Small Craft Advisory 391, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
835 AM AKDT Tue May 16 2017

.ANALYSIS AND UPPER LEVELS...

There is a 75-125 knot Pacific subtropical jet stream stretching
from 40N to 50N. The strongest PV anomaly is centered south of
the Alaska Peninsula this morning with a well defined dry
airstream trailing this feature as seen on Water Vapor satellite
imagery. The associated warm front is quickly approaching Kodiak
Island while the main surface low is centered north of Dutch
Harbor. Across Southern Alaska, shortwave ridging is building
ahead of the warm front, and this has allowed for mostly quiescent
weather inland. Weak coastal ridging along the North Gulf Coast
along with weak interior low pressure has allowed for increasing
southeast gap winds through Turnagain Arm. Some of these winds
have clipped the west side of Anchorage early this morning, as
well. Finally, overnight polar orbiting nighttime microphysics
satellite imagery also captured an area of low stratus and fog in
the North Gulf.

&&

.MODEL DISCUSSION...

The models have settled into excellent agreement with all the
major weather features for the next two days. The main area of
discrepancy over the past few days was the timing of when the
North Pacific front would reach the North Gulf Coast, but the
models have all come into a consensus that will occur by Tuesday
night before falling apart. In this pattern, the greatest
discrepancies among models tends to be the showery and convective
precipitation patterns inland, and all the models show various
biases. However, forecast confidence remains high through the week
before uncertainty increases for the weekend. The GFS was favored
for this forecast along with some hi-res NAM guidance for gap
winds.

&&

.AVIATION...
PANC...VFR conditions will dominate the forecast period. The
bigger challenge will continue to be the timing/onset of southeast
winds at Anchorage airport in the afternoon and evening periods.
There should be a lull in southeast wind by this morning before
they increase again in the mid afternoon. These gusty southeast
winds will likely continue through the overnight hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level high over the area will slowly slide north today.
Surface high pressure will build along the north Gulf coast ahead
of a front moving into Kodiak Island. The thermal trough will move
north of the Alaska Range. These two factors will allow gusty
winds to develop through the Copper River basin, the Knik River
Valley and Turnagain Arm. The front will reach the Gulf coast
early Wednesday morning, but weaken considerably by that time. The
remnants of the front will then work inland during the day
Wednesday, with some lingering showers through Wednesday night.
Another front will move into Kodiak Island Thursday morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tue and Wed)...
The main focus in the the short range forecast is wind associated
with a front that moves inland across the Southwest today.
Southeast winds are expected to rapidly increase this morning as
the pressure gradient tightens between a surface low near the
Alaska Peninsula and a ridge across Southcentral. A low level jet
max in the southeasterly flow aloft, orients from the western
Gulf to Bristol Bay. Strong gusty winds will mix down to the
surface at day break and continue through the evening before
tapering off tonight. Southeasterly flow continues through the
end of the week, however gusts will mainly be confined along the
Aleutian Range. Rain associated with the front will bring the
highest accumulations along the eastern Alaska Peninsula and
Western Capes today. Rain tapers off with showers remaining in
the forecast through Thursday night as the surface low meanders
near the Peninsula.

&&

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

A broad occluded surface low meanders near the eastern Aleutians
through Thursday night as an upper low over the western Aleutians
merges with the low lifting over the western Alaska Peninsula. The
frontal boundary associated with the surface low focuses rain
across the central and eastern Bering today then diminishes on
Wednesday as the surface low drifts south, aligning with a strong
wave rounding the base of the upper trough. A ridge builds across
the the western waters on Wednesday bringing a stable air mass
and patchy fog for the western and central Aleutians through mid
week.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The forecast period commences with two closed lows influencing the
Aleutians...one south of Kiska Island and the other west of
Kodiak. Both the GFS ensemble mean and the ECMWF are tightly
clustered at 500 MB with both features...while the Canadian GEM
pings into the lead storm but continues to have run to run
continuity issues with the second storm. Due to the handling of
the overall pattern...have better confidence in the ECMWF and GFS.
The coastal ridge will set up over SE AK and block the first
system. Expect the surface low to fill but definitely ushering
moisture into the AK Peninsula and Kodiak Island. While the second
low pushes into the Gulf of Alaska, robust easterly winds and an
impulse will get ejected from the system bringing the next wave of
moisture to the region. This is looking to be a pretty wet period
for the AK Peninsula and Kodiak...with ridging over the Bering
and ridging over SE AK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 138 160 174 175 176 177 178 180 411 412.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JRA
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...PS