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FXAK02 KWNH 042359

PMDAK 



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



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