Small Craft Advisory 171, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKST Fri Feb 23 2018

.ANALYSIS AND UPPER LEVELS...
Much of Alaska is under the influence of a moderate upper-level
jet for the first time really this winter. This is keeping the
southern half of the state in an active wave pattern with 110 to
130 kt aloft moving things along. First off, snow around
Southcentral Thu did not disappoint with just about everyone
seeing a couple to several inches (see the local storm report
summary for details). This will be followed by a transient ridge
of high pressure today. The building heights and associated
pressure rises are making for gusty conditions in the west and
north oriented gaps (Seward, Whittier, Barren Islands). This high
will then quickly yield to another modest area of low pressure
late tonight, set to bring another round of snow to much of the
southern half of the state. Then yet another snow maker will move
in by Mon (see the Long Term Discussion for more details).

Over the Bering, yet another gale-force low is pushing a decaying
warm front from west to east. This warm front is bringing plenty
of rain and gusty southeast winds along the Eastern Aleutians and
the Alaska Peninsula. This front will be responsible for bringing
another round of snow from the Western Capes all the way through
the Copper River Basin by Sat.

&&

.MODEL DISCUSSION...
Models remain in very good agreement in the short term. This is
lending to moderate to high forecast confidence for the synoptic
pattern. The areas of uncertainty with this package are the
potential for fog development and precise snow amounts with the
system tonight into Sat. First, microphysics satellite imagery
reveals fog and low stratus lingering around many areas, to
include Cook Inlet. Until the ridge axis passes overhead and we
start to feel the influence of the next front, inland locations
will remain vulnerable to fog/stratus. However, it does seem to be
waning this morning. For tonight's snow, there is a little
uncertainty on exactly how far north in the Gulf the surface low
tracks. Further north would likely lead to slightly higher
(another inch) snow amounts for much of Southcentral. The exact
timing of when the snow ends is more uncertain than Thu's system.
Another weak low will remain upstream, so this could help to keep
light snow falling through Sat evening.

&&

.AVIATION...
PANC...Satellite shows fog and stratus breaking up across the
area as the offshore gradient ushers in some drier air. However,
there remains the potential for it to move in briefly at any time
this morning. It should not be long-lived as the effects of the
next front should help us to go VFR this afternoon. Then there is
fairly high confidence in the timing of the next front bringing
snow and at least IFR conditions back to the terminal late this
evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
After the widespread snow that pushed through Southcentral
yesterday, the primary forecast challenge this morning will be
with determining how long the fog/stratus that developed
throughout Northern Cook Inlet yesterday evening will continue
today. Nighttime microphysics passes overnight indicated that
this area is beginning to break up, so the worst of the fog has
likely already occurred, however with high pressure moving in
from the west there may still be some patchy areas persisting
through this morning. Otherwise gap winds along the North Gulf
Coast, which have been strongest in the Barren Islands and Passage
Canal areas will begin to diminish this afternoon as a ridge of
high pressure slides into the Gulf. This high pressure will
however quickly move through the Gulf as yet another low will
track into the Western Gulf by this evening.

This second low will be the focus for another round of widespread
snow throughout the Gulf Coast and Southcentral beginning late
tonight and continuing into Saturday. The models are starting to
come into agreement with a low track just to the south of
Middleton Island which will help keep much of the moisture from
reaching the Gulf Coast. As a result there is good confidence that
this system will generally bring less moisture to most areas than
the snow that just occurred on Thursday, however snow ratios are
expected to be higher with cooler air associated with it. As a
result in the increasing confidence in a low track slightly
further to the south than initially expected, accumulations were
also dropped slightly for the event with the morning package. Gap
winds will then develop again behind the Gulf low on Saturday,
however with the westerly jet setting up well to the south these
are generally expected to be lighter than today's with the
strongest winds focused on westerly gap wind areas (such as
Passage Canal and the Barren Islands).

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The next system is rapidly approaching from the west this morning
as high pressure retreats to the east. Cloud cover will increase
west to east as an elongated front approaches the coast, making
landfall a little before noon. Temperatures will warm up as the
front arrives, but the majority of the area should remain cold
enough to stay snow with 2-6 inches accumulating by late Friday
night. Along the Kuskokwim Delta coast, gusty east winds will
accompany the period of heaviest snowfall this afternoon and
evening leading to blowing snow and periods of decreased
visibility below one-half mile, thus the Winter Weather Advisory
remains in effect, though the timing was bumped up 3 hours as the
system is arriving a little sooner than originally expected.

By Friday night, winds behind the front will switch around to the
west and begin to increase as cold air advects in from the north.
Gusts are expected to be as high as 40-50 mph along and near the
western coast line from Bristol Bay up to the Kuskokwim Delta. These
gusty west winds will continue through much of the weekend and
allow some rough surf to develop along the coastlines Friday night
through Saturday night.

&&

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

A surface low is currently tracking north across the Western
Bering Sea with an associated front sweeping across the Central
Bering and Aleutians traipsing towards the Western Alaskan coast.
Southerly gale force winds with the front will switch to westerly
today as the front passes and the low center stalls out over the
Northern Bering Sea, which will allow long-fetch westerly gales to
set up over almost the entire area through the first half of the
weekend. This will allow fairly significant wave run-ups over the
eastern 2/3 of the Bering, which will combine with widespread snow
shower activity to impact most locations from the Pribilofs to
the eastern 1/2 of the Aleutians with unsettled weather. The
progressive pattern continues Saturday afternoon as the next low
rapidly moves in from the SW towards the Western Aleutians
bringing another round of strong gale force winds with rain and
snow.

&&

.LONG TERM FORECAST (Sunday through Thursday)... The active and
progressive pattern will continue Sunday through Thursday. This
means we will see another snow producing system departing the
southern mainland Sunday morning followed by weak ridging to see
yet another upper trough and the resultant surface feature move
through the southern mainland again late Sunday and Monday. By
Thursday a large low with a front from the northern Bering draping
through the western mainland will likely bring strong warm
advection into the mainland Thursday. However, Southcentral
Alaska will remain on the cooler side keeping precipitation frozen
inland.

Over the Bering Sea and Aleutians we have a large strong low
north of Amchitka Sunday with the front arcing to St Matthew
Island then to the Pribilofs and crossing the Aleutians at the
triple point near False Pass. High end Gales will be in advance of
the front and storm force wind is likely in the southern quadrant
of the low along the Central Aleutians Sunday. The storm barrels
into the mainland Monday followed by weak high pressure across the
Bering Sea. Yet another storm then moves into the picture again
Wednesday likely bringing gale to storm force wind the the
central and western Aleutians along with ample warm advection.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155.
MARINE...Heavy Freezing Spray Warning 185.
Gale Warning 119 120
125 130 131 132 138 139 141 150 155 165 170 172 173 175 177 178
179 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...SA

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>