement and intensity.
Hamrick
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