Small Craft Advisory 606, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 AM AKDT Tue Aug 7 2018

.ANALYSIS AND UPPER LEVELS...
An upper level longwave trough is in place over the state. This
trough covers a wide swath of area and extends across the Bering.
South of the Aleutians, a strong ridge of high pressure continues
to retreat southward in response to the impinging southbound
trough. Upper level flow across the Bering remains zonal, with an
upper level 140 knot jet still advecting moisture onshore across
the southern half of the state.

At the surface, radar imagery indicates rain moving in across
Southwest and the northern AKPEN. Some isolated showers are also
observed moving across the Mat-Su Valleys and along the coastal
areas of Prince William Sound this morning. Satellite imagery
shows plenty of low level stratus over the Bering with surface
observations confirming fog along the Aleutians, AKPEN and coastal
areas of Southwest Alaska.

&&

.MODEL DISCUSSION...
Models remain in good agreement through Thursday morning.
Disagreement begins to arise by Friday, as solutions are showing
discrepancies with regard to the location of an Arctic low
approaching the Seward Peninsula. The NAM and GFS have a more
eastward track while the EC keeps the low center over Russia at
this time. Models also disagree on the timing and location of an
inbound North Pacific surface low over the southern Bering through
the early weekend.

&&

.AVIATION...
Winds will become light early this morning and then remain light
through the TAF period. The ceiling will drop this morning as rain
moves in before rising to VFR conditions this afternoon. However,
there remains some question with the timing of the rising ceiling
and there is a chance it could occur earlier than predicted.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Thursday)...
The upper level trough centered over the Southwest Mainland will
deepen slightly as it progresses towards the Gulf this morning.
This will allow for showers to develop from the Susitna Valley
into the Cook Inlet region, while widespread rain (which will be
heavy at times) form from the North Gulf Coast into the Copper
River Basin. A compact and intense shortwave that developed over
northern Bristol Bay overnight will continue heading eastward into
the Susitna Valley this morning, which may produce a few morning
lightning strikes as has already been observed just to the west of
Mt Susitna. Further to the east, deep and moist southwesterly
flow that will continue to tap into the tropical Pacific Moisture
that has now entered the Gulf. This will bring the heaviest
rainfall to S-SW facing slopes such as the Prince William Sound
the southern Talkeetnas, and the Alaska Range including the
Denali Highway area where an additional 1.5-2" of rain is expected
today. This heavy rain over the same area than 1-2" of rain that fell
on Sunday will produce continued river and stream rises possibly
into moderate flood stage to the Susitna River and Maclaren River
drainages, as well as waters draining from the Talkeetna Mountains
including Hatcher Pass.

The incoming upper level trough will move very slowly through the
South Mainland, bringing continued rain from the Talkeetna
Mountains eastward into Wednesday. This will allow for cloud
cover and gradual cooling for the rest of Southcentral as cold air
associated with the upper level trough move in it's wake. Skies
should then slowly begin to clear later in the day Thursday as a
ridge of high pressure begins to move in. This ridge will provide
the best chance of a break in the weather for most of the area for
the end of the work week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Today through Thursday)...
A broad upper level trough over Alaska continues to deepen as a
series of shortwave troughs move through the general upper flow
into the Gulf of Alaska. The main shortwave to note is now
tracking across Kodiak Island after crossing the Alaska Peninsula,
so precipitation is becoming less organized and more showery this
morning. The low level flow is onshore, so there will be plenty
of low stratus and some fog moving into Southwest through
Wednesday. Precipitation will slowly diminish with most of
Southwest drying out (precipitation wise) Wednesday night as weak
upper ridging tries to build in. However, expect cooler
temperatures to prevail given the stable northwest flow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Broad upper troughing extends through most of the Bering Sea this
morning with the remnants of a once stout warm front tracking
over the Eastern Aleutians this morning. Northwest flow behind the
front will keep precipitation along the Eastern Aleutians through
Wednesday. The next low of consequence will bring 15-30 knot
winds to the Western Aleutians Tue night with the low making slow
eastward progress and eventually reaching Unalaska by early
Thursday.

&&

.LONG TERM FORECAST (Thursday through Monday)...
Thursday will continue the stretch of relatively dry weather for
much of Southern Alaska as weak upper level ridging turns the
upper level flow northwesterly, which will allow for some drying
of the atmosphere. The one exception will be through the Copper
River Basin and Prince William Sound, where lingering showers from
the rain events earlier in the week will continue through the day
with instability highest in that area. Everywhere else should be
dry throughout the day, though some afternoon showers over the
Kuskokwim Mountains can't be ruled out. Meanwhile, a relatively
weak area of low pressure moves into the southern Bering and
Aleutians Thursday, and tracks northeastward into the central
Bering Thursday evening.

Then on Friday morning as most of southern Alaska continues drying
out, including points east into the Copper River Basin, the
aforementioned Bering low will move into the coast of Southwest
Alaska, starting up the next round of rain there. The rain will
move inland through the day, such that all of Southwest Alaska is
into the rain by Friday evening. Further east, the rain will start
in Kodiak midday Friday, then move into the Kenai Peninsula by
Friday evening. The models differ somewhat on whether or not any
rain gets to Anchorage and points north and east much before
sunset, but regardless, the models continue to agree the vast
majority of the day will be dry. They also agree on the rain
moving into much of the rest of Southcentral during the overnight
hours Friday night.

Showers with perhaps more steady rain currently looks to linger
through much of the day Saturday, though the models disagree more
and more with time as to the intensity. They generally agree on
the bulk of the rain moving east into the Copper Basin by Saturday
night. Meanwhile out west over the Bering, the models are
expectedly having a very difficult time handling the strength and
path of the extratropical remnants of Typhoon Shanshan, as it
makes its way into the western Bering on Saturday. The GFS is much
stronger, slower, and further west than the EC. The GFS track
would confine any associated precipitation to Southwest Alaska
overnight Sunday night into Monday morning, with a pair of
generally dry days for Southcentral. Meanwhile the EC track brings
associated rain into Southwest Alaska Saturday night, into
Southcentral Sunday morning, and the rain continues with varying
intensities into Monday. Thus, uncertainty beyond Saturday is very
high. What is agreed upon though regarding the storm is while out
over the Bering, even the weaker EC brings storm force winds
around the storm to locales along the track, so this storm is
certainly one to watch over the upcoming week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 141 145.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...KO/DK
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...JW