Small Craft Advisory 304, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
531 AM AKDT Fri Apr 19 2019

.ANALYSIS AND UPPER LEVELS...

A broad upper level low is centered over the Kuskokwim Delta with
the flow encompassing much of the Bering and Southcentral. Water
vapor satellite imagery displays this low well as a textbook curl
over the Kuskokwim Delta. Several shortwaves are moving about the
flow with the most predominate shortwaves extending along the
Aleutian Islands from Cold Bay west, and a second over the
northern Gulf of Alaska. Cold air continues to wrap around this
low over southwest Alaska, pushing eastward into Southcentral.

At the surface, a deformation zone stretches from the Kenai
Peninsula over Anchorage into the Copper River Basin, producing
widespread snow over those areas. The atmosphere is generally
unstable through the Gulf and Southcentral. This is evident by the
increased lapse rates from the Anchorage 12Z sounding. In
addition, water vapor satellite imagery is detecting convective
cumulus clouds over the central Gulf behind the deformation zone.

&&

.MODEL DISCUSSION...
Synoptically, models have come into generally good agreement
through Sunday as the low over the Kuskokwim Delta tracks eastward
toward Southcentral. On Sunday, a Pacific low moves into the
southern Gulf and this system is not well handled by the models.
There is variation not only with the placement of the low center,
but also with the strength of the front it pushes into the
northern Gulf Sun morning. For the low and local effects, the
NamNest was the model of choice.

&&

.AVIATION...
PANC...Light snow will persist at the terminal for the next few hours,
before improving CIGS and visibilities are expected. Gusty south
winds will continue through the evening, before dropping to less
than 10 knots. Given the fresh snow cover, with afternoon
temperatures climbing into the mid to upper 30s, sublimation
should allow some light fog to persist, likely in the MVFR range.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Fri and
Sat)...
The potent upper level low is tracking through Bristol Bay this
morning. It is allowing an impressive shortwave to lift from south
to north across the northern Gulf Coast this morning. This will
keep widespread snow falling across the Cook Inlet up through the
Matanuska and Susitna Valleys this morning. Temperatures should
remain at or below freezing through the morning, so we are
expecting most of the inland precipitation to fall as snow.
Instability is already present on the 12z upper air sounding at
Anchorage and should only increase as we go through the day today.
This could aid in another quick couple of inches of snow to fall
at sea level this morning with higher amounts at elevations above
1,000 feet. The Special Weather Statement remains in effect to
address this.

The upper level low itself will move overhead this afternoon. This
will bring the peak of instability with 500 mb (18,000')
temperatures dropping to near -40C. There is even some jet support
at 30,000' to further support shower development. Thus, we have
added a slight chance of thunderstorms over the Northern Gulf of
AK and gulf coast this afternoon. There will also be some gusty
south-southeast winds across the area in our post-frontal
environment. As usual, these will be strongest in the usual places
such as the Turnagain Arm and the Copper River Valley.

More active weather is in store as we head into the weekend.
Strong southerly flow/upslope over the Northern Gulf will bring a
return to steady precipitation across much of the Prince William
Sound by late tonight. The one area of particular interest in this
pattern is Seward. They typically see fairly high precipitation
amounts with southerly flow and strong instability. While they
could see some snow mix in at times and possibly even some
accumulation, we do expect the April sun and warmer air off
Resurrection Bay to result in mostly rain. A new large low will
develop over the Central Gulf Sat. This system will reinforce the
precipitation along the Gulf Coast (with no perceivable break for
most) and bring another shot of snow to inland areas such as the
Cook Inlet and Mat-Su Valleys.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Today
through Sunday)...

An upper level low currently centered near Bristol Bay will
continue to bring isolated snow showers across the region through
this evening. This low will track northward and merge with a
second upper level low on Saturday creating a broad upper trough
over all of western Alaska. Cool temperatures and isolated snow
showers (particularly along the coast) are likely to continue
through the day.

A surface low will track into the Gulf this weekend. It is still
difficult to say exactly how much this will affect Southwest. It
will depend on how much the ridge upstream will amplify due to the
series of shortwave troughs in the Bering. Nonetheless, confidence
is at least moderate that some areas could see accumulating snow
starting early Sunday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Today
through Sunday)...

The overall pattern experienced over the past several days is
expected to continue under an upper level trough. This combined
with light northerly flow (though gusts will be stronger in
channeled areas) will constitute onshore showers and/or flurries
and cooler temperatures over the Aleutians through the weekend.

&&

.MARINE (Days 3 through 5)...The forecast still looks likely for a
low pressure system to make its way up into the Gulf of Alaska on
Saturday, then move northward and linger there through Monday. A
relatively large swath of gales is expected through the Gulf of
Alaska on Saturday and Saturday night. The higher winds will
become confined to the North Gulf Coast by Sunday, but will remain
at gale strength possibly as late as Sunday afternoon. Northerly
winds on the west side of this low will also bring gale force
winds to Cook Inlet Saturday and Sunday. In the Bering Sea and
Aleutians, northerly flow is expected to continue through Monday.
Overall, this will be less than gale force, except for Bristol
Bay and south of the Alaska Peninsula, where winds will reach gale
force mainly on Sunday.
&&

.LONG TERM FORECAST (Days 3 through 7)...
High pressure continues to hold strong in the Western Aleutians on
Sunday, meaning that most of the weather action will be happening
east and south of the Alaska Range through early next week. The
low that moves into the Gulf of Alaska on Saturday will bring a
front onshore on Sunday, then linger, bringing generally unsettled
weather for Southcentral and the Copper River Basin throughout
the long term period. This system will initially bring some
warmer air into the state, but by Monday, cooler air will
infiltrate from the west and temperatures closer to average or
slightly below will return. On Tuesday it starts to become less
clear as to how this pattern will break down across the region.
Some forecast solutions show a low pressure system developing
south of the Aleutians and then moving into the Gulf. Other
solutions show the high pressure system moving eastward and
bringing better weather to the mainland, while a storm develops in
the western Aleutians and tracks northward. Expect change in the
long term forecast over the next several days as things start to
shake out.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...LF

PRELIMINARY LOCAL STORM REPORT...SUMMARY...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
1213 PM AKDT FRI APR 19 2019

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

1150 AM     SNOW             UPPER HILLSIDE          61.10N 149.72W
04/19/2019  M6.0 INCH        ANCHORAGE          AK   PUBLIC

            6 INCHES OVERNIGHT. DANGEROUS ROAD
            CONDITIONS ON THE HILLSIDE AT HIGHER
            ELEVATIONS.

1127 AM     SNOW             BIG LAKE                61.51N 149.95W
04/19/2019  M1.5 INCH        MATANUSKA-SUSITNA  AK   PUBLIC

            REPORTED AT BIG LAKE, AK.

1120 AM     SNOW             LAZY MOUNTAIN           61.61N 149.06W
04/19/2019  M6.5 INCH        MATANUSKA-SUSITNA  AK   PUBLIC

            6.5 INCHES SINCE LAST NIGHT AT 879 FT IN
            LAZY MOUNTAIN COMMUNITY.

0915 AM     SNOW             2 MILES ESE POTTER HILL 61.08N 149.78W
04/19/2019  M4.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            4 HEAVY WET SHOW OVERNIGHT, STILL SHOWING.

0830 AM     SNOW             17 NNW HOPE             61.14N 149.77W
04/19/2019  E7.0 INCH        ANCHORAGE          AK   PUBLIC

0824 AM     SNOW             8 NW ANCHORAGE          61.27N 149.33W
04/19/2019  E5.0 INCH        ANCHORAGE          AK   PUBLIC

            STILL SNOWING.

0803 AM     SNOW             NEAR ARCTIC AND 76TH    61.15N 149.91W
04/19/2019  E2.7 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0751 AM     SNOW             18 S KNIK               61.19N 149.78W
04/19/2019  E3.3 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0730 AM     SNOW             NEAR MM. 61.5           61.80N 148.80W
04/19/2019  E2.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

            STILL SNOWING.

0711 AM     SNOW             EAST ANCHORAGE NEAR LOR 61.15N 149.81W
04/19/2019  E4.0 INCH        ANCHORAGE          AK   PUBLIC

0618 AM     SNOW             GIRDWOOD                60.94N 149.11W
04/19/2019  M10.0 INCH       ANCHORAGE          AK   NWS EMPLOYEE

            SNOW STARTED AFTER 10 PM ON THURSDAY. LIGHT
            TO MODERATE SNOW FALLING AT THE TIME OF THE
            OBSERVATION.

0612 AM     SNOW             O'MALLEY TRAILHEAD      61.12N 149.71W
04/19/2019  M5.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            5 IN ACCUMULATION SINCE 6AM THUR NIGHT.
            LIGHT S WIND WITH VISIBILITY LESS THAN 0.25
            MI.

0612 AM     SNOW             NATIONAL WEATHER SERVIC 61.16N 149.98W
04/19/2019  M2.5 INCH        ANCHORAGE          AK   OFFICIAL NWS OBS

            SNOW AMOUNT FROM 4PM THURSDAY THROUGH 4AM
            FRIDAY. SNOWING AT THE TIME OF THE
            OBSERVATION.

0530 AM     SNOW             KLATT RD & OLD SEWARD   61.12N 149.88W
04/19/2019  M2.5 INCH        ANCHORAGE          AK   PUBLIC

1224 PM     SNOW             2 SW CAMPBELL           61.16N 149.94W
04/18/2019  M2.0 INCH        ANCHORAGE          AK   PUBLIC

            2 IN OVER NIGHT WED THROUGH THUR AFTERNOON.
            TOTAL FOR SNOW EVEN WAS 5.25 IN FROM WED AM
            THROUGH THU AFTERNOON.

1204 PM     SNOW             2 SW CAMPBELL           61.16N 149.94W
04/19/2019  M5.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            TOTAL OF 5 SO FAR SINCE LAST NIGHT. WET
            SNOW. STILL LIGHTLY SNOWING.

&&