Small Craft Advisory 256, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 AM AKDT Mon Mar 26 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low centered just east of St. Paul Island
continues to be the focus for the most active weather across
southern Alaska this morning. Southeast of the upper low extends a
shortwave trough that is currently enhancing the precipitation
ongoing across much of Southwest Alaska, Kodiak Island, and the
far southern Kenai Peninsula. A Winter Weather Advisory for
Bristol Bay from the Nushagak River west remains in effect until
noon, primarily for the falling snow, but for localized visibility
restrictions from blowing snow as well. Since the parent low near
the Pribilofs is barely moving, the snow showers have been
affecting many of the same areas, rather than continually moving
through. Meanwhile, further southeast along the front, a new low
is developing southeast of Kodiak Island as it takes advantage of
the warmer air there. This low is referred to as a triple-point
low, as it forms at the intersection of the warm air to the
southeast, cool, precipitation-modified air to the north, and
fresh cold air to the southwest. Since it too is slow moving, a
very wet, persistent easterly wind continues to move into Kodiak
favoring low ceilings and visibilities in the falling rain and
snow. For much of Southcentral away from the southern Kenai
Peninsula, conditions remain quiet with only associated cloud
cover moving into the area from the south, but with little to show
for it in terms of precipitation.

&&

.MODEL DISCUSSION...

The models have all latched on to the sprawling, slow moving low
near the Pribilofs. Agreement is very high drifting the center of
the low slowly towards Southwest Alaska, making landfall Tuesday
morning just west of Kuskokwim Bay near Kwigillingok. Since the
models usually handle the well- developed slow moving systems
well, this is not surprising. Track shifts continue in regards to
the newly formed triple point low just off the coast from Kodiak
Island this morning. The current low is only forecast to move very
little, and yet another triple- point low is forecast to develop
east of it late Monday night into Tuesday morning. There is large
disagreement on where that forms, however, with the upper level
ridge over eastern Alaska unlikely to go anywhere, it is favoring
southward shifts in the eventual track of the low. Precipitation
is now looking like it will struggle to even reach Prince William
Sound and most of the Gulf Coast, including Seward and Cordova.
Thus, the latest forecast updates continue to decrease chances of
precipitation associated directly with that low. Many of the
models now show a northward moving upper level wave that will
bring chances of snow to interior Southcentral Alaska Tuesday
afternoon and evening. Since it is weak and with limited moisture,
only a chance of snow is forecast. With weaker gap winds,due to
the low remaining further offshore, forecast challenges are
minimal except for who sees precipitation Tuesday into Wednesday.
There still remains time to hammer those details out, and as such,
forecast confidence is high once again.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...A
front over the southern Gulf will continue to lift slowly
northward today while a triple point low near the south end of
Kodiak Island drifts northeastward. A sharp upper level short-wave
trough just south of the Gulf will track rapidly northeastward to
the eastern Gulf this afternoon, leaving a weakening low and
frontal boundary sitting over the northern Gulf. Thus, the vast
majority of wind and precipitation with this system will remain
over the Gulf and Kodiak Island. The southern Kenai Peninsula will
see a bit of light snow while the remainder of Southcentral can
expect just some high clouds.

Beginning tonight, focus will shift to the west as a vertically
stacked low spinning over the eastern Bering Sea makes its way
toward Southwest Alaska. Cyclonic flow out ahead of the upper
level low will make its way to Southcentral beginning overnight
tonight, with a series of weak waves crossing Southcentral Tuesday
through Tuesday night. This will bring a chance of light snow to
the much of the region Tuesday night into Wednesday. With deep
southwest flow, the most likely area to see a light snow
accumulation will be the Mat-Su Valleys and the west side of the
Talkeetna Mountains. Stuck with chance PoPs in Anchorage for now
until get a better idea of timing and placement of individual
upper waves.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...The low
near the Pribilof Islands will be the primary driver of short term
weather impacts for the southwest mainland as it makes slow
northeastward progress today and eventually moves inland over the
Kuskokwim Delta on Tuesday. The occluded front currently pushing
inland will continue to bring snow, primarily to western portions
of Greater Bristol Bay, through early this afternoon. Blowing snow
concerns have largely been inhibited by temperatures hovering
near or above freezing, however the Winter Weather Advisory was
left in place through noon as areas west of the Nushagak River may
see several additional inches of accumulation this morning. Snow
will rapidly taper off to showers this afternoon in the wake of
the front, making for a relatively quiet overnight period. Showers
will then quickly reintensify on Tuesday as the low begins to
move ashore, with strong cold advection in southwest flow aloft
helping to destabilize the air mass and keeping a showery regime
in place through Tuesday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Widespread snow showers will continue across the eastern Bering
through tonight in cold northwest flow on the back side of the
low as it makes slow progress towards the southwest coastline.
Showers will then trend downward through the day on Tuesday as the
low pushes inland, leaving much of the Bering in a benign westerly
flow regime. Showers will not shut off completely as another wave
dropping in from eastern Russia keeps weak cyclonic flow over the
region, supporting isolated to scattered snow showers in the
forecast into the middle of the week.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The extended forecast covers late Tuesday through the weekend and
will have multiple weak features moving through the pattern. In
addition, the majority of the activity will stay south of the
southern mainland. However, the models generally struggle with the
handling of weak features further out in time and this forecast is
no exception leading to higher uncertainty with any new systems
moving into the area. The result was that ensembles were
preferred over the deterministic solutions increasingly toward the
weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 161.
MARINE...Gales: 119 120 130 131 132 137 138 139 173 351 352 413
                414.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...DWK