Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
406 PM akdt Tuesday Jun 2 2015
Short term...marine layer is slowly being replaced by more of a
synoptic cloud cover as the afternoon progresses, although a large
portion of the interior of Southeast Alaska is enjoying another
mostly sunny day. Today will be the end of that for a while as an
upper trough rotates north spreading middle level clouds and rain
over the northern half of the Panhandle by late tonight. To the
south, rain will push north from southern British Columbia. By
tomorrow, categorical probability of precipitation will be everywhere except zones 27 and
28, and portions of zone 26. 24 hour rain totals ranging from less
than 0.25 inches to slightly more than 1 inch with the highest
totals in the Yakutat and Juneau area. These totals are supported
by a fairly weak but well organized band of vorticity that will
push inland through the day Wednesday. Low and middle level jet
strength not very impressive and is not expected to enhance precipitation
rates or surface winds much at all. Moderate Omega values will
aide in higher rainfall rates, though, especially over westward
facing upwind slopes where orographic lifting will be maximized
Winds still not much of a concern, as the forecast area will still
be under the influence of a surface ridge of high pressure that
will first become amplified by the upper trough, and then shifted
east by any approaching front that will be a factor early on in
the extended range forecast period. The only area where winds will
be impacted by the ridge will be the far northern Inner Channels
and public zones. Look for southeast winds 10 to 20 miles per hour in the
Haines zone near Lynn Canal, southwest winds 10 to 20 miles per hour or
possibly higher in Skagway, and small craft southerlies over
northern Lynn Canal by middle morning tomorrow. Over the Gulf, winds
will form up out of the southeast tonight and gradually increase
through the day tomorrow. Have a Gale Warning in place for the
western portion of offshore zone 310 by late tomorrow afternoon.
Weak low level cold air advection from the west may result in
daytime highs a degree or two cooler than today across the
Pressure grids unchanged from last night with minor tweaks to wind
speeds and directions. Pop and quantitative precipitation forecast from Gem, GFS, and European model (ecmwf). Also
blended in some rfc quantitative precipitation forecast. No significant changes to temperatures.
Overall forecast confidence is average.
Long term...fairly strong occluded front for this late in the
season will affect the area Wednesday night and Thursday. Models have sped up
the northward movement of the front some. Then...a low pressure
wave looks like it will move north-northeast into the far eastern Gulf Thursday night.
This will hold up the occluded front over the main part of the
Panhandle. Once the wave moves past the area early Friday...the front
will be able to push east of the Panhandle during Friday. Ended up using
the 12z GFS for Wednesday night-Friday as it was a good compromise on the
main features during this time. Used latest wpc further out as
models diverge somewhat on other systems that may affect the area
for the weekend and early next week.
For Wednesday night-Thu...rain will diminish over the main part of the
Panhandle as system from short term pushes NE of the area Wednesday
evening...but rain from the occluded front will push into the area
for later Wednesday night and Thursday...with the far southeastern area seeing the
rain last. Rainfall could become locally heavy over the northwest third of
the area especially Thursday. Will have gale force winds for the eastern
Gulf ahead of the front...with strongest winds over the far northwest
marine area over the eastern Gulf.
For Thursday night-Friday night...occluded front will get hung up near the
outer coast Thursday night as a low pressure wave rides north-northeast along the
front. The wave should pass inland Friday morning...and this will
push the front to the east of the area later Friday. The wave will
help keep locally heavy rain going mainly over the northwest half of the
area Thursday night. The rain should diminish from the west Friday as front
moves inland...but onshore flow behind it will keep some rain in
the area Friday night.
For the weekend and beyond...threat for rain will continue with
more systems potentially affecting the area. The GFS and European model (ecmwf)
suggest another low and front may affect the area during the
weekend but the track of the system differs. At this
point...maintaining generally chance probability of precipitation seems like best way to
handle it and allow later shifts to adjust this time frame as
Aviation...low visual meteorological conditions with occasional imc conditions due to low
clouds are expected across Southeast Alaska through the short term
forecast period. Visibility reductions in heavier precipitation
down to 3 statute miles at times.
Hydrology...suicide basin near Juneau is draining and Mendenhall
Lake continues to rise with a stage of 6.9 feet as of late Tuesday
afternoon. A forecast crest around 8.7 feet is expected late
Wednesday morning. Flood stage for Mendenhall Lake is 9 feet.
There is much uncertainty with this event however...so the
forecast water level on the Lake May need adjusting. A Special
Weather Statement has been issued for this event. Elsewhere,
Panhandle creeks, streams, and rivers are forecast to rise
significantly with expected moderate to heavy rains from a front
on Thursday. No floods are currently forecast, however, the taiya
river near Skagway is forecast to reach action stage of 16 feet
around 8 PM Thursday evening.
Marine...Small Craft Advisory for pkz012.
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