Small Craft Advisory 261, NWS Juneau, AK

Southeast Alaska Forecast Discussion...Corrected
National Weather Service Juneau AK
257 PM AKDT Mon May 1 2017

.SYNOPSIS...A weather front over the far northern gulf will move
onshore the northeast gulf coast Mon evening. A storm force low
will move north into the southern gulf on Tue, then drift into the
northwest gulf while slowly weakening Wed.

&&

.SHORT TERM...We increased the expanse of strong, gusty winds
through the Panhandle from Tuesday afternoon into Tuesday night
and have added headlines advising of these winds into the third
period. Aside from Pelican to Sitka to Hydaburg, we have expanded
into Ketchikan, Metlakatla, Thorne Bay, Kake, Wrangell, Juneau,
and Yakutat through the night. The higher resolution NAM fell in
line close enough with other models' positioning of the low to be
able to use as a guide.

We also warmed temperatures for Tuesday night. Mixing and warm air
advection from a marine and very saturated air mass should aid in
keeping temperatures very mild overnight.

Local patchy fog and some clearing late tonight from the southern
Panhandle still looks likely.

Most changes were reserved for Tuesday/Tuesday night. The
NAM/NAMDNG5 along with some help from the ECMWF was used for
guidance on changes.

.LONG TERM.../Wed night through Mon night/ as of 1030 PM Sunday /
Somewhat quieter weather pattern in the works as a strong upper
level trough over the gulf with an embedded upper low stays
relatively stationary through Fri. During this period it will
acquire an increasingly negative tilt. The trough will then get
split in two and push east on Sat as a new ridge with a strong
trough behind it advances from the W. The ridge proves to be very
transient as it quickly moves into Canada and the trough behind it
sets up shop over the gulf by early next week.

At the surface, the strong low from Tue remains mostly stationary
under the barely moving upper trough in the western gulf through
Thu night. During that time period it will weaken nearly 25 mb as
it is cut off from its upper level support by this point. Still
long range models are vary agreeable on gale force winds
persisting around it into Wed night with 25 kt or higher winds
lasting into Thu. Increased winds in the gulf as a result. Most of
these winds expected to remain well offshore so most areas of the
panhandle will see lighter winds during this time. As for precip,
onshore flow behind the front will keep the panhandle fairly wet
into Wed night. However, the increasing negative tilt of the
upper trough will cause surface flow to turn more easterly through
Thu gradually shutting off the precip.

Farther out, long range models are in rather good agreement for a
somewhat drier forecast for the late week period and into the
weekend. Favored WPC for updates. Building ridge over the gulf
Fri into Sat will help dry things out especially over the north.
It will also help keep a weak low coming up from the south mainly
over British Columbia on Fri. Southern panhandle may see some rain
out of it but most of the moisture stream is expected to stay in
BC. Next system not expected until the late weekend as a front
coming from the west, but here models start to differ on timing.
Elected to keep pops at a chance to account for the timing
differences.

&&

.AVIATION...Areas of MVFR ceilings are occurring with weak
onshore flow and weak front moving in. May get occasional IFR
ceilings and vsbys this evening at PAYA. Am expecting the clouds
to lift from S to N tonight and Tue morning as low level backs to
a more offshore direction. Think some fog will occur over southern
TAF sites late tonight, with potential IFR ceilings/vsbys
especially at PAPG. Increasing winds aloft ahead of approaching
front Tue will induce wind shear especially over the south.
Ceilings/vsbys should be VFR from late Tue morning through 00z.
Later TAFS will likely have lower conditions move in along with
stronger low level winds/wind shear for Tue night.

&&

.MARINE...Gale to storm force winds of 35 to 50 kts still look
good for the big system moving into the central gulf Tuesday and
Tuesday night. No changes were made for the Gulf of Alaska. Some
winds were increased through the Inside channels particularly
Tuesday afternoon/night.

&&

.HYDROLOGY...With a little more southeasterly gradient in the
steering flow of low level moisture, and with models lessening
rainfall amounts, we did decrease the forecast from 0.5 to 1 inch,
mainly during a 6 to 9 hour time frame from Tuesday afternoon
through evening. Once the front passes, rain rates should diminish.
We expect some stream rises, but nothing significant during this
early part of the season and plenty of snow locked in the mountains.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from Tuesday evening through late Tuesday night
for      AKZ026.
     Strong Wind from Tuesday afternoon through Tuesday evening for
     AKZ027.
     Strong Wind from Tuesday evening through late Tuesday night for
     AKZ025.
     Strong Wind from Tuesday afternoon through late Tuesday night
     for AKZ021-022.
     Strong Wind from Tuesday afternoon through Tuesday evening for
     AKZ023.
     Strong Wind from Tuesday evening through late Tuesday night for
     AKZ017.
     Strong Wind Tuesday evening for AKZ028.
MARINE...Gale Warning for PKZ022-041>043-051-052.
     Small Craft Advisory for PKZ021-033-036-053.

&&

JWA/EAL

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