Area Forecast Discussion
Issued by NWS

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

008
FXAK67 PAJK 301301
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
501 AM AKDT THU JUN 30 2016

.SHORT TERM...Upper low will remain over the gulf through
tonight. Upper trof will move north across the area today. A low
pressure area will move northeast into the southeast gulf by late
tonight. Used blend of the 06z NAM and 00z GFS to handle the low
moving in...with GEM close as well. ECMWF was further west with
the low track than the others and was considered an outlier.

Main forecast concerns will be precip potential...winds...and
potential for flooding on the Mendenhall Lake/River system due to
glacial dammed lake outburst from Suicide Basin...which will be
discussed in the hydrology section below. Right now...have showers
over much of the area associated with minor shortwaves aloft and
the main upper trof moving north through the area. As the upper
trof moves north...expect shower activity to decrease from the
south today into this evening...although there may be a short
redevelopment of showers over the higher terrain this afternoon
during max daytime heating. With lower levels somewhat cooler
than yesterday...not expecting any TSTMS at this point...but the
northern 1/4 of the inner channels could be a close call with
cooler air aloft still there this afternoon. As showers diminish
early tonight...rain from the approaching low pressure area will
spread into the far south by late evening...then reach the south-
central area overnight. Looks like a decent period of dry weather
for the north beginning by late evening.

Wind wise...currently strongest winds are over the northern inner
channels. 15-20 KT southerly winds are occurring there. The far
north will increase to 20-25 KT this afternoon due to thermal
enhancement. Winds will drop off over the north tonight as
pressure gradient weakens then shifts to a weak northerly
orientation. Over the south...strongest winds will be ahead of the
low...especially over the southeast gulf. Expect 25-30 KT easterly
winds to spread into the far southeast gulf tonight. The southern
inner channel area will see 15-20 KT winds develop tonight.

.LONG TERM...Models continue to struggle with the evolution of
the upper level pattern, namely the persistent upper level low
that has been anchored in the gulf for the past several days. And
as expected this is also leading to differences in model solutions
at the surface.

The upper level low is expected to gradually shift to the
southeast through Saturday before quickly pushing east into
British Columbia on Sunday. At the surface, the low pressure
system mentioned in the short term section will weaken into an
open trough on Friday. A secondary piece of energy will induce
another wave of low pressure which will quickly race across the
far southeast gulf early Saturday. There is model disagreement on
the exact track and strength of this feature. The NAM is the
strongest and farthest north with the system while the ECWMF and
GFS are indicating a weaker and farther south track. Regardless,
the main impact from this secondary wave will be enhanced lift and
a period of moderate rainfall for the southern panhandle Saturday
morning and into the afternoon.

Rain will be ongoing across a large portion of the panhandle at
the beginning of the long range period on Friday morning. This
will continue for much of the day before the second wave of low
pressure moves near the southern panhandle on Saturday. As
mentioned above, this will enhance rainfall rates across the
south. For the north, expect precipitation chances to decrease
significantly by Saturday evening. For the 3rd and 4th, weak
impulses embedded within the flow around the upper low which will
now be located in British Columbia make for a tricky rain
forecast. Current feeling is most of the precipitation will stay
on the east side of the Coast Mountains. However, periods of
rain will still be possible into early next week. Models diverge
rapidly into next week with the ECMWF depicting a much more
amplified solution than the GFS. Therefore, continued with
climatology...mostly cloudy with a chance of rain.

Generally light wind regime through the extended period.
Strongest winds will remain over the far southeast gulf. For the
inner channels, 10-15 kt will be the main rule of thumb. The
exception will be in Northern Lynn Canal where some enhancement to
the southerly pressure gradient will lead to 20-25 kt over the
weekend.

Used the 06z NAM/GFS for updates through day 4. The remainder of
the forecast was left as is.

&&

.HYDROLOGY...Based on Suicide Basin gage...water levels are
falling in the basin. They began to drop just after 15z
yesterday...and as of 12z this morning has dropped about 4.5 feet
from peak water level. Mendenhall Lake began to rise after 18z
yesterday...although some of that was likely due to enhanced
runoff from melting snow and ice during the warm day. But it
appears that the glacial dammed lake release is now main factor in
Mendenhall Lake level rising. As of 12z this morning...the lake
has risen to a stage of 5.7 feet. Will coordinate with the RFC
about this early this morning. Stay tuned for possible issuance
of flood watches or warnings for Mendenhall Lake and river later
today.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-041-042.
&&

$$

RWT/TPS

Visit us at http://www.weather.gov/Juneau



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