Small Craft Advisory 376, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 PM AKDT Tue May 8 2018

.ANALYSIS AND UPPER LEVELS...
A very amplified weather pattern between a nearly stationary low
over the Central Bering and broad area of high pressure over the
Yukon Territory into the Eastern Interior continues to dominate
the weather over the region. The most significant feature in this
pattern is a deep southerly jet that stretches from the North
Pacific into the Southwest Mainland, bringing widespread rain to
the Southwest Mainland as well as areas of the Cook Inlet regions.
This rainfall has mostly been limited to the Southern Kenai
however as southerly downsloping winds across the Chugach have
limited rainfall elsewhere to just a few passing showers. On the
west side of the Bering low, gale force northwesterly winds have
spread from the Western Bering into the Central Aleutians, which
are weakening as the low begins to slowly fill.

&&

.MODEL DISCUSSION...
The models remain in very good synoptic scale agreement through
mid week in this very persistent weather pattern, so the primary
forecast challenge surrounds rainfall chances in the downslope
prone areas of Southcentral and the Southwest. The NAM in
particular was too aggressive in downsloping this morning as
showers passed through Anchorage and the Matanuska Valley this
morning, while the GFS and many of the high res models resolved
this fairly well. As a result changes to the precipitation
forecast today favored the GFS and high resolution models, and
largely ignored the very dry NAM. Looking further into Thursday
and Friday, there are some minor differences in the placement of
the next low that will move northward from the North Pacific into
Bristol Bay, however these are small enough not to have a
significant impact on forecast confidence.

&&

.AVIATION...
PANC...VFR conditions with light winds should continue into
Wednesday with persistent downsloping from the Chugach Mountains.
There may be a few rain showers that pass through the area,
especially on Wednesday, however these are expected to be brief
with ceilings remaining above 5000 ft.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Not a whole lot of change in thinking with the afternoon forecast
package. The main issue has to do with the ability of rain to make
it to the lee sides of the mountains Wednesday and Friday. While
the Southeast flow should be strong enough to keep most of the
rain on the windward side of the mountains, but there is enough
deep moisture that it could still cause some rain as it did this
morning. The GFS model is going with heavier amounts of rainfall,
but that does not seem very likely as flow is still strongly
Southeasterly. Increased to POPs on Wednesday and Friday to
reflect a better chance for some rain those days.

&&

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

The remnants from the occlusion and the warm front have moved into
Southwest. The King Salmon radar (PAKC) is active from Dillingham
to southwest of Port Alsworth. The current temperature right now
in King Salmon is 47F and Bethel is 49F. The PABC radar has had a
few showers on it this afternoon but most of the action has been
on PAKC where the deepest fetch of moisture has into southwest.
The moisture plume will continue to impact portions of southwest
again tomorrow. Drier conditions anticipated on Thursday day. By
Thursday evening the next precipitation shield will be north of
the AKPEN. Friday will be wet and cloudy again.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 though 3)...
The forecast period commences with a closed low over the Bering
and the remnants of the frontal boundary slowly decaying. All of
the models depict showers lingering across the Bering as the
upper low wobbles and slowly fills. The thickness values from the
NAM and the GFS suggest that there is a possibility for snow
showers for portions of the Bering in addition to rain showers,
but significant accumulations are not anticipated with the remnant
wrap around moisture. A second lobe of vorticity will enter the
region from the southwest and get entrained into the circulation
of the Bering vortex. This second low will become a well-organized
system and make landfall somewhere between Cold Bay and the
AKPEN. Expect another round of gale force winds with new system.
Gusty winds expected in passes along the Eastern Aleutians as
this low closes in Thursday and into Friday. This storm will be
moving up from warm source region and will be an all rain event.

&&

.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...Gale 130 150 155 None.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...JW