Gale Warning 684, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
717 AM AKST Sun Dec 22 2019

.ANALYSIS AND UPPER LEVELS...
On satellite imagery there is a frontal boundary that stretches
across the Bering to the Alaska Peninsula and pushing into the
Western Gulf. There is a discernible wave developing along the
front near Kodiak. On ASCAT (Advanced Scatterometer) imagery
widespread gale to storm force winds are being observed over the
Bering and the North Pacific. There are two jet cores south of the
Aleutians, the strongest being 150+ knots. The minimum temperature
observed at the National Weather Service in Anchorage for the
past 24 hours was negative 2 degrees Fahrenheit.

&&

.MODEL DISCUSSION...
The models are in phase today regarding the major systems.
However, the models are not in good consensus starting Monday. The
bane of the model solutions for this week will be honing in on
the track of the surface lows especially as they move towards
Southcentral Alaska. Expect updates and amendments to the forecast
package the next 5 days.

&&

.AVIATION...
PANC...As of 1320z on "Nighttime Microphysics" satellite imagery
there is a patch of fog from Knik Arm to Hope. Fog moved into PANC
this morning. Added VCFG and a TEMPO for the fog. VFR conditions
and light winds will persist. The high resolution models have very
light winds through 18z.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

A triple point low center is forming along the frontal boundary
approaching Kodiak Island this morning. This gale force front will
continue to move north along the North Gulf Coast tonight while the
surface center will lag behind. There will likely be a secondary low
center which moves into Prince William Sound late tonight. Models
still differ on the finer details on the movement and life cycle of
these centers. That said, it looks like moderate snow will spread to
the eastern Kenai Peninsula tonight. Gusty easterly winds will
accompany the snow and likely reduce visibilities in blowing snow
from the Turnagain Pass to Portage Valley corridor early this
evening through early Monday morning. Light overrunning snow will
likely spread inland of the Kenai and Chugach Mountains as moist
southeasterly winds ride over the cold air. The upper trough is slow
to move through and generally maintains some south to southwesterly
flow aloft. So for now it looks like the lions share of the
precipitation(mainly snow) will be confined to the North Gulf
locations and higher elevations along the Richardson Highway north
of Thompson Pass through Monday. Lessor amounts elsewhere. However,
there is still model uncertainty so stay tuned for updates.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Cold air continues to remain in place for southwest Alaska. A
front stemming from the strong low in the central Bering is seen
on satellite approaching the southwest coast early this morning.
While the core of the low is strong, the front extending from it
looks rather weak. It will be enough to drop a few inches of snow
mainly around Bristol Bay, with some blowing snow as well.

As a result, we did continue the winter weather advisory for
blowing snow today. Winds out of the northeast will be strong
enough to reduce visibility to advisory criteria (one half mile),
though the worst of it looks to be confined to coastal areas northwest
of Dillingham, so reflected this in the forecast. Also, since the front
is weak and low begins to decay today, winds look like they will diminish
enough this afternoon to end the blowing snow threat, so curtailed
the advisory from ending tonight to late this afternoon at 4PM.

As the low center comes ashore tomorrow, it will have decayed
enough to just be producing light snow in most places. However,
areas around Dillingham will catch another round of snow that
could give them another few inches. Tomorrow afternoon weather
will begin to quiet again as the low moves ashore and falls apart.

&&

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

The Bering Sea low that has garnered much attention from us in the
Bering continues on its northeasterly track early this morning.
The high wind warning for the central Aleutians also remains in
effect as winds are as forecasted...more on this below.

The mature low is well defined on satellite, with cold weather
stratocumulus observed wrapping into the low, indicating cold air
arrival as expected. We also received multiple scatterometer
(winds derived from satellite) passes directly over the center of
the low, which enabled us to verify wind speeds. This does not
happen often, so it was exciting to get them as we don't have many
weather observations to work with and it hampers our ability to
verify forecasts. Anyway, scatterometer wind speeds showed around 60
kts for max speeds, which correlates well with the few observations
we do have. It also lines up with the forecast...this means the
forecast is on track so did not make many changes to it.

The low will continue to progress eastward through today and will
begin to weaken as it does so. It will weaken to a storm force low
this evening and further reduce to mostly gales by tomorrow
midday, when it reaches the Bristol Bay Coast.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...
Tuesday will be the quietest of the extended period days for
Southern Alaska waters as a whole, with no strong features to
speak of. A weak spinoff triple-point low will be over the western
Gulf, the last remnants of the giant hurricane force low currently
out over the Bering. Thus, except for some small craft winds south
of this low the Gulf will be quiet on Tuesday.

Meanwhile a developing North Pacific low will begin producing gale
force winds north of the low center on the Pacific side of the
Alaska Peninsula and eastern Aleutians Tuesday afternoon. The low
will strengthen as it moves into the southwestern Gulf Wednesday
morning, producing storm force winds on the west side of the
center. It strengthens even further as it moves to between Kodiak
and Middleton Island Wednesday evening. There are some model
differences, but barrier jet winds to hurricane force are not
outside the realm of possibility at this time. Regardless of local
hurricane force winds, widespread storm force winds are likely over
large swath of the Gulf starting Wednesday afternoon and continuing
into Thursday with the return flow behind the low.
Storm force winds persist through the day Thursday through the
Barren Islands, with widespread gales across the central and
southern Gulf.

A separate gale force front will move into the western Aleutians
on Wednesday. While it will maintain its strength through
Thursday, it will stall out its movement over the central Bering,
and thus, impacts from the front should be minimal.

&&

.LONG TERM FORECAST (Days 3 through 7: Tuesday through Saturday)...
A very interesting, active weather pattern is in store across
Southern mainland through the long term. Uncertainty begins right
from the get-go. A deep polar upper level trough will be over
Southwest Alaska Tuesday morning. An associated shortwave trough
extending southeastward from the low will attempt to phase with a
second upper level low over the far southern Gulf. The models are
in poor agreement as to how much phasing occurs. The better the
phasing, the more likely accumulating snow across Southcentral,
including Anchorage, will be. Snow lovers will like the EC, which
begins steady snow as early as Monday evening. The rest of the
models aren't quite as bullish with the moisture until Tuesday,
when the main trough axis approaches. Even with the driest models,
all of them show some snow during the day, a very fitting pattern
for Christmas Eve. The GFS holds off on the heaviest snow until
overnight Christmas night and into Christmas Day. For now that is
an outlier, as the general model consensus has the snow stopping
overnight.

As mentioned in the marine discussion, on Christmas Day, all eyes
will turn to a strong low tracking northeastward through the Gulf
towards Prince William Sound. The center is decently agreed upon
in the models to track to near Cordova late Wednesday night into
Thursday morning. This would normally be a dud for Anchorage
snow-wise. However, there is good agreement that the supporting
500 mb trough axis remains west of Anchorage, which would support
moisture being drawn northwestward from the low and into Anchorage
and the Mat-Su. Thus, potential for another snowfall event on
Thursday is certainly there. Elsewhere, blizzard conditions are a
significant threat with the low, including for Kodiak, Seward,
Thompson Pass, and potentially Cordova if they can stay cold
enough. All of Southcentral will want to stay particularly attuned
to the forecast over the coming days. One other thing that can be
said for certain is that everywhere away from the immediate coast
will stay plenty cold enough that precipitation type will not be a
problem, snow will be the name of the game.

Model agreement fades quickly on the details beyond the low on
Friday and Saturday, but, it seems likely that an even colder air
mass, colder than the one we have in place presently, will move
into Southcentral, likely causing some areas first sub-zero
temperatures of the winter.

Outside of Anchorage-proper, interesting weather will also be
occurring. For Southwest Alaska, the forecast is actually simple.
The storm track will stay out over the Gulf, with a very
persistent strong upper level trough or low that sits over
Southwest Alaska for much of the long-term period. This means dry,
windy, but most importantly, bitterly cold temperatures. Many
interior areas won't even get to zero for highs, and lows will
routinely flirt with 20 below across the Kuskokwim Valley.
Bitterly cold wind chills will be a significant threat for several
days next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 145. Winter Weather Advisory 141 161. High
Wind Warning 187. Blowing Snow Advisory 145.
MARINE...Hurricane 173 175 177 178 413. Storm 155 165 170 172 174
176 185 411 412 414. Gale 119 120 125 130 131 132 138 139 351 352
150 160 171 180 181. Heavy Freezing Spray 140 180 181.

&&

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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
153 PM AKST SUN DEC 22 2019

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0200 AM     FLOOD            DENEKI BRIDGE           61.77N 149.96W
12/22/2019                   MATANUSKA-SUSITNA  AK   EMERGENCY MNGR

            RECEIVED A REPORT OF FLOODING WEST OF THE
            DENEKI BRIDGE ALONG WILLOW CREEK (EAST OF
            PARKS HIGHWAY). SPOTTER INDICATED THAT THE
            CULVERT AT THE BRIDGE GOT BLOCKED BY ICE AND
            THAT THERE WAS FLOODING UPSTREAM AND SOME
            RESIDENTS WERE BEING EVACUATED.

&&

ED