Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 260003
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
403 PM AKDT Mon May 25 2020

.ANALYSIS AND UPPER LEVELS...
An easterly wave moving west across the Copper River Basin
is spreading clouds and rain from the Copper River Basin to the
north Gulf and MAT-SU Valleys. Temperatures and freezing levels
are such that rain is likely mixing at times with snow in the
passes surrounding the Copper River Basin. Broad cyclonic flow
around the upper low out west continues showers and/or rain from
Southwest ALaska to over the Bering Sea and Aleutians.

&&

.MODEL DISCUSSION...
Models are reasonable and consistent in the handling of the
synoptic pattern. The easterly wave moving west across
Southcentral seems to be well handled. Models are also consistent
in moving the near resident upper low over the eastern Bering Sea
to the Gulf of Anadyr Wednesday and dropping another upper center
south to over the western Bering Sea. Looks like a weak Rex Block
starts to set up by mid week over the Mainland with an upper high
over the eastern Mainland/western Canada and a upper low over the
southern Gulf.

&&

.AVIATION...
PANC...VFR conditions and generally light winds will persist
Ceiling may however may occasionally drop in the 3500 to 5000 ft
range in showers.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An easterly wave over the Copper River Basin today is heading
into the Susitna Valley through Kenai Peninsula this evening. This
is a little farther southwest than models have been showing but
radar shows rain into Turnagain Arm and continues to fill-in in
over Knik Arm. The main lifting with the wave is farther north so
the Mat-Su valleys are expected to see more rainfall than
Anchorage southward.

An upper level trough will traverse the region from south-to-
north Tuesday keeping some rain showers over the area.

A warming trend will begin Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Thursday evening)

An area of low pressure near the Pribilof Islands will continue
to slowly move north over the next couple of days. As it does so,
a series of disturbances will lift north across the region,
providing bursts of ascent (lift) while also helping to increase
instability, which could allow for a few thunderstorms to
develop.

Speaking of convection, we decided to pull the thunderstorm
mention for the Kuskokwim Mountains as the latest instability
progs show MUCAPEs staying below 100 J/kg, total totals (TTs)less
than 50, lifted indices of +2 or higher, and mid-level lapse
rates staying below 7 deg. C/km. With these meager values and
surface observations paltry moisture values on the upwind side of
the mountains, this seemed to be a prudent move. Will continue to
monitor webcams to see if change in thinking occurs, but, thus
far they`ve show flat "pan-cake like" cumulus with little vertical
growth.

Heading into Tuesday, temperatures will gradually warm at the
surface and 850 mb, with moisture best tomorrow. For Tuesday the
models show MUCAPEs increasing (one instability parameter we look
at) into the 300 J/kg with TTs climbing above 55 and mid-level
lapse rates around 7 deg. C/km with lifted indices near 0. This
is very marginal, but perhaps sufficient for storms to develop
given the lift from a vort lobe passing overhead. Should they
develop it would be across the lower Kuskokwim Valley (Sleetmute
east).

Then on Wednesday, the main low and attendant disturbances aloft
move away from the region, with models showing instability and 700
mb RH (relative humidity) values decreasing. With MUCAPEs of only
100-200 J/kg, and a weaker trailing disturbance passing overhead,
couldn`t rule out a few low-topped afternoon showers. Dry weather
is expected on Thursday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Thursday evening)

An area of low pressure southwest of Shemya will continue to lift
towards Siberia through Thursday, sending a frontal system north
and eastward over the next few days. Widespread rainfall will
accompany this front, with just a few hit and miss showers
possible elsewhere.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...
Gulf of Alaska: A front wrapped around the north side of a low
may possibly produce gales from Thursday morning through midday
over the far southern Gulf`s offshore areas. The low center will
move into the eastern Gulf through Saturday, but as it weakens,
the chance of gales will remain low. Confidence is average.

Bering Sea/Aleutians: Gales continue to be most likely Thursday
morning over the central Bering from Adak through to west of Saint
Matthew Island. The chances of those gales materializing in that
area have decreased since yesterday. Confidence quickly diminishes
as to whether a new round of gales will impact the southern
Bering and Eastern and Central Aleutians Friday into Saturday, as
model agreement on the track of a new North Pacific low that would
cause the gales is very low.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Tuesday)...
The long-term forecast starts out with a stagnant weather pattern
in place. An upper level Rex Block of sorts starts out the long
term on Friday across mainland Alaska, with an upper level high
over the northern Yukon, with a stationary upper low over the
Gulf. A narrow area of ridging is over Southwest Alaska, with a
large trough over the far western Bering. An upper low associated
with the western Bering trough moves east to around the Central
Aleutians by Sunday. Whether or not that low gets close enough to
the still-stationary Gulf low to absorb it remains in question,
with more solutions than not suggesting that one does not absorb
it until early next week.

Either way this pattern will leave much of southern mainland
Alaska, especially Southcentral, on the wet side of things. This
is because pulses of energy circulating around the upper level
Gulf low will periodically swing east to west, or easterly waves
as we meteorologists call them, around the Gulf low and across
Southcentral. Since the waves will not be originating from
interior Canada, but rather from the Gulf, they will frequently
have lots of moisture associated with them. Thus, cool, cloudy,
and wet conditions, with potential for thunderstorms if any
prolonged clearing can occur in between waves, are expected. The
upcoming weekend may allow the Copper River Basin in particular to
end what had been a very dry May prior to last night well above
normal for total precipitation.

For Southwest Alaska, since the mountains of Southcentral will
wring out much of the moisture and the waves will be moving
towards the southwest, it`s far more likely to be dry for much of
the long-term. Any stronger waves could still set off showers and
thunderstorms in the afternoons, depending on how much
instability and sunshine can develop earlier in the day. Sunday
afternoon appears to be the best chance of precipitation as of
now.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 155 (Johnson River).
MARINE...NONE.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...JW


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