Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Mon Jul 22 2019
Valid 12Z Fri Jul 26 2019 - 12Z Tue Jul 30 2019
Over and near the mainland the primary forecast uncertainty
regards the evolution/path of the upper low (and surrounding flow)
forecast to be just west of the mainland at the start of the
period Fri. Low confidence details of the southern periphery of
an Arctic ridge may have a significant influence on the path of
the upper low itself while progressive North Pacific flow will
play a role in energy that may elongate to the southeast of the
low and cross the Gulf of Alaska. For days 4-5 Fri-Sat the 06-12Z
GFS and 00Z ECMWF/CMC cluster fairly well with each other and 06Z
GEFS/00Z ECMWF-CMC ensemble means so those operational models
provide the foundation for the early portion of the forecast.
After Sat guidance steadily diverges for upper low due in part to
differences in the degree to which the Arctic ridge extends into
the mainland and northwestern Canada. What trends exist seem to
be pointing toward slower upper low progression. The 00Z ECMWF
mean reflects this trend and has support from the 06Z GEFS/00Z CMC
mean, while the 12Z GFS is slower than the 06Z run and the 00Z CMC
has the upper low meandering over the far northern Bering for the
entire rest of the period. Interestingly operational ECMWF runs
have been fairly consistent but with three of the past four runs
becoming faster than the means. This leaves the 06Z GEFS/00Z
ECMWF means as the best starting point for the late-period
forecast given the increasing spread with time, with a model/mean
blend providing the transition between the operational
model-focused day 5 Sat forecast and the all-means forecast by day
8 Tue.
Farther upstream the guidance diverges over the North
Pacific/Aleutians/southern Bering late in the period, especially
after 12Z Mon. The most likely scenario as reflected by the
latest ensemble means and GFS runs would have a warm front
approaching if not reaching the western Aleutians. Based on the
ensemble mean position of the upper trough by this time there is
the possibility that anchoring low pressure could end up being a
little farther east than depicted by the means, but not nearly to
the extent of the 00Z ECMWF. Thus preferences lean to the means
exclusively by day 8 Tue as with the evolution over the
mainland/Gulf of Alaska.
The overall pattern should support fairly broad coverage of
precipitation potential for at least portions of the period. Best
signal for highest 5-day accumulation exists over the southern
mainland and Panhandle but some northern areas may see meaningful
totals as well. Especially after Sat, guidance spread for
specifics is sufficiently great to keep confidence low for
pinpointing the position and timing of any locally significant
rainfall on a particular day.
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