Small Craft Advisory 598, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 AM AKDT Fri Aug 7 2020

.ANALYSIS AND UPPER LEVELS...

A broad 992 mb surface low is sitting near King Salmon this
morning with an occluded front stretching across the northern
Gulf. A moist, southeasterly flow along and ahead of the front is
bringing widespread rain from Prince William Sound and the
eastern Kenai Peninsula west to the Alaska Range. Downsloping
across the Kenai and Chugach Ranges is limiting precipitation
from Cook Inlet to the Mat-Su Valleys, where only scattered
showers are falling. The latest ASCAT scatterometer pass confirms
southeasterly small craft advisory winds across the northern Gulf
this morning with lighter southwesterly winds across the central
and southern Gulf. Gap winds are also prevailing this morning
through places like Turnagain Arm due to the increased pressure
gradient between the aforementioned system and a coastal ridge.
These winds, however, are currently bending down inlet with only
light and variable winds for locations such as Anchorage and
Palmer.

For Southwest Alaska, a trough is extending northward from the
AKPEN low north along the coast and out over the eastern Bering.
This feature, along with an associated upper-level shortwave
extending from the upper-level low stacked over the surface low,
are both helping to enhance shower activity across Bristol Bay and
the Kuskokwim Delta this morning. Gusty northwesterly winds and
showers also extend across the eastern Bering south to the
southern AKPEN in response to the upper-level low and cold air
advection on the back side of the low helping to increase
instability.

Conditions are more stable over the central and eastern Aleutians
north to the Pribilofs where a ridge has built in between the
AKPEN low and a new low beginning to spin up in the Gulf of
Anadyr. This system is extending a front into the far western
Bering as seen on infra-red and water vapor satellite imagery.

&&

.MODEL DISCUSSION...

Models continue to remain in decent agreement through about 00z
Sunday, especially with the front over the northern Gulf
weakening and stalling along the Southcentral coast. Guidance then
breaks down quickly regarding the development of a surface low
south of Kodiak Island and its track across the Gulf through
Monday morning. Specifically, the 00z and 06z GFS runs remain
the western-most and strongest solutions. The EC and GEM are the
farther east and are in best agreement with each other. The 00z
NAM had the eastern most track, with the most recent run, the
06z, even farther east. At 00z Sun, there is a 200 mile spread and
8 mb difference between the GFS and NAM. This continues through
00z Monday when the GFS has a 992 mb low near Middleton Island,
the EC and GEM have a 1000 mb west of Yakutat, and the NAM has
already transitioned the low to an open wave and pushes it
onshore east of Yakutat. Bottom line, there is average agreement
of widespread precipitation along the Southcentral coast,
especially around Prince William Sound, with easterly gales likely
across the marine zones. However, there is low confidence on
whether areas to the west like the Kenai Peninsula and Kodiak
Island will see heavier rainfall and gusty winds. The GFS and GEM
solutions would also bring a wider area of gales and even some
storm-force winds along the coast.

&&

.AVIATION...

PANC...Expect VFR conditions through this TAF package with an
isolated shower possible this morning. Variable winds this
morning are expected to turn southeasterly early this afternoon
with gusts between 15 and 20 kts developing by late afternoon.
The southeasterly winds are expected to linger through the
overnight hours with gusts dropping off later tonight.

&&

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

The frontal system remains across the northern and eastern Gulf
this morning. Radar returns show continued showery activity across
Prince William Sound and the Kenai Peninsula. Lighter showers are
seen around the rest of Southcentral with the Anchorage Bowl
seeing very light returns due to downsloping.

Decreasing gap winds and precipitation will occur today as the
upper level trough weakens as it moves inland. The best chances
for rainfall will be across coastal regions and over higher
elevations.

Tomorrow, chances for rain will increase for lower elevations as a
low moves into the Gulf from the North Pacific in the afternoon to
evening timeframe. The exact strength and track of this low is
still uncertain, as models are proposing differing solutions that
would lead to different outcomes. These details will be further
refined with future forecast packages.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Sunday)

Early this morning, satellite imagery shows a pair of mid level
lows near Bethel and King Salmon, with the associated surface lows
seen easily on the radar mosaic loops. Although the low further
south will slide off into the Gulf, a shear axis was observed
just to the northeast of these points, lying in a northwest to
southeast orientation. The net affect will be sufficient lift and
moisture, with bursts of ascent from vort maxes rotating around
these features, to maintain scattered shower development across
all of southwest Alaska through the evening.

In fact, a few thunderstorms have been observed during the late
late evening/early morning hours between McGrath and Lime
Village. This is a result of the factors monotoned above and
sufficient elevated instability. With little change seen in the
pattern, isolated thunderstorm mention was added to the forecast
for this afternoon east of a Crooked Creek to Iliamna line, with a
similar setup across the far eastern portions of the lower
Kuskokwim Delta for Saturday afternoon/evening. Otherwise, look
for a drying trend along the western coast on Saturday, before
showers return there on Sunday, while remaining possible through
the period for points eastward.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Today through Sunday)

The upper trough currently situated across the Bering Sea and
Aleutians will get reinforced by another short-wave trough diving
southeast from Siberia this morning, reaching the Bering by
Saturday morning, and the Alaska Peninsula on Sunday. The net
affect will be periods of showers through the period, with winds
remaining below gales.

Across the far western Bering and Aleutians, a front approaches
Saturday night, moving eastward to near Adak on Sunday. Widespread
rainfall will accompany this feature, with some uptick in winds
and waves expected. However, winds here will also remain below
gales.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Gulf of Alaska: Models are having a hard time handling a North
Pacific low moving into the Gulf on Sunday. Most models have the
system tracking into the western Gulf, while the NAM solution is
keeping a weaker solution farther east. There is also a bit of
spread on the strength of this low pressure system, but most
models are in consensus that winds will be in the gale force
range, with the GFS trending towards a higher solution. By Monday
morning, this system weakens, with quieter conditions expected
for Tuesday. Stay tuned for future forecasts as this system
continues to be evaluated and updated.

Bering Sea: Models are in good consensus with a front tracking
over the western Bering and Aleutians on Sunday into Monday
morning, with winds likely to stay below gale force level. A week
front associated with the system will continue to track eastward
towards the eastern Bering and ALeutians through Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday through Thursday)...

The overall synoptic pattern on Monday keeps a low approaching
the western Aleutians and Bering, with a low over the Gulf of
Alaska. The system out west has models in pretty good agreement
keeping showers in the forecast as the front moves eastward
through Tuesday stretched across the Bering and Aleutians.
Southwest Alaska will remain relatively dry until this front
approaches the coast for Wednesday, but models are still
struggling with timing and position of precipitation moving in
through Thursday.

For Southcentral, a weakening low in the Gulf will keep a chance
of showers in the forecast on Monday, before tapering off on
Tuesday. However, the uncertainty is this low position keeps
precipitation a forecast challenge. A front approaching late
Wednesday associated with a low south of the Aleutians will be
the next chance for widespread rain, but models are having a much
harder time handling this system the farther east it travels.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...AH