Small Craft Advisory 84, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 PM AKST Fri Jan 26 2018

Updated long term discussion

.ANALYSIS AND UPPER LEVELS...
An upper-level low in the Gulf of Alaska and another over the Sea
of Okhotsk are working with a building ridge located along the
International Date Line to lock in an Omega Block at the jet
stream level. The strongest southerly flow remains over the
Kamchatka Peninsula with roughly 100-knots of North-Northeasterly
flow working along the West Coast and out of the Alaska Interior.

At 500-mb, an elongated region of sheared positive vorticity is
found along the West Coast.

At the surface level, a weak low sits over the southern Susitna
Valley and lies along a stationary front which stretches from
Shelikof Strait to Western Canada. The pressure gradient remains
tight between this feature and a 1041 mb high just west of St.
Lawrence Island.

Results are seen at the surface in the form of gusty north winds
and passing snow showers over Southwest Alaska. These are most
notable along the Alaska Peninsula (primarily Cold Bay) where
blizzard conditions have been ongoing since last night. And where
deep cold meets north winds producing wind chill values have been
in the -30 range most of the morning.

Garden-variety snow showers are found along other Gulf coast
locations.

&&

.MODEL DISCUSSION...
The overall synoptic pattern is still in good agreement between
all models and that is expected with the blocking ridge in the
Bering Sea helping to stabilize the pattern. Models initialized
much better on the 12Z runs than they did yesterday when they were
6 to 8 hours late on the movement of the shortwave moving through
Southcentral Alaska. The issue for models remains in the Gulf of
Alaska where the complex low is producing small low centers that
are of a scale at which even the higher resolution models are
having difficulty properly resolving - with poor run-to-run
consistency. This is making the Kodiak snow and wind forecast of
particular difficulty through the weekend.

&&

.AVIATION...
PANC...VFR conditions and northerly winds will persist through
Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Southcentral will continue to be under the influence of a deep
upper level trough extending down from the arctic and south
through the Northern Pacific. An associated surface low over the
Gulf will continue to move south while still bringing increased
cloudiness and scattered snow showers to the coast. An embedded
shortwave trough will spin around the top of the low tomorrow
morning bringing another quick shot of accumulating snow to
Kodiak. The rest of Southcentral should stay cold and dry with
light offshore flow on Saturday.

By Sunday, a high amplitude upper level ridge currently over the
Bering will tilt east into Southcentral bringing clearing skies
and cold morning temperatures. As the ridge continues to build
through the day a broad low pressure system will begin to move
north across the North Pacific at the same time. This will rapidly
increase the pressure gradient between the cold southern mainland
and the relatively warm Gulf of Alaska, leading to the
development of strong northerly offshore winds. The winds will
ramp up through the day on Sunday, peaking by the evening, and
look to be strongest through the gaps and passes including out of
the Copper River Delta, Thompson Pass, and down through Valdez.
Thompson Pass will likely see the worst conditions Sunday
afternoon/night as gusts are expected to be near 70 mph which will
lead to blowing and drifting snow as well as very cold wind
chills, making travel through the area hazardous.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Tonight through Sunday night)
A rather persistent pattern looks to provide continued dry
offshore flow through the weekend. An area of high pressure across
the North Slope will allow increasing heights and temperatures
aloft to continue. The latest numerical guidance shows 500mb
heights climbing from current ~ 528 dam east to 540 dam west
range, rising to a 540 dam to 550 dam range by Sunday afternoon.
What this means for us is increased subsidence aloft, leading to
warming in the lower levels of the atmosphere. This can be seen in
the 950 mb temps, as they warm from current values of -12 C to -8
C over the next 36 to 48 hours. Thus, a continued slight moderating
trend looks to continue through the period.

However, given the orientation of the high to our north and a
strengthening low in the North Pacific, winds will gradually veer
to the northeast, maintaining an offshore flow regime. This
allows the pressure gradient to tighten somewhat near the
mountains, where winds may be a little more breezy than near the
coast. Otherwise, look for dry conditions to prevail.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Tonight through Sunday night)
An area of low pressure continues to move further away from the
region, as it continues its southward journey south of the chain.
This system brought windy and snowy conditions to the central and
eastern Aleutians last night and this morning, aided by a jet max
rounding the back side of the low. With its departure, the
pressure gradient between this storm and the Arctic high will
rapidly decrease through this evening, allowing for decreasing
wind and precipitation while allowing visibilities to improve.
Thus, the previous Blizzard Warning has been cancelled.

This has also allowed cold air advection to stop while allowing
warming temperatures in the 850 mb to 500 mb layer. In fact,
taking a close look between the 12Z and 00Z soundings from area
RAOBs, one can easily see this. This will allow for slightly
warmer and dry conditions to prevail through the weekend with
generally light winds in place. The exception to this will be
across western portions of the Aleutian chain and Bering, where
another front will try to enter the region, with showers and gale
force winds developing for Saturday afternoon into Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The Omega Block feature will be on-going into the extended period
with the ridge axis over the Eastern Bering and Eastern Aleutians.
This configuration will keep Southwest Alaska in North-
Northeasterly flow through at least Tuesday. This pattern,
combined with tight low-level surface pressure gradients, will
keep a risk of strong wind flow through channeled terrain
especially along the Alaska Peninsula, Thompson Pass and the
Copper River Delta.

Tuesday, a cold front drops out of the Arctic, but bypasses
Southcentral thanks to tall Alaska Range mountains. The deep cold
spills into Southwest and is supported by a newly formed upper-
level low that undercuts the blocking High over the Chukchi Sea.

The remainder of the week will feature a gradual return to
southerly flow over the Eastern Aleutians and Southcentral as the
new upper-level low drifts westward through the Southern Bering
Sea. Timing between the ECMWF and GFS shows up with a difference on
the scale of days. Since the models are typically too fast to erode
the riding and cold air over the Mainland in the extended forecast,
at this point the forecast favors the slowest EC solution that keeps
below normal temperatures and dry conditions for the interior South
Mainland through the middle of next week. It will be worth watching
the Pacific jet to monitor any subtropical connections and potential
atmospheric river effects into Northern Gulf Coast communities if
the ridging erodes as quickly as the GFS has been indicating.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray 127 160 180 181.
         Gale Warning 155 165 170 172 178 180.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...DS/DEK