Small Craft Advisory 448, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 AM AKDT Wed Jun 6 2018

.ANALYSIS AND UPPER LEVELS...

Two weak upper-level disturbances are moving east to west across
the forecast region this morning. The first is transiting across
the northern AKpen and is producing showers in a line from
Iliamna to just south of Dillingham. The second upper-level
easterly wave is sliding through the lower Susitna Valley and is
triggering a line of showers from Anchorage southwest into the
Cook Inlet.

Farther west, a fair bit of mid-level cloud cover remains over the
lower Kuskokwim River Valley, the remnants of widely scattered
showers and thunderstorms (some severe) which initiated yesterday
afternoon from the interaction of an easterly wave and a
persistent thermal trough stretching from the Tanana Valley to
the Yukon Valley.

Low stratus and areas of rain continue to affect the Aleutians as
a surface trough remains draped over the region. Farther north,
high pressure is nosing into the northern Bering.

&&

.MODEL DISCUSSION...

Models continue to be in generally good agreement through the
short term. Tracking weak upper waves and timing and location of
convection continue to be the main forecast challenge. By day 3,
the models begin to diverge on the location of a developing low
south of the Aleutians, with the NAM and GFS farther east and
stronger with the low than the EC.

&&

.AVIATION...
PANC...MVFR conditions will continue through 18z today as low
clouds linger over the airport complex. VFR conditions are
expected from mid-morning through the evening. Showers over PANC
early this morning will come to an end by 12z today. Gusty
southeast winds will increase this evening.

&&

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

The overall pattern and therefore the weather is not changing
much the next few days. Flow aloft will remain from the northeast
and there will be a number of short waves that move across the
area in this northeast flow which will kick off some showers. A
short wave in the northeast flow aloft this morning is keeping
showers over a large part of southcentral Alaska.

The unstable airmass in the Susitna Valley and Copper River Basin
will make afternoon and evening thunderstorms a good possibility
in these areas both today and tomorrow. The northern parts of the
Susitna Valley and Copper River Basin are most likely to see
thunderstorm development.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

It was quite the interesting day across the area yesterday, as
multiple storms showed some hail signatures, with even a hook echo
having developed which was associated with a low-topped but
organized supercell. Heading through today, the thermal trough
will strengthen some with another wave aloft crossing the region
from east to west during the peak heating cycle. This wave does
look somewhat stronger than what we saw yesterday.

As for instability, soundings show SBCAPEs in the 500 to 1500
J/kg range, with lifted indices down to -7, mid-level lapse rates
around 7.6 deg. C/km, and freezing levels in the 6500 to 7500
feet range. All of these parameters are similar to yesterday, with
the difference being the 0-6km bulk shear, which drops to around 10
knots versus 20 on Tuesday. So what this means is storms should
reach similar coverage and strength as yesterday with a slightly
slower movement. Conversely, with the thermal trough strengthening
while pushing further inland, greater storm coverage should be seen
into the greater Bristol Bay region today, with locations north of a
Dillingham to Iliamna line having the best chance of seeing a
thunderstorm.

Additional disturbances will continue to affect the region
Thursday and Friday, with good shower and some thunderstorm
coverage.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days through 3)...

High pressure will continue to build southeast from eastern Siberia
through the Bering into the north Pacific. This will maintain
relative light light winds and dry conditions, with just a few
showers possible further east towards the AKPEN through Thursday
evening. Friday is where things get a little more tricky, as the GFS
model wants to keep a developing storm system south of 50N as it
strengthens, while the NAM model tracks it further north with
precipitation affecting portions of the Aleutians. Neither model has
really budged on this with other guidance offering a compromise
between the two. For now we stayed with persistence in the forecast
and will await future model runs.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The long term forecast begins on Friday night with an upper level
ridge moving into the Gulf of Alaska, bookended by two low
pressure systems, one in the Southeastern Gulf and the other just
south of the Central Aleutians. A weak to no-flow pattern will set
up over most of mainland Alaska leading to pleasant and warm
weather for the weekend. Rising 1000-500 mb thicknesses will
promote above average temperatures for most of the southern
mainland Saturday and Sunday, with Sunday looking to be the
warmest day of the weekend. A thermal trough will develop over the
interior promoting diurnally driven convection over the higher
terrain in the afternoon.

One kink to the forecast is the possibility of an
upper trough digging south across the state from the North Slope
over the weekend. Models are consistent in the fact that there
will be a trough sweeping across the northern half of the state
but are not in agreement as to how far south the trough will get.
The GFS has the trough splitting west as it reaches the AK Range
while the EC is more bullish and keeps the system fairly intact as
it crosses the mountain range. If the EC solution verifies,
widespread thunderstorms will likely develop over the Northern
Susitna and Copper River Valleys Saturday night, possibly
exasperating recent fires that have popped up in those areas.
Those with a keen interest in the fire weather conditions should
pay close attention to the forecast this weekend as the situation
develops.

Looking into next week, models are in agreement with an area of
high pressure building over the mainland, resembling a typical
pattern associated with very warm summertime temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...KVP