Small Craft Advisory 215, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
502 PM AKDT Fri Mar 15 2019

.ANALYSIS AND UPPER LEVELS...

There is a large vertically stacked low in the southwest Gulf that
is moving slowly northward. The leading front is situated along
the north Gulf coast, with gale to storm force winds over much of
the northern Gulf. There is also a significant amount of
precipitation occurring along the coast and coastal mountains.
The air mass is fairly warm and precipitation is generally in the
form of rain near sea level. For inland areas, there is strong
easterly gap flow and down-slope drying with very little
precipitation. Further west, there is a low in the northern Bering
and another in the western Bering. There is slightly cyclonic
westerly flow aloft over the southern Bering, with showers are
occurring. Most of these showers are in the form of snow with
fairly cold air over the area.

&&

.MODEL DISCUSSION...

The numerical models are in good agreement through Saturday night.
On Sunday, the models agree that there will be another low
approaching the Gulf from the south, but differ somewhat on the
timing and path. It looks like the Nam is the outlier, with one
consolidated low while the other models split it into two lows
that are north and south of each other.

&&

.AVIATION...
PANC...VFR conditions will persist. Wind shear is expected at the
airport, with north winds at the surface and east winds aloft.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Sunday evening)

The next in a series of low pressure system and fronts is
currently making its way north towards the region. A ~975 mb low
will reach the southwest Gulf tonight with a front training to its
east. Sustained gale force winds and widespread precipitation
will precede the frontal arrival, with storm force winds possible
from near Cordova eastward. The low will tend weaken to ~983 mb
over Shelikof Straight tomorrow afternoon, with the associated
front having reaching the Gulf coast. Some locally heavy rainfall
will develop for the coastal mountains, with heavy mountains snow
likely, as the snow level looks to be around 1000 feet. Increasing
sustained winds look to top out in the 45 to 60 mph range for the
upper Hillside of Anchorage, Turnagain Arm, and across the
Chugach mountains. Some fairly strong gusts up to 70 mph will also
occur for these area as well. Locations west and north of the
Chugach Range will likely see down-slope effects, with just a few
sprinkles or flurries expected. Winds will switch around to the
south to southwest following frontal passage, with some breezy
conditions continuing, but not to the extent as tonight.

Both of these features will largely weaken as they exit the area
Saturday night, but, another low moves into the Gulf for Sunday,
with a renewed surge of moisture heading for Southcentral.
Temperatures should also rise through the period, given the
southerly fetch of warm air and moisture aloft.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tonight
through Sunday afternoon)...

A decaying North Pacific low continues to move northward toward
the Gulf of Alaska. Its associated front will continue to lift
inland over the AKPEN and into Southwest Alaska from east to west
through the evening. Strong northwesterly winds on the back side
of the low will become channeled through the usual gaps and
passes along the southern AKPEN. This front will also generate
precipitation across Southwest beginning over the AKPEN, and
spreading westward across Bristol Bay and the Kuskokwim Valley and
into the Kuskokwim Delta. By Saturday, the front interacts with
an upper level low originating from over the central Bering that
moves eastward over Southwest.

One challenging factor with this forecast was determining the
rain/snow line. As the front tracks westward, precipitation will
transition to rain from east to west. However, the incoming upper
level Bering low stifles the westward progression of the (now
very weak) front by Saturday afternoon. This will effectively
inhibit warm air from reaching west of Dillingham, keeping
precipitation in areas such as Togiak, Platinum, Quinhagak, and
Bethel all snow.

Precipitation will retreat back eastward Saturday night through
Sunday as the Gulf low opens into a trough, allowing the Bering
trough to track eastward into coastal Southwest. Colder air will
accompany the trough leading to a transition back to all snow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tonight
through Sunday afternoon)...

An unstable convective pattern continues across the Bering for the
weekend. An upper level low over the northern Bering will move to
the central Bering by Saturday morning. This will generate gusty
northwesterly winds over the western and central Bering for
Saturday afternoon. A brief period of snowfall is also expected
over the Pribilofs Saturday afternoon as the low moves through.
On the backside of the low, cold air advection will continue and
a generally showery regime will remain over the Bering through
Sunday afternoon.

&&

.MARINE (Days 3 through 5/Sunday Night through Wednesday)...

Minimal changes are expected in the overall flow pattern in the
next 3 to 5 days. The weather in the Bering will remain relatively
quiet, while Gale force fronts and low pressure systems will
continue to periodically lift north into the Gulf. While models
are still not in great agreement regarding the timing and track of
individual features, confidence is good the general persistence
of the large scale pattern.

&&

.LONG TERM FORECAST (Days 3 through 7/Sunday through Friday)...

The weather pattern for the long term sees little shift with
continued troughing over the Bering Sea and southerly flow through
Southcentral AK. This means continued weak flow and quiet weather
for the Bering Sea; Southcentral AK will continue to receive low
pressure systems every few days. As a result, the warm and wet
pattern will be sustained over southcentral AK for the foreseeable
future. Models hint at a larger low developing in the Gulf of
Alaska late next week. However, there is little agreement by this
time in the long term so we will have to wait and see what develops.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 120 125 129 130 131 139 155 351.
         Storm Warning 119.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/SEB
MARINE/LONG TERM...BB