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FXAK67 PAJK 271342
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
542 AM AKDT Thu Apr 27 2017

.SYNOPSIS...A weakening gale force low west of Port Alexander will
move inland this afternoon near Sitka. Widespread rain and showers
will continue as this low moves inland through Friday before high
pressure builds in Friday night. A low will move toward southern
Southeast early Saturday with more chances for rain into Sunday.


&&

.SHORT TERM.../Thursday through Friday night/ The low discussed
above can be seen moving east on satellite imagery toward the
central outer coast. The colder, higher cloud tops associated with
the front have cleared all but the far northern inner channels
and more cellular, convective clouds can be seen moving in off the
gulf and over the central/southern zones. A robust second wrap
has formed and will move ashore over the central outer coast later
this morning. Soon afterwards, the low itself will move inland
and begin to weaken quickly. Rain shower activity today could be
heavy at times over the central and southern panhandle. A still-tight
pressure gradient will result in small craft winds over the
central inner channels today with rapidly changing wind directions
possible as the low approaches the coast. Westerly or
southwesterly marginal gales are forecast for the southern coastal
marine zones today. Look for winds inside and outside to diminish
significantly overnight as the low weakens. Lingering shower
activity will diminish slowly but steadily through Friday night,
but chance PoPs remain in the forecast through the short term as
the next low approaches from the southwest. Existing forecast
remains on track so changes limited to PoP and temps tonight.
Blended SREF and NAMDNG5 for PoP. Max temps for today adjusted -
mostly down - per GFS MOS. Overall forecast confidence is average.

.LONG TERM.../Saturday to Thursday/ Ridging over the eastern gulf
and panhandle expected to shift into British Columbia as an upper
level trough/closed low moves into the southeastern gulf. 500 mb
vort max band associated with this trough may lift northward late
Saturday as ridging builds again over the central gulf and then
into the eastern gulf through Sunday. Upstream, a large deep
upper low moves over the Aleutians and remains quasi-stationary
through mid week with the ridge remaining over the eastern gulf
and panhandle. By the end of the week this broad upper low will
likely shift east but uncertain on the exact track.

At the surface on Saturday there will be a weak low in the
northwest gulf with a developing low tracking south of the eastern
gulf. Lots of model spread on the track of this system, and with
the position of this low track differences will result in large
forecast variation. Best estimate is the low will move near Haida
Gwaii through Saturday and likely dissipate by early Sunday
morning. Seems to be a trend of models moving the low center
further south which will have more offshore flow for the northern
regions, thus drier weather, compared to the south which will have
greater chance for rain. Nudged winds from this system toward
small craft levels in the southeast gulf. GFS/Canadian have
trended further north with stronger winds than the ECMWF/NAM which
also dissipate the low sooner. Decided best option was a
compromise blend until we get better model resolution. Beyond the
weekend models fell more in line with the next surface low near
the Aleutians the the associated occluded front moving into the
gulf Monday into Tuesday. Later in the week a much stronger low
appears to track up from the southwest and will have more impact
with respect to winds/pop/qpf.

Once again the word of the long range is uncertainty, especially
for the weekend. RMOP values below 30% with the weekend low.
Values improve later in the week as the next systems move in and
model align. Used a blend of GFS/ECMWF for Saturday even though
they were not in great agreement. Did this to get a smoother wind
field and moderate any very low or high wind speeds. For the rest
of the long range utilized WPC for its use of ensembles. Forecast
confidence is below average.

&&

.AVIATION...Rain and showers along with some MVMC conditions
will continue across most TAF sites as a weakening low moves
inland near Sitka. Visibility should be mostly VMC but occasional
MVFR may occur in heavier rain bands. Expect gusty winds across
most of the northern TAF sites as the low moves inland through
this afternoon before the pressure gradient weakens this evening.

&&

.MARINE...The compact low will result in limited fetch and produce
a steep, confused sea over the eastern gulf. Coastal buoys south
of Cape Spencer indicating combined seas of 8 ft as of early this
morning. Seas forecast to build to as high as 15 feet west of Cape
Decision later today before beginning to subside. Southwesterly
swell with a period of 9 to 12 seconds.


&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Gale Warning for PKZ042.
Small Craft Advisory for PKZ022-031>036-041-043.
Small Craft Advisory due to Seas for PKZ051.

&&

$$

Fritsch/JRA/PRB

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