Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
756 PM EDT Fri Apr 28 2023
Valid 12Z Tue May 02 2023 - 12Z Sat May 06 2023
...Overview...
Low pressure and associated upper dynamics expected to reach near
the southwestern mainland as of early Tuesday, along with the
leading front, should produce areas of enhanced precipitation
along the southern coast and Panhandle from the short-range time
frame into Tuesday and perhaps Wednesday. Most guidance continues
to show a mean trough aloft persisting over the western mainland
through the end of the week but with a fair amount of uncertainty
for embedded details, while upper ridging over Canada may try to
build in from the east to some degree by late week. Meanwhile
there is still a lot of spread and variability for system details
across the North Pacific/Aleutians/Bering Sea, though with some
trends over the past couple days to guide the forecast.
...Guidance Evaluation/Predictability Assessment...
Over and near the mainland, as of early Tuesday the 12Z CMC joined
what has been the dominant cluster with the core of the upper
system and surface low pressure reaching the southwestern mainland
as opposed to a farther south track. However, there are lingering
signals that fine-scale shortwave energy (with low predictability)
within/around the upper trough could lead to a second wave that
tracks along the southern coast. Then there is the new 18Z GFS
that has come in with the southern scenario for the primary low,
more like some earlier CMC runs, tempering confidence somewhat.
Still the large majority of guidance favoring the mainland track
for the primary low into Tuesday and a second wave that may be
near the Kenai Peninsula by early Wednesday recommended that
solution, which provided good continuity in principle. The
leading wavy front has continued to trend faster in most guidance
(except for the 12Z UKMET that is closer to consensus from 24
hours ago). Individual operational model runs show an increasing
variety of possibilities for specifics within the mean trough
aloft expected to remain aligned over the far western mainland for
the rest of the week. These differences involve what happens to
energy already within the trough as well as upstream Arctic flow
that could drop into the trough, with both the GFS and ECMWF
varying quite a bit over consecutive runs. The ensemble means
thus far have been agreeable and consistent with the trough axis
while not committing to any specific embedded upper low(s) by the
latter half of the period. This recommends more ensemble mean
weight later in the period as model details diverge.
Guidance continues to display spread and trending for evolution
across the North Pacific/Aleutians/Bering Sea. There is some
relative stability in the forecast from Tuesday into Wednesday
with a system tracking toward the western Aleutians while a
leading frontal wave takes shape as it tracks along the Aleutians.
After early Wednesday, primary trends over the past day or so
have been toward quicker shearing out of the dynamics supporting
the western Aleutians low, leading to weakening of that feature
while the leading wave gains greater emphasis as it toward/into
the eastern Pacific. ECMWF/ECMWF mean/CMC mean runs were first to
suggest these trends and the 12Z/18Z GFS have adjusted more in
that direction versus prior GFS runs that were slower and deeper
Bering Sea low pressure. This leaves the 12Z CMC more on its own
with its Bering Sea system. Even with some clustering for general
trends, there are still question marks with system latitude south
of the Alaska Peninsula after midweek, with no clear consensus
yet, while flow details to the north could have some influence as
well.
Today's forecast considerations led to starting with a blend
consisting of the 12Z ECMWF, GFS, and CMC/UKMET in order from more
to less weight during the first half of the period. This solution
reflected the most common ideas of guidance while tempering lower
confidence regional specifics of any particular model. After
early day 6 Thursday, increasing detail spread and disagreement of
the CMC with other guidance led to increasing 12Z GEFS/00Z ECMWF
mean weight and phasing out the CMC. Total ensemble mean weight
reached 40 percent on day 7 Friday and 70 percent on day 8
Saturday.
...Weather/Hazard Highlights...
The system forecast to reach the southwestern mainland by Tuesday,
along with associated upper troughing plus a surface wave that may
track along the southern coast, will likely spread enhanced rain
and higher elevation snow across portions of the southern mainland
into the Panhandle. Faster timing of the leading wavy front may
bring some of the heavier precipitation to the southeast corner of
the mainland and northern Panhandle just before the start of the
extended period, with this moisture pushing southward over the
Panhandle into Tuesday. Meanwhile the upper trough/surface wave
may focus another area of heavy precipitation over areas near
Prince William Sound on Tuesday. Persistence of the upper trough
over the western mainland should maintain an unsettled pattern the
rest of the week, but with gradually decreasing precipitation
amounts across the south. Low predictability details within the
upper trough will determine specifics of this general lighter
trend. The upper trough will also support areas of mostly light
precipitation over other portions of the mainland. The Aleutians
should see mostly light rainfall with one or more weak waves
tracking across the region, per recent majority trends.
Confidence in specifics is not yet very high though, with a lower
probability scenario involving more rain with stronger low
pressure still existing.
The forecast pattern should bring increasing coverage of below
normal high temperatures, with cooler anomalies spreading
northward into Wednesday-Thursday and then stabilizing or possibly
moderating a bit. Morning lows will tend to be above average into
Tuesday and then some moderately below normal readings will likely
begin to move into some southern/western areas by mid-late week.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Tue, May 1-May 2.
- Heavy precipitation across portions of mainland Alaska, Tue, May
2.
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