Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
553 am akdt Tuesday Mar 31 2015
Short term...at 09z Tuesday morning...WV imagery showed an upper low
positioned over the scntrl Gulf. Upper ridge was located
downstream from this feature over northwestern Canada...while a weakening
shortwave trough was lifting north over the Panhandle. Model guidance
shows the upper low over the Gulf will move east toward the coast of
southeastern Alaska by 12z Wednesday...with only minor differences in position noted
amongst suite of solutions due to variations in how the low
phases with a vorticity streamer approaching it from the west. At the
surface...low pressure will enter the eastern Gulf by
afternoon...while a ridge of high pressure builds north-northwest over the
southern/central Panhandle. This will result in a tightening pressure
gradient over Lynn Canal and cause southerly winds to strengthen to
small craft. Elsewhere...small craft seas will continue over the
outside waters...and west-southwesterly winds to 30 knots will eventually develop
over marine zone 41 along the southern periphery of the Gulf low.
Otherwise...stratiform precipitation will diminish over the northern
Panhandle as the day progresses...while bands of showers spread
inland ahead of the upper disturbance during the afternoon and
night...with the highest probability of precipitation expected to
occur over the southern half of the Panhandle. Sufficient cape will
exist for thunderstorms from the southern outside waters into the southern
Panhandle today/tonight. This has apparently been confirmed by an
isolated lightning strike located near Annette at 1230z.
However...the overall thunderstorm environment is forecast to be
marginal due to weakening ascent associated with the upper
low...and cold boundary layer temperature profiles /LCL
temperature at or below 3c...warm cloud depth less than 500 meters/.
Thus...will continue to monitor short term trends and update the
forecast for thunderstorm potential if confidence in additional
No change was made to pressure grids. Pop and quantitative precipitation forecast were updated
using a combination of GFS/ECMWF/arw/nmm.
Long term...the phasing of the upper level low and the vorticity
streamer described above will be complete Wednesday morning
extending showers for southeast into Wednesday. However, rainfall
amounts will begin to lighten as the day wears on, in part, due to
more anticyclonic curvature building with the building ridge over
the eastern Gulf. Probability of precipitation will be higher inland as the instability is
accentuated by the accent over the Coast Mountains.
All eyes will then focus on the storm south of Dutch Harbor this
morning. This storm swings a front across the Gulf
Wednesday afternoon into Wednesday night and into the Panhandle
Thursday with widespread 30 knots winds and steady rains moving
northeast, eventually into the Panhandle. New runs of the European model (ecmwf)
left the pack of models by advancing it faster. With the
uncertainty on speed, kept the forecast as is, but we must
communicate some uncertainty here. The inherited precipitation
field was blended with the 00z GFS and 00z ecwmf to advance it
farther east a little. Wednesday night, all models with the
exception of the NAM showing precipitation over even the the
interior Panhandle. Confidence not great on this with the
orientation of the advancing front, and tried to counter this
incorporating a little of the NAM. However, the 700 mb flow still
carrying more onshore component, and thus with gradual upglide,
felt we had to leave it in. The GFS suggesting rain bands may be
heavy at times on Baranof Island late Wednesday night and through
the southern Inner Channels Thursday. Winds will strengthen in
turn to small craft from the central Panhandle southward late
Wednesday night/early Thursday morning. But parallel gradients
should keep the far northern passages relatively light from the
The low begins to slide to the southeast into Friday, thus drying
out the northern Panhandle. Residual instability will persist over
the south enough to extend shower activity into the early weekend.
Some colder air will slide south of the Alaska Range in northwest
flow behind the low which may be enough to reintroduce some snow
mixing in with the dissipating showers on the northwest edge of
the precipitation field. The European model (ecmwf) is suggesting a healthy short-
wave riding eastward from the Bering Sea into the northern Gulf
late into the weekend bringing the possibility of some snow mixed
with the rain for the northern Panhandle Sunday night. GFS not as
keen. Thus was careful to both raise probability of precipitation and moderate temperatures
just a tad. Still left the chance of snow with rain for Juneau and
points north and northwest.
GFS and European model (ecmwf) used to update from late Wednesday night into
Friday. Only minor tweaks made for Wednesday into Wednesday
evening. Forecast confidence average for period.
Marine...Small Craft Advisory for pkz012-041.
Small Craft Advisory due to seas for pkz042-043-052.
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