Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
430 am akdt Thursday Oct 23 2014
Short term...satellite imagery this morning showing two 985 mb
low pressure systems in the norther Pacific rotating around each
other. The low located west of haida gwaii will affect our area
with a weak frontal band currently over the southern Panhandle
moving northward. This low will continue to progress westward
merging with the the second low later in the day. The resulting
circulation will weaken as it moves off the coast of haida gwaii
again and become quasi stationary.
Clear breaks over the northern Panhandle due to offshore flow has
resulted in fog development for locations north of Frederick Sound
and temperatures dropped down to freezing due to radiational cooling.
Local visibilities have dropped below 1 mile at times. Clouds will
increase through the morning as the front advances northward.
Rain showers over the southern Panhandle have been light but do
expect more shower bands to move over the southern region today
and again tonight as the reformed low moves back near haida gwaii.
Showers will develop over the central and northern Panhandle later
in the day as front moves north. The front will be shearing apart
while it moves north and forcing mechanisms weaken so showers will
be scattered. With cooler air aloft and warm sea surface
temperatures producing convective atmosphere with cape around 200
to 400 j/kg and lifted index of 0 to -2c south of the low center so have
included slight chance of thunder storms over the Southeast Alaska Gulf.
Over the southern coastal waters small craft winds and seas around
10 feet will diminish through the morning as the low pulls off to the
west. Winds and seas will pick up again late tonight as the
reformed low moves back in. Northerly small craft winds over
northern and southern Lynn Canal early this morning due to
pressure gradient between the low in the southeast Gulf and high pressure
over the Yukon. Elsewhere winds in the 20 knots range with some
easterly gap and outflow winds between the southeast Gulf low and high
over British Columbia. Winds will weaken through the morning as the low
pulls off the west. Weak gradient tonight will result in winds
generally less than 15 kts but with increasing winds over the far
southern Panhandle as the reformed low nears haida gwaii.
Models have not been handling these two low pressure systems in
the North Pacific very well so forecast confidence is below average. Lots of
shifting in strength and position of the low centers as they rotate
around each other. Model choice was mainly picking ones that were
in better agreement and have shown less change run to run. Decided
to use a blend of 00z European model (ecmwf)/NAM for the short term forecast which
did shift the low back east slightly this morning and altered some
wind speeds. Models also had a large spread in timing precipitation bands
so went with a blend of NAM/GFS/sref, with bit more confidence in
Long term...Middle Range models struggle a little on Friday into
Saturday as the consolidating low becomes quasi-stationary near the
coast of haida gwaii in conjunction of another weak low forming
over the northern Gulf on Saturday. A surface trough will become a
bit active through the weekend between the northern Gulf low and
the low off the coast of haida gwaii. Scattered showers will
continue along the trough Saturday...then expect increasing
offshore low level flow across the northern areas for generally
dry weather Sunday while the southern areas stay under scattered
showers. None of the model solutions given for Friday and Saturday
handles the synoptic pattern well. Decided to make a temporal
transition from the short term forecast of NAM and European model (ecmwf) based
solutions and made smooth connection to the wpc ensemble guidance
Models continue diverting for next week as they handle the
Tropical Storm Ana...currently moving slowly north-northwestward
over northwest of Hawaii. The jtwc predicts Ana will weaken and
re-curve northeastward by Friday morning near 30n/170w...then
heading toward haida gwaii through Monday evening. Ana will
continue to weaken and become an extratropical system by Sunday
evening. The wpc ensemble solution will keep the extratropical
system moving over haida gwaii with wet weather across the
southern Panhandle Tuesday and Wednesday. No significant
winds/seas are expected...except Friday night when the near-
stationary low off the coast of haida gwaii produces small craft
level winds/seas over the southern outer waters. Temperatures will
remain within normal range over the northern Panhandle while
extratropical system maintains southern areas warmer than normal.
Forecast confidence remains below average.
Marine...Small Craft Advisory for pkz012-013-041-042.
Small Craft Advisory due to seas for pkz022-043-051-052.
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