Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
446 am akst Wednesday Mar 4 2015
Short term...models over the last several days have erred on
precipitation amounts and coverage tonight and low temperatures for the
Panhandle. Though trends were correct...only a few hundredths of
rain have been recorded at a few locations (ahn aya) and snow is
not a factor when repeatedly for the last several days models
predicted temperatures at or near freezing for most areas in the
northern Panhandle. Our corrections were generally in the correct
direction for the most part but in particular NAM quantitative precipitation forecast has been way
off...hence the "rain likely" and mixed precipitation scenario has erred
for Juneau and most areas, first period. Some minor isolated foggy
conditions have accompanied some drizzle/light rain areas tonight.
Situation appears to evolve into a more serious precipitating
scenario later today and tonight as forcing is stronger. Elongated
north-S closed low migrates into the central Gulf and associated
frontal forcing becomes stronger later today and tonight. Stronger
onshore flow leads to a model consensus on significant precipitation
amounts although still only in the half inch range at most
locations. As front progresses east through the Panhandle precipitation
should migrate towards the south end at 12-15z Thursday.
With this in mind we are proceeding into a rainy period initiating
16-20z today through tonight. Winds won't be an issue except for
Clarence Strait where southerly flow is expected to reach Small Craft Advisory
level tonight. With no source of cold air for the low levels
(except for a bit of advection and initial cooling to wet bulb)
snow won't be a factor anywhere at sea level. Raised min temperatures
further by a degree or two at several locations...the result is
another day and night of temperatures 3-4 degrees above average
for the date. Dynamic component changes in the inherited forecast
were insignificant as very little changes were apparent in the
00z/06z model runs...ec/GFS/NAM new fields were very similar to
previous runs. Made a few wind tweaks offshore and over some of the
channels on the southern half. Pop/quantitative precipitation forecast and other thermodynamic updates
were made with an ec/NAM blend along with inherited grids.
Long term...weak elongated trough extending southward from the western Gulf
into the northern Pacific is forecast to deamplify and eject east-northeast toward the
Panhandle by Thursday afternoon as a second trough digs east-southeast from the
Bering Sea and Aleutians. Rain chances will increase over the
Panhandle with the ejecting trough...highest pop and quantitative precipitation forecast are
forecast to occur over southern areas where moisture flux is greatest.
Rain then diminishes Thursday night as the wave exits into British
Columbia. Quasi-zonal flow aloft then develops over the Gulf into
southeastern Alaska Friday and Sat.
Potentially significant airmass change is forecast to occur sun
and Monday...as -40c air at 500 mb aids in strong midlevel height
falls resulting in trough amplification over the northern Gulf.
Midlevel trough will be accompanied by a cold front in the low-
levels that is forecast to enter the northern Panhandle Sun
morning...and then progress east-southeast across the remainder of the
Panhandle by Sun night. 850 mb temperatures are forecast to be at or below
-20c over interior Alaska and the Yukon...and -10 to -20c over southeastern
Alaska. Model guidance shows this airmass persisting over the region
through the remainder of the long term period.
Given consistency in model guidance...began a modest downward
trend in temperatures behind the cold front for the sun-Wednesday time
period. The cold airmass will make snow probable during this time.
However...large-scale ascent is not forecast to be strong...and
low-level trajectories will generally be from the interior of Alaska
and Yukon resulting in a relatively dry environment.
Thus...snowfall is currently expected to be light. In
addition...cold midlevel temperatures and steepening lapse rates
combined with weak Post frontal ascent prompted shower/convective
wording Sun night through Tuesday.
Nudged inherited pressure grids toward the Gem/ECMWF/NAM through
12z sun...then maintained wpc guidance thereafter. Pop/quantitative precipitation forecast was
refreshed with GFS/ECMWF...though changes to previous forecast
Marine...Small Craft Advisory for pkz036.
Small Craft Advisory due to seas for pkz043-051.
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