Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Thu Jul 08 2021
Valid 12Z Mon Jul 12 2021 - 12Z Fri Jul 16 2021
...Some precipitation likely to linger over the southern coast and
Panhandle into next week...
...Overview...
Guidance continues to show a fairly blocky pattern across the
Bering Sea, Mainland Alaska, and the Gulf/Northeast Pacific. A
slow-moving upper low should track from southeast or east of
Kodiak Island toward the Panhandle through the week while an upper
ridge over the northern/east-central Bering Sea extends into
portions of the mainland. Initial weak energy farther west over
the Bering will either go around the western side of the upper
ridge or possibly combine with North Pacific/Aleutians energy that
is more likely to flow to the south of the ridge. Meanwhile the
best consensus and continuity show fast and low-amplitude flow
aloft across the Arctic, tending to evolve toward a broad trough
whose axis reaches the Canadian Archipelago later next week. This
flow will likely support a mean frontal boundary that should be
near the northern coast early in the week and then push southward
into the north-central mainland by mid-late week.
...Guidance/Predictability Evaluation...
Various aspects of flow over the Arctic and the blocky pattern to
the south are proving to be challenging to resolve in the models.
In what has become a multi-day oscillating theme, trends over the
past 24 hours have led to a farther west position for the Gulf
low. The GFS adjustment is particularly dramatic as the 12Z/07
run strayed to the fast side while latest runs are on the western
side of the spread. The GEFS mean does offer some support for the
western idea though. The ridging that extends into the mainland
at least favors slow movement, regardless of exactly how far
westward it starts out. A composite of 12Z model solutions seems
best at this time--reflecting the westward adjustment versus
yesterday while avoiding the more extreme sides of the envelope
given multi-day inconsistency. Farther west, latest trends have
been toward a farther south track for a weak wave that reaches the
Aleutians early-mid week and then likely dissipates. Meanwhile
the models have been quite varied among each other and in
consecutive runs for Arctic flow details. 00Z-06Z models had
generally suggested more amplified upper troughing that would push
the northern mainland cold front farther south than continuity,
but new runs have reverted back to higher heights. Averaging the
varied solutions of recent ECMWF runs and other 12Z guidance along
with the ensemble means yielded a forecast close to continuity for
the front that drops into the northern mainland, along with
moderate-strength high pressure to the north. Finally,
models/means are showing a decent signal for upper dynamics and
low pressure to consolidate over the western Aleutians/Bering
toward the end of the week. A model/mean blend provided a good
starting point for this potential system.
Today's forecast blend started with various components of the 12Z
GFS/ECMWF/CMC/UKMET early in the period and then transitioned to
half models (00Z-12Z ECMWF and 12Z GFS/CMC) and half means (12Z
GEFS/00Z ECens) by day 7 Friday. Offsetting differences among the
models in the Arctic helped to guide the forecast even more toward
the means than the explicit weight over that region.
...Weather/Threats Highlights...
Today's westward adjustment for the Gulf low leads to longer
persistence of precipitation over areas from near Kodiak Island
across the southern coast and into portions of the Panhandle.
There could be some localized 24-hour totals of at least one inch
in the early part of the week. However magnitude/location will be
very sensitive to the exact position of the low at any particular
time and so far the model spread and inconsistency do not provide
much confidence in refining important specifics. Meaningful
moisture could extend somewhat farther into Southcentral as well.
Elsewhere, some rainfall may accompany the front that should drop
into the northern half of the mainland by mid-late week while a
weak wave should spread light-moderate rainfall across the
Aleutians mostly during the first half of the week. Late in the
week an area of rain and somewhat stronger winds may extend ahead
of a system reaching over or near the western Bering Sea.
Clouds and precipitation will keep many areas along the southern
coast and Panhandle below normal for temperatures during the
period. Cool air behind the front dropping into the northern
mainland will drop readings over the North Slope to below normal
levels by Wednesday-Friday. In-between, expect highs to trend
toward modestly above normal during the first half of the week and
then settle back to within a few degrees on either side of normal,
while lows tend to remain a little more above normal.
Rausch
Hazards:
- Heavy rain across portions of southern mainland Alaska, Sun, Jul
11.
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
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