Storm Warning 15, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
251 PM AKDT Sat Apr 20 2019

.SHORT TERM.../Through Monday night/ Very active weather expected
through the short range period. An occluded front will move into
the southern gulf this evening, then continue N through the
eastern gulf and SE AK tonight and Sun. Low pressure will rapidly
develop over the NE Pacific Sun and move NNE into the eastern
gulf Sun night. There is still some uncertainty on the exact track
and strength of the system, but it appears to be very impactful
for the area. The low should reach the Yukon territory by Mon
afternoon. A smaller low and trof will move across the gulf into
the area Mon night, with some model differences with the system.
In general, preferred the 12z NAM/ECMWF ideas with the main
systems, especially the potentially powerful low Sun night and
Mon.

The main impacts during the short term will be potential for
strong to high winds, and significant rainfall. The biggest period
of concern for both is Sun night and Mon. There may be very strong
winds for most of the area as the powerful low moves through the
eastern gulf Sun night and Mon morning, then reforms in the
Yukon in the afternoon. The initial burst of wind will be with the
associated occluded front moving NE across the area Sun night and
early Mon morning, then a second burst of wind is likely Mon
afternoon along the central outer coast as strong W-SW wind moves
onshore to the S of the low, and a tight southerly pressure
gradient develops over the N by early afternoon once low jumps
into the Yukon. Decided to issue a high wind watch for most of the
area for Sun night into Mon, with wind gusts to 60 mph or more
possible. Looks like most places will get up to 6 hrs of high
wind, but some timing uncertainty caused the watches to be longer.
Marine winds may reach storm force over much of the inner channels
and far eastern gulf Sun night into Mon (or possibly hurricane
force around Cross Sound after low moves past them Mon morning).
Needless to say, Sun night and Mon will not be a good time to be
out on the water.

The heavy rain threat will be Sun night into Mon morning as the
occluded front moves onshore. Could see 1-3 inches of rain with
local amounts over 4 inches possible through Mon morning. Highest
rainfall should be across the southern panhandle, which should
help out with diminishing the effects of the long term drought.
River levels are on the low side, so not expecting flooding at
this point, unless rain rates stay heavy enough for small streams
to rise out of their banks.

Otherwise, have strong winds for the central and southern outer
coast late tonight and Sun, and gales for the eastern gulf and
parts of the inner channels, as first occluded front moves in. The
showers going on late this afternoon will be replaced by a
steadier rain tonight from S to N as the front moves in. But as
front gets further N late tonight into Sun, the rain could become
more localized due to possible downslope flow across the northern
areas. There could be a short break in the rain along the outer
coast Sun before the much stronger system moves in for Sun night.

For Mon night, the smaller system will likely bring in more
showery weather, and with a colder airmass also moving in,
potential for some snow is there across the N, especially the NE
gulf coast, late Mon night. Too early to tell how much snow would
fall due to the model differences on track of the main sfc low
and supporting upper wave.

.LONG TERM.../Tuesday through Saturday/ as of 10 pm Friday.
Strong upper trough/low over Western Alaska, and the short waves
rotating around it, continue to dictate the weather for the
panhandle for the long range. This pattern is rather persistent
but does show signs of braking down by late week. The favored
scenario is an upper high from the Arctic cutting into the
northern part of the trough and causing the trough to positively
tilt before cutting off the low completely by Fri. However, there
are some very different ideas on where the upper low will be by
that point (scenarios range from the Bristol Bay area to the SE
gulf), so forecast confidence for the late week period is low to
moderate.

For mid to late week, A few more short waves will be pushing
through the area bringing more precip and some enhanced winds. For
most of these, timing is something guidance has yet to work out,
so confidence on when any of these short waves move through is
rather low. However, there is one short wave that guidance has
more of a handle on. That one comes in from the SW on Tue. Feature
is somewhat weak however, so at most expect more organized
showers and a brief wind increase as it moves through.

The other feature of note for the mid to late week period is the
area of cooler air that will be moving in behind the strong Mon
low. 850 mb temps are still expected to drop down to around -7C
over much of the area. Likely will see snow levels drop to only a
few hundred feet above sea level, and some areas in the north may
see snow mix in with the rain showers mid week, particularly at
night. No snow accumulation expected at sea level at this time but
higher elevations could see some accumulations.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...High Wind Watch from late Sunday night through Monday morning
     for AKZ026.
     High Wind Watch from Sunday evening through Monday morning for
     AKZ027-028.
     Strong Wind from late tonight through Sunday afternoon for
     AKZ023-027.
     High Wind Watch from late Sunday night through Monday afternoon
     for AKZ024.
     High Wind Watch from late Sunday night through Monday afternoon
     for AKZ021-025.
     High Wind Watch from Monday morning through Monday evening for
     AKZ018>020-022.
     High Wind Watch from Monday morning through Monday afternoon for
     AKZ023.
MARINE...Gale Warning for PKZ036-041>043-051-052.
     Small Craft Advisory for PKZ021-022-031>035-053.

&&

RWT/EAL

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