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FXAK67 PAJK 271353
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
453 AM AKST Mon Feb 27 2017

.SHORT TERM...Active but uncertain weather pattern starting Monday
night. The northerly trough digging down from the AK interior
that has been discussed in previous long range forecasts will
finally move over the northern AK Gulf tonight. The high pressure
ridge over the gulf that has been providing mostly clear and dry
weather to the panhandle will shift to the west with an upstream
blocking pattern developing. At the surface a closed low will form
by Monday evening over the northern gulf then begin to slowly
shift to the southeast. Associated frontal band will cross over
the eastern gulf by this afternoon then track over the panhandle
this evening. Far northern areas including Yakutat expected to
avoid majority of precip from this first event. As temperatures
will cool with northerly flow, precip for the region will be
mainly snow, with still some rain mix over the southern panhandle
in the evening. As of now precip for the central regions should
have highest intensity late tonight then southern areas in the
early morning. Calculated snow accumulations in the 2 to 6 inch
range with 15 to 20 to 1 snow ratios and lower ratios for area
with temps at or near freezing. Model soundings have a deep moist
layer in place and good dendritic growth zone tonight but a start
a drying trend at the surface early in the morning which would
begin to diminish snow rates. Generally light winds over the inner
channels today but then increasing northerly winds for locations
north of Frederick Sound tonight and increasing southerly winds
to the south. Gale force winds on south side of the gulf low over
the off shore water and increasing outflow winds for the
northeast gulf coast and Yakutat Bay.

Still high uncertainty with the exact track of the low center and
timing of the front, even less than 24 hours into the forecast
period. In addition have a fair spread with QPF and thus a large
spread in potential snow amounts. With the low moving in from the
north not getting a sustained feed of moisture for the central
panhandle, which would limit snow amounts, and slightly warmer
temps with SW onshore flow for the southern panhandle, limiting
snow accumulation there. But overall deep layer moisture, cold
air in place, dendritic growth zone and a few model runs putting
out almost twice the 3 hour QPF could produce higher snow
accumulations. While models initially in good agreement have more
of a spread on position of the front between the ECMWF/Canadian
and GFS/NAM by Monday evening. Decided to use an ensemble approach
by making nudges to models with higher winds/QPF but limiting the
changes. Forecast confidence is below average.

.LONG TERM...High confidence in an accumulating snow event extends
into the long range forecast period beginning Tuesday morning.
Lower confidence in snow totals, but feel at least advisory
amounts will continue for much of the panhandle through the day
Tuesday. At this time, think the far northern inner channels as
well as the Yakutat area will not see heavier accumulations.
Trouble spot is the Juneau area, where accumulations are less than
advisory amounts, but not by much.

Upper level circulation will continue to draw cold air from the
interior out over the gulf and then over the panhandle with little
to no airmass modification. Thus, the typical transition from snow
to rain that often occurs with an overrunning event is not
expected except for the far southern zones. Much later in the
week, another system will approach the panhandle from the
southwest on Thursday, but warmer low level air will probably only
affect the far southern zones as the parent low will be fully
occluded and cut off from the warm sector by the time it reaches
southeast Alaska. Net effect is snow or snow likely for most of
the panhandle for most of the coming week.

Low confidence in the position and track of the low early on in
the extended range forecast as evidenced by very poor model
agreement. Consequently, low confidence also extends to the wind
forecast for the eastern gulf through Wednesday night.

No changes to previous pressures or winds. Updated PoP and QPF
with a blend of the Canadian, GFS, and SREF early on, then
Canadian and ECMWF through Friday night. Temps adjusted using both
ECMWF and GFS MOS through Friday, then GFS Ensemble thereafter.
Overall forecast confidence is below average.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory from 3 AM to 6 PM AKST Tuesday for
     AKZ026-028.
     Winter Weather Advisory from midnight tonight to 6 PM AKST
     Tuesday for AKZ021>024-027.
     Winter Weather Advisory from midnight tonight to 9 AM AKST
     Tuesday for AKZ020.
MARINE...Small Craft Advisory for PKZ022-041>043-051-053.
     Small Craft Advisory due to Seas for PKZ052.
&&

$$

PRB/Fritsch

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