Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
703 PM EDT Mon Jul 27 2020
Valid 12Z Fri Jul 31 2020 - 12Z Tue Aug 04 2020
...Overview and Guidance Evaluation...
The most uncertain part of today's forecast appears to be the
evolution of energy to the west and south of an upper ridge
expected to build over the mainland and western Canada. In
varying ways the GFS/ECMWF/UKMET still take a decent proportion of
initial eastern Bering Sea upper low energy into the northeastern
Pacific/Gulf of Alaska. However they are generally slower to do
this than seen 24 hours ago. The ensemble means had been hinting
at trending in the operational model direction but now they have
reverted to earlier runs that tracked most of the eastern Bering
low directly to the north. The 12Z CMC does this as well. What
happens with this feature and surrounding flow has an impact on
northward ejection of the initial upper low over the eastern
Pacific (near 50N 140W early Fri). One positive aspect of the
forecast even with these combined uncertainties is that most
guidance shows an upper low reaching or evolving over the
Northeast Pacific/Gulf in some fashion, followed by ejection
across or south of the Panhandle.
Over the past day most guidance has been much more consistent and
agreeable with the fairly strong system expected to track across
the Bering Sea during the weekend into early next week. The most
notable difference is that the 12Z/06Z GFS runs are faster than
the model/mean consensus, favoring the 00Z run for the GFS
component of the forecast. By later in the period a model/mean
compromise looks reasonable to resolve latitude differences, with
operational runs generally near or south of the ensemble means
(which are all very similar).
In order to yield a more modest adjustment from continuity between
the eastern Bering and Northeast Pacific given the high
sensitivity/low predictability for specifics, the first half of
the forecast used a blend of the 12Z/00Z cycles of both the ECMWF
and GFS along with a little 12Z UKMET input. Then the latter half
of the period featured an even blend of models (both ECMWF runs
and 00Z GFS) and means (12Z GEFS/00Z ECens).
...Weather Highlights/Threats...
Some focused precipitation should persist into late week over the
southwest mainland/Alaska Peninsula with the low pressure and wavy
front associated with the upper low initially over the eastern
Bering Sea. The ultimate path of this upper low is quite
uncertain. Current forecast that pulls the low southeast would
keep moisture confined to the southwest but a northward track
(only modestly less than even probability) would extend organized
precip farther northward over the western mainland. Then by this
weekend Northeast Pacific/Gulf upper low evolution from one or
more sources may bring a surge of moisture into the southern
mainland/Panhandle with significant precip totals possible at some
locations depending on specifics of flow aloft. A portion of this
moisture may continue northward with time. A front settling near
the central mainland could help to focus some activity as well.
The fairly vigorous system tracking across the Bering Sea during
the weekend into next week will likely bring a period of
precipitation and stronger winds, with effects reaching the
mainland late weekend/early next week. Best potential for highest
totals with this system will be near the southern coast but other
locations may see meaningful precip as well. From late this week
into the weekend expect below normal high temperatures across the
northern and southern thirds of the mainland along with the
Panhandle, while warmer readings versus normal will prevail over
the central mainland. With time the negative anomalies should
expand and yield below normal highs over most of the state by next
Mon-Tue, but still with greatest negative anomalies over the far
north and south. Clouds/precip should promote broader coverage of
above normal min temps. The North Slope may still see below
normal lows 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
Hazards:
- Heavy rain across portions of mainland Alaska, Thu, Jul 30.
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Sat-Sun, Aug 1-Aug 2.
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