Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
306 PM EDT Sun Mar 10 2019
Valid 12Z Thu Mar 14 2019 - 12Z Mon Mar 18 2019
Confidence is relatively good with respect to the large scale flow
pattern across Alaska and the North Pacific during the extended
period, with persistent mean upper-level troughing near the
Aleutians, and an upper ridge axis rebuilding across
western/northwestern Canada, into portions of mainland and
Southeast Alaska by late in the period. ECENS and GEFS D+8
standardized anomalies provide strong support for the persistence
of this large scale pattern, with maximum positive 500 hPa height
anomalies centered over central British Columbia, and minimum
negative anomalies centered between the Aleutians and Hawaii. This
setup will keep the preferred storm track near/south of the
Aleutians and into the Gulf of Alaska, with persistent southerly
flow into Alaska. Models/ensemble members showed good clustering
surrounding a low pressure system moving onshore near Kodiak
Island or southern mainland Alaska by Thu. By Thu night-Fri this
system should weaken as it moves inland and merges with a broader
area of shortwave energy farther north. The forecast during days
4-5 was based heavily on a blend of the ECMWF/GFS solutions, which
showed relatively good consensus during this early portion of the
extended period.
The flow continues to look a bit more chaotic in the wake of the
initial Thu system, with a number of smaller scale shortwaves
expected to traverse the north central Pacific upper trough and
eventually move into the Gulf. The ECMWF/GFS along with their
ensemble means show some consensus that another surface low should
move north into the Gulf Fri night-Sat, with perhaps a triple
point low development farther east across the Gulf on Sun, and
finally another potentially deeper low pressure system following a
similar track by Mon. Spread with respect to timing, intensity,
and structure of these systems gradually increases through time,
but model spread by the end of the period is perhaps slightly less
than average. Nonetheless, the chaotic nature of the flow pattern
with multiple smaller scale waves makes for the potential for a
highly changeable forecast. Additional uncertainty continues to
arise in the handling of a couple areas of arctic shortwave energy
exiting eastern Siberia during the middle to latter portion of the
week, and whether they amplify across the Bering Sea or remain
confined closer to eastern Russia as a consolidated upper low.
Given the increased spread, the forecast was trended toward
heavier ensemble mean (ECENS/GEFS) starting on day 5 (Fri), with
majority ensemble means during days 7-8 (Sun-Mon).
Ryan
Bullet points from the WPC Days 3-7 Hazards chart:
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 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