Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
653 PM EDT Wed Jul 08 2020
Valid 12Z Sun Jul 12 2020 - 12Z Thu Jul 16 2020
...Overview and Guidance/Predictability Assessment...
Some aspects of the forecast for the Arctic and portions of the
mainland have been changing considerably over recent
model/ensemble runs. In the past day an overwhelming consensus of
guidance has trended surprisingly fast with the retrogression of
an Arctic high now expected to reach close to the northeastern
mainland by the start of the period early Sun and possibly Siberia
as soon as Tue. Early in the period it is hard to argue with
adjusting to the 12Z model consensus, while the relative newness
of the fast retrogression scenario leads to a somewhat more
conservative approach--toward an average of the 12Z GEFS/CMC and
00Z ECMWF means by late in the forecast. This is still a dramatic
westward trend compared to yesterday's guidance. The new 12Z
ECMWF mean has come in faster though not quite to the extent of
the operational run, so there is some added support for the fast
scenario. There are some elements of continuity preserved though.
The surface front expected to be draped across the north-central
mainland briefly changes its position as the Arctic upper low
passes by but otherwise the lingering weakness aloft to the east
of the upper low ultimately keeps the boundary in a location
similar to yesterday. Albeit faster, leading energy that reaches
the mainland in the short range period appears to be taking a
trajectory similar to prior preference.
Across the North Pacific into the Gulf of Alaska, the eastern-most
question mark arises from the evolution of shortwave energy over
and near the mainland at the start of the period. The 12Z ECMWF
becomes the most concentrated with it thus resulting in a fairly
strong/northward Gulf wave. The CMC has some similarities but is
southeast with the wave while remaining guidance (including the
12Z ECMWF mean) is more open with the upper feature and
weaker/southward with the surface wave. Preference leans away
from the northern extreme for this wave. Farther west the general
evolution has held up fairly well over the past day. Low pressure
initially south of the Aleutians should very gradually weaken as
upper dynamics open up. Individual waves feeding into the overall
area of low pressure or tracking northeast ahead of it have very
low predictability due to their small scale though. A preferred
model/mean blend ultimately maintains continuity with a weak
lingering wave reaching Bristol Bay by early Wed but the 12Z
ECMWF/CMC offer a more suppressed alternative so confidence is
still not particularly high. So far there is decent clustering
for a stronger system approaching/reaching the Aleutians by next
Wed-Thu. The 12Z CMC/GFS are fairly close to the ensemble means
with some modest latitude variance. The 00Z ECMWF was farther
south and the new 12Z run is a little northwest.
Today's starting blend used the 12Z operational models on day 4
Sun and then steadily trended toward an evenly weighted
model/ensemble mean solution by day 8 Thu. Some manual editing of
the resulting output was required to provide better definition of
the Arctic upper low. Due to the 12Z ECMWF lower-confidence
solution over the Gulf and becoming faster than most other
solutions for the Arctic upper low, ECMWF input changed to the 00Z
run after a transition on Sun.
...Weather/Hazard Highlights...
Some of the specifics are not resolved but there is still decent
agreement that a majority of the state will see below normal
temperatures during the period, with more extreme anomalies for
the daily highs relative to the lows. Coolest readings versus
average should be across the North Slope during the weekend into
early next week as the Arctic upper low passes by. The
south-southeast and Panhandle should be another area of relatively
greater negative anomalies. Some locations near the western coast
may see one or more days of above average temperatures while more
of the south/west could see above normal lows. Gradually rising
heights aloft/flatter mean flow may support a moderating trend by
midweek.
The best potential for highest five-day precipitation totals will
be over the southern mainland near the Alaska Range. Shortwave
energy passing through early in the period should provide the best
focus for activity followed by somewhat lighter/more scattered
rainfall. Also expect periods of rain farther north with the
front over north-central areas possibly providing a focus at
times. Uncertainty with wave details from the North Pacific
through the Gulf keeps confidence low for exactly how much
precipitation occurs over the Aleutians and Panhandle. A
better-defined storm system coming into the picture from the
western Pacific should bring an area of organized rainfall and
strengthening winds to the Aleutians by next Wed-Thu.
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