Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
509 am akdt Sat Jul 26 2014
Short term...water vapor imagery shows upper low continuing to
spin over the Gulf early this morning. Sensible weather will be
very similar to Friday with rain persisting through much of the day.
Blended ec/NAM with inherited package resulting in a slight shift
west of the surface low over the northern Gulf.
The main forecast challenge will again be the timing of heavier
rains and strength of winds over the Inner Channels. Satellite
imagery and surface analysis show one wave moving across the
Panhandle at this time with another wave evident on WV imagery
rounding the base of the trough over the Gulf. Expect MDT-heavy
rains to continue pushing north-northwest across the Panhandle this morning
with rains easing until the next shortwave increases precipitation over
the southern Panhandle late this morning into the afternoon.
Models in very good agreement with a third disturbance rotating
around the parent trough tonight. Rain will ease and increase a
couple of times through the period as each wave passes.
Winds across most of the Panhandle will continue to be moderate
to strong southerlies with the tightest pressure gradient again
across the southern Panhandle and along the outer coast. With the
surface low expected to be slightly further west...the gradient
will be slightly weaker today so kept near gales today over Clarence
Strait with advisories north into southern Chatham and Cross Sound.
Forecast confidence is very high in another wet and breezy day
across Southeast Alaska.
.Long term...Blocking pattern that is keeping the upper level
low over the Alaska Gulf will remain in place through the week then
begin to break down by next weekend. Models are in good agreement
for this upper level synoptic pattern with the low between the North
Pacific/Bering Sea and Continental U.S. High pressure ridges through Friday
but then diverge with the timing of the ridges breaking down. The
cyclonic rotating surface lows/shortwaves over the Gulf will
continue to keep the southeast Panhandle with wet weather and cloudy
skies with some breaks as the waves pull off to the west.
Longest breaks occurring toward the end of the week before The
Block breaks down. New model runs are in better agreement through
84 hours with the position of these features but not in great
agreement with the resulting pop fields. At this time the Sunday
shortwave looks to pull west with the next wave moving in on
Tuesday. Each wave will bring in precipitation and increasing winds but
location of the low could keep things further off shore.
Indications that the next low will bring in stronger winds with
coastal waters and southern Inner Channels reaching small craft
levels, but for now keeping the southerly wind speeds in the
20 knots range. Little change done to temperatures with limited
diurnal range through the week due to rainfall and cloud cover.
MOS guidance still showing a larger diurnal spread but with
expected cloud cover the warmer temperatures may be over done.
Keeping a watch on the track of a former typhoon and if it will
be pushed north and kept well west of our area or passes far
enough south to be able to track east and then get caught up in
the upper level low circulation over the Gulf bringing wide
spread heavy precipitation over the Panhandle. For now models
leaning toward keeping this feature to the north and west.
Stayed with the trend of using ensemble blend with 00z European model (ecmwf)/GFS
and some NAM for press and winds then transition to wpc for the
extended forecast. While the position of the low on sun was
further west overall changes to the forecast were minor. Although
the models are in good consensus for the synoptic scale pattern
still enough variation at the surface for less confidence in the
practical forecast. As models usually have difficulty with timing
large pattern changes confidence drops with the timing of the
blocking pattern breakdown.
Marine...Small Craft Advisory for pkz022-033-035-036-041>043.