Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
626 am akdt Friday Sep 4 2015
Short term...the latest satellite imagery shows a low pressure
system over the western Gulf with an associated weather front
over the central Gulf. This low and front will drift east through
the short term period with the front moving over the coastal areas
late this afternoon and through the Panhandle tonight. This
system has tropical moisture associated with the front as seen
from the precipitable water imagery precipitable water values of 1.2 to 1.5
inches are around 150 percent normal for this time of year.
Light rain has started in the Yakutat area which will increase
through the day, becoming heavy at times by the late morning
excessive rain rates will continue through the evening. Total rainfall
will be near 1.5 inches before the front drifts east
of Yakutat late tonight. Local rivers and streams will rise
significantly but at this time are expected to remain in their banks.
Rain will spread over the rest of the forecast area through this
morning and move south through tonight. The rain will the
heaviest along the outer coast with moderate rain over the north central
areas. Rain amounts will range from 0.5 to 1.25 inches through
A barrier jet associated with the front is producing gale force wind over
the northern Gulf at this time. As the front drifts east through today,
gales to small craft winds will spread into the eastern Gulf with seas
building to 8 to 10 feet. As the front approaches the eastern Gulf, small
craft winds will develop near Cross Sound by early this evening. The
Inner Channels will see relatively light winds through today but the
surface ridge will sharpen up this afternoon as the front moves east.
Minimal small craft winds will develop over northern Lynn and 15 ktn over
southern Lynn as a result. These winds will diminish in the evening and
switch to the north as the ridge gets pushed off to the east by the
front. Wind speeds will be less than 15 knots overnight as the pressure
gradient will be on the lighter side with parallel flow. Lighter winds
will persist over north/S channels tonight generally less than 15 knots but
the east/west channels will increase to 20 to 25 knots through tonight and
these conditions will spread over the southern half of the Panhandle
later on tonight.
Land winds will be the strongest along the coastal areas with some
higher gusts this afternoon over the northern Gulf and tonight
over the eastern Gulf. The Inner Channels will see some increase
in wind speeds but generally be 10 to 20 miles per hour.
Overall there is average forecaster confidence through the short
term period and a mix of the 06znam/GFS and 00z ec was used.
Long term...Hurricane Ignacio is still causing major divergence
in model solutions. Most recent best track from the central
Pacific hurrican center has the storm tracking across haida gwaii
late Monday night or early Tuesday. NCEP GFS ensemble track
guidance from 4pm yesterday afternoon fans out significantly east
of 150w, so confidence in the current forecast in terms of low
track and associated wind field beyond Monday afternoon is
decidedly below average.
Ahead of Ignacio, a weather front that moves ashore during the
short term forecast period will move south on Saturday. Tropical
moisture connection unrelated to Ignacio will result in moderate
rain rates for the central and southern outer coast Saturday
morning. Wrap around shower activity will keep much of Southeast
Alaska wet for Saturday night through Sunday night. A weak low
will bring more rain in on Monday and it current models are
merging this precipitation field with some of the rain associated with
Ignacio. Current forecast reflects moderate rain over the northern
half of the Panhandle, but widespread heavy rain for locations
south of Angoon Monday night.
Used a blend of GFS and European model (ecmwf) for updates to pressure and wind
over the Gulf through Sunday night, then trended towards wpc on
Monday. Pop and quantitative precipitation forecast from GFS, Canadian New Hampshire and European model (ecmwf). Little change
to temperatures. Overall forecast confidence is average through Sunday,
then falls to below average.
Marine...Gale Warning for pkz043-051-052.
Small Craft Advisory for pkz012-022-033-035-036-041-042.
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