Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 211250
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKDT Tue May 21 2019

.ANALYSIS AND UPPER LEVELS...

The upper levels primarily consist of an upper level low over the
eastern Bering, a secondary, weaker, smaller low over the Gulf,
and strong ridges over both the far western Bering and over
western North America. In this pattern, much of Southcentral
remains under upper level southeasterly flow, which is
transporting moisture northward from off the Gulf. Much of
southcentral is between systems, which is allowing for some areas
with relatively clear skies. The low southeast of Kodiak is
largely a swirl of low clouds and is not producing much in the way
of precipitation.

&&

.MODEL DISCUSSION...

The models are in generally good agreement through Wednesday, with
some disagreement creeping in Wednesday night regarding the track
of a low moving northward out of the Pacific into the Alaska
Peninsula and Shelikof Strait. The models have been in general
agreement about the track of the low in recent days, but today
have all trended much further west with the track of the low. The
current track takes the low a bit west of due north, moving it
near Chirikof Island Wednesday evening, then predictably
struggling where the center of the low emerges over Southwest
Alaska by Thursday morning. The only difference that change in
track would make would be the timing of the start and end of
associated winds and precipitation. Other than that, some patchy
fog development is possible along some of the coasts, which will
be a forecast challenge. Instability will be diminishing over the
coming days with the increasing cloud cover associated with the
low, so thunderstorm threats will be diminishing. Over the next
couple days sea breeze strength and timing are also small features
that frequently pose challenges for the forecast. Overall though,
forecast confidence is high.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. The sea breeze
will change the direction of those winds to onshore for much of
the day. The Turnagain Arm wind may approach the airport tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Wednesday night)...

A remnant weak upper trough axis will produce a few clouds and
possibly isolated showers today, especially this morning over the
western Kenai and Cook Inlet. Otherwise, expect a fair day with
considerable sunshine for much of the area and warmer temperatures.
The Copper River Basin will start out nice, but clouds will be on
the increase this afternoon ahead of some upper waves moving in
from Southeast Alaska. The origination of these is from convection
over British Columbia yesterday. Thus, expect warmer air and
increased instability move into the eastern Copper River Basin.
Models have been struggling with figuring out the track of the
waves as well as extent of precipitation. Based on the latest
data, expect numerous showers near McCarthy by late this
afternoon, spreading northwest across the Wrangell Mountains and
along the Alaska Range this evening. Clouds will spread westward
across the basin, with a few showers possible in the Glennallen
area this evening. This will then all exit to the Interior Alaska
tonight.

The next big weather maker will be a strong spring-time storm
system moving up from the Pacific. Expect strong winds and
widespread moderate to heavy rain to spread northward across the
western Gulf and Kodiak Island Wednesday morning with the leading
front. Ridging out ahead of this system will lead to another
fairly nice day across Southcentral on Wednesday. The front will
then lift to the north Gulf coast by Wednesday night, weakening
as it does so. There is some uncertainty in the track of the low
center, but it appears it will dissipate in the vicinity of
Bristol bay and the Alaska Peninsula. Thus, the bulk (if not all)
of the rain over Southcentral will remain along the coastal zone,
with inland areas remaining dry. There will be some gusty winds on
Wednesday as the surface ridge builds along the north Gulf coast
ahead of the front. Expect the strongest winds through the typical
gaps, including Turnagain Arm into Anchorage, the Knik River
Valley into Palmer, and along the Copper River.

&&

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

Low pressure over the eastern Bering pushes a front along the
Southwest coast today...with the highest chances for rain along
the Kuskokwim Delta. The next storm system quickly organizes
tonight with the upper level jet nudging the low center to near
Kodiak Island Wednesday afternoon. Light southerly winds today
shift northeast tonight with gusty conditions across Bristol Bay.
On Wednesday, strong easterly winds develop across Bristol Bay as
the low center nears the Alaska Peninsula. With strong low level
flow out of the east, downsloping across mountains will greatly
limit accumulations on the lee side of the ranges across the
Southwest. This storm weakens considerably on Thursday as the low
tracks over Bristol Bay. Southerly flow returns across the region
as the low fills and lifts north of the Kuskokwim Delta.

&&

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

Low pressure remains dominant across the eastern Bering as low
pressure currently over the eastern waters is followed by another
system tonight through Thursday. This next system will be a gusty
gale force storm as the upper level jet noses across the Alaska
Peninsula tonight. Strong northerly flow across the western Bering
tapers down later this afternoon...however gusty conditions
persist through Thursday as an upper ridge amplifies across the
western waters tonight through Thursday. Another front develops
across the North Pacific on Thursday and pushes toward the
central and eastern Aleutians during the afternoon.

&&

.MARINE (Days 3 through 5)...

A weakening low will drift north towards Bristol Bay, leaving
mostly benign conditions and broad cyclonic flow over the Bering
and Aleutians Thursday through Saturday. Winds will also diminish
in the Gulf during the day Thursday with a high building in the
North Pacific. That said, expect combined seas around 15 feet in
the central Gulf in swell originating from the weakening low. Seas
will subside late Thursday into Friday. South to southeast flow
will prevail in the Gulf Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The long term commences with a compact, but weakening low in the
western Gulf shifting northward towards Bristol Bay. The leading
front will bring precipitation to Southwest early Thursday. As it
heads northeastward, precipitation will spread to the northern Gulf
Coast, but likely not make it far inland. The front will be
relatively fast moving, and expect downsloping to limit
precipitation chances in the Anchorage Bowl.

The next low will approach the central Aleutians late Thursday into
Friday. There is some uncertainty with its strength and track, but
the general consensus is that there will be a wetter trend for
Southwest Friday into Saturday as the associated front passes over.
Deep southerly fetch ahead of the low will support precipitation in
southern mainland Alaska over the weekend. It`s a bit soon to say
with confidence where the heaviest precipitation will be, but we are
leaning towards the focus being in Southcentral. While the
operational GFS projects a much deeper low heading into Bristol Bay,
thus keeping more of the energy farther west, the ECMWF and FV3
continue to maintain a weaker low. This is diminishing confidence
in the GFS solution. So, the current expectation is for
persistent wet weather over Southcentral Saturday and Sunday.
Precipitation will taper off Sunday night into Monday and another
low will develop over the western Aleutians, potentially bringing
unsettled weather to the southern mainland by mid next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 130-138, 150, 155, 178, 351, 352
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...MM


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