Small Craft Advisory 584, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
429 PM AKDT WED JUL 20 2016

.ANALYSIS AND UPPER LEVELS...
One final sunny, warm day concludes today in Southcentral as an
upper level ridge of high pressure is pushed south out of the
area. A pattern change is underway as a front is moving into
Southwest Alaska. Currently, Bethel and King Salmon radars show
a solid area of rain having pushed largely through Bethel and
Dillingham, and moving through King Salmon northward through
Aniak. This morning's 12Z Anchorage sounding shows a cooling of
the upper levels has already begun with freezing levels falling
below 12,000 feet. Temperatures are in the 70s once again over
much of Southcentral, while 50s and 60s are predominant across
Southwest Alaska where rain and cloud cover are in place.

A vertically stacked low over the far western Bering is producing
gusty winds over much of the western and central Bering with Small
Craft Advisories in effect for some of the offshore zones of the
Bering and along much of the Aleutians. This low will be one of
the catalysts for the pattern change on Friday. More details on
that below.

&&

.MODEL DISCUSSION...
All models are in fair synoptic agreement. They all handle the
front moving through Southwest AK with similar timing, but they
are struggling with some of the finer details. These details could
prove critical over the next few days. First, with the timing and
precipitation associated with this front, they all show something
a little different. As the front approaches, southeast winds
should increase through the usual gaps (Turnagain Arm, Copper
River Basin, Mat-Valley). But the timing of these winds varies by
an hour or two depending on which model you choose. The Hi-Res ARW
was preferred in handling this. The other interesting aspect with
the front is the precipitation. Some of the global models seem to
be painting too much QPF across much of the Cook Inlet. This is
likely due to their lack of resolution because with the southeast
flow, the terrain often times can "shadow" places on the leeward
side. The interesting aspect though is some possible weak instability
along the mountains. If the front was able to tap that, it could
make for more rain than otherwise might be expected. The NAM and
GEM Regional were preferred in handling the lower QPF amounts.

Lastly, the system that develops in the Gulf Fri remains a big
question mark. Models are trying to figure out the exact track
of the upper level low that moves in from the Bering. The subtle
differences in track could spell drastic differences in rain
amounts across Southcentral. If the low stays far enough north,
it could make for significant 1 inch + rain for many areas. If it
drops south as per the 12z NAM, most areas might actually stay
dry. This forecast package leans towards an EC/GFS combination,
which is a moderate middle ground. Needless to say, forecast
confidence with this system in low at this time.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A chance of precipitation tonight will be associated with the
decaying front as it passes across the region. This front will
also induce a wind shift as northwest flow shifts to the
southeast. This will occur by midnight for Turnagain Arm, which
means smoke from the McHugh Creek fire will begin to drift back
into the Anchorage Bowl, and continue to do so through Thu night.

As the front decays and stalls over the forecast area on Thu
light precipitation is expected over most of the area. This will
become more widespread Thu night as the next system approaches.
Significant rainfall is then expected on Fri with the phasing of
surface front and upper support.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Rain will continue to spread eastward across the Lower Kuskokwim
Valley and Bristol Bay through this evening as the front pushes
inland, with steady rain continuing along the Alaska Range through
Thursday morning. Cool and wet conditions will continue on Thursday
into Thursday night in the wake of the front as deep southwest
flow remains focused over the region and favorable upper level jet
energy moves over the southern mainland, although rain will likely
be lighter and more sporadic in nature. Friday will bring the
passage of an upper level trough and a transition to northwest
flow, keeping the cool and wet pattern going into the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Rather benign cloudy conditions and gusty west to southwest winds
will persist across much of the Bering through Thursday in broad
southwest flow ahead of a closed upper low gradually shifting
eastward from its current position north of Attu. An embedded
disturbance over the North Pacific will bring a brief shot of
potentially heavy rain to Central Aleutians late tonight, with
rain then spreading to the Eastern Aleutians and Alaska Peninsula
through Thursday evening as the feature quickly transits the
southeast Bering and a weak surface low develops south of the
Pribilofs. The upper low will then bring another round of rain to
the central and eastern Bering on Friday as it opens up into a
trough and dives southward towards the Gulf of Alaska. This will
yield drier conditions and partial clearing by Friday evening as
northwest flow develops in the wake of the trough.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Friday evening, a rather dynamic low moving from the
Eastern Bering across the Alaska Peninsula will quickly move to
the Northern Gulf coast during the overnight. Rain will have
already overspread much of Southcentral with showers over
Southwest Alaska. This low will deepen Saturday morning then
move eastward, bringing rain to an end from west to east across
Southcentral, although showers will likely form in the afternoon
across the mountains. Another shortwave trough will dig into
Southwest Alaska during the afternoon and evening, quickly
spreading more rain into southern Alaska Sunday and Monday.
Thereafter, all the global models are in good agreement that a
large trough sets up over the Eastern Bering Sea and Southwest
Alaska by Tuesday, keeping cooler and wetter weather in place
through the end of next week.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...MO/JW
SOUTHCENTRAL ALASKA...DS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...JA