Gale Warning 544, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
708 AM AKDT Tue Oct 6 2020

.ANALYSIS AND UPPER LEVELS...

An upper-level trough remains over Southcentral and the Gulf of
Alaska this morning. Within this trough, there are a few embedded
shortwaves tracking east to west across the region. The first of
these sits near Yakutat and is transiting away from the region.
The second is tracking along the Alaska Range and is producing a
wide area of mid and upper-level clouds, as well as a few high-
elevation showers across the Copper River Basin. The third is
moving across the Kenai Peninsula and helping spin up a weak
surface low. This feature is also enhancing precipitation and an
easterly wind flow across Prince William Sound.

To the west, satellite imagery shows little in the way of cloud
cover from Cook Inlet north across the Mat-Su Valleys as a
transient mid-level ridge moves over the area. Temperatures from
Kenai and Wasilla to Talkeetna and Cantwell have dropped into the
upper 20s to mid 30s due to strong radiational cooling from the
clear skies and light winds.

For Southwest Alaska, showers continue to linger across parts of
the Kuskokwim Delta and Bristol Bay coast. This is due to the
combination of a moist, northwesterly flow, cold air advection,
and instability as a shortwave moves onshore behind a cold front
that pushed through the region yesterday.

Farther west, water vapor imagery shows a large upper-level low
trekking across the Kamchatka Peninsula with a shortwave
undercutting the main low. These features are driving a double-
barrel surface low east from the Kamchatka coast. The first of
these lows is pushing a weak front into the central Bering while
the second is driving a storm-force front, as seen on the latest
ASCAT scatterometer pass, toward the western Aleutians. Surface
observations from Shemya have indicated peak wind gusts around 50
kts all night. This complex system looks to strengthen and become
the main weather focus as the trailing low becomes absorbed into
the first and deepens as it moves east over the Bering.

&&

.MODEL DISCUSSION...

Models are in good agreement overall. There are some subtle
differences in the position of the shortwaves embedded in the
upper-level trough, but these differences do not affect the
sensible weather. Guidance is all showing the development of a
weak surface low in the northern Gulf and the resultant easterly
flow bringing precipitation over Prince William Sound. Models
also differ slightly in the timing of a shortwave crossing the
Alaska Range and moving over Southcentral on Wednesday, with the
NAM being the fastest. Again, the difference in timing does not
yield a difference in the sensible weather, with a period of light
rain possible from Cook Inlet into the Susitna Valley beginning
late morning/early afternoon Wednesday.

Models are also clustered well on a deep low moving from the
Kamchatka Peninsula east to Saint Matthew Island by Thursday, as
well as its associated gale-force front as it sweeps over
southwest Alaska by Wednesday. As this front sweeps east and
enters the western Gulf, guidance is keying in on the development
of a triple-point low. Again, this is well depicted on all models,
but there are differences in position and strength. Nonetheless,
it does appear that a surface low will spin up somewhere near
Kodiak Island by Thursday and push a front to the Southcentral
Coast by early Friday morning. There is good model agreement with
this, even three days out. Thus, forecast confidence is above
average through the short term.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. With a ridge
building in overnight, there is a chance for fog to develop near
or over the airport complex early this morning, lingering through
mid-morning today. Confidence in this solution is very low,
however, with only VCFG highlighted in the TAF package.

&&

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

Overall, relatively calm weather for today and tomorrow. A
remnant trough with weak surface rotation and various shortwaves
is set up over Southcentral and the Gulf today. This will keep the
North Gulf Coast and parts of the Copper River Basin in light
rainfall, and allow the rest of Southcentral to remain dry and
see some clearing. This pattern will persist until Wednesday when
a new front attached to a strong low out over the Bering
approaches Kodiak Island from the south.

Wednesday night the elongated front pushes into the Southern
Gulf. Impacts with this should remain minimal, as it has weakened
considerably by this point. Widespread small crafts can be
expected out ahead of the front, as well as some gusty winds
through Turnagain Arm and the Upper Hillside on Thursday morning.
Additionally, coastal areas can expect moderate to heavy rainfall
late Wednesday night through early Friday morning as the front
pushes towards the coast. Flooding should not be an issue, this
time around, as this system is weaker and does not have the same
tropical moisture feed we saw with storms last week. Downsloping
will likely keep eastern Kenai Peninsula and Anchorage relatively
dry under southeasterly flow. However, by Thursday night this
flow weakens, thus increasing the chance for both areas to receive
some light rain.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Today
through Thursday)...
A weak upper level short-wave will make its way across southwest
Alaska today, producing a few rain showers. Otherwise, expect a
mix of sun and clouds and overall quiet conditions. This will all
change tonight into Wednesday as a strong storm system makes its
way across the Bering Sea. The leading front will reach the
Southwest Coast by around midnight, with rain and strong
southerly winds spreading inland overnight through Wednesday
morning. There could be a brief period of mixed rain and snow as
precipitation first reaches the Kuskokwim Valley, but generally
expect temperatures to jump into the upper 30s as gusty winds move
in and mix out any residual cold air. The front will stall over
southwest Alaska Wednesday afternoon and then slowly weaken
through Wednesday night, with rain and winds slowly diminishing.
However, clouds and showers will persist all the way through
Thursday as upper waves rotate in from the Bering low.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Today
through Thursday)...
A strong front over the western Bering and Aleutians will march
eastward across the Bering Sea today through tonight, then push
inland across southwest Alaska on Wednesday. This will lead to
widespread gale force winds, with smaller areas of storm force
winds as well as moderate rain. With prevailing southerlies along
the front, expect some stronger gap winds as the front crosses the
eastern Aleutians and southern Alaska Peninsula. Conditions won't
improve all that much behind the front, as the low follows
behind, ultimately tracking to around St. Matthew Island Wednesday
night. Showers and gusty winds will persist through Thursday due
to upper level short-waves rounding the low center and cooler
unstable air wrapping into the low.

&&

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

Bering/Aleutians

A low is expected to stall near Saint Matthew Island on Thursday.
Northerly and westerly gales are expected on its western and
southern sides, respectively. These will gradually weaken to sub-
gales through Friday.

Gulf of Alaska

Easterly gales are possible along the Gulf Coast Thursday evening
under a front. Otherwise sub-gales and lower seas will persist
through the weekend.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through
Sunday)...

An upper low will stall near Saint Matthew Island Thursday. The
associated broad surface low will bring widespread rain and gusty
winds to the entire Bering/Aleutians. It will slowly weaken and
again stall near Bristol Bay through the weekend. While there are
model discrepancies, all of them show generally northerly flow
and widespread showers continuing through the weekend. Farther
east, a weakening front will track across the Gulf Thursday
bringing gusty easterly winds along the coast. Both the GFS and
ECMWF indicate a stronger compact low tracking northward along
the Panhandle, though it appears to weaken as it approaches
Southcentral. These models remain in generally good agreement
through Sunday. Both show another low tracking well south in the
Gulf. This low will not directly impact Southcentral, but will
continue to drive the overall showery pattern along the Gulf Coast
and higher terrain in the Chugach Mountains. Temperatures are
expected to fall slightly into the weekend, however highs will
remain seasonal near 50.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood advisory 121.
MARINE...Storm warning 174-178 412 413.
Gale warning 150 155 160 165 170-173 179-181 185 411 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...KM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SEB
MARINE/LONG TERM...AP