Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
641 PM EDT Thu Jun 18 2020
Valid 12Z Mon Jun 22 2020 - 12Z Fri Jun 26 2020
...Overview and Guidance/Predictability Assessment...
The latest guidance offers similar ideas for the large scale
evolution during the period. An area of low heights (one or more
embedded upper lows) initially near the Alaska Peninsula/Gulf of
Alaska should weaken during the first half of the period, followed
by a North Pacific system that should reach near the eastern
Aleutians/extreme southwest corner of the mainland by late next
week. At higher latitudes expect a slow moving upper low over the
Arctic to anchor a mean trough whose axis should extend at least
as far south as the Bering Strait. Embedded energy will likely
push a cold front into the northern mainland early next week, with
the front stalling and persisting on the leading side of the upper
trough the rest of the week.
The most difficult part of the forecast involves the Northeast
Pacific/Gulf of Alaska pattern during the first half of the week.
Through the 00Z-06Z cycles there had been a split in guidance with
long-term continuity in favor of a fairly slow and deep system,
best represented by the 00Z ECMWF/ECMWF mean (GFS runs erratic but
tending to be faster/eastward). However in varying ways all
solutions that had advocated for a slower system trended
faster/eastward in the 12Z run, with the full array of new
models/means tracking low pressure anywhere between the Gulf and
Haida Gwaii by day 5 Tue. Typically greater confidence in a
pronounced trend early in the extended range favored an adjustment
to the 12Z guidance in principle--along with a multi-wave approach
(parent and triple point lows) to account for the broad latitude
spread in the guidance.
Elsewhere guidance was at least somewhat more stable and agreeable
aside from low-confidence shortwave uncertainties within the high
latitude mean trough aloft and typical differences with precise
track and strength of the North Pacific system approaching the
southwestern mainland. With the high latitude trough the means
have trended noticeably deeper since yesterday while over the past
couple days the ECMWF has been more consistent with the shape of
embedded energy versus the GFS. Finally, there has been a hint of
a trailing western Pacific system which could begin to have some
influence on the Aleutians very late in the week. The average of
12Z guidance adjusted a little slower with this feature.
Today's forecast started with a 12Z model blend and very modest
ensemble input for about the first half of the period. Then it
trended quickly to an even model/ensemble (12Z GEFS/00Z ECens)
blend as details became more uncertain within the more agreeable
mean pattern.
...Weather/Threats Highlights...
12Z model changes for the leading system expected to track over
the the Northeast Pacific and possibly into the Gulf during the
first half of next week have lowered the coverage and intensity of
associated precipitation along parts of the southern
coast/Panhandle. There is still significant track uncertainty,
keeping confidence in precip specifics low. The trailing North
Pacific system will likely bring moisture across portions of the
Aleutians and then eastward through the Peninsula/southern
mainland and possibly to the Panhandle. Some enhancement will be
possible over favored windward terrain. Interior areas will see
scattered convection during the period with some added focus from
shortwaves within the mean trough aloft aligned just northwest of
the mainland as well as frontal boundaries over/near the North
Slope region. Expect a large percentage of the state to see below
normal highs during the period. The greatest negative anomalies
should be over the North Slope and extreme south/Panhandle.
Anomalies for lows should be more mixed with the best potential
for some above normal readings away from the aforementioned
regions with coolest high temp anomalies.
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
Hazards: 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