Small Craft Advisory 722, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
642 PM AKDT Sun Sep 29 2019

ANALYSIS AND UPPER LEVELS... There is an upper level trough over
the central and eastern Bering Sea and Aleutian Islands. There is
an upper level ridge over the Western Bering, and another ridge
over the Panhandle and extreme northwest Canada. There is a long
front stretching from the Susitna valley south through Cook Inlet
across the western Gulf of Alaska. This front has a long moisture
fetch into the tropics near Hawaii. This is bringing a decent
swath of rain to much of Southcentral. There is a surface low
forming along this front just south of the Kenai Peninsula.

MODEL DISCUSSION...
Overall, the numerical models are in good agreement through the
short term portion of the forecast (Tuesday afternoon). There are
some minor disagreements on the location and movement of a surface
low in the Gulf Monday night and Tuesday, but this has minimal
impact on the forecast. Therefore the forecast confidence is
higher than normal.

AVIATION...
PANC...MVFR ceilings are expected through late Monday morning as
considerable moisture remains over the area, then will improve to
VFR by the afternoon. Winds will be fairly light.

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
(Tonight through Wednesday evening)

Widespread rainfall continues to fall across all of Southcentral,
thanks to a deep tropical feed with Hawaiian origins. This is
being fed north from a deep southerly to southwesterly flow
aloft, anchored by a weak surface low of 1005 mb near the Barren
Islands. This fetch of moisture looks to continue through the
evening. As the low treks ever so slowly to the east through the
night, rainfall will become more showery in nature after midnight.
Then for your Monday, with the low centered south of Cordova, the
a drying offshore flow pattern will develop as much of Southcentral
will be on the backside of the low, with any rainfall being located
near the Alaska range and Copper River Delta.

This drying trend will be short-lived though, as strong shortwave
energy, currently diving southeast through the central Bering,
reaches our current storm system by early Monday afternoon. As
this occurs, the low retrogrades (moves westward) towards the
Barren Islands once again. How far west and how much does it
deepen remains the main questions, as bombogenesis (at least a
24 mb drop in 24 hours) commences. This afternoon's model runs
aren't in good agreement with the track and strength, as the GFS
deepens it to ~977 mb. Conversely, the European (ECMWF) and NAM
numerical models are weaker at about 987 mb, and the Canadian
Global near 984 mb. While this has minimal effect on the overall
precipitation amounts, this difference could have impacts with
northerly flow on the western flank of the storm.

The main consensus though is for widespread rainfall to develop
for Monday night and Tuesday, with amounts generally in the one
half inch to one inch range a good possibility, as some residual
tropical moisture gets entrained into the storm. As of right now,
locations south and east of a Houston to Paxson line stand to see
the heavier/more widespread rainfall, with amounts tapering off
the further north and west one goes. This storm will then move off
to the south and east for Wednesday, with a renewed trying trend
developing.

SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/tonight
through Wednesday evening)...

The front that brought ample rainfall to Southwest has moved
eastward, leaving showery, cooler flow in its wake. Some patchy
morning fog is possible along the coastal areas as onshore flow
remains weak during the early morning hours Monday. A ridge will
move over the eastern Bering, bringing northwesterly flow to much
of Southwest by Monday night. Northwesterly winds are expected to
gust through the usual gaps and passes along the Alaska Peninsula
and across the greater Bristol Bay area Monday night into Tuesday.
Another weakening front arrives to the Southwest coast by Tuesday
night bringing showers and westerly onshore flow.

SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/
tonight through Wednesday evening)...

An upper level ridge continues to build over the western Bering
through Monday. As the ridge moves eastward, winds will strengthen
across the eastern Bering and Alaska Peninsula Monday night into
Tuesday. Meanwhile, a front associated with a Kamchatka low
arrives over the western Aleutians Monday evening bringing gusty
southerly flow and rain. This front weakens as it traverses
eastward and brings generally westerly flow across the Bering for
Wednesday.

MARINE (Tue afternoon Thu night)...
The strong low pressure system in the Gulf will keep gale-force
winds along the back side of the low, mainly in the southern and
western Gulf. There could be some storm force gusts, especially
out of bays and passes on Tuesday afternoon, but sustained winds
should be max-gales. Those winds will taper off quickly Wednesday
night into Thursday as the system weakens toward the Panhandle.
The Bering Sea will see a period of relatively benign conditions
through mid-week as broad westerly flow sets up, but not very
strong. However, the fetch and time of westerly winds could stack
some larger waves/swell than the weaker winds would suggest. The
next significant system enters the Bering on Thursday with a gale-
force front.

LONG TERM FORECAST (Tue afternoon Sat night)...
The extended forecast begins with a Gulf of Alaska low near peak
intensity. However, that system quickly weakens while the pattern
shifts a little bit to a zonal jet stream with several strong
disturbances. The first disturbance spins up a very progressive
low that tracks into the Bering Strait region. The wind direction
and wind duration hint at some possible surge/coastal erosion
issues, but the worst should be north of the Kuskokwim Delta
area. However, there is some uncertainty associated with the
broader forecast as the remnants of Typhoon Mitag will make an
extratropical transition and phase with the jet over east Asia.
Both long range models (ECMWF and GFS) show a little different
progression of this feature. The ECMWF delays the trough into
which the tropical system phases, and hence not a great phase,
which results in a flat ridge downstream. The GFS looks to fully
phase the feature which really amplifies the downstream north
Pacific ridge. Details should become more clear early to mid week.
Stay tuned.

AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES: 119, 125, 130-132, 150, 351, 352
FIRE WEATHER...NONE.

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...ML

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