Small Craft Advisory 370, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKDT Sun May 6 2018

.ANALYSIS AND UPPER LEVELS...
The upper level pattern has a cut-off low centered just north of
Attu Island and a deep trough that extends northeastward across
the Alaskan Arctic coast. Somewhat higher pressure is over the
northern Pacific that is keeping a rather tight southwest gradient
from the eastern Aleutians through Southcentral and the Alcan
border. There is also a short wave trough over the Bristol Bay
area which is moving northeastward. From the trough eastward there
will be some showers this evening to the overnight hours as there
is also cold air aloft associated with it which will enhance
lift. An area of baroclinicity over the Gulf of Alaska will move
north-northeastward tonight which is helping to provide
additional moisture into Southcentral Alaska and the north Gulf
coast.

&&

.MODEL DISCUSSION...
Models remain in excellent synoptic agreement through the middle
of the week. A feature with the large low in the western Aleutians
that has been tricky for the models to resolve has been the two
centers the low has: the original one and triple-point low.
Models are now all consolidating the energy around the triple-
point low tonight which will produce a very tight pressure
gradient west of Adak which will produce storm force winds in some
marine areas.

&&

.AVIATION...
PANC...Gusty southeast winds are expected to taper off this
evening as the pressure gradient eases and becomes more
southwesterly. Monday should see a light sea breeze over the
terminal for the afternoon hours before winds switch back toward
the southeast Monday evening. While VFR conditions are expected,
some light rain moving through the region this evening into the
overnight hours may drop ceilings into the 3500 to 5000 ft range.
Visibility is expected to remain 6SM or greater.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A shortwave trough pushing into Southcentral Alaska will
increase shower coverage over the area from west to east this
evening, while breezy southeasterly winds diminish. Showers will
taper off through the early morning hours, lingering the longest
over the Copper River Basin. Skies will clear as a transient
ridge builds over Southcentral Alaska on Monday for a brief
sunnier break before clouds from the next incoming frontal system
push over Kodiak Island Monday night and over the Kenai Peninsula
Tuesday. Additionally, as the frontal system approaches,
southeasterly gap winds will increase again through the day on
Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

The radars at King Salmon (PAKC) and Bethel (PABE) have been very
convective this afternoon. The atmosphere has become very
destabilized and there is no shortage of moisture. The derived PWs
over southwest range from .3 to .5 of an inch which is well over
100 percent of normal PWs for this time of year. There is even an
area between Aniak and Lime Village that is over 200 percent of
normal by the NESDIS TPW scheme. Factor in the atmospheric
moisture with the position of the shortwave trough pushing into
western Alaska and the net result is a round of airmass
thunderstorms. Once the trough lifts, the convection will get cut
off. There is a strong low brewing near Adak and this storm will
move eastward. The edge of the precipitation shield will move into
Kipnuk and Togiak by tomorrow afternoon with the lion's share of
the QPF anticipated on Tuesday for Southwest.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

There is a mature low with an occluded front sweeping the Chain.
The current surface pressure is near 975mb west of Adak and the
models show it deepening a skosh more tonight. Both the NAM and
the GFS bring this low due east along the chain Monday and then it
starts to trek northeast on Tuesday. The latest ASCAT/advanced
scatterometer pass has detected gale force winds north and south
of Atka which is not much of a surprise considering the extent of
the tight pressure gradient force associated with the deepening
low. The models are even hinting at the possibility of storm force
winds tonight. Southerly gale force winds will accompany the
front as it tracks eastward. These winds will persist into
Wednesday and then front will loose its gusto and stall.
Additionally, rough seas and precipitation will accompany this
storm.

&&

.LONG TERM FORECAST (Days 3 through 7: Wednesday through Sunday)...

The weather pattern on Wednesday starts out with a stalled front
aligned southeast to northwest from the central Bering into the
Barren Islands and the mouth of Cook Inlet and across much of
Southwest Alaska, making for a very wet day in those areas. Its
parent low is stalled out over the middle of the Bering about 100
miles west of the Pribilofs. With the exception of the Canadian,
the rest of the models agree the associated precipitation will
remain from Chenega westward, with relatively little precipitation
expected during the day in Prince William Sound or interior
Southcentral. A new northward moving upper level wave finally
pushes the front northward into Prince William Sound and much of
the rest of the northern Gulf late Wednesday night into Thursday.
Meanwhile over the Bering, the stalled out low will slowly move
south towards the central Aleutians Wednesday night, as it begins
to rotate around a new North Pacific low tracking northward
towards the southern Alaska Peninsula and Kodiak. Building ridging
over much of Southcentral will allow a much warmer air mass to
move in.

On Thursday, the north Pacific low will slow down as it
approaches the Alaska Peninsula due to the interaction of that low
with the one tracking south out of the Bering. The model
discrepancies continue to increase, but it seems likely by the end
of the day that Kodiak City will get into the associated rainfall,
as the low's front moves into the southern Gulf. The precipitation
along the north Gulf Coast will diminish to showers as the first
front on Wednesday dissipates. Thursday should be a mainly cloudy
day with scattered showers across Southwest Alaska, but shouldn't
be nearly as wet as Wednesday.

By Friday the model disagreements become even greater, with the
EC taking the new low north up the west Gulf Coast into the Alaska
Range, while the GFS, the older EC runs, and the previous
official forecast turn the low westward into either Bristol Bay or
along the Pacific side of the Alaska Peninsula, bringing the
center near Cold Bay Friday afternoon. On this latter track, which
is the preferred once again, the associated strengthening front
will plow into the north Gulf Coast during the day Friday, which
despite the discrepancies in forecast low track, remain in good
agreement. Thus, nearly all of southern Mainland Alaska will have
a wet day on Friday, with the exception of the western Kenai
Peninsula, Anchorage, and the Matanuska Valley, which due to
downsloping should only see isolated to scattered shower activity.

By Saturday and Sunday, the models become so divergent from each
other that little can be said about the forecast with any kind of
certainty, but the pattern looks to remain unsettled.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 155.
MARINE...Storm 178. Gales 130 131 155 165 170 177 179-181 411-414.

&&

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