Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EDT Sun Jul 26 2020
Valid 12Z Thu Jul 30 2020 - 12Z Mon Aug 03 2020
...Overview and Guidance Evaluation...
Today's guidance has carried over some aspects of the forecast
pattern evolution from continuity but with some meaningful
adjustments for details within an area of expected
troughing/closed low(s) between the Bering Sea and Northeast
Pacific, while providing some improved clarity for what could be a
fairly strong system tracking across the Bering Sea next weekend.
The ideas of an east-west upper ridge building over the mainland
and northwestern Canada along with eastward departure of an
initially deep Arctic upper low have maintained reasonable
consistency.
Since yesterday the operational GFS/ECMWF/UKMET runs have all
trended toward a more pronounced southeastward pull of Bering Sea
dynamics into the Northeast Pacific, leading to an upper low and
surface system where prior consensus had suggested a somewhat
farther north and east-west elongated pattern at the surface and
aloft. The ensemble means hint at lower heights reaching into the
northeastern Pacific but appear to be a cycle or two behind in
depicting the full detail of this trend. Corresponding to this
adjustment the latest majority cluster has trended a little
stronger with the east-west upper ridge over the mainland. The
12Z CMC differs somewhat from the other models so there is still
some lingering uncertainty. Another question mark is how energy
within a leading upper low off to the southeast ejects from its
initial position. Solutions range between a northward track that
would affect portions of the southern coast/Panhandle to a more
sheared/eastward evolution that would produce lesser effects.
Meanwhile the guidance has improved its clustering for how upper
dynamics (likely in the form of a closed low) should track from
Siberia/Kamchatka into the Bering Sea, supporting a fairly strong
surface system crossing the Bering next weekend. Among the
ensemble means available up to the time of forecast preparation
only the 12Z GEFS mean reflected the operational model evolution
while the 12Z CMC/00Z ECMWF means provided at best a vague hint of
this system--though ultimately leading to an eastern Bering upper
low like the other solutions late in the period. The new 12Z
ECMWF mean has adjusted to the operational model cluster to
provide further confidence in the majority scenario. Typical
guidance error currently suggests an intermediate timing, between
the slow 12Z ECMWF and fast 12Z CMC. Ahead of this system there
is still low confidence in specifics of a western Pacific wave
that may get picked up and brush the Aleutians around Fri or so.
Latest trends are toward a faster wave that may become embedded
along a warm front versus a slower pickup that could allow the
wave to merge with the parent low.
Preference to reflect the majority operational model cluster for
specific details led to using the 12Z GFS/ECMWF/UKMET for the
first part of the forecast period. The blend transitioned to
nearly even model/ensemble weight by days 7-8 Sun-Mon, with a
higher proportion going to the 12Z GEFS mean versus the 00Z ECMWF
mean due to comparisons for the system tracking over the Bering
Sea next weekend. By the latter half of the period the
operational ECMWF component was split between the 12Z run and
prior 00Z version due to some aspects of the new run becoming less
appealing at that time.
...Weather Highlights/Threats...
Initial southeastern Bering Sea low pressure and wavy frontal
system extending to the south and southeast should bring some
focused precipitation to parts of the Alaska Peninsula/southwest
corner of the mainland early in the period. Some moisture may
reach the rest of the southern mainland and Panhandle late week
into the weekend but precip intensity and duration are quite
uncertain due to low predictability of medium-smaller scale
aspects of flow aloft. Some precipitation may also accompany a
front that should drop down across the northern half of the
mainland and then stall. Low pressure expected to track across
the Bering Sea next weekend may bring a period of precipitation
and stronger winds with some effects possibly reaching the western
mainland by late in the period. Expect below normal high
temperatures over approximately northern and southern thirds of
the mainland as well as the Panhandle. Above normal highs should
prevail over central latitudes but likely with a gradual cooler
trend over time. A larger portion of the state should see above
normal lows with the North Slope most likely to be the exception
with some below normal lows.
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
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