Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Mon Jul 12 2021
Valid 12Z Fri Jul 16 2021 - 12Z Tue Jul 20 2021
...Overview...
The majority of guidance shows a similar large scale evolution
during the Friday-Tuesday period. Expect separate mean troughs
over the Arctic/Siberia and to the south over the western
Pacific/Bering Sea, with some phasing of these streams possible by
late in the period. Mean ridging ahead of these troughs should
align over the eastern Aleutians/southern mainland by the weekend
with gradual weakening/suppression early next week. A mean trough
with one or more embedded lows should persist over or a little
south of the southern Panhandle/Haida Gwaii. As has been the case
in recent days, there are a number of uncertainties for embedded
features though some details may be starting to come into better
focus.
...Guidance/Predictability Evaluation...
One aspect of the forecast that has improved this cycle is the
handling of initial energy from a short-range Bering Sea upper
low. The 12Z ECMWF abandoned its multi-run idea of bringing the
energy into the southwestern mainland this weekend (joined briefly
by the 00Z UKMET before reverting back in the 12Z run). While the
12Z GFS combined this energy with a portion of flow ejecting from
the upper low emerging from Kamchatka to yield a similarly-timed
shortwave, the new 18Z run has trended much more toward the 12Z
model consensus that is weaker/faster--keeping the leading ridge
stronger as has been advertised by most ensemble mean runs.
Within the expected storm track from Kamchatka/far northwestern
Pacific into the western Bering Sea, there is decent agreement
upon a leading system from late this week through at least the
weekend (albeit with some lingering timing spread but generally
trending toward an intermediate solution) and a moderate signal
for a second system during the first half of next week. Details
of Arctic flow continue to go astray after about the first couple
days of the forecast, which has been typical lately. Of
particular note in this cycle the 12Z ECMWF becomes faster than
other guidance by day 6 Sunday with a compact upper low that
ejects from the mean trough northwest of the state.
The first part of the forecast emphasized 12Z operational
guidance, though with the GFS component divided among the model
and the GEFS mean to tone down the GFS handling of Bering Sea
energy. Divergence for specifics later in the forecast maintained
the preference of recent days to tilt the blend 70 percent toward
the 12Z GEFS/00Z ECMWF means by day 8 Tuesday. This solution
provides decent continuity with typical day-to-day adjustments.
While the leading western Bering system dissipates by next Monday
per this blend, model clustering is getting close to supporting
better definition at that time.
...Weather/Threats Highlights...
Northern parts of the mainland should see some precipitation late
this week and weekend with the passage of an Arctic system that
lifts away a leading warm front and drops a trailing front that
will also stall and return back as a warm front. Low pressure
tracking from the northwestern Pacific into the western Bering Sea
should spread a broad area of precipitation and brisk winds across
the Aleutians and Bering Sea. Moisture should begin spreading
into the western mainland around Sunday with precipitation
persisting and expanding gradually eastward thereafter. There
could be some locally enhanced pockets of activity but it will
take additional time to refine the specifics. As it is guidance
is just starting to show better agreement on the timing of the
overall moisture shield. Meanwhile precipitation over the
Panhandle should tend to favor southern areas and trend lighter
with time.
The North Slope and portions of the Panhandle/southeast corner of
the mainland should see the most persistent below normal highs
during the period, though northern areas may see some day-to-day
variability depending on frontal position. Moisture arriving from
the west during the weekend into next week will bring a cooling
trend from west to east, with increasing coverage of below normal
highs. On the other hand low temperatures will tend to be more
above normal with only smaller areas of below normal anomalies.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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