Small Craft Advisory 519, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
906 PM AKDT Sat Jul 8 2017

Updated Long Term Discussion

.ANALYSIS AND UPPER LEVELS...
The upper levels across the state are being dominated by three
primary features. Out West, a strong upper level low is moving
Eastward along the Aleutian Chain. A nearly stationary secondary
low is churning in the Gulf of Alaska. This low has flung a few
lobes of shortwave energy that have been the spawn of precip over
the last few days. The strongest shortwave is moving across the
Alaskan and Canadian border, moving West (as we aptly call these
systems 'Easterlies') and is enhancing upward motion. The last
feature of note aloft is a ridge of high pressure that spans much
of the central interior and stretches southeast through much of
western Canada. Features at the surface are not all that different
than aloft as weakening low pressure is moving near Adak. A
weather front extends from this low to the east, near Dutch and
then south through the pacific. A weak low is evident under the
Gulf low as high pressure is being pinched off along the Gulf
coast. As the upper level easterly is tracking towards the area,
blossoming radar returns are showing across much of southcentral.
As of 2030z, rain has overspread Palmer/Wasilla and is encroaching
on Anchorage.

&&

.MODEL DISCUSSION...
Models are, and have been, in good agreement with the evolution of
this potent easterly wave. Probably the biggest disagreement they
have is on the longevity of the rain through tomorrow and into the
morning hour. Some high resolution guidance is showing a relative
sharp cutoff to the precipitation, but thoughts are that as the
models are ending rain - the upper level support has not passed.
Beyond this system - model agreement is pretty good through much
of the week, but falls apart into the weekend. We note almost
daily changes in whether south central will see ridging into next
weekend, or some weather system moving across. With no strong
longwave pattern, it makes long term forecasts lower confidence as
we head into the next week.

&&

.AVIATION...
PANC...MVFR conditions to remain as rain overspreads the terminal.
There is a chance for cigs to drop to IFR in the rain this
afternoon in the heaviest of showers. Not expecting vis to drop
below MVFR. Conditions to improve tomorrow morning as rain exits.

&&

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

The next "car" in the train of easterly waves is upon us here in
Southcentral this afternoon. A number of ingredients are in place
for Southcentral to pick up its highest rainfall totals of 2017 so
far through this evening:
1) A very moist air mass that already in place in the pre-event
atmosphere 2) An amplifying inverted trough moving westward across
the region 3) Upper level inverted ridging west of the Alaska
Range further increasing the jet stream winds 4) A branch of the
jet over western Alaska splitting from the main jet, causing
increased divergence/diffluence over the area, further adding lift
to the atmosphere 5) Strong veering wind profile on the 00Z
Anchorage sounding, which is indicative of warm air advection
favorable for lift.

The only negative influence for high rainfall totals is the fast
moving nature of the easterly wave, so all of the above
ingredients will only be in place for a 12-18 hour period. Thus, a
rapid end of the heavy precipitation from east to west will occur
overnight, though lingering showers behind the wave may add
modestly to rainfall totals into Sunday morning.

The best forcing and lift will be north of Anchorage, across the
Copper River Basin and Susitna Valley. Expected rainfall totals
will range from nearly 2 inches through the Talkeetna Mountains,
1-1.5 inches in the Mat-Su Valleys and the Alaska Range, 0.75 to
1 inch through the Anchorage Bowl, around a half inch through
Kenai, Soldotna, and Seward, and the lowest rainfall totals in the
southern Kenai near Homer, with around 0.2 inches expected.

Clouds will decrease through the day Sunday from east to west,
with many areas of Southcentral ending the day with mixed sun and
clouds. The exception will be over the eastern Copper River Basin,
as the next wave already encroaches into the area. Unlike the
current wave though, the lion's share of the energy associated
with the wave will pass north of the area, so the northern Copper
River Basin west through the far northern Susitna Valley will be
the areas most likely to pick up additional rain through Monday.

At the same time Sunday afternoon and night, a front will move
into Kodiak and stall there, making for an extended period of time
of rain, moisture, and easterly winds across the island. In
between the two systems, a squeeze play may lead to pleasant
weather from Anchorage south with a fair amount of sunshine on
Monday. Clouds increase from north to south ahead of a third wave
Monday evening, though that next wave looks to favor areas east of
Anchorage primarily.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level wave will move into the Kuskokwim Valley from the
east overnight and continue slowly westward through the day
Sunday. This wave is being forced into the ridge along the Bering
Sea coast which will force it a little farther northward but it
should still make it into the northern Kuskokwim Delta by Sunday
afternoon. It remains questionable as to whether this rain wil
make it as far south as Bethel and areas south of there will see
some sunny periods under the ridge so there will be a large
difference in weather over a small distance tomorrow with these
features.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Wet and foggy. The low in the south-central Bering Sea will
continue to move slowly eastward as it is hindered by the weak ridge
over the southwest mainland. This will keep the front stalled over
the Alaska Peninsula tonight before the low scoots farther to the
south and under the ridge Monday. When this happens it will allow
a stronger ridge to build over the central Bering Sea Monday
night. There is a bit of warm air aloft with this ridge which will
keep widespread fog over the Bering Sea and Aleutians for the
next few days.

&&

.LONG TERM FORECAST (Days 3 through 7...Tue through Sun)...
Confidence is slowly increasing that the middle to end of next
week could feature some fairly potent ridging across much of the
southern half of AK. To start the period, the remnants of yet
another stacked low in the eastern Gulf of AK will be moving off
to the east into SE AK. Meanwhile, a ridge will begin to build up
from the North Pacific over Western AK while yet another cold
trough digs over the Western Bering. Tue could be another day of
afternoon-evening showers across SW AK and some rain stretching
from the Copper River Basin into the Cook Inlet as the last bit
energy wraps around the Gulf Low. But then it does look
increasingly likely that dry conditions will set-up for the back
half of the week over much of the southern mainland courtesy of
the ridge. There are some indications that this could be one of
the warmest airmasses of the year so far as well. There is still
plenty of uncertainty regarding temperatures though, especially
since they are highly dependent on how much cloud cover is in
overhead. However, for places over the Eastern Bering and the West
Coast, the ridge will likely bring widespread stratus and some fog
as per the climatological normals for July. Over the Central and
Western Bering, the cold trough will keep rain and sub-small craft
southerly winds in the forecast.

The biggest question will be how long the longwave pattern
features a dominant ridge. The GFS shows energy impinging upon it
from the west and breaking it down by next weekend. The EC shows
it holding firm a few days longer.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...EZ
LONG TERM...MSO