Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Sat May 04 2024

Valid 12Z Wed May 08 2024 - 12Z Sun May 12 2024

...Wet/snowy pattern along the southern coast and Panhandle,
especially mid-late week...

...Overview...

The majority of guidance is suggesting that a mean low aloft
should meander over the southeastern Bering Sea/Alaska
Peninsula/North Pacific region during the period, favoring periods
of precipitation along the southern coast and Panhandle. 
Currently expect the heaviest rain and higher elevation snow to be
from the Southcentral coast through the Panhandle with two systems
tracking into the Gulf of Alaska Wednesday-Thursday.  Forecast
details become increasingly uncertain beyond Thursday, with
models/ensembles offering various ideas for upstream North Pacific
features--ultimately affecting how much easterly flow and moisture
the Alaska Peninsula may see by next weekend.  Farther north the
majority of guidance shows an upper trough axis near the western
mainland, possibly transitioning to a col region at times.  Also,
western Canada upper ridging could periodically extend back into
parts of the mainland.

...Guidance Evaluation and Preferences...

Latest solutions are coming into better agreement for a rapid
succession of two waves forecast to track into the Gulf of Alaska
during Wednesday-Thursday.  A westward trend in the 12Z
ECMWF/UKMET versus their 00Z runs has led to improved clustering
for the first wave.  Up to this point there has been a fair amount
of spread for details of the second wave.  12Z dynamical models
have significantly narrowed their east-west spread, as the GFS
adjusted deeper/westward and the UKMET shifted eastward (though
representing the deep extreme).  The ensemble means tilt a bit
closer to Kodiak Island than most of the operational runs but this
is a minor difference given the prior spread and typical
predictability for five days out in time.  Interestingly, a
majority of 00Z ECMWF-initialized machine learning (ML) models
were closer to the weaker/eastward 00Z-06Z GFS in principle but
the average of new 00Z ML runs seemed to be a little deeper and
stronger.  An operational model composite with greater emphasis on
the GFS/ECMWF provided a reasonable starting point for this part
of the forecast.

The remainder of the forecast hinges on what becomes of a
potentially elongated wave/frontal system that may reach the
northwestern Pacific and vicinity by day 5 Thursday.  There is a
majority scenario, including the GFS/ECMWF and their means along
with most of the 00Z ML models, that would have this system or at
least a breaking-off leading part, continue rapidly eastward near
or south of the Aleutians and ultimately track to a position some
degree south of the Alaska Peninsula by next weekend.  However
within this cluster there is significant north-south spread with
corresponding differences in wind/moisture effects across the
Aleutians and Alaska Peninsula/southern coast.  ECMWF/ECens mean
runs are on the northern side with the average of 00Z MLs almost
as far north.  The 12Z/18Z GFS runs shifted well south after the
06Z run was more like the ECMWF.  Preference sided with an
intermediate solution perhaps 2/3 toward the ECMWF/ECens mean,
closest to the 00Z ML average.  New 12Z ML runs offer more
differences in how the initial system may elongate and where best
low pressure emphasis may be within the overall axis.

In sharp contrast, recent CMC/CMCens runs have been holding the
Thursday western Pacific system westward behind an initial Pacific
into Siberia ridge instead of letting the dynamics pass through
the ridge per the majority cluster.  This leads to a completely
different surface pattern from the Bering Sea/Aleutians/North
Pacific into the mainland from Friday onward.  This evolution is
low enough in probability not to include in the deterministic
forecast but there is certainly potential for future trends away
from the majority solution in one way or another.

Finally, there is a signal for another system that could brush the
Aleutians or track farther south by next weekend.  Thus far the
GEFS/ECens means are closer to the Aleutians than their
operational counterparts.  The CMCens derives its Aleutians system
from some combination of the late week system and trailing energy.
 Preferences that tilt somewhat toward operational guidance keep
this system south of the Aleutians for the time being.

Forecast considerations led to starting with 70 percent total 12Z
GFS/ECMWF and 30 percent UKMET/CMC early in the period, followed
by incorporating 30-40 percent 12Z GEFS/00Z ECens means plus
splitting GFS input between 12Z/06Z runs (day 6 onward) and the
ECMWF between 12Z/00Z runs (day 7 onward) to account for detail
differences that develop later in the period.

...Weather/Hazards Highlights...

Latest guidance trends appear to be focusing on the mid-late week
period for heaviest precipitation along the southern coast and
Panhandle, with rain over coastal/low elevation areas and snow at
higher elevations.  A leading system tracking into the Gulf of
Alaska on Wednesday will produce some moderate to locally enhanced
amounts, while a stronger system on Thursday should bring a period
of heavier activity to the Southcentral coast and Panhandle. 
Currently the GFS/ECMWF show potential for this second system to
bring in precipitable water values up to 2-4 standard deviations
above normal for a brief time.  Brisk to strong winds may
accompany this system as well.  Areas of lighter precipitation may
extend northward over parts of the mainland, with uncertain
influence on any river flooding due to ice jams and snowmelt. 
Confidence decreases for forecast specifics behind this second
surface low.  It looks like a close call as to whether the
Aleutians see any meaningful wind/rain from a potential system
tracking out of the western Pacific.  Then the uncertain latitude
of this system as it reaches south of the Alaska Peninsula by next
weekend keeps confidence fairly low regarding how much easterly
low level flow and moisture may affect the Alaska Peninsula, and
with even lower confidence for areas farther east along the
southern coast.

Cloud cover and precipitation will tend to keep daytime highs
below normal across southern areas, though the southern Panhandle
may trend warmer toward the weekend.  Mean troughing aloft near
the western coast should keep highs on the cool side in its
vicinity as well.  The Interior will have the best potential for
above normal highs.  Expect greater coverage of above normal
morning lows.  Exceptions will likely be along the western coast
and a few localized pockets elsewhere.

Rausch


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