Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
750 PM EDT Fri Jun 23 2023
Valid 12Z Tue Jun 27 2023 - 12Z Sat Jul 01 2023
...Overview...
The best clustering of guidance shows an initial southeastern
Bering Sea upper low drifting into and across the northeastern
Pacific/Gulf of Alaska while upstream energy digging through
Siberia forms another Bering Sea upper low that persists into next
Saturday. Farther northeast, upper ridging will likely prevail
from western Canada into the northern mainland and parts of the
Arctic. This evolution should yield a fairly wet pattern across a
large part of the state during the period, with some variability
for which regions see relatively higher totals on a day-to-day
basis.
...Guidance Evaluation/Preferences...
Leading into the start of the period, the latest GFS runs do not
seem to be depicting a weak wave that other models are showing
with a track that may take it into or near the northern Panhandle
around or shortly before early Tuesday as it dissipates. Behind
this feature, latest ensemble mean runs show better than average
agreement for the leading upper low (or implied low in some means,
as definition decreases across the Gulf later in the period) as
well as the next low settling into the Bering by next
Thursday-Saturday. Early-mid period, operational runs show some
variability among each other and consecutive runs for the path of
the initial upper low. Lack of pronounced trends/clustering would
recommend an average approach, with reasonable support from the
means. By day 8 Saturday there is a noticeable divergence between
the models and means as most models (aside from the 06Z GFS) track
the Gulf low well inland versus the slower means. However the new
12Z ECens mean, arriving after forecast preparation, has adjusted
faster to be closer to the operational runs.
The ensemble means, an average of the 12Z/00Z ECMWF runs (the
newer run perhaps a bit fast for a time), and an average of
12Z/06Z GFS runs trending more to the 06Z version with time,
provided the most reasonable intermediate depth and track for the
second Bering Sea upper low. By late in the period the 12Z GFS
has an upstream Siberia upper low that ultimately overwhelms the
pattern versus the Bering low which consensus maintains through
Saturday. The 12Z CMC becomes a slow and deep extreme by days 7-8
Friday-Saturday. The new 18Z GFS has yet another minority
scenario for the features of interest, leading to a more
persistent upper low near the Alaska Peninsula.
Farther north over the mainland toward the end of the week, the
12Z GFS develops stronger retrograding energy aloft than most
other solutions while the 12Z CMC does not extend the axis of
northern coast/Arctic ridging as far westward as consensus.
Based on the distribution/behavior of latest guidance, the days
4-6 (Tuesday-early Thursday) forecast started with a blend of the
12Z-00Z ECMWF/12Z-06Z GFS/12Z UKMET to yield the preferred
intermediate solution for the features of interest. This blend
also ended up being close to a refined version of continuity from
yesterday. Days 7-8 Friday-Saturday incorporated 25-40 percent of
the 12Z GEFS/00Z ECMWF means while maintaining some input from the
last two ECMWF runs and transitioning the GFS input more toward
the 06Z run. This led to a compromise progression of the Gulf low
and represented a somewhat stronger version of the means for the
second Bering low.
...Weather/Hazards Highlights...
Flow ahead of the upper low tracking from the southeastern Bering
Sea across the Gulf of Alaska will likely spread an area of
focused rainfall along the southern coast from the Alaska
Peninsula to the Panhandle, especially during Tuesday-Wednesday.
Some lingering activity may persist through late week over the
southeastern coast/Panhandle. Rain could be locally moderate to
heavy at times but should not be intense enough to reach hazardous
levels. The surface system associated with the second Bering Sea
upper may spread another area of rainfall into the Alaska
Peninsula and vicinity toward the end of the week. There is more
uncertainty with rainfall totals for this event due to guidance
not yet resolving important details for the strength and
orientation of low level flow. Elsewhere across the mainland,
expect fairly cloudy and damp conditions during the period though
locations along the northern coast may see a drier trend after
Tuesday. Most rainfall to the north of the southern coast should
be on the light side during Tuesday-Wednesday. Then by late week
into the weekend, most guidance shows an increasing amount of
moisture (precipitable water anomalies highest in the 12Z ECMWF,
lowest in the 12Z GEFS mean) reaching the northern/eastern
portions of the mainland. Individual model runs vary with details
of location and magnitude of associated enhanced rainfall over
parts of those areas, so additional time will be needed to assess
this potential threat.
Widespread cloud cover should lead to a more narrow diurnal range
than average for temperatures, with lows tending to be near or
above normal and highs generally below normal. The most extreme
anomalies should be with cool highs along and near the southern
coast. Some pockets of above normal highs will be possible over
eastern parts of the Interior around midweek and for multiple days
over portions of the northwestern coast and Seward Peninsula.
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 outlook 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