Gale Warning 336, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
410 PM AKDT Sun May 5 2019

.ANALYSIS AND UPPER LEVELS...
The pattern continues to be dominated by a broad trough west of
the mainland and the North Pacific Ridge holding over SE AK. The
broad trough is ferrying plenty of lobes of energy around in a
cyclonic (counter-clockwise) fashion. Each of these has the
potential to bring a round of wind and precipitation along with
it. One of these is reflected at the surface as a warm occlusion
near the Gulf Coast. Light rain has already made its way into
parts of the southern Kenai Peninsula and most of Bristol Bay.
The pressure gradient is also increasing in response to the front
and gusty south to southeast winds are developing through many of
the usual places (Knik Arm, Turnagain Arm, the Copper River
Valley, etc).

The upper level 300 mb (30,000') jet is primarily south of the
area. It is supporting the longwave trough as it runs basically
west to east parallel to the Aleutian Chain at about 140 kt. A
weak 50 kt finger is helping to bring some of the energy up
toward Southcentral, but the finger is so weak and disorganized
it is really only acting as steering flow.

The next wave is tracking along 50N south of Sand Point. This
system will develop and take a nearly identical track to the
current front off to the NE over the next day or so.

&&

.MODEL DISCUSSION...Models are in very good agreement. This is
making for higher than normal forecast confidence for at least the
first half of the week. The few areas of uncertainty surround a
brief shot of light rain around Cook Inlet Mon AM, and the exact
strength and location of the winds along the Anchorage Hillside
starting early Tue.

&&

.AVIATION...
PANC...VFR conditions should persist. A brief period of light rain
is probable overnight into Mon morning which could spell MVFR
ceilings for a few hours. The southeasterly Turnagain winds are
just barely making it into the terminal. This shouldn't last more
than 3 or 4 hours. At that point in time, down Inlet pressure
gradients will increase once again and the winds should shift
south of the terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Tuesday evening)

A front will continue to approach the southern coast of Southcentral
Alaska through tonight, before undergoing frontolysis tomorrow as
it moves ashore. Fairly widespread precipitation will affect the
coast through the next 24 hours, some of which may be heavy at
times. This feature will also bring at least small craft criteria
winds, with some sustained gales likely east of Montague Island
as a weak barrier jet persists.

Further inland, precipitation chances will also be on the increase
for most locations, with the traditional downslope holes seeing
mainly just some sprinkles through tomorrow morning. However, from
just after sunrise through late morning, the 850 mb flow will
veer to a more southerly component. This should allow these rain
shadows to fade with at least a few showers filling in these
voids. The tight pressure gradient associated with the front has
already caused the Turnagain and Knik winds to kick up. This looks
to continue until tomorrow morning, with improving conditions for
the afternoon expected as the pressure gradient slackens due to a
weakening front. However, another front will lift up toward the
region on Tuesday, with a surface low moving into the Bristol Bay
area. This next front will once again cause winds and precipitation
chances to increase.

For those locations in the downslope-induced rain shadows the next
couple of days, afternoon temperatures could be tricky. Warming
temperatures at 850 mb will work in tangent with frictional
heating, due to the descending air. Add into the mix the
potential/likelihood for thinning of the clouds (and perhaps some
breaks in them), all points to a higher bust potential in the
model guidance for highs. As such, we trended up these forecasted
values, with Tuesday being warmer due to increased flow aloft.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3; Tonight
through Wednesday)...

Rain currently moving northward along the Bristol Bay coast will
track over the Kuskokwim Delta this evening. Scattered showers
will continue across all of Southwest Alaska through Monday in a
moist, southerly flow east of a broad low centered over the
Aleutians. A stronger, compact low will approach the AKPEN late
Monday night into Tuesday. This will lead to gusty winds near 50
mph and moderate to locally heavy rain along the Bristol Bay
coast. The low will weaken as it moves north over the Kuskokwim
Delta. Thus winds will gradually diminish. However, moderate rain
will continue across the entire region through the day. Moist
southerly flow persists Wednesday, with scattered showers
remaining the story for the foreseeable future.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3;
Tonight through Wednesday)...

A broad low currently centered near Atka will bring light winds
and scattered showers to much of the region through Monday. A ~980
mb compact low will move over Sand Point early Tuesday morning
bringing gales and locally heavy rain to the eastern Aleutians.
Rain will taper to showers by late morning, though gusty winds
will continue on the south side of the low. As it moves further
north, it will gradually become more broad leading to likely
showers over the western and central Aleutians Wednesday in cold
air advection.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

No significant weather concerns are expected Wednesday with a few
areas of small craft winds over the eastern Bering associated with
a departing low. Starting Thursday however, a deepening low
centered well south of Kodiak Thursday afternoon is expected to
produce gale force winds across portions of the western Gulf, with
widespread small-crafts over nearly all of the western Gulf by
Thursday evening. Gales are most likely on easterly winds down
Shelikof Strait Thursday evening. Gales are also expected around
the eastern half of the low as the center tracks to near Kodiak
City for much of the day on Friday. Meanwhile northerly winds may
approach gale force across the eastern Bering from Kuskokwim Bay
south to the Alaska Peninsula during the day Friday into Friday
night as well.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through
Sunday)...

The long-term forecast features a rather stagnant weather pattern
across Southern Alaska and the Bering. Due to the upper level
pattern remaining roughly the same throughout the long-term, the
models are in much better agreement than they usually are, even
through Sunday. That said, and as usual, the models still disagree
on many of the smaller details, especially the exact tracks of the
lows. Therefore, the good model agreement does not necessarily
mean there won't still be significant changes to the forecast
between now and when the weather happens.

A trough over the eastern Bering Thursday morning is expected to
slowly drift westward and become less amplified through Sunday.
The strong ridging over Southeast Alaska will similarly weaken and
become more zonal with time. The storm track is expected to set up
in between these two features, across Southern Mainland Alaska. A
developing low is expected to form well south of Kodiak Thursday
evening. The low will track northward over the western Gulf
through Friday night, weakening as it moves into Southcentral.
This weekend, as the pattern breaks down and becomes more zonal,
there are increasing disagreements with the storm track and where
the active weather will be. However, it's likely the weather
remains unsettled, especially over Southwest Alaska. A new low
will approach the western Aleutians on Sunday, but should be
weaker than Friday's western Gulf low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 150 155.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...JW