Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 210513 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
913 PM AKDT Sat Oct 20 2018

.ANALYSIS AND UPPER LEVELS...
Satellite imagery beautifully shows the next storm that will be
the director for much of the weather in southern Alaska. A strong
vertically stacked low sitting just south of Kodiak Island is
bringing warm, moist air from the central Pacific into the Gulf of
Alaska. Cold air aloft is wrapping around the low which is
causing it to deepen and intensify as it moves north. Within the
flow of the trough around the new low, a short wave extends over
the northern Gulf coast. Coupled with the shortwave, isobaric
packing along the coast line will ramp up wind speeds at the
surface, becoming quite robust through the gaps and marine zones.
An occluded front extending ahead of the surface low is pushing
over Kodiak Island bringing rain and gusty conditions to the area.
With cool air aloft and warmer air near the surface, instability
rises and the potential for convective activity ahead of the front
develops. Due to the strength of this next front and the immanent
heavy coastal rainfall, the Flood Watch previously in effect for
the Seward area has been upgraded to a Flood Advisory. The west
remains quiet with little rainfall and fairly calm winds.

&&

.MODEL DISCUSSION...
Models are in generally good agreement through Monday afternoon
with the handling of the large low south of Kodiak Island and its
associated front. Mainly will be doing minor adjustments using
the NAM/NAMNEST to current grids. Models are struggling with the
next low pushing north into the Gulf Monday night and Tuesday. The
interaction of the surface low and upper jet area causing some
significant differences in timing and to a lesser extent intensity
of the low. The ECMWF is the faster of the the solutions. Will
probably trend toward the slower GFS/NAM solutions.

&&

.AVIATION...
PANC...Generally VFR conditions expected. LLWS is expected through
Sunday morning with north winds at the surface and strong easterly
winds aloft. Gusty southerly winds will then develop around noon
Sunday.

&&

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

A very interesting period of weather is in store for portions of
Southcentral over the next 48 hours. A storm-force low will skirt
the western end of Kodiak Island Sunday morning before trekking
across the eastern Bristol Bay/AKPEN region before stalling as it
fills/wobbles around itself. We`re already beginning to feel the
affects of this system and its associated tropical moisture feed,
as showers continue to affect the southern coast, with even a
thunderstorm having developed across southern parts of Prince
William Sound.

The associated occluded front will lift northeast across the
western into the central gulf through the night before washing out
Sunday morning. Widespread rainfall, some of which will be
moderate to heavy at times, will accompany this feature. This
storm has a good pocket of cold air aloft, with the 500 mb
temperatures down to -30 C, which will overspread the region as a
dry slot moves in behind the front. This will cause precipitation
to become more showery in nature. The cooler air aloft combined
with relatively warm sea surface temperatures and added moisture
quality/depth via the tropical connection will generate a pretty
respectable amount of instability for this time of year. Current
model forecasts and point soundings show MUCAPEs up to 600 J/kg
with lifted indices down into the 0 to -4 range. Even surface-
based parameters show similar values (SBCAPE is a little lower,
but still up to 400 J/kg). This all points to a good chance of
seeing at least a few thunderstorms across all of the Gulf and
even to coastal locations.

As for rainfall amounts, the heaviest will be along the eastern
Kenai Peninsula and coastal locations near Prince William Sound,
where a general storm total of 2 to 4 inches (with locally higher
amounts) are possible. Thus, the previous Flood Watch for the
Seward area has been upgraded to a flood advisory. We`ll also be
keeping an eye on things farther east towards the Valdez region,
but flooding appears to be less of a concern as the better
moisture fetch and dynamics are juxtaposed farther west.

For our mariners, a strong barrier jet is in the process of
forming and we expect sustain storm force winds, with some
hurricane force gusts possible from Shelikof Straight eastward
towards the Sound. A long fetch length will aloft offshore seas
to build upwards of 35 to 40 feet as well. Additional storms are
seen to follow this one in the later in the week, so the active
weather will continue.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
There is a fierce low with an occlusion south of the AKPEN that
will continue to move north-northeastward. This system will
influence the sensible weather across Southwest most of the
forecast period. Currently, north of this low the winds
predominately offshore/north and east. As time elapses expect more
moisture to advect into the region. The radar at King Salmon
(PAKC) is already pinging into precipitation. The temperatures are
still pretty warm given that we are in the third week of October.
Expecting rain and rain showers in the low elevations and the
higher elevations will get a fresh blanket of of snow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
The longwave pattern features a deep trough over the Eastern
Aleutians and AKPEN with a secondary trough east of Kamchatka. The
latest ASCAT (advanced scatterometer) pass has detected a weak
circulation south of Nikolski another weak circulation near near
St. Paul. There is a tight low with a swath of Gale and Storm
force winds south of Sand Point. The low south of the AKPEN will
continue to move north-northeastward, and then its track will re-
curve later in the period. The water temperatures are near their
climatological max and with that cold pool of air aloft in
association with the low, expect showery/convective precipitation.
Expect wet and unsettled weather across the Central and Eastern
Bering in association with this low. For the Western Bering and
Aleutians, a ridge will slowly move towards the region Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday Night through
Saturday)...

Two lows remain in the forecast headed for the Gulf during the
long-term forecast, and colder, more seasonable temperatures may
finally be on the way to Southwest Alaska by the end of the
workweek.

The low on Tuesday continues to be a struggle for the forecast
models regarding the low`s eventual strength and intensity.
Compared to yesterday, the models are trending weaker with the low
intensity, with the NAM and EC showing a weaker low that barely
produces gales along the north Gulf Coast, with the low center
tracking into Prince William Sound Tuesday afternoon. The
Canadian has widespread gales and takes the low inland near Seward
Tuesday evening, and the strongest solution, the GFS, has storm
force winds (55-70 mph) with the barrier jet and brings the low
center inland near Seward Tuesday evening. Thus, considerable
uncertainty still remains on the impacts. The NAM is the middle-
ground model for track, but in deference to previous runs having
stronger winds, gales will be kept along the Gulf Coast for this
forecast package. This also is a positive trend for rainfall along
the Gulf Coast, especially in Seward, where the NAM and EC
solutions both suggest under an inch of rain falling there from
this low. Even with the stronger and possibly wetter solutions,
the track likely keeps Seward relatively dry than if the storm
passed west of there. None of the guidance suggests prolonged
heavy rain due to the fast movement of the low.

The next low moves into the Central Gulf on Thursday morning. The
models remain unanimous that this one will be by far the stronger
of the two lows. Thus, more rain is likely with this second low at
all points along the coast. Uncertainty remains high as to how
fast the low moves as well, with the GFS the fastest and EC the
slowest. The difference between the two is a full 12 hours.
Regarding track, the GFS and EC both take the center of the low
over the far southern Kenai Peninsula, tracking west, but the GFS
has it there Thursday evening while the EC waits until Friday
morning. The Canadian makes landfall over Afognak Island Thursday
evening. On the low`s approach, easterly winds will mean
relatively little rain in Seward. Most of the rain there will
occur after the center passes their longitude and the winds turn
out of the south. The east to west track across the central Gulf
should keep winds along the coast lower, but widespread gales
(35-50 mph winds) are likely, especially closer to the low center.
The model solutions diverge greatly over the weather pattern
behind the low Friday and Saturday, but none of them as yet shows
any more strong lows in that timeframe.

Well above normal temperatures are likely to persist area-wide
through Thursday as each low brings in reinforcing warm air to
mainland Alaska. However, the lows will help bring colder, far
more seasonable temperatures to the western Bering on Wednesday.
The cold air slowly tracks east to the central Bering late on
Thursday, then perhaps tries to encroach on the Kuskokwim Delta
late Friday into Saturday. However, since by late Friday into
Saturday the models diverge greatly, there is no consensus on how
or even if the cold air makes it that far east. The very warm
October continues.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 125.
MARINE...Storm Warning 119 120 125 13>132 138 139.
 Gale Warning 136 137 140 141 150 155 160.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RA/RC
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...JW



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