Small Craft Advisory 887, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
433 PM AKST Sun Nov 11 2018

.ANALYSIS AND UPPER LEVELS...
A broad baroclinic zone with abundant moisture extends north from
below 40N to across the Gulf and Southcentral Alaska. Widespread
rain is occurring at lower elevations across the Gulf with snow
mixing in at higher elevations along the road system. Rain has
been pretty steady across the Anchorage Bowl and the Matanuska
Valley where temperatures hovering around freezing with road
temps below, producing widespread areas of freezing rain. Rain has
even started spreading as far north as Talkeetna. Precipitation
is generally snow across the Copper River Basin with a mix of
freezing rain over the southern basin. Precipitation was pretty
sparse over Southwest Alaska. However, rain associated with a
frontal wave was starting to work into Bristol Bay this afternoon.

Northerly storm to gale force winds on the backside of the surface
trough were observed over the eastern and central Bering Sea.
Gales were evident across the northern and western gulf ahead of a
frontal wave.

&&

.MODEL DISCUSSION...
Models are in relatively good agreement trough tonight. There is
still some disagreement on the handling of the next low heading
into the Northern Gulf Monday/Monday night. The ECMWF continues
to be the fastest and further West solution, with the NAM as the
slowest (though a little faster on the 18z run). Will lean toward
a middle ground GFS solution for now which still maintains
another batch of moderate to heavy rain along the Southern Kenai
Peninsula and and areas of storms over the GUlf as well in the
Bering Sea.

&&

.AVIATION...
PANC...MVFR conditions should slowly improve to VFR during the
night as rain diminishes. Ceilings, though, may hold at or just
below 5000 ft.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The active weather will continue through the next 48 hours, as
our current storm is moving towards the AKPEN (pressure at 983mb).
Several disturbances seen in satellite imagery and numerical data
are heading north, crossing our neck of the woods overnight and
through Monday morning. The precipitation will become more showery
in nature from south to north as we head through the evening,
with temperatures holding steady to slowly climbing as we head
through the night. This will allow most precipitation to change
over to all rain, with the exceptions being the northern Susitna
valley and the Copper Basin, where the cold air will remain
entrenched a bit longer. In fact, some of the Copper Basin valleys
from the Glen Allen area and north may see a prolonged period of
freezing rain and snow potential through the day tomorrow, as
essentially the colder air remains locked in place even longer
(almost like a cold air damming effect). This looks to remain
below Advisory thresholds, however.

Then as we head through Monday, multiple pieces of energy diving
into the backside of the trough will allow several surface
lows to rotate around each other in a Fujiwhara-like fashion,
while along one disturbance embedded in the southerly flow to
rapidly deepen to below 970 mb.

So what does this mean for us? Overall, look for steady to slowly
rising temperatures to continue through Tuesday for most areas,
with precipitation mainly falling as rain. With strong southeast
flow aloft, the Anchorage Bowl will likely see the "rain shadow"
effect during the afternoon Monday, as the related down-sloping
winds help wring out the low-level moisture due to compressional
warming. However, as the next storm approaches Monday night, look
for rainfall coverage and intensity to ramp up once again.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Disorganized low pressure systems will continue to pinwheel
through the area for the next couple of days. These smaller
systems and associated fronts are embedded within a larger, broad
area of low pressure that will remain largely stationary the next
few days, keeping weather active throughout SW Alaska.

The next round of weather associated with this system will move
through SW Alaska tonight. This round will be in the form of a
warm front, which is expected to bring precip between about
midnight and mid morning tomorrow. Warm air will override cold in
in place at the surface, so winter weather advisories mainly for
freezing rain have been issued for zones 152, 161 (north of
Dillingham) and 155. Snow is also in the forecast with these
advisories, with specific amounts depending on location. However,
the main emphasis is the freezing precipitation. Gusty off shore
winds will also continue for SW Alaska. A brief break in the
weather will occur after this system passes through. Tuesday looks
like the day for the next weather event, though the forecast for
this is uncertain as of now and still being dialed in.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
As with the past few days, weather over the eastern Bering will
be affected by the same low broad pressure system that is
impacting the rest of souther Alaska. Several smaller systems will
spin through the eastern Bering as they exit SW Alaska, bringing
rain shower and Gale to Storm force winds for the eastern Bering.
When it comes to precip, it should be noted that due to warm
temps with the low it will fall mainly as rain. The actual low
center sits right along the eastern Aleutians, so winds will be
lighter there.

The western Aleutians are on the backside of this low, which means
they will continue to see strong northerly (storm force) winds and
high seas with a cold air push from the north. Winds taper and
conditions improve Monday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The long-term forecast begins Tuesday night with a double-barrel
surface low situated in the Gulf with one center of circulation
south of the AKPEN and the second drifting slowly northeast toward
the Alaska Panhandle. As has been the theme, there are subtle
differences in placement and strength of these features; however,
the various guidance (NAM, EC, GFS, and Canadian) show the double-
barrel system and indicate a weakening trend with said features.
Support for these features is also lacking in the upper levels,
with the jet is well south of the surface lows and little if any
thermal gradient or advection in the mid-levels.

Guidance begins to breakdown further heading into the late
Wednesday/early Thursday time frame. The GFS and EC both indicate
an upper-level shortwave digging south across the Aleutians,
helping amplify the pattern over the north Pacific. This, in turn,
strengthens and steers another strong north Pacific low northward.
The GFS and Canadian are the more bullish solutions, with a low
approaching Kodiak Island late Thursday. The EC, however, slows
the track of the low, stalling it in the southern Gulf until
Friday before a low upstream finally pushes the Gulf low toward
the southcentral coast.

Upstream, guidance is holding tight to the building of a ridge
over eastern Russia and keeping it in place through the long term.
This will keep the jet, and surface low track, south of the
Aleutians. Here, the GFS, Canadian, and EC all develop a low
southwest of Shemya Wednesday with a front across the western half
of the Aleutians. Solutions then diverge greatly with the forward
progression of not only the low, but the front and development of
a subsequent triple point low Thursday into Friday. Again, the GFS
and Canadian are more similar than the EC.

By the weekend, guidance is all over the place with the upper-
level and surface features. However, the common denominator
between each solution is the continuation of the broad low
centered over the eastern Aleutians. This setup would keep a
moist, southerly flow over the Gulf and across southcentral and
southwest Alaska with clouds and precipitation wrapping around
the low over the Bering and Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory (mixed precip) 145 152 155 161.
MARINE...Storm Warning 119 131 132 138 175 176 179>181 185.
 Gale Warning 120 125 130 136 137 139 150 160 165 170 173 174
                177.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...TM