Small Craft Advisory 774, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
404 AM AKDT Tue Oct 9 2018

.ANALYSIS AND UPPER LEVELS...

In the upper levels, a broad low centered over the western Bering
has extended a long wave trough east across Southwest Alaska and
into the western Gulf. A second upper level trough rotating
around the south side of the low is currently swinging through the
central and eastern Aleutians, while a deeper third trough
further to the west but well south of the Aleutians tracks east.
An upper level ridge extending from the eastern Gulf through
southcentral Alaska and then tilting off to the northwest is being
overrun by the frontal system rotating out of Southwest Alaska.
In the Gulf, the front currently stretching from the Barren
Islands off to the southeast into the Central Gulf is lifting
north towards the Kenai Peninsula with a broad cloud shield
extended out ahead of it.

&&

.MODEL DISCUSSION...

Models continue to be in good synoptic agreement, with the
precise timing of the various shortwave rotating through the main
challenge. In general, the shortwaves have been arriving slightly
faster than depicted so think the trend of speeding up the
systems is a reasonable one. Other changes from past model runs
include a slightly more southern track for the next incoming low
as it passes to the south of the western Alaska Peninsula this
afternoon. It does however then still curve to the north following
its earlier track so the net result of this change is rather minor.

&&

.AVIATION...

PANC...VFR conditions will continue as southeasterly flow
increases ahead of an approaching frontal system. Downslope drying
should keep precipitation out of the area through late evening.
Early Wednesday morning some light rain may make it through the
downsloping at times with a much better chance of rain as well as
breezy southerly winds making it to PANC Wednesday afternoon and
evening.

&&

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

Windy and wet will be the general theme through Wednesday. The
culprits, a surface low/front lifting north across the Gulf today
and a second and stronger low/front moving north tonight and
Wednesday.

The first low near Kodiak Island this morning moves north and
weakens in the vicinity of the Barren Islands later today. Its
frontal trough continues north and stalls along the North Gulf
coast this evening. Gales are expected with the low and along the
front. The brunt of the rain will fall along the coasts and
western Cook Inlet. Strong cross-barrier flow (southeast) will
keep areas immediately to the lee of the Chugach mountains on the
drier side through tonight.

The stronger of the surface lows (remnants of Typhoon Kong-rey)
currently over the north Pacific intensifies and moves to over
eastern Bristol Bay Wednesday morning, then weakens as it heads
over the western interior Wednesday afternoon. A secondary triple
point center will likely develop along the frontal trough in the
vicinity of Kodiak Island tonight then head up Cook Inlet
Wednesday afternoon carrying what is left of the front with it.
Expect generally east/northeast gales to storm force winds over
the western Gulf tonight with these features. Winds will shift to
a southerly direction, due to strong pressure rises on the
backside of the triple point, Wednesday morning and trail behind
this low as it heads up the Inlet. Very strong gap flows are
expected in favored locales, Turnagain Arm, Portage Valley,
Matanuska Valley, Knik Arm, and along the Copper River, to name a
few. Winds should diminish across most areas Wednesday
evening/night. As with the first low today, the precipitation
pattern should be about the same. The exception will be rain
should get into the traditionally downsloped areas to the lee of
the Chugach as the triple point works its way up Cook Inlet
Wednesday afternoon/early evening.

&&

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

Widespread showers have developed across the western half of the
area. This is due to strong low-level convergence along an
approaching front, aided by a weak tropical connection helping to
boost precipitable water values, with vort maxes in the flow aloft
providing increased bursts of ascent. This looks to continue
through the evening with rainfall spreading further inland from
west to east. By late evening, the coverage may drop off somewhat
as the front weakens due to the occlusion process, but this will
be short-lived.

Our next feature of interest is a rapidly strengthening surface
low, with even more of a tropical feed being reinforced across the
region. This storm (the remnants of former Super Typhoon Kong-Rey)
will reach the AKPEN around midnight, with a significant vort max
rounding the base of the Bering Sea long-wave trough, with a 65 to
70 kt low-level jet developing, helping to advect higher moisture
values into the region. By morning, southwest Alaska will be
juxtaposed between the arriving surface low, upper disturbance,
and left exit region dynamics of a 135 knot jet stream. This
combined with the low-level moisture advection will allow for
fairly robust rainfall to develop. It would be interesting to
watch the radar morphology for this event, but, unfortunately the
King Salmon Doppler radar is down for now. In any event, widespread
rainfall will develop with some of it being moderate to heavy at
times. This will be especially true on the east and south-facing
slopes where orographic lifting will increase rain rates
significantly.

The other story with this second storm will be strong wind gusts,
both ahead of the low as some of that 850 mb jet energy mixes
down, aided by down-sloping affects in the lee of the mountains,
and also on the backside of the low as strong disturbances
rotating around the low and dry advection enhance downward mixing
potential. All of this will spread northward through the day
Wednesday. So in short, look for widespread light rainfall
Tuesday, followed by moderate to heavy rainfall with strong wind
gusts developing on Wednesday, with rainfall continuing into
Thursday as yet another disturbance enters the picture.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Today through Thursday)

Deteriorating conditions will rapidly ensue over the next 24
hours for the AKPEN and adjacent coastal waters. The remnants of a
former super typhoon will interact with a strong vort max
rounding the base of the persistent Bering Sea trough. This will
allow for rapid intensification of a surface low as it enter the
region, aided by strong moisture transport at 850 mb and left exit
dynamics of a intensifying upper-level jet stream. Widespread
rainfall, some likely heavy at times, will develop. Also,
sustained storm-force winds, with hurricane-force gusts are
expected. This will allow seas to build into the 19 to 24 feet
range, with the worst conditions expected from late Wednesday
morning into the afternoon for locations south and east of the
Shumagin Islands. Between the peninsula and the mainland,
sustained gales are about as strong as the winds look to get for
Bristol Bay.

For the remainder of the region, disturbances rotating around the
western Bering low will bring periods of sustained gales with seas
occasionally reaching 14 to 17 feet, with a general showery
pattern remaining in place.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The mid to long term begins Thursday with the models in overall
agreement with a low over the northern Bering Sea and a ridge of
high pressure extending from the Gulf of Alaska into the north
Gulf Coast. Moving into Friday we have a weak low, short wave that
gets caught in the strong southerly flow into the Gulf of Alaska
that rapidly moves to the north Gulf Coast by Saturday morning.
This will push the ridge of high pressure out of the Gulf of
Alaska into British Columbia. Meanwhile, out west, we have
another strong low moving into the western Bering Sea and
Aleutians late Friday night. The models are struggling to unify on
a solution as the storm track and forecast pressure have
significant spreads between the deterministic models and the
ensemble blend. Where the models are consistent is the location of
the front as it sweeps into the western Aleutians Friday night to
Saturday morning. As we move into Sunday the spread in the model
solutions continues to grow with significant differences in the
sensible weather depending on which solution is used. The ensemble
solution and WPC have a faster track to the east northeast than
the latest deterministic models. Therefore, we will maintain the
current forecast due to low confidence in any one.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 119 120 125 129 136 137 139 140 141 155 160 177 178
         Storm Warning 130 131 132 138 150

&&

SYNOPSIS AND MODEL DISCUSSION...JER
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...SA