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FXAK67 PAJK 271209
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
409 AM AKDT FRI MAY 27 2016

.SHORT TERM...The weather over the next two days will be
influenced by the evolution of a complex upper trough over the
southern panhandle and western BC. The upper shortwave
responsible for enhancing the brief thunderstorm activity over the
northern panhandle yesterday afternoon will carve out a closed
upper low over northern Haida Gwaii this afternoon. As this
happens, a remnant shortwave trough over northern BC will be drawn
westward and across the central panhandle on Friday night. In
response to this feature, an east-west surface trough is forecast
to develop across the coast range Friday afternoon through Friday
night.

Models are indicating surface based CAPE values in excess of 500
J/kg over the southern panhandle this afternoon. The instability
combined with steep lapse rates underneath the cold upper low and
convergence associated with the surface trough prompted the
inclusion of a slight chance of thunderstorms south of Frederick
Sound for Friday afternoon. Some of the hi-res guidance including
the experimental AK HRRR are producing some decent pulse-type
convection Friday afternoon which lends some confidence to the
forecast. For the northern and central areas, offshore boundary
layer flow should be the rule Friday with scattered clouds and
warm temps into the middle 60s. Northerly pressure gradient does
increase into Friday evening so some of the northern inner
channels will experience an increase in the winds to near 20 kt by
late Friday night. As the above-mentioned shortwave passes the
coast range late Friday night, hi-res NMM and ARW indicate a quick
burst of cross barrier flow so included a brief period of gusty
conditions near downtown Juneau late Friday night.

Overall there was decent model agreement. Main changes were
outlined above utilizing hi-res guidance. Forecast confidence
about high with lower confidence in the details of the convective
coverage over the southern panhandle.

.LONG TERM...Long range forecast begins with a prominent upper
ridge over the western gulf and western Alaska with two upper
level lows pinching the ridge off over the gulf through the
weekend. The low on the east side will be dropping south out of
the Yukon and eventually stall over the southern panhandle this
weekend before weakening and moving east early next week. The
western low near the Aleutian Islands will remain there through
Monday while weakening. Remnants of this low will eventually move
into the gulf by mid next week and strengthen. Exact track this
low remains uncertain as solutions are still in poor agreement
with spread from 40n 140w to the eastern Aleutians.

Little change to the surface forecast through the weekend.
Overall dry weather for the northern panhandle due to offshore
flow with wetter weather farther south under the stalled upper
low. Embedded in the offshore flow will be several weak
disturbances that will try to bring showers over the coast
mountains and into the northern panhandle. Offshore flow and
downsloping should be enough to keep these showers at bay but
clouds will still linger so kept mostly cloudy skies through most
of the weekend. Bumped up outflow winds over gulf for Saturday but
for now still kept them at 20 kt. For the rest of the panhandle
and inner channels no significant winds expected as n inner
channel winds diminish. Differential heating even with cloud cover
still enough for some sea breeze circulations.

Models initially in good agreement with some small differences in
precip over the n panhandle. By next week model spread increases
for track of the upper low from the Aleutians making the forecast
tricky and low confidence. Current scenarios: ECMWF; moves low up
thorough the central gulf on Tuesday then curves it back to
Kodiak. Associated front then impacts the panhandle Tue night.
GEM; low stays in the SW gulf and the associated front stays
offshore of the panhandle. Used some NAM DNG5 and hi-res ARW for
winds Saturday then left grids as is with WPC ensemble due to
model spread.


&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...None.
&&

$$

DEL/PRB

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