Small Craft Advisory 995, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKST Tue Dec 22 2020

.ANALYSIS AND UPPER LEVELS...

The longwave pattern features a broad trough over the Bering and a
ridge axis bisecting SE Alaska and the Yukon Territory. There are
numerous shortwaves riding up the back side of the ridge and
distinct upper level closed lows near the Pribilof Islands and
over NE Russia. The leading edge of the jet is entering the Gulf
Of Alaska while the jet core of 170 knot is south of the
Aleutians. At the surface there is a an mature front over the
Bering and another mature front making headway across the Gulf.
On satellite imagery, there is a distinct tropical connection with
the low in the Gulf. The deep fetch of moisture extents from just
east of Kodiak to an area north of Hawaii.

&&

.MODEL DISCUSSION...

Synoptically, models are in good agreement through 36 hrs. There
are some minor difference with the onset and end time of
precipitation. As usual, the thermodynamic parameters of the
models are slightly off which could have implications for p-type
(snow vs. rain). This is most noticeable over Cook Inlet and
Prince William Sound. In the past 4 days, the model agreement has
improved significantly over the Bering, the North Pacific and in
the Gulf Of Alaska.

&&

.AVIATION...
PANC...There is a frontal boundary moving across the aerodrome.
Expect several decks of cloud layers. Even though the cigs will
decrease, the current thinking is that the cloud coverage will
stay above VFR minimums. The main impact to the airfield will be
low level wind shear today.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today
through Wednesday Night)...

The very active storm pattern across Southcentral continues into
Wednesday, with a much needed and well-deserved break in the
weather expected starting Wednesday night.

For the Anchorage area, the High Wind Warning remains in effect
until 6pm this evening. While winds have been slow to come up so
far this morning, there are already indications upwind across the
eastern Kenai Peninsula that the period of highest winds is
arriving with Portage Glacier recently reporting an 82 mph gust.
Nevertheless, the model guidance has backed off a little bit on
the peak winds, and as such the expected peak wind gusts have also
been refined down as well. Nevertheless, this still is expected
to be a very impactful event with frequent gusts well over
hurricane force for Turnagain Arm and higher elevations, with even
a few hurricane force gusts possible over the Lower Hillside. The
peak winds still look to be between now and noon, as the front
causing the winds barrels north across Southcentral. This
afternoon into tonight, the winds will diminish a bit, but will
still remain strong with gusts approaching 75 mph still possible
at higher elevations.

The gusty winds are also expected to increase across much of the
Matanuska Valley today. Palmer has already had a peak wind gust
earlier this morning of 50 mph. The gusts here too will peak
later on this morning, but will remain gusty into tonight.

The High Wind Warning also remains in effect for the Eastern Kenai
Peninsula, specifically through Portage Valley and Eastern
Turnagain Arm, with Portage Glacier already getting hurricane
force gusts with the peak winds still to come later this morning.
The Blizzard Warning has been an interesting story. It remains in
effect, but has been trimmed back to 9am. Most of the observing
stations along the Seward Highway were well above freezing
earlier yesterday evening, suggesting rain was occurring far
sooner. However, since then temperatures have been dropping and
Portage Glacier has changed back over to snow. Nevertheless,
warmer air is making its way towards the area behind the front
causing the strong winds, and since temperatures along nearly all
of the Seward Highway are at least at, and in many cases still are
above freezing, it stands to reason the warmer air coming in with
the front later this morning should turn all of the precipitation
over to rain up through Pass level. That said, it's likely there
will be localized changes back and forth between rain and snow
through this morning, and the blizzard conditions are still likely
occurring through Turnagain Pass, so travel remains strongly
discouraged through this morning.

A secondary area of low pressure still well south of the Gulf will
race northward towards Southcentral, reaching the coast of the
Kenai Peninsula somewhere between Seldovia and Seward this
evening. The low center will then hug the coast and move into
Prince William Sound late tonight, then move inland and dissipate
Wednesday morning. This will have a number of effects as it
happens: First, the High Wind Warning for the Cordova area remains
in effect, because that low will push the hurricane force barrier
jet inland for a few hours before the strongest winds rapidly
diminish. This is likely to occur within a few hours of midnight
tonight. Second, this low track will keep the Cook Inlet region
on the "cold" side of the low. However, by tonight there will be
very little in the way of cold air to work with, as the warming
flow expected all day today will scour it all out.

Therefore, as with a few storms already this winter season,
temperatures will be right near freezing tonight into Wednesday
morning with a good likelihood of occasionally heavy
precipitation. However, what form that precipitation takes is
still very much in doubt, as a degree or two will make all the
difference between blowing snow and a driving cold rain. The
eastward trend in the low track does favor a colder solution
and snow, and thus, the forecast reflects the still highly
uncertain expectation that most of the precipitation tonight into
Wednesday morning falls as snow across the Cook Inlet region.
1 to 3 inches of snow are possible for a few hours, but due to
the fast moving and heavy nature of the snow band producing the
accumulation, they will come down quickly. Below freezing
temperatures will also likely cause many slick areas, due to
refreeze from today's melt combined with the snow.

The snow rapidly shifts north through the day, such that by
lunchtime Wednesday, most of the snow should be up in the Susitna
Valley with Anchorage and points south drying out and maybe
clearing the skies a bit Wednesday afternoon. The low may produce
periods of heavy snow through Thompson Pass and the mountains
north of Cordova tonight through Wednesday as well, with the
upslope persisting into Wednesday night in those areas. For the
rest of Southcentral, Wednesday night and beyond is looking pretty
quiet, with seasonable to slightly above average temperatures
heading into Christmas.

&&

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

The evolving progress of the mix rain/snow precipitation type
(P-type) is still valid as southerly winds continue to bring warm
temperatures over the area, leading rain across AKPEN, Bristol
Bay and farther inland but well south of Kuskokwim Valleys, and
portions of Kuskokwim Bay before noon today. Meanwhile, steady
light snow will continue north of Chuilnuk Mountains and around
Sparrevohn this morning. The cold air advection from the stacked
low over the Bering will reach the coastline by this evening if
not sooner. The snow trends into Wed as the freezing level
advances eastward and across Southwest Alaska, and the amount of
snow will decline after tonight. Essentially, the amount of snow
accumulation in the next two days will range from 1 to 4 inches
near surface, but a bit high at higher elevation between 6 to 8
inches today. Fresh to strong southerly winds and higher gusts
inland will gradually diminish by this afternoon, but not so for
AKPEN. A shortwave embedded within the westerly flow south of the
low from the Bering will maintain elevated winds and snow showers
over much of AKPEN through Wed night.

&&

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

No new news since yesterday about the steepening stacked low over
the Bering. Limited forward motion is expected of the low until
Wed morning when it finally picks up speed while moving toward
Kuskokwim Bay through Wed afternoon. A stubborn upper level ridge
northeast of Southcentral will stick around for another day until
it starts weakening by Wed morning, which leads the circulation to
pass right through Y-K Delta by Wed night. Light freezing spray
will cover much of central Bering and a portion of Kuskokwim Bay
through Wed night. Heavy freezing spray will likely be found
farther west of Nunivak and Pribilof Islands, with only a minimal
coverage south of Kipnuk in the next couple days.

&&

.Marine (Days 3 through 5: Thursday through Saturday)...

Gulf of Alaska
Models have come into better agreement on the track of low moving
out of the North Pacific and then towards the AKPEN on Thursday,
but there continues to be disagreement on the strength. There
remains upside risk to the southerly winds currently in the
forecast, with gales looking possible across the western Bering.
On Friday/Saturday fairly quiet conditions are expected with good
forecast confidence.

Bering Sea:
On Thursday models continue to struggle with the placement of a
compact storm force low, with the GFS bringing this low through
the central Aleutians, while other models track this system mostly
south of the Aleutians. Our forecast sides with the further south
track,keeping lighter winds in across the Aleutians. On Friday a
front pushes into the western Bering/Aleutians, and a few areas of
Gales appear possible. On Saturday a deepening storm system looks
to set up across the western Aleutians, but there are significant
model differences as to how quickly this storm strengthens. There
may be significant upside risk to winds across the eastern Bering
on Saturday if the stronger solutions work out.

&&

.Long Term (Days 4 through 7/: Friday through Monday)...

On Friday models have come into better agreement on the track of a
North Pacific low, with good agreement on this next storm system
heading towards the AKPEN, bringing another wintry mix to
southwest Alaska and Kodiak. A gale force front looks to push into
the western Aleutians at the same time. On Saturday the AKPEN low
mostly dissipates. Meanwhile, a storm system begins to rapidly
intensify across the western Aleutians with the potential for some
storm force winds setting up across the central Aleutians along
with plentiful rain. On Sunday most models show this
Bering/Aleutians storm becoming fully mature and retrograding
north and west towards the Kamchatka Peninsula while unsettled
weather continues across the Bering and Southwest Alaska.

Model consistency disintegrates heading into Monday, but the
general idea of an active pattern across the Bering and relatively
quiet weather across Southcentral should hold.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 125. High Wind Warning 101 125 135.

MARINE...Hurricane Force Wind 119 125. Storm 413 351 120. Gale 411
412 414 165 170 172 173 174 175 185 352 128 129 130 131 132 139
140 141. Heavy Freezing Spray 180.

&&

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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
936 AM AKST TUE DEC 22 2020

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

0930 AM     NON-TSTM WND GST BEAR VALLEY             61.08N 149.71W
12/22/2020  M106 MPH         ANCHORAGE          AK   PUBLIC

            MEASURED AT 1500 FT ELEVATION. WINDS JUMPING
            UP AND DOWN WITH PEAK GUSTS TO AROUND 106
            MPH. LOGS BLOWN OFF WOOD PILES.

&&

GJERDINGEN

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1105 AM AKST TUE DEC 22 2020

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

1056 AM     NON-TSTM WND GST ELMENDORF AFB           61.25N 149.80W
12/22/2020  M64 MPH          ANCHORAGE          AK   AWOS

            PAED AWOS/ELMENDORF AFB.

&&

GJERDINGEN

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1120 AM AKST TUE DEC 22 2020

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

1107 AM     NON-TSTM WND GST FORT RICHARDSON         61.26N 149.65W
12/22/2020  M66 MPH          ANCHORAGE          AK   AWOS

            REPORTED AT FORT RICHARDSON AWOS.

&&

GJERDINGEN

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
235 PM AKST TUE DEC 22 2020

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

0920 AM     NON-TSTM WND GST 3 WNW WHITTIER          60.79N 148.82W
12/22/2020  M124 MPH         ANCHORAGE          AK   MESONET

            RWIS STATION WRTA2. WHITTIER ACCESS ROAD AT
            TUNNEL.

&&

GJERDINGEN