####018004818####
FXAK02 KWNH 162358

PMDAK 



Alaska Extended Forecast Discussion

NWS Weather Prediction Center College Park MD

758 PM EDT Tue Apr 16 2024



Valid 12Z Sat Apr 20 2024 - 12Z Wed Apr 24 2024



...Overview...



Guidance agrees that the period will start with a strong upper

ridge covering western Canada and Mainland Alaska with a

deep-layer low to the south of the Alaska Peninsula/Kodiak Island.

 With time, models/ensembles diverge regarding the evolution of

the North Pacific low (affecting moisture flow along the

southwestern coast) as well as for what becomes of initial

troughing/embedded upper low over Siberia, with implications for

the ultimately persistence and strength of the upper ridge. 

Meanwhile there are also various ideas for low pressure that may

reach the Aleutians and vicinity by the early-middle part of next

week, with a dependence on both the Siberia energy and another

system expected to emerge from the western Pacific.





...Guidance Evaluation and Preferences...



The relative majority of solutions, including the 00Z

ECMWF-initialized machine learning models (MLs), at the very least

support better maintenance of an east-west upper ridge over the

mainland than the 12Z GFS or GEFS mean, which bring the Siberia

upper upper trough/low northeastward (though at least slower in

the GEFS).  The new 18Z GFS has at least dampened the effect of

the upper trough/low, holding onto more ridging across the

southern mainland.  On the other hand, consensus suggests that the

12Z ECMWF could lean a bit to the strong side with the upper ridge

by the latter half of the period when there is somewhat of a

signal for the northern part of the ridge to get eroded.



Meanwhile the MLs lean toward the quickest southward progress of

the Siberia upper low, which ultimately leads to developing a

decent surface low in the vicinity of the central Aleutians by

Monday, versus most other solutions in the cluster that are still

to the south of the western Aleutians at that time.  Recent ECMWF

runs offer other options, including some splitting of the Siberia

energy with some going south and the rest going around the

mainland ridge.  It is a low-confidence endeavor to resolve the

potential interaction among this energy and a separate system that

may emerge from the western Pacific.



There does not appear to be any well-defined clustering for the

exact evolution of the low initially south of the Alaska

Peninsula/Kodiak Island.  The most common theme is for gradual

weakening of the system and possible renewed emphasis farther

south as a separate Pacific wave tracks underneath it.  This

evolution would lead to a steady decrease of wind/moisture focus

between the Alaska Peninsula/Kodiak Island and the Kenai Peninsula.



Guidance inputs for today's forecast were constrained not only by

solution comparisons but also by local unavailability of data. 

The cluster representing the apparent most likely scenario along

with availability led to starting with an even blend of the

00Z/12Z ECMWF for the first half of the period, followed by 30-40

percent inclusion of the 12Z CMCens mean for days 7-8

Tuesday-Wednesday.



...Weather/Hazards Highlights...



Flow around initial North Pacific low pressure should support

ongoing precipitation focus along the eastern Alaska Peninsula,

Kodiak Island, and possibly into the Kenai Peninsula.  However the

intensity of this activity should gradually decrease with time and

lighter precipitation may spread along other parts of the southern

coast.  With very uncertain details, the Aleutians and vicinity

should see an increase of precipitation and wind by the

early-middle part of next week with low pressure that may affect

the region.  The most likely pattern evolution should favor above

normal temperatures on average over most of the state.  Some

erosion of the northern part of the mainland upper ridge should

allow a front to settle over the northern/north-central mainland,

with the North Slope and vicinity likely trending somewhat cooler

north of the front.  Otherwise localized parts of Southcentral and

the northern Panhandle could see one or more days of temperatures

close to normal.



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



$$