Area Forecast Discussion
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593
FXAK67 PAJK 222255
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
255 PM AKDT Mon May 22 2017

.SHORT TERM.../through Tuesday night/ A front stalled over the
southern panhandle will move E of the area by late tonight. A weak
low will move into the SE gulf tonight, then across southern SE AK
Tue. A high pressure ridge will build in behind the low Tue
afternoon and Tue night. Used mainly 12z GFS to handle things
through Tue night, with main adjustment to shift the weak low a
little bit to the S of previous forecast. Did use 12z NAMnest to
help with some of the inner channel winds.

Main forecast concerns will be precip potential and winds.
Steadier rain with the front across the S should transition to
showers tonight as front moves E of them. The weak low will bring
more showers to the area late tonight and Tue, especially over the
S half of the area. Once the weak low moves E of the panhandle Tue
night, the shower threat should decrease from the NW. May see some
showers move into the far NW part of the area late Tue night as a
weak trof moves into the NW gulf.

Winds have dropped off over much of the area, with below SCA level
winds going on. Still around 20 KT over N Lynn Canal and Clarence
Strait this afternoon, but these will drop off tonight. More
significant winds will move into the area Tue and Tue night
especially along the outer coast as the high pressure ridge builds
in. Am expecting much of the E gulf to increase to NW 25-30 KT
Tue afternoon with some gales likely for areas around Cross Sound
and S of Port Alexander. The nearby western inner channels will
pick up to 20-25 KT by Tue evening as the NW flow increases ahead
of the ridge building in. Could be locally windy along the
southern outer coast as well Tue night.

Otherwise, some clearing should develop from the NW Tue into Tue
night over the panhandle as the NW flow gets going. May have
shallow marine layer clouds remaining over most of the gulf
though. Kept in mention of patchy fog for the far N area late tonight
as they may see more breaks in the clouds then. Think the fog over
the S will break up somewhat quickly this evening as front moves
out so will not mention it at this point.

.LONG TERM.../Wednesday through Sunday/ As of 10 pm Sunday. For
the start of the long range, at upper levels main features will
be high pressure ridge over the eastern gulf with a low north of
the Bering sea with associated trough extending over the
Aleutians. As the upper low begins to shift to the east the gulf
ridge will narrow and sharpen. By Friday the upper level trough
will have moved over the central gulf with the ridge axis now
moved over British Columbia. For the rest of the weekend uncertain
if a wave will form over the northeast gulf.

At the surface the high pressure ridge will remain over the gulf
through the week. Main difference between previous forecasts is
indications that the trough from the Bering sea low will move a
short wave into the north central gulf and this wave may ride
over the surface ridge and impact at least the northeast gulf, if
not the north central panhandle. At most thinking this would be
scattered to isolated showers mid week but may just move in mid
level cloud cover. As this wave dissipates, a return to high
pressure being the dominant feature. Will be watching for marine
layer formation under this gulf high.

The other trend for the long range is warming temperatures aloft,
with 850 mb temps reaching 8 to 10 C by the weekend. Have
continued to keep warm surface temps in the forecast, but with
uncertainty of marine layer formation have held off on getting
above the upper 60`s, however 70 and higher is not out of the
question. Did drop Yakutat temps a bit due to the incoming short
waves and onshore flow from ridge axis position.

Small craft winds on lee side of the ridge will diminish as the
axis moves east. Indication of tip jet formation near Cross Sound
and Cape Decision so keeping winds at least in the 15 to 20 kt
range for these locations. For rest of the inner channels not
expecting winds to be much more than 15 kt.

Used a blend of GFS/NAM, which were in good agreement for the mid
week wave in the northeast gulf. Tempered down any pops from these
models due to initial uncertainty. Otherwise forecast confidence
is average.

&&

.AVIATION...Less wind and no wind shear reported today but wide
spread MVFR conditions across the panhandle with some scattered
IFR for the southern half due to a lingering moisture stream aimed
at the area. Low vis and ceilings expected to continue through
the night as onshore flow continues. By late tonight and tomorrow
vis and ceiling will be improving up to MVFR or better from N to S
as a low moves across the southern gulf turning flow more
offshore. Showers will also be shutting off from N to S at the
same time through the south will see some showers linger into Tue
evening. Some gusty winds in places this evening but they are
expected to lighten up through the night and remain mostly low
tomorrow.

&&

.HYDROLOGY...Most of the small streams and smaller rivers are
falling at this point. Larger rivers are still rising though, but
they should begin to drop Tue. We are watching Mendenhall Lake
closely though as it began rising this morning even though there
was little or no precip in the area from last night through early
afternoon. Latest image this afternoon from Suicide Basin showed
it had risen compared to yesterdays image, so believe that is not
the cause of this rise.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Gale Warning for PKZ041-042.
     Small Craft Advisory for PKZ022-033-043-051-052.
     Small Craft Advisory due to Seas for PKZ053.

&&

$$

RWT/PRB

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