Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 180012

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
412 PM AKDT Mon Jul 17 2017

Two key features are evident on the charts today: a shortwave
passing over Southcentral and a much more robust shortwave and
circulation moving over the Bering Sea.

The first feature was responsible for a low deck, drizzle and
showers over Southcentral as it drove atop a weak surface ridge. The
second feature was moving a cold front across the Bering which
had not yet reached the Pribilofs. The tail end of the front stretched
south to just west of 170W.

The fetch of moisture wrapping into the low was widespread over
the Bering with the coldest cloud tops moving between St. Paul and
Amchitka. This region was also producing cloud-to-ground/sea
lightning strikes overnight which tapered off around 17z.
Satellite-derived moisture measurements indicated this area had
200%+ the normal level of precipitable water.


The bowling ball of a vorticity maximum combined with healthy
divergence aloft will make this weather system the driver for the
week to come for Southwest and Southcentral Alaska.

Models were in good agreement this morning and continued to dance
in step with each other until 21z Tuesday when we start to see
timing issues along the triple point near Kodiak Island and
further placement of the cold front over the Gulf of Alaska.

Challenges from this entire system will be timing the onset of
steadier rainfall over Southcentral and determining if there is
enough wind for any rain shadowing effects in the lee of the
Kenai and Chugach Mountains. The NAM was used as the main model of
choice for Southcentral.

In Southwest, timing of the front, associated winds and visibility
will be the overall challenge for the remainder of the week. The
NAM was used as the main model of choice here.


PANC...VFR conditions and light southerly winds will prevail until
the rain arrives early Tuesday. After midday, MVFR will be more
likely with occasional gusty winds.


The transition to a more active weather pattern is underway
tonight, as a front begins to push into the western Gulf and Cook
Inlet region overnight into Tuesday morning. This will usher in
rain from west to east over much of the area on Tuesday and
persist through Wednesday night. The only area that will escape
most of the precipitation will be the Copper River Basin which
only has a chance of rain. Winds along the north Gulf coast will
also increase to Small Craft Advisory levels with this front.
Winds look to remain under Gale force but it is not out of the
question for a barrier jet to develop along the southern Kenai
Peninsula and produce some areas of Gale force winds on Wednesday.
Gap winds through Turnagain arm are also expected to ramp up
Tuesday morning with the arrival of the front, and persist
through early Thursday morning.


The Southwest Alaska region continues with abundant moisture with
onshore flow as a weather front tracking west to east across the
eastern Bering before pushing through the AOR tonight. This
synoptic feature is producing widespread rain/stratus accompanied
by gusty southerly winds, and will exit the region by Tuesday
morning. Look for the Kuskokwim Delta to receive another swath of
moisture with rain/stratus by Tuesday morning through Wednesday
morning time-frame. As a weak low pressure system drops down from
the Bering Strait by Tuesday morning near Nunivak Island before
exiting into the Bristol Bay area by early Wednesday morning.


There is a 995 mb low tracking into the northern Bering with an
associated cold front draped through the eastern Bering extending
through the Eastern Aleutians this evening. This system is
tracking eastward, and the frontal boundary tracks inland over the
Southwest Alaska region this evening. The rest of the Bering, and
the Aleutians will have southwesterly wind on the backside of the
aforementioned synoptic system. This ushers in a showery weather
regime. By Tuesday a ridge moves into the western Bering, and
gradually builds into the northern Bering by Wednesday evening.
Look for marine layer stratus with fog to develop under this high
pressure ridge. Meanwhile, a low drop down from the northern
Bering by Tuesday morning, and tracks along the Southwest Alaska
coastline before crossing the Alaska Peninsula by Wednesday


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

The models start out the long term in fairly good agreement, with
a surface low south of Kodiak and it`s attendant upper level low
gradually moving south away from mainland Alaska. Thus, across the
mainland, typical summer ridging indicative of a developing
thermal trough develops. This will result in offshore flow across
Southcentral. Depending on how strong a northerly wave moving
eastward across northern Alaska gets, will determine thunderstorm
coverage. Most likely all it`s energy misses the area to the
north, so any storms will likely be the typical mountain storms
that remain nearly stationary. Weak ridging persists away from the
coast, which should help make for a period of tranquil weather due
to a slowly changing pattern that keeps the active weather away
from the area right through the weekend. Temperature-wise, most
of the warmest air looks to remain over the Bering, so seasonable
conditions are likely with gradually increasing threats for
mountain storms expected through the weekend.





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