Area Forecast Discussion
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FXAK67 PAJK 242308
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
308 PM AKDT Sun Sep 24 2017

.SHORT TERM...Tonight through Tuesday...As of 1500 this afternoon,
current WV satellite imagery shows a shortwave lifting across the
northern gulf and northern portions of the SE Alaskan panhandle,
with a ridge of high pressure currently positioned over western
Canada. This shortwave developed from a parent trough that is currently
positioned over the Aleutian Islands of mainland Alaska. In
addition, WV imagery continues to show a very moist subtropical
jet advecting moisture with the flow between the ridge and the
deeper trough over the western gulf, which will impact our weather
for the early half of the work week. Current precipitable water
analysis even shows anomalously high PW values with this stream of
moisture.

As this shortwave continues to exit the area tonight, we should expect
PoPs to diminish during the overnight hours. The ridge will then
begin to build back in over the eastern gulf early tomorrow and
possibly help in decreasing cloud cover, mainly across northern
portions of the panhandle. This should aid with fog development
overnight tonight into tomorrow morning due to relatively stable
conditions and a very moist ground. The ridge will be weak and
should quickly propagate east before an additional shortwave
begins to lift out of the southern gulf and across our area during
the Tuesday timeframe. As mentioned previously, a subtropical jet
is assisting with bringing additional moisture to the area, so
this new system on Tuesday will contain subtropical
characteristics. This means that additional precipitation, a much
higher snow level, and slightly above average temperatures with
not a large diurnal spread can be expected for most of the area.

Generally, adjustments to the forecast were made using a
combination of the ECMWF and the GFS, as there was generally good
agreement between the two.

.LONG TERM...Big concern in the longer range is for a likely
atmospheric river event around mid to late week. Models are still
sorting out details on this but all indications are a significant
rain event is in store for at least part of the area. GFS and
ECMWF ensembles show precip water values 2-3+ standard deviations
above average with maximum on Wed and Thu. Ended up using 18z GFS
to start long range for Tue, then went with mainly WPC for Tue
night onward.

With details such as where main frontal band will hang up while
some low pressure waves move N along it, confidence in where
heaviest rain will occur is no more than average, but it appears
somewhere over the NW half of the area will be the wettest during
midweek. Models hint that far SE may even dry out for a bit as
waves moving along the front push precip band back to the N
around Wed night and Thu. Would expect significant rises on rivers
and streams where heaviest precip occurs, but too early to tell if
any flooding may occur. Will also need to watch out for potential
mudslides with ground fairly saturated from rain events occurring
before the midweek period.

For latter part of the long range, models differ on how fast main
trof moves out of the area, which will affect how quickly precip
diminishes. Keeping near climo POPs for much of the area for next
weekend attm.

&&

.AVIATION...Phone problems continue at Yakutat. Conds low enough
to warrant NIL TAF. Very low cigs/vis many TAF sites. Transition
to drier and more stable atmosphere except at very low levels/sfc.
Very difficult forecast problem Mon morning about whether low
stratus will be several hundred feet aloft for good visibility or
surface based resulting in dense fog. TAFs try to convey a little
of both.


&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012.
     Small Craft Advisory due to Seas for PKZ042-051>053.

&&

$$

Voveris/RWT

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