Gale Warning 498, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
440 PM AKDT Mon Oct 14 2019

.ANALYSIS AND UPPER LEVELS...

Two major systems continue to dominate the active weather pattern
across the domain today. The first is out of the Bering in what
is left of the former Typhoon Hagibis. The second is a deepening
low drifting to the south out of the Gulf of AK. Much of mainland
Alaska remains under the influence of high pressure in between
these two systems leaving sensible weather mostly dry, cool, and
quiet.

Over the Bering, the low that was formerly Hagibis is nearing
peak intensity. It is sitting in the favored left exit region of a
strong 150 kt jet at 300mb (30,000'). The lowest recorded central
pressure to this point has been 956.3 mb on buoy 46035. Models
show the potential for it to deepen a bit more this afternoon
dropping as low as 950 mb, but it may miss any platform that could
sample the deepest value. Winds have peaked between 70-80 mph so
far, mostly along the Western Aleutians.

The other system moving into the North Pacific has completely
occluded on satellite this afternoon. It is spinning some cold air
and some upper level energy around its center. We will monitor its
associated front for the possible formation of smaller
circulations.

&&

.MODEL DISCUSSION...

Models continue to do a fine job with the overall synoptic
pattern. There is high confidence in this portion of the forecast.
However, there are discrepancies in the finer details the could
prove to be important. The first of these is regarding the
"trailing" low coming into the Western Bering quickly on the heels
of Hagibis. By Tue afternoon, a compact, but strong low will
quickly traverse along the Aleutian Chain. The forecast models now
seem in much better agreement with this system than 24 hours ago.
It is worth noting that the ECMWF was the first to catch onto it.
This forecast package will trend towards a stronger, consolidated
low that could impact the Pribilof Islands. The other uncertain
arena is across the Northern Gulf of AK. The forecast models
continue to really struggle with some smaller circulations
wrapping through that area from east to west. The GFS has been the
most aggressive with this feature, but it has backed off (and
dropped further south) on recent runs. This will be watched
closely as it could bring some light snowfall to portions of the
Western Prince William Sound and possibly as far west as Anchorage
Wed morning.

&&

.AVIATION...

PANC...VFR conditions and light northerly winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Monday
night through Thursday)...

High pressure exists over the mainland while low pressure spins in
the Gulf of Alaska. Movement of air flowing from high pressure to
low pressure is keeping gusty offshore flow through gaps and
passes in Southcentral and keeping conditions generally cold and
dry for the mainland. Offshore, the low in the gulf is creating
gale force winds over marine areas.

One interesting aspect of the forecast is a weak low pressure that
will generate along the gulf coast late tomorrow into Wednesday.
This will bring precipitation to the eastern Kenai Peninsula and
move northwest toward the Anchorage area overnight. As of now, it
looks like the cold dry air in place will keep any precipitation
out of Anchorage, with the low stopping short around Whittier. If
things do change any precipitation that reaches Anchorage will
fall as snow or a rain/snow mix. However, the chance of this
happening is slim and any accumulation will not amount to much.
Models are still figuring this system out though, so it bears
watching the next couple of days.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Thursday evening)

A fairly strong front associated with post tropical cyclone
Hagibis will approach the coast this evening, with widespread
rainfall developing along it. This front will weaken as it pushes
inland overnight and through the day Wednesday morning, as it
outruns the better upper-level support.

Then, another front approaches the coast by Wednesday afternoon,
as another storm system slides across the central Bering and
through the AKPEN. Widespread rainfall will once again develop
along and ahead of this feature, with coastal communities seeing
the best potential for more rainfall.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Thursday evening)

The tale of two storms best sums up the weather for the region.
Currently, the remnants of Super Typhoon Hagibis have helped
initiate rapid cyclogenesis, with the low forecast to bottom out
around 950 mb later this evening. Given a long fetch length and
hurricane force sustained winds expected, wave heights will
continue to build. This has already been observed, with buoy 46070
showing a 5 foot increase during the last 3 hours. This will
continue through the evening, with maximum wave heights of 45 to
50 feet expected. As for the winds, earlier this morning winds
near Adak were gusting to just shy of 80 mph, with current
observations showing gusts near 60 mph across the central
Aleutians and Pribilof Islands. These winds will gradually
diminish for the Aleutian chain, with it taking longer to subside
for St. Paul and St. George Islands, as they'll remain closer to
the storm for a longer duration.

Then, as one storm exits the region to our north, a new system
quickly moves in on its heels. This next feature will be much
more compact in size, but it'll also tap colder air aloft that
will get ingested into the storm from Siberia. This allows for a
much steeper lapse rate environment to develop. As a result, we
expect the precipitation to have a more vigorous showery type of
appearance. The models also show some modest instability
developing, with lifted indices down to -2 and SBCAPEs of 150 to
300 J/kg. This should be sufficient for some isolated
thunderstorms to develop.

The bigger story will be a return of sustained storm force winds
for a good portion of the Bering Sea, with some winds reaching
hurricane force near the southern flank of the low. Given the more
compact nature, wave heights are anticipated to be lower, as they
top out around 35 feet.

&&

.MARINE (Days 3 through 5)...

Bering Sea/Aleutians: Models continue to show discrepancies for
the synoptic setup in the Bering in the Thursday timeframe. In any
case, there continues to be indication that a double barrel low
will set up in the Bering, with one center near Cape Newenham and
the other center in the northeast Bering. It's tough to say which
will be stronger, but models have shown a slight trend deepening
the Bristol Bay/Cape Newenham low. This would likely result in
some gales in the southern Bering and eastern Aleutians Thursday.
Expect mixed seas with combined wave height in the 15 to 20 foot
range... subsiding Friday. A series of disturbances will likely
rotate around the northeast Bering low, thus areas of showers and
small craft advisory conditions are likely through the end of the
week.

Gulf of Alaska: An upper low will cross the AKPEN Thursday and a
surface low will develop near Kodiak Island. The range of
solutions is a bit broader than ideal... the main question is how
far north the low propagates. It's possible that it will bring
gales to the southern outer waters Thursday. Or, it could track
far enough south that stronger winds will be concentrated in the
North Pacific. Either way, elevated winds are likely out of
Kamishak Bay as pressure drops in the Gulf. The pattern looks
generally benign in the Gulf by Saturday, but model discrepancies
compound so it will be important to monitor the forecast.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Details in the long term remain murky, as models continue to
struggle with the preceding evolution of post-Typhoon Hagibis.
However, there are some general trends apparent. Low pressure is
likely to settle in the Bering Sea (or perhaps centered off the
west coast) through early next week, and cold air advection in
north flow around the west side of the low will continue showery
unstable conditions over the Bering/Aleutians. The location of the
low center is uncertain, but if it sets up near the west coast,
precipitation and increased southwest winds are likely in its
southern periphery. High pressure in the low levels will persist
over northeastern Alaska, leading to cold, generally dry
conditions inland. However, enhanced cyclogenesis is likely as
modified cold, dry air originating in eastern Siberia spills into
the North Pacific. Thus, expect a series of lows to approach the
Gulf from the west. This will likely bring precipitation and
increased cloud cover to the North Gulf Coast, possibly making it
inland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Hurricane Force Wind Warning: 412
		 Storm Warning: 173, 177-179, 181, 185 411, 413, 414.
 		 Gale Warning: 131, 132, 138, 155, 165, 170-172, 174-176, 180,
					   351, 352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...MM