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FXAK02 KWNH 022357

PMDAK 



Alaska Extended Forecast Discussion

NWS Weather Prediction Center College Park MD

757 PM EDT Thu May 02 2024



Valid 12Z Mon May 06 2024 - 12Z Fri May 10 2024



...Overview...



An upper low initially over the northeastern Pacific/Gulf of

Alaska (associated with an impactful system during the short range

period) should steadily weaken and open up as its energy lifts

northward with time, pushing into mean ridging over Canada into

the northern mainland.  This energy may produce some areas of

meaningful precipitation over Southcentral and vicinity. 

Meanwhile, the combination of a northern Bering Sea upper low and

a system emerging from the northwestern Pacific should ultimately

support an Aleutians system whose surface reflection with mean

upper low may settle over or near the Alaska Peninsula by late

next week.  This system will likely bring increasing precipitation

to the southern coast and Panhandle during the middle to late part

of next week.  With lower confidence, another system may have some

influence on portions of the Aleutians by next Friday.



...Guidance Evaluation and Preferences...



A blend approach looks good for resolving the weakening upper low

expected to drift north and the northeast from the Northeast

Pacific/Gulf of Alaska, with the initial surface reflection

weakening in place.  Regarding the Bering Sea through North

Pacific evolution, there is at least the common theme of overall

upper troughing with an embedded low by mid-late week, along with

best emphasis for surface low pressure near the Aleutians and into

the Alaska Peninsula.  However dynamical and machine learning (ML)

guidance has been showing a lot of detail differences as can be

expected with such a scenario involving uncertain interaction of

separate features.  Adding in the 12Z guidance there is a notable

trend that favors trimming away the western side of the envelope

for the Bering Sea upper low by around midweek.  This leaves the

12Z GFS as a modest western extreme given adjustments in the other

guidance including the new 12Z ECens mean.  Beyond this aspect of

the forecast, there is better than average agreement toward the

overall upper/surface low reaching near the Alaska Peninsula by

Friday (while weakening at that time) but individual dynamical and

ML solutions offer minimal agreement on finer details that will

take a while to get resolved.



Between the initial Northeast Pacific feature and Aleutians

system, guidance shows a very wide spread for the potential track

of a mid-latitude wave forecast to be well south of the Aleutians

as of early Monday.  The majority of solutions would keep the wave

near or south of 50N latitude and have no effect on Alaska.  On

the other hand the 12Z CMCens mean tracks it to Kodiak Island

while the past couple CMC runs have brought it into the northeast

Pacific.  Such a solution would yield an earlier increase of

moisture along the southern coast and/or Panhandle.  The ML models

do not add any clarity, as their envelope generally spans the

range of dynamical guidance.  At this time preference the

maintains continuity via the southern scenario, keeping the

CMC/CMCens out of the forecast.



Thus far there has been some spread and trending in the guidance

for a western Pacific system that may affect parts of the

Aleutians by late next week.  Ensemble means have been on the

slow/ill-defined side while the ML models show a faster/northward

depiction that would have more influence on the Aleutians.  The

12Z ECMWF is closest to the 00Z ML idea while the 12Z may stray a

little north and the GFS runs (like the 00Z ECMWF) track somewhat

south but have similar timing.  Latest signals favor tilting the

forecast as close as possible to the 12Z ECMWF for now.



Above guidance comparisons led to starting today's forecast with a

40/30/30 blend of the 12Z ECMWF/12Z GFS/12Z UKMET early in the

period.  By day 6 Wednesday the blend started to add a little 12Z

GEFS/00Z ECens input while splitting the GFS among 06Z/12Z runs to

help center the Aleutians system.  Days 7-8 incorporated only 30

percent total of the means as the 12Z GFS/ECMWF were fairly close

to them in principle near the mainland and given the ECMWF

preference near the western Aleutians for Friday.



...Weather/Hazards Highlights...



Portions of the southern and eastern mainland may see one or more

episodes of locally enhanced precipitation early-mid week as the

initial upper low offshore the southern coast/Panhandle gradually

opens up and lifts northward/northeastward. Currently the details

are too uncertain for evaluating how this activity may influence

any river flooding due to ice jams and snowmelt.  Lingering mostly

light precipitation is possible along the southern coast and

Panhandle early in the week.  The system tracking near the

Aleutians should bring that region mostly light precipitation

during the first half of the week.  As the surface low reaches the

eastern Aleutians and Alaska Panhandle, leading flow will likely

bring increasing moisture to the southern coast and eventually

Panhandle.  By next Wednesday-Thursday the forecast rainfall is

sufficient to merit a heavy rain area from portions of the Alaska

Peninsula through Kodiak Island and into some of the Southcentral

coast in the Days 3-7 Hazards Chart.  The extent to which rainfall

may reach hazardous criteria into the Panhandle will be evaluated

tomorrow.  The Aleutians may see another episode of rainfall by

next Friday with another system but confidence in specifics is

fairly low at this time.



Expect below normal high temperatures over most of the eastern and

southern parts of the state early in the week, with the eastern

Interior tending to have the best chance for some above normal

readings.  The forecast pattern should keep the southern coast and

Panhandle below normal for highs through the rest of the week

while other areas trend toward a mix of above/below normal highs. 

Morning lows will tend to be more above normal over most of the

state (highest anomalies over the North Slope), though still below

normal over the far west early in the week.  The Southcentral

coast and Panhandle may see some localized below normal lows as

well.



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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