Small Craft Advisory 378, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 AM AKDT Wed May 9 2018

.ANALYSIS AND UPPER LEVELS...

The stalled low pattern that we have been seeing over the Bering
still remains and is responsible for the low to mid-level
moisture that is streaming inland from the Gulf across the
southern Mainland. The subtropical jet south of the Aleutians
continues to bring moisture up to this stationary low, which is
likely to stick around for a while. WV imagery shows a shortwave
lifting north towards Kodiak and southwest Mainland along the
frontal boundary. A second stronger shortwave south of the central
Aleutians along the base of the low will be a key feature later
this week.

There is a stalled front associated with the low pressure system
extending from the western Gulf to the Kuskokwim Delta. Gusty
southeast to easterly flow is occurring along the frontal
boundary with a few showers continuing to move ahead of the front
over the western Gulf and southwest coast.

&&

.MODEL DISCUSSION...
Models are generally in good agreement with the overall synoptic
pattern. However, there are minor differences on the track of the
low expected to quickly develop over the North Pacific on
Thursday which then tracks to the eastern Bering early Friday. The
GFS was used to make edits through Thursday night, then some use
of the NAM was incorporated across the Gulf and Southcentral to
capture slightly stronger pressure gradient winds along the coast
and east to west oriented channeled terrain.

&&

.AVIATION...
PANC...A weak disturbance moving from the gulf will be enough to
bring a few showers across this morning. A front along the gulf
pushes across tonight with light rain expected to develop around
midnight through early Thursday morning before diminishing. VFR
conditions are expected to prevail, however brief MVFR conditions
are possible along the passing boundary. Light variable winds
shift to the southeast along the passing front.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level short-wave trough and surface cold front will lift
northward across the Gulf today. Thus far, the bulk of rain has
been falling on Kodiak Island, with around 2 inches of rain in
Kodiak City the past 24 hours. That will change today as steadier
and heavier rain spreads northward to the Kenai Peninsula and
Prince William Sound areas while rain tapers off from south to
north on Kodiak Island. Low level southeast flow will maintain
drier conditions west of the Kenai and Chugach Mountains, though
expect sprinkles or a brief period of very light rain as weak
upper level impulses embedded in the flow ahead of the trough
cross the mountains. The upper level short-wave will lift
northwestward tonight into Southwest Alaska, with just the tail
end crossing Southcentral. Therefore, the surface front will
dissipate as it heads northward across the Kenai Peninsula. Rain
will spread into the western Susitna Valley, while the rest of
interior Southcentral stays mostly dry. Temperatures, which peaked
out in the upper 50s to lower 60s for many areas on Tuesday,
should be fairly similar today. While there will be more cloud
cover, temperatures will start out warmer this morning and a
warmer air mass will continue to advect in from the southeast.

Right on the heels of this first system, an intensifying low over
the North Pacific will turn northward and head for Kodiak Island
and the Alaska Peninsula tonight, bringing another round of heavy
rain and strong winds to the western Gulf and Kodiak. While the
low will ultimately curve northwestward into Bristol Bay on
Thursday, a strong upper level southerly jet and short-wave
will head north across the Gulf toward Prince William Sound
Thursday through Thursday night. Model guidance is also trending
toward development of a surface low. Expect widespread rain and
strong winds along the coast, with rain likely spreading inland
Thursday night into Friday as flow shifts to more of a southerly
direction behind the frontal system.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...Wed
through Fri)...
A weakening frontal boundary is slowly drifting across the area.
This is allowing showers to continue across the area, especially
along southeast facing terrain. A shortwave rounding through the
base of the trough will push over this boundary this morning
serving to reinvigorate the precipitation. This should help to
make the rain a bit more steady basically from King Salmon up
through Bethel and over towards Aniak. Temperatures will remain
quite mild today with a warmer airmass, even with abundant cloud
cover. Then tonight, this boundary starts to finally lift north
and east in response to a much more compact low pressure center
approaching the south side of the Alaska Peninsula.

The new low will track quickly through the Alaska Peninsula Thu
afternoon through Thu evening. Before the precipitation associated
with it arrives, gusty southeast winds will develop and help to
dry the area out the early part of Thu. But by mid-day, the rain
should reach Bristol Bay and then spread quickly north towards the
YK-Delta. With the track of this system, moderate cross-barrier
flow will help some of the population centers in the lee of the
terrain (King Salmon, Bethel, etc.) to remain on the drier side.

Friday the northward progress of the low slows significantly as
it hugs the West Coast. This will leave much of the area
vulnerable to showers with mild onshore flow and weak cold air
advection.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3...Wed
through Fri)...
The vertically stacked low over the Central Bering will remain the
dominant feature to start the forecast period. It is embedded
within a longwave trough. Expect broad, weakening, cyclonic
(counter- clockwise) flow across the region with widespread
showers. The airmass is quite warm on the eastern side of the low,
so expect mostly rain showers there. While to the west side of
it, some cold air could bring passing snow showers. A shortwave
coming around the base of the longwave trough will serve to
reorient the pattern over the next couple of days. This shortwave
pushes up towards the Eastern Aleutians/Alaska Peninsula from the
North Pacific early Thu. The associated surface low reaches peak
intensity (upper 960 mb) just before crossing the chain into the
Eastern Bering Thu evening. So showers and winds will diminish
across the western half of the area while they increase to gale-
force levels in response to this low over the eastern half. By
Fri, this low starts to fall apart over the extreme Eastern Bering
while another decaying warm occlusion moves into the Western
Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Tuesday)...

The long-term forecast starts out with a front moving into the
north Gulf Coast late Thursday night into Friday morning. Winds
along the typical barrier jet areas from Prince William Sound
eastward will be at gale force with storm-force gusts as the front
moves in. Heavy rain all along the coast from Seward eastward
will be ongoing to start out the day. For Anchorage, the Matanuska
Valley, and the western Kenai Peninsula, rain shadowing should
keep any precipitation with the front limited to some scattered
showers. However, the winds will be strong out of the gaps of
Turnagain Arm and the Knik River Valley as well. Meanwhile, out
west, a weakening, nearly stalled-out low centered off the coast
of the Kuskokwim Delta will keep the conditions windy and
unsettled for most locations north and west of Dillingham.
Onshore flow throughout Friday afternoon may allow for the usual
instability showers to develop through Bristol Bay and the Lower
Kuskokwim Valley. The warm air mass that is over much of
Southcentral for Wednesday and Thursday will retreat to the north
and east on Friday in favor of a cooler, more seasonable air mass
for the area that will persist into early next week. Rainfall
rates will diminish along the Gulf Coast Friday night, but steady
light rain will persist through the night. Meanwhile the Bering
low's slow northward drift will keep the weather largely steady-state
across the Bering and Southwest Alaska with occasional rain and
snow. Low ceilings will remain prevalent.

On Saturday, with the surface low having drifted north of Nunivak
Island, most of Southwest Alaska will be in broad southwest
onshore flow through much of the day, keeping the area quite cool
for this time of year. On Saturday afternoon, the approach of a
weak upper level jet streak may further invigorate shower activity
throughout the Kuskokwim Delta and Bristol Bay. Showers will also
be common through much of Southcentral on Saturday, as a separate
much stronger upper level jet streak over eastern Alaska puts most
of Southcentral in a region of broad lift, which promotes shower
development. The gap winds will be weaker on Saturday than Friday,
which in turn will diminish the rain shadow effect and allow
Anchorage, the Matanuska Valley, and the western Kenai Peninsula
to get in on the afternoon shower activity expected on Saturday.

For Sunday, the upper level jet will be long gone off into Canada
by Sunday morning, leaving Southcentral with seasonable, mostly
cloudy conditions. Some showers are expected to redevelop once
again during the afternoon, but are not expected to be nearly as
widespread as on Saturday as high pressure builds over the Gulf.
Cool conditions will also continue across Southwest Alaska as the
core of the coldest air remains over the eastern Bering. Mostly
cloudy conditions with instability showers develop once again
through that region as well Sunday afternoon, but, like in
Southcentral, should not be as widespread nor as heavy as Saturday.

A slow-moving low brings more unsettled weather to the western
Aleutians Sunday. This low will slowly track eastward, keeping
much of the southern Bering unsettled right into Tuesday, as model
differences on track and speed increase by then. Monday and
Tuesday both look to be decently dry for Southcentral with weak
high pressure persisting, though isolated showers both afternoons
can't be ruled out. Temperatures should also be on a warming trend
as the cool air mass over much of southern mainland Alaska
modifies.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 131 138 150.
         Gale 130 132 137 155 160 165 180.

&&

SYNOPSIS AND MODEL DISCUSSION...ALH/KH
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...JW