Small Craft Advisory 723, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKDT Mon Sep 30 2019

.ANALYSIS AND UPPER LEVELS...
There is a front tracking east through the Copper River Basin
following behind ridging to the east that is also pushing to the
east. The front brought heavy rain to areas along the north Gulf
coast and moderate rain to inland locations. The band of heaviest
precipitation is now east of Glennallen. To the west, a trough
extends south through southwest Alaska and through the Alaska
Peninsula. In the base of the trough, south of the Alaska
Peninsula, a low is developing and beginning to move to the
northeast. Farther west, ridging is over the western Bering is
also moving to the east.

&&

.MODEL DISCUSSION...
The models are in good agreement with the main synoptic features
into the mid term, but start to struggle with a low in the Gulf
Tuesday. There are minor differences in the strength and position
of the low resulting in differences with how strong the winds will
be. The EC and NAM have been in good agreement and were preferred.

&&

.AVIATION...
PANC...Light winds will persist into the afternoon and then shift
to the north and increase to around 10 kt. Ceilings will remain
low through the early part of the day, but will gradually improve
into the evening. There is some uncertainty with the timing of VFR
conditions returning to the airport.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Mon and
Tue)...
Monday will bring a short reprieve from the wet weather before
another strong low moves in on Tue. Rainfall amounts from Sun
ranged from about 0.50" around the Anchorage Bowl to closer to
1.0" closer to the coast. The front has just about cleared the
ALCAN border this morning and just a weak surface low remains over
the Northern Gulf of AK. This surface low will be the key to the
forecast for the next 48hrs. First, it is helping to produce a
weak offshore gradient at the surface. For now, this is keeping
just enough dry air flowing across the surface to keep widespread
fog from forming. However, localized areas will still see fog this
morning and it could become more widespread, especially as the
sun rises and helps to mix out some of the moisture from Sunday's
rain.

By tonight, a strong upper level shortwave will move towards the
Western Gulf and give plenty of support to this surface low. The
shortwave is currently tracking along the Aleutians just south of
Dutch Harbor. This system will cause the weak low to slide towards
Kodiak Island and rapidly deepen through Tue morning. This will
bring yet another round of widespread moderate rain to nearly all
of Southcentral. This rain will carry from southeast to northwest
across the area associated with a weak warm front. The best
chance for rain around the Anchorage area will be early Tue
morning (daybreak) through the midday. The most substantial impact
from this system will be the winds over the marine areas. As the
low deepens and pivots just east of Kodiak Island, it will be able
to tap into some colder air from SW AK. This is a fairly classic
set up to bring some very gusty N-NW winds through Shelikof
Strait, the Cook Inlet, the Barren Islands, and much of Kodiak
Island. We have issued a storm warning for marine areas just east
of Kodiak. There is still some uncertainty with the exact strength
and track of the low center, so winds and waves may need to be
increased with future packages.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Upper level troughing along the Kuskokwim Delta coast will
continue to bring scattered showers onshore today. As the flow
across Southwest Alaska turns northwesterly in response to ridging
building in along the coast, the north side of the Alaska
Peninsula will become more favored for upslope showers.
Northwesterly winds will increase rapidly tonight through Tuesday
morning both through the typical gaps and passes in the Alaska
Peninsula and also across Southwest Alaska as a strong surface low
in the western Gulf rapidly intensifies. The northwest winds will
start diminishing Tuesday afternoon through Tuesday night. A
shortwave trough will move onshore over the Kuskokwim Delta
Tuesday evening followed close behind by a second one Wednesday
morning and a broader upper trough Wednesday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Upper level ridging in the western Bering will shift east and into
the central Bering today and then over the eastern Bering
tonight. Northwesterly winds will increase over the eastern Bering
and across the Alaska Peninsula tonight through Tuesday morning in
response to the tightening of the surface pressure gradient
between the ridge and a low deepening rapidly in the western Gulf.
Following the ridge, a shortwave trough and accompanying surface
ridge will move from the western Aleutians and Bering this
afternoon and evening to eastern Bering by Tuesday. The next
shortwave will swing across the northern and central Bering
Tuesday afternoon through Tuesday night followed by another one
Wednesday and Wednesday night.

&&

.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 delay 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...120, 130, 131, 136, 137, 138, 139, 140, 150,
				 155, 160, 351, 352.
		 Storms...132.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
MARINE/LONG TERM...ML