Small Craft Advisory 594, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
433 PM AKDT Fri Aug 11 2017

.ANALYSIS AND UPPER LEVELS...
A large closed upper level low near Kodiak Island is tracking
slowly to the east. Short-waves rounding the east side are headed
northward across the Gulf of Alaska and into Southcentral. At
the surface, a series of surface lows are strung out from the
northeast Pacific to the northern Gulf. All of this is bringing
widespread rain to coastal areas, with some rain crossing the
mountains into inland areas.

All eyes are on a potent "autumn-like" high amplitude upper level
trough digging out of the arctic southward across the northern
Bering Sea and along the west coast of Alaska. This trough is
supported by a 100kt+ upper level jet streak on the back side.
Baroclinicity is strong with 850 temps ahead of the trough around
+10 degrees Celsius and temperatures as cold as -10 degrees
Celsius behind the trough. An elongated surface low has formed
on the front edge of the trough, with an area of moderate to
heavy rain pushing onshore along the west coast. Northerly winds
are rapidly strengthening behind the trough across the northern
Bering Sea. Conditions are quite benign out ahead of this trough
across much of Southwest Alaska and westward across the southern
Bering Sea and Aleutian chain.

&&

.MODEL DISCUSSION...
Numerical guidance is in excellent agreement with handling of
large scale features through the weekend and overall forecast
confidence is high. The models are struggling a bit with details
of where precipitation is falling across Southcentral today,
especially inland. With weak cross barrier flow across the coastal
mountains tend to prefer the wetter solutions. There are also
some differences in rainfall amounts over Southcentral this
weekend due to small differences in exact placement of smaller
scale features moving through the trough (or around the upper
level low that forms). However, the storm total QPF fields look
pretty similar as far as average area-wide precipitation totals.
With such a dynamic and progressive system expect there will be
embedded areas of higher precipitation, but it will be a challenge
to hone in on them at this point in time. The other forecast
challenge is falling snow levels as cooler air sweeps into
Southcentral Saturday night and Sunday. The air mass will modify
as it moves eastward, but heavy precipitation will help locally
drop snow levels lower than forecast temperatures may suggest.

&&

.AVIATION...
PANC...Continuous rain this afternoon and evening will allow the
low levels to slowly become more and more saturated. Thus, expect
ceilings below 5000 ft. With low level flow out of the east
ceilings should remain mostly VFR, though there will likely be
some shorter periods of MVFR ceilings in the area. It looks like
there will be a pretty good break in rain overnight tomorrow
through Sat morning which will allow ceilings to lift a bit.

Heavier rain will then move back in around mid-day Saturday and
continue into Sunday. Expect both ceilings and vis to drop to MVFR
for periods of this rain event. With a convective aspect to the
precipitation would expect quiet a bit of variability (with ups
and downs in ceiling and vis).

&&

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

The upper low over the western Gulf will elongate southeastward
through tonight, with the southern portion breaking off and
ejecting into the panhandle on Saturday. A plume of moisture on
the northern side of this low will push through Southcentral
Alaska this evening, bringing rain to much of the area. There will
be a brief "break" (showers instead of steady rain) later tonight
through Saturday morning before the next system arrives Saturday
afternoon. This system will push in from the west and is
associated with an upper low that is riding the polar jet. Thus
there is a fairly strong cold front with this system, that will
push to Anchorage Saturday night. Some snow is possible on the
higher peaks late Saturday night into Sunday as the snow levels
could drop to around 4000 feet. The upper low itself will cross
the Kenai peninsula early Sunday afternoon, after which
precipitation chances will quickly diminish.

&&

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

A potent low pressure system in the northern Bering will drive
much of the weather across the Southwest Mainland over the next
several days. This low is associated with a potent low level jet
that will bring gale force winds along the Kuskokwim Delta coast
especially Saturday. This low is associated with a strong PV
anomaly and downward vertical motion, which will allow much of
these strong winds to reach coastal areas at the surface as the
low pushes inland. In addition, this low is associated with a
plentiful moisture supply. This will result in a soggy weekend
across much of the Southwest mainland as the low slowly pushes
inland and southward.

&&

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

High pressure will remain in place across much of the central and
western Bering in the short term. As a result, widespread fog and
relatively light winds will dominate weather conditions. A front
will bring slightly stronger winds in the small craft category
Sunday to the Central Aleutians. Meanwhile, winds will remain
gusty along the eastern Bering as a low drifts inland to the
southeast.

&&

.LONG TERM FORECAST (Days 3 through 7)...
One important change in the models early next week has been a
trend towards a more progressive upper level trough over the
mainland late Sunday. As a result there is now general consensus
that weak high pressure will develop over the South Mainland Mon-
Tue between the departing low and the next low diving southward
into the Eastern Bering. This high pressure should bring a brief
break in the active weather pattern to much of Southcentral early
next week, however there is very good agreement that a series of
arctic troughs will move through the Mainland and Eastern Bering
into late next week. There are some timing/intensity issues in
the models from these trough, so forecast confidence is low on the
exact details of the long term forecast. Forecast confidence is
however good with the overall trend of continued cool, wet, and
fall-like conditions.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 180 181 185
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ELN
LONG TERM...DEK