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FXAK67 PAJK 122314
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
214 PM AKST Tue Dec 12 2017

.SHORT TERM...Persistent, anomalously deep troughing continues
over the western Gulf of Alaska through Wednesday night. Deep
layer moist southerly flow will continue to direct a series of
fronts across the Panhandle. The first in the series of fronts
will track across the central and southern Panhandle tonight. Rain
will be widespread and heavy at times south of Frederick Sound
tonight with small craft level winds over the outside waters and
spreading into Clarence Strait later this evening.

More substantial shortwave and associated strong front will
slowly approach the panhandle Wednesday and Wednesday night. This
front will once again be accompanied by strong moisture transport
with NAEFS ensemble integrated moisture transport values between 4
and 5 standard deviations above normal. Southerly flow in the H85
to H7 layer progged around 50-60kt and these favorable
orographics will combine with strong WAA to produce yet another
period of heavy rainfall from late Wednesday through Wednesday
night.

Winds will also be on the increase Wednesday especially over the
outside waters. Barrier jet formation Wednesday will lead to gale
force easterlies along the northeast Gulf coast. As the front
pushes east on Wednesday night, gales will spread into the
remainder of the offshore waters with small craft SE`ly winds on
the inside waters. Gusty winds will also develop along the outer
coastal land areas with 40 mph gusts likely in places such as
Yakutat and Sitka Wednesday evening.

Model spread becomes more significant as we head into Thursday
regarding the position of the front but still looks to be a very
warm and wet period.

.LONG TERM...Thursday through Tuesday...The upper level ridge
extending into British Columbia, which has contributed to the
blocking pattern and warmer temperatures we have seen as of late,
will finally begin to break down and push east. This should
provide an opportunity for additional shortwaves to transition
east and impact the panhandle, leading to a fairly active pattern
and one that should remain wet; the position of these shortwaves
will continue to pull in subtropical moisture. Model analysis
continues to show anomalously high precipitable water values with
each of these passing systems, so we`re not seeing any significant
periods of drying out in the long term.

Most of the major models agree that a strong shortwave will
develop near the Aleutian Islands on Friday, and an associated
weather front will lift northeast and impact the panhandle on
Saturday. As this shortwave continues to lift northeast, an upper
level trough will begin to build in aloft over our area and
should provide some assistance with lowering temperatures across
our area. The NAEFS currently shows a decreasing trend in
temperatures continuing into the early part of next week, with
temperatures nearing normal or even slightly below normal for
this time of year. The GEFS is also following this trend with
temperatures as well, which led to higher confidence in adjusting
our temperature forecast.

After the Saturday timeframe, most of the major model solutions
disagree with one another, so we opted not to make any
significant changes to the forecast after this point. However, we
decided that we would lower temperatures a few degrees in the
early part of next week to fit with the pattern shift, as
mentioned previously. For the Thursday through early Saturday
time period, we used a 50/50 blend of the GFS and ECMWF to make
adjustments to the winds and PoP fields.

&&

.HYDROLOGY...Strong warm advection on Wednesday will result in
freezing levels once again rising to between 4 and 6 kft. While
recent warm temps have likely decreased the mountain snow pack
significantly, there will likely be at least some additional snow
melt which will combine with storm total rainfall amounts of 2 to
4 inches over the northern panhandle to produce rises rivers and
streams. At this time no flooding issues are expected but will
need to monitor smaller streams such as Jordan Creek near Juneau
should heavier rainfall rates develop.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Gale Warning for PKZ041>043-051-052.
     Small Craft Advisory for PKZ012-013-021-022-031>036-053.

&&

$$

DEL/JJV

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