Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
403 PM EDT Wed May 15 2019
Valid 12Z Sun May 19 2019 - 12Z Thu May 23 2019
Once again forecast details are quite uncertain across a broad
area covering the North Pacific, Aleutians/Alaska Peninsula, and
the Bering Sea. Most guidance suggests this region will be under
relatively low heights aloft with multiple embedded low centers,
but agreement and continuity are quite poor regarding details.
For example energy initially dropping into the Bering could become
part of the overall cyclonic circulation which ensemble means show
near the eastern Aleutians/southwestern Alaska Peninsula--or
alternatively just meander over the Bering or drop farther south
into the Pacific. At the same time various possibilities exist
for one or more northeastern Pacific waves. Yet another major
uncertainty involves the ultimate track and timing of mid-latitude
low pressure which will be supported by an amplifying central
Pacific trough and may eventually affect the Aleutians/Alaska
Peninsula.
Given poor model/ensemble member clustering and run-to-run
changes, the forecast maintains a conservative approach over the
above area by way of a model/mean blend, consisting of the 06Z
GFS/FV3 GFS and 00Z ECMWF along with the 06Z GEFS and 00Z
NAEFS/ECMWF means. The blend removes the 00Z ECMWF after day 6
Tue due to its run becoming farther west than consensus for the
overall central-eastern Pacific pattern. Also the forecast leans
slightly toward the operational runs early and 80 percent toward
the means by days 7-8 Wed-Thu. Even the ensemble means have
varied with their weak reflection of late period central Pacific
low pressure. Overall trend from the past day is for slower
arrival of this low but the new 12Z ECMWF adjusted faster from its
questionable 00Z run and by day 8 Thu is not far from the manual
forecast's timing. In spite of the detail uncertainties, around
days 4-5 Sun-Mon there is relative agreement for a period of
easterly low level flow that would enhance precipitation somewhat
from the Kenai Peninsula to Kodiak Island and the Alaska
Peninsula.
Farther northeast the ensemble means have been fairly consistent
in showing a Canadian ridge building over the northeastern
mainland and possibly merging with a short range Arctic ridge.
Operational runs show different ideas for ridge details,
supporting a solution that transitions from a model/mean blend
more toward the means with time. Based on stronger ensemble mean
trends from yesterday and recent operational runs, the manual
forecast enhanced the strength of the ridge relative to the
initial blend described above. The ridge axis is close to
continuity though.
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
No significant hazards are expected over Alaska during this
forecast period.
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