Small Craft Advisory 822, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 PM AKDT Tue Oct 23 2018

.ANALYSIS AND UPPER LEVELS...
A vertically stacked, fast moving and maturing low in the
northern Gulf of Alaska is pushing inland this afternoon. Colder
air is wrapping around this low, similar to the last several
storms. The jet stream still sits on the southern edge of the Gulf
with an arm running up along the panhandle which is migrating
northeastward. Embedded in the flow of the low are several
shortwaves which are moving along the North Gulf Coast. Increased
vorticity is present around the closed low center and in the
periphery of the system. The northwesterly movement of a front
ahead of the low is increasing instability and creating heaving
banded precipitation throughout the area. While most of
Southcentral is experiencing widespread rainfall, the Copper River
Basin saw snow this morning. Southerly flow continues to dominate
which is increasing winds through the typical gaps throughout
Southcentral.

Meanwhile, low pressure persists in the eastern Gulf as a
secondary low, embedded in the flow around the Gulf low shifts
southeastward toward the central and eastern Aleutians. The
associated front is bringing rain and higher gusts to the central
Bering Sea.

&&

.MODEL DISCUSSION...
The models still struggle some with the next low to enter the
northern Gulf on Thursday. They all place the low center generally
in the northern Gulf by Thursday afternoon, but differences in
position have a significant impact on the precipitation pattern
and wind fields. The good news is that the central pressures are
about the same. For now increasing winds in general over the Gulf
marines from what was previously in the grids.

&&

.AVIATION...
PANC...Could see some occasional MVFR conditions this evening with
rain and showers. Winds should shift from gusty northerly to
Southerly early this evening and remain relatively strong and
gusty into the early morning Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Portions of Southcentral will continue to see strong winds this
evening and tonight. The surface low is currently over the Kenai
Peninsula with an approximate pressure of 963mb. There is a tight
pressure gradient from Cordova to Whittier to Anchorage. This low
will head north and jump over the Cook Inlet tonight and the
gradient will slacken a bit but remain enhanced for tomorrow too.
The radars at Kenai and Middleton Island remain active. Still
expecting rain in the low elevations this evening and
tomorrow...with the snow level as low at 3,000 feet. Thursday will
be drier for Anchorage but showers will linger over Prince William
Sound and the Kenai Peninsula.

&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Showery activity and mostly cloudy skies continue to be the
pattern for southwest Alaska, due to a broad upper level trough
over southwest Alaska that is allowing surface low pressure
systems to rotate into the region. Temperatures in the lower
Kuskokwim Valley have stayed cold today, so cooled off forecasted
temperatures in that area. A rain/snow mix was also added in
valley areas for this evening and tomorrow morning, with snow
overnight, to reflect these cooler temperatures. Some clearing
will again occur in the lower Kuskokwim Valley as winds shift to
north the next couple of days, which will keep minimum temperatures
just below freezing.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The low pressure system moving south through the eastern Bering
Sea is progressing as expected and continuing to produce rain
showers as it goes. The low will push south of the Aleutian
Islands tonight, with strong northerly flow behind it. This
northerly flow will bring a colder airmass over the central and
western Bering. However, due to warm sea surface temperatures, a
warm marine layer will keep temperatures up and mitigate the
intrusion of cold air. Winds are still expected to elevate, with
gale force winds in the central and western Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...(Friday through Tuesday)
The forecast remains complicated as model continuity continues to
be in poor agreement starting this weekend. The main issues are
related to how quickly the models break down the persistent Bering
trough, as a series of compact and strong Arctic vort maxes tries
to make inroads further south. Yesterday, the general consensus
was that this would occur towards the end of the forecast period
as they were essentially trekking from the North Slope towards the
Yukon border, allowing the trough to deamplify some as it
repositions eastward.

However, today's runs show these disturbances taking a more
meridional approach as they dive south, basically emerging from
eastern Siberia and diving straight into the Bering trough. These
disturbances will allow the trough to persist longer by re-
energize it, but also add uncertainty the resulting 500mb height
fields. The same is true (albeit with slightly less uncertainty
than aloft) with the low-level pressure fields in relation to
surface low placement and timing (as one would expect). Finally,
with the majority of these systems remaining over water where the
can't be sampled by our land-based radiosonde network, which is a
large part of the model initializations, it's no surprise their
forecasts continue to go awry.

One should also keep in mind that typically the models have a
fast bias in pattern change events, so it wouldn't be surprising
if this wet pattern of multiple systems lifting north into the
Gulf continued into at least the middle of next week. Stay tuned!

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 119 120 125 131 170 172 173 174 175 176 177 178 185.

&&

SYNOPSIS AND MODEL DISCUSSION...BC/RA
SOUTHCENTRAL ALASKA...PS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...PD