Small Craft Advisory 457, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
346 PM AKDT Mon Jun 1 2020

.ANALYSIS AND UPPER LEVELS...
A vertically stacked low south of the Alaska Peninsula has a
front stretching out of it into the Gulf of Alaska. A triple point
low is developing in the central Gulf along this front today and
can be seen in satellite imagery.

A weak upper level wave brought some light rain to the Copper
River Basin early this morning and is continuing to weaken as it
moves westward. Convective potential over Southcentral and
Southwest Alaska remains low this afternoon and evening with the
only moderately unstable air close to the Alaska Range.

A weak ridge of high surface pressure is enhancing along the
mountains of the north Gulf of Alaska coast. This is bringing in
the gusty southerly-to-easterly winds to the typical channeled
areas such as Turnagain Arm, the Knik River, and along the Copper
River.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement through most of the
week. As has been the case recently, the synoptic agreement is
good, but timing the propagation of upper level waves and
assessing the convective parameters is where the model differences
creep in. Southwest mainland Alaska is an area where there is a
large difference in timing of an upper level wave moving from
north-to-south Wednesday where the NAM is faster and pushes the
wave to the south much farther as well. This will impact the
convective potential due to the way this wave develops and moves.

&&

.AVIATION...
PANC...VFR conditions will persist. Gusty southerly winds are
expected during the afternoon and evening both today and Tuesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level trough remains in place along the northern Gulf
coast. This will allow the easterly wave pattern to continue
through the middle of the week with on and off showers expected.
Southeasterly winds through the usual places including Turnagain
Arm, Knik River Valley, and the Copper River Basin will pick up
this evening before diminishing overnight as a front approaches
the northern Gulf coast. A weak surface low will move over the
Cook Inlet area late Tuesday while a coastal ridge in place along
the northern Gulf coast strengthens. This will lead to another
round of gusty southeasterly winds Tuesday night for the usual
places including Turnagain Arm, Knik River Valley, and the Copper
River Basin. Meanwhile, diurnal heating on Tuesday may generate
enough instability over the northern Copper River Basin and
Talkeetna mountains to produce an isolated thunderstorm or two
Tuesday afternoon and evening. By Wednesday, a low meandering in
the Gulf begins to move toward the southeastern Gulf. This will
allow several upper level waves to pass over Southcentral from
east to west, resulting in continued unsettled and showery
conditions for midweek.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A relatively quiet few days are on tap for southwest Alaska.
Residual moisture across the region, coupled with weak transient
short waves should spark a few showers this afternoon. Most of the
activity should remain across the Alaska Range and Kusko
mountains, but non-mountain showers look to be confined to a
corridor between Stuyahok and Sparrevohn, through the Kilbucks.
With the thermal trough deep into the interior of Alaska, and no
appreciable upper level dynamics, thunderstorm activity should
remain at bay.

Tuesday afternoon is will sing a different song across the Lower
Kuskokwim Valley (generally west of Stony River) as a weak
shortwave will drop in from the Yukon and Upper Kusko Valleys.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The stationary low south of Dutch Harbor is continuing to bring
rain and gusty winds to the central and eastern Aleutians. This
low will continue to drift east and weaken. Winds will taper off
and rainfall will continue with this eastward track towards the
Alaska Peninsula Tuesday.

&&

.MARINE (Days 3-5: Thursday through Saturday)...
Generally quiet weather is expected throughout the forecast
period for all marine areas with light winds and low waves.
Westerly flow will prevail across the Bering Thursday through
Saturday. A weak low pressure system located near the southern
Gulf of Alaska Thursday morning will continue moving southeastward
while dissipating with general northerly winds expected for much
of the Gulf Thursday afternoon through Saturday with waves
diminishing Thursday evening.

&&

.LONG TERM FORECAST (Days 4-7: Friday through Tuesday)...
Models remain in general good agreement for Friday and Saturday
that a weak upper level ridge of high pressure will be present
over the mainland of Alaska with a vertically stacked low present
over the northern Bering Sea. This will bring unsettled weather to
Southwest Alaska with the chance for showers leading to slightly
below normal temperatures for Friday and Saturday. Generally quiet
weather is expected for Southcentral and the Copper River Basin
Friday and Saturday, although low pressure over the Gulf may
allow for weak easterly waves to move across the mountain ranges
across Southcentral leading to the chance for showers each
afternoon. Temperatures are expected to be above normal for
Friday and Saturday across Southcentral and the Copper River
Basin.

Differences emerge beyond Saturday with the ECMWF trying to
develop a cut-off low over the Bering early Sunday and then moving
it slowly to the south and the east. The ECMWF is faster in
developing this low and stronger. The GFS on the other hand is
stronger with the ridge building over the interior of Alaska and
doesn't develop the low the ECMWF was showing until later and
further south. The forecast period for Sunday through Tuesday
remains uncertain in regards to the sensible weather leading to
low confidence in the forecast. The strength of these upper level
features and placement will be key to the sensible weather for the
second half of the weekend into early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 120.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SS
MARINE/LONG TERM...MV