Area Forecast Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
FXAK67 PAJK 182309

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
309 PM AKDT Sun Mar 18 2018

.SHORT TERM...According to current WV satellite imagery, an upper
level ridge continues to build over the gulf, with the ridge axis
in an almost east-west orientation and extending through the
northern gulf. A broad upper level trough over the Bering Sea is
helping to further strengthen this ridge. In addition, onshore
flow has returned to the panhandle, allowing a lower cloud ceiling
to develop over most locations. Some non-accumulating
precipitation has been reported near coastal areas, namely Elfin
Cove and Gustavus. Precipitation coverage should increase tonight
and eventually transition to more of a stratiform nature, as a
weak low moves across the northern gulf and lifts northeast into
Canada by early Monday morning.

On Monday, the ridge will continue to pack against the Coast
Range as a shortwave begins to dig southeast into the eastern
gulf. A front associated with a developing surface low with this
feature will track southeast, impacting the panhandle beginning
Monday night, again increasing precipitation chances. Early
Tuesday morning, cold air advection will increase behind the low
as it moves further south. Precip coverage will shift to more of a
convective, or showery, regime. Models also continue to indicate
steeper lapse rates, which supports this shift from stratiform to
convective precip type. The increased cold air will bring in the
concern for some snow, most likely a rain, snow mix, but QPF does
not appear to be significant, so little to no snow accumulation
is expected with this system.

For the short term forecast, we increased winds throughout the
inner channels and a few areas on land, where we now have a Strong
Wind headline out for Skagway beginning Tuesday midday. We also
increased our PoPs throughout the panhandle because of increasing
confidence with this next system moving in beginning Monday.
Minimum temps were adjusted up and maximum temps were adjusted
down through the short term to account for a weak diurnal change
because of increased cloud cover and substantial moisture at the
mid and lower levels from stronger onshore flow. We chose to use a
combination of the NAM and GFS deterministic models as well as
MOS Guidance to adjust the forecast accordingly.

.LONG TERM.../Tuesday through Sunday/ of 10 pm Saturday. The
long range forecast starts out with a strong upper trough/low
dropping S through the area while an upper ridge builds over
western Alaska. This pattern persists through late week for the
most part as the upper low stalls briefly W of Haida Gwaii. The
upper level pattern then appears to shift early next weekend as a
strong trough from Russia destroys the western Alaska ridge
establishing a broad trough over much of Alaska by next weekend.
The resulting trend is toward warmer SW flow over the Gulf and
Panhandle by that point.

Overall changes were rather minor especially in the Tue through
Thu period as current model guidance seems to be painting a
coherent picture with little difference noted between individual
models (ECMWF and WPC were the main choice for guidance). Strong
cold air advection behind the upper trough will cause outflow
winds to increase again shutting off showers across the northern
panhandle through Tue night due to combo of dry cold air intrusion
and downslope flow. Farther south, showers will persist as the
upper low lingers around Haida Gwaii. Temperatures should be warm
enough that most areas will see these showers as rain but some
snow could mix in during the cooler nighttime hours. Low to no
snow accumulation expected.

As for winds, Northerly outflow will be increasing Tue night into
Wed as a cold air mass drops into the area. Winds could reach gale
force in some areas (like Lynn Canal) during this time. Pressure
gradient forecasts are starting to suggest that even that might
be too weak for some areas so future forecasts may increase winds.
Surface temperatures will also be affected as 850 mb temps drop
to -8 to -10 C across much of the area through Fri. Overnight
lows will have the most noticeable drop with many areas dipping
into the mid to upper 20s during this time (this is a drop of 3
to 5 degrees over the previous forecast). High temps will not cool
down as much (if at all) as the lows will, as the Spring sun
should help warm temperatures up during daylight hours.


.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from Tuesday afternoon through Tuesday evening for
MARINE...Small Craft Advisory for PKZ012-013-051-052.




Visit us at is the U.S. government's official web portal to all federal, state and local government web resources and services.