Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
427 am akdt Tuesday Oct 21 2014
Analysis and upper levels...
The upper air pattern continues to be dominated by a large upper
low centered over Bristol Bay. A strong 155 knots North Pacific jet
stream anchors the base of the trough near 42n...with a strong
leading shortwave trough tracking into the southeast Gulf of
Alaska. Numerous lightning strikes (from GOES satellite data) have
been noted along and behind the occluded front with this system. There
is an embedded shortwave trough wrapping around the parent low
over the Kuskokwim Delta...with slowly rising heights building
into southcentral. Eventually...the low in the southeast Gulf will
wrap west-northwest toward the western Gulf...with the Bristol
Bay low slowly tracking south as ridging builds into the Bering
midweek. This will setup a dry offshore flow pattern across
southern Alaska by Wednesday.
Models are in excellent agreement with the general large scale
pattern headed into the weekend. There are some remaining small
discrepancies associated with the exact track of the Gulf
low...but they are much smaller and less impactful that previous
days. The European model (ecmwf) and GFS were favored across the region through the
forecast. Forecast confidence is very high.
Short term forecast...
a band of moisture continues to spread showers across portions of
Prince William Sound...the eastern Kenai...and into inland
locations from Anchorage to the western Kenai. This band of
moisture will pivot south through the day...with showers ending
from north to south. Eventually this band will pivot over Kodiak
Island this evening...with rain and snow impacting mainly the
eastern portion of the island before drying out by Wednesday
morning. Fog and low stratus will remain entrenched across the
Copper Basin until drier air filtering in from the north moves in
this evening. A strong low moving north into the Gulf will set up
a dry offshore flow pattern this evening through Thursday...with
the strongest gap winds peaking generally around late Tuesday evening
through Wednesday afternoon.
steady snow across the lower Kuskokwim Delta will diminish by middle
to late morning...with a general drying trend through Wednesday as
a northerly flow pattern develops. A front will approach from the
west late Thursday afternoon into Friday...spreading rain and snow
from west to east.
strong northerly winds and rain and snow showers will continue
across the eastern and central Bering and Aleutians through
Wednesday. A front will track into the western Bering Wednesday
then spread through the Bering while weakening Thursday and
Long term forecast...
The models are in decent agreement with the upper trough as it
moves over the Bering Strait on Saturday...but then quickly
diverge on Sunday and Monday. The European model (ecmwf) in particular as been the
most sporadic with this feature...digging deep troughs as far
south as the southern Gulf and bringing very active weather to the
southern Mainland. The GFS and GFS ensemble on the other hand has
been much more consistent in keeping a much weaker upper trough
over the central interior...with more typical cloudy and showery
weather over the southern Mainland. After coordination with wpc
this morning...the long term forecast after Saturday heavily uses
ensembles (ec and naefs)...which currently more closely resemble
the operational GFS. Future runs (in particular of the ec) need to
be closely watched however...as a deeper Gulf low would result in
a significantly different long term forecast into next week.
Marine...gale 150 165 170-174 178-179.
Ahsenmacher Oct 14