Small Craft Advisory 351, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 PM AKDT Tue May 2 2017

.ANALYSIS AND UPPER LEVELS...
The upper level low centered just south of the Alaska Peninsula
remain the main upper level feature over southern Alaska guiding
the weather pattern. The jet stream is running around the
periphery of this low with the strongest portion moving though
the southeastern Gulf of Alaska fueling the strong surface low
entering the southern Gulf. This low is the big surface feature
bringing storm force winds to the Gulf of Alaska tonight and
rainfall to the Gulf coast. Strong downsloping should keep keep
inland areas mostly rain-free from this system as it stalls in the
Gulf tomorrow and then moves back to the Southeast after that.

The airmass over Southwest mainland Alaska...especially the
Kuskokwim Delta...is unstable today with an area of CAPE strong
enough to develop thunderstorms late this afternoon and evening.
The big question is whether there will be enough surface heating
to trigger the convection.

&&

.MODEL DISCUSSION...
After the last few model runs started moving the major Gulf low
westward with each run, models are aligning quite well now with
both low track and strength. The low should peak in intensity
around 965 mb as it enters the Gulf this evening and then begin
to weaken as it moves north-northwestward and then becomes
stationary tomorrow. The outlier right now is the ECMWF which is
slower moving it back to the southeast on Thursday. This may make
a small difference in wind speed over the Gulf, but should not be
more than a 5 KT difference between the model solutions.

In the Bering Sea region, the models all have a nearly identical
solution with the way a low develops in the trough of the jet
stream and rapidly intensifies as it moves eastward...just to the
south of the Aleutians. The NAM and GEM models are hinting at a
weak secondary low developing and then rotating around the main
center, but that will only have a minimal impact on forecasts if
it does.

&&

.AVIATION...
PANC...VFR conditions will persist. Winds will become northerly
and a little gusty this evening as the strong low moves into the
Gulf of Alaska. Winds at mountaintop level will be from the east
to southeast bringing in a wind shear level. At this time it looks
to be above 2000 ft so LLWS is not mentioned in the TAF, but it
may be in a layer above the 2000 ft level through much of the
night.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Winds will increase to storm force over the Gulf coastal waters
by later this evening as the unseasonably strong North Pacific low
tracks into the central Gulf. This system will bring widespread
rain to Kodiak Island and locations along the coast through
Wednesday night as the low stalls and gradually weakens in place.
Inland locations will remain fairly strongly downsloped through
Thursday, keeping any shower activity limited in coverage. Despite
the drier conditions, cloud cover will increase through tonight
and remain fairly extensive for the next several days as what's
left of the front pushes inland. The advance of the front will
also help to shift the gusty northerly winds around to the east on
Wednesday afternoon through favored areas such as Turnagain Arm.
Conditions will begin to improve during the day on Thursday as the
low continues to weaken and moves off to the east.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

An interesting weather pattern is setting up across Southwest
Alaska. Off the top, there is some isolated thunderstorm potential
from Bethel west towards the coast late this afternoon into this
evening as a wave of moisture currently crossing the Kilbucks
moves southwestward. A lack of cloud cover over the Kuskokwim
Delta this morning allowed for sunshine across the area to
increase the instability. The forcing for thunderstorms cleared
Bethel before the storms initiated, so the threat should be
confined to the area between Bethel and the coast.

A strong low in the Gulf is making its way northward towards
southern Alaska this afternoon. While most of the impacts will
stay on the east side of the Alaska Range, the one element that
will affect much of Southwest Alaska are increased winds. Ridging
over the Bering in conjunction with the wind field around the Gulf
Low will cause gusty winds area-wide, but particularly across
Bristol Bay, from this afternoon through the day tomorrow.  The
winds will diminish Wednesday night as the ridging over the Bering
weakens but more importantly, the low over the Gulf quickly
weakens.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The winds gusting across the eastern Bering are largely due to the
influence of the strong Gulf low. Since the winds are northerly,
expect stronger winds downwind of bays and passes that open to the
Pacific side. As the low spins and gradually weakens across the
Gulf over the next couple days, expect that the area of northerly
winds over the eastern Bering will gradually diminish.

For the western Bering, a front is sweeping across the western
Aleutians this afternoon, its eastward progress will stall by
tomorrow as the front occludes. It is not until a stronger front
moves in from the south starting Wednesday night over the central
Aleutians that active weather will continue through the area,
eventually reaching Dutch Harbor by Thursday evening. The second
stronger front will cause gale force winds to lash the central
Aleutians on Thursday morning.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The models remain in good agreement with the vertically stacked
weakening low over the Gulf of Alaska Friday weakening and exiting
to the southeast through Saturday. The big fall like storm is
handled similarly by the models through the weekend weakening the
low as it moves over the Alaska Peninsula and southwest of the
Gulf of Alaska. As we move into early next week the remnants of
the low drift into the Gulf of Alaska through Tuesday.

We expect a ridge to build in along the Gulf Coast through Friday
with the interior trough of weak low pressure fueling the somewhat
typical south and southeasterly gap and cross barrier winds across
the Chugach Mountains and Alaska Range.

Overall this pattern will hold the southern mainland in a cooler,
cloudier and more moist scenario through this weekend.

Further west another low is on tap to enter the western Bering
Sea Sunday and Monday, although the models diverge on resolving
the location of the low and the associated front. Generally this
will still result in continued cloudy and wet weather for the
Central and Western Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storms 119 120 131 132 351. Gales 130 138 139 176-178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...SA