Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EDT Thu Jun 04 2020
Valid 12Z Mon Jun 08 2020 - 12Z Fri Jun 12 2020
...Overview and Guidance/Predictability Assessment...
To the south of 55-60N latitude the majority of guidance continues
to show a similar large scale evolution though with some moderate
adjustments/detail spread. Over the past day the guidance
consensus has trended westward with the upper low initially near
the Alaska Peninsula as of early day 4 Mon. After that time there
is still good agreement upon moderate progression across the
northeastern Pacific and interaction with the surface system that
starts the period to the southeast of the upper low.
ECMWF/UKMET/CMC runs and ECMWF/CMC means offered the best
clustering over the Northeast Pacific, in contrast to the slow
scenario of the 12Z GFS. The 06Z GFS was much closer to the
majority. The 12Z GEFS mean reflected some influence from its
corresponding operational run but ultimately trended more toward
the consensus. Continuity is good with the idea of the surface
low reaching no farther north than 50-55N latitude. Upstream
there is still decent agreement and continuity in principle
(especially among the means) for the system tracking into the
Aleutians next week. There is some debate among the operational
models whether a separate frontal wave may form ahead of the
parent low. At the extended time frame such a feature has low
predictability. Thus a blend approach that does not explicitly
depict such a wave but leaves ample room for one to evolve seems
to be optimal at this point.
Important specifics over some areas of the mainland have become
more uncertain within an upper pattern whose forecast over recent
days has been showing a relative lack of definition though with a
tendency for some degree of ridging. Today there is a more
pronounced divergence of solutions for energy that may or may not
separate from the southern fringe of progressive Arctic flow. The
12Z GFS/CMC both dropped a fairly deep and compact upper low along
the western coast of the mainland Tue onward, after the 06Z/00Z
GFS runs deposited a compact upper low over the
northern/northeastern mainland. On the other hand ECMWF/UKMET
runs and ensemble means so far have not signaled much potential
for meaningful energy to be left behind. Given the lack of
agreement, relatively new nature of the compact upper low
scenario, and low predictability of such a small-scale feature,
preference was to maintain a more progressive Arctic shortwave
through mid-period but with a minimal spot low aloft left behind
over the northeastern mainland Wed-Thu--which could plausibly
exist based on the forecast blend's shortwave position on Tue. In
association with the Arctic shortwave there is now a better
reflection of a front arriving into the northern mainland
early-mid period. Later in the period the 12Z ECMWF brought
another piece of Arctic energy close to the northwestern mainland
but with little meaningful support from other guidance.
Above considerations led to starting with an operational model
blend that incorporated the most common ideas among the 12Z
ECMWF/CMC/UKMET and 06Z GFS for about the first half of the
period. Then the blend transitioned to a model/ensemble mean
combination, and included the 00Z run along with the 12Z version
for ECMWF input, as various details diverged later in the period.
...Weather Highlights and Hazards/Threats...
Periods of convection will be possible over favored locations
across the southern mainland. Some activity may extend across
eastern areas depending on shortwave energy that may lift
northward near the eastern border with Canada. The upper low
departing from near the Alaska Peninsula may produce some
lingering precipitation early in the week. What happens with the
southern periphery of Arctic shortwave energy during the first
half of the week provides a significant element of uncertainty for
a possible added source of rainfall enhancement over the northern
and/or western mainland. Meanwhile moisture ahead of the system
crossing the Northeast Pacific may extend into parts of the
Panhandle. The Aleutians should see an increase of precipitation
and winds with the system approaching/reaching the islands next
week, though enough detail differences exist to temper confidence
in specifics at this time. For temperatures expect negative
anomalies for highs to be most prominent across portions of the
southern mainland and Panhandle. Warmest anomalies for highs
should be over parts of the western mainland. The front reaching
the North Slope region should bring a brief cooling around Tue.
The majority of the state will likely see above normal anomalies
for low temperatures.
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