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Area forecast discussion 
National Weather Service Upton New York 
655 am EST Thursday Nov 20 2008 


Synopsis... 
unseasonably cold air will remain through the weekend...as a series 
of upper level disturbances move across. The first of these will 
move through early this morning...followed by another late tonight 
into Friday...and the last in the series moving through during 
Friday night. Strong high pressure pushing southeast from central 
Canada into the plains states through Friday will then build 
eastward this weekend. 


&& 


Near term /until 6 PM this evening/... 
approaching middle level vorticity with accompanying 800 mb trough is currently 
sparking a band of snow showers across central New York and NE PA. Ewr 
tdwr showed some of these making it into northern New Jersey...and think 
these will move across the rest of the County Warning Area early this morning...with 
low chance pop (and a possible quick dusting) over inland sections 
and slight chance elsewhere. After this disturbance moves through... 
cyclonic flow and weak positive 800 mb Theta-E advection beneath an 
inversion should help maintain partly to mostly cloudy skies this 
afternoon. For maximum temperatures have undercut mav...which was off by about 
2-5 degrees yesterday... with middle 30s inland and upper 30s for NYC 
metropolitan and the coast. 


&& 


Short term /6 PM this evening through Saturday/... 
flow aloft turns west and then SW tonight as a strong middle level 
shortwave digs southeast toward the upper Great Lakes...with weakly 
cyclonic 800 mb flow and middle level inversion helping maintain partly 
cloudy skies. Another middle level vorticity with accompanying surface-800 mb 
reflection moving toward the area in advance of the amplifying Great 
Lakes trough will bring increasing clouds after midnight...and 
perhaps a period of light snow late tonight into Friday morning... 
mainly to the NYC metropolitan area and portions of the lower Hudson 
Valley. With the strongest forcing passing just south of NYC and 
Long Island...have only forecast chance pop at this time...with any possible 
accumulation on the light side...a dusting to possibly an inch... 
possibly impacting the morning commute. 


The amplifying trough to the west should also initiate cyclogenesis 
off the middle Atlantic coast Friday afternoon and night while becoming 
negatively tilted...and forecast models are in close agreement that 
the developing surface low should pass well southeast of the 40/70 
benchmark...so do not expect anything more than a slight chance pop 
for the afternoon across NYC and Long Island. As this upper trough 
swings across Friday night...expect there could be a few widely 
scattered snow showers across the interior. Temperatures on Friday should be 
held close to those of Thursday due to cloud cover and any precipitation....once 
again middle/upper 30s. 


Winds pick up Friday night into Sat as the offshore low intensifies... 
pulling down yet another batch of unseasonably cold air...with lows 
from the upper teens to middle 20s both Friday/Sat night...and only rising 
to within a couple of degrees either side of freezing on Sat. 


&& 


Long term /Saturday night through Wednesday/... 
cold air advection with northwest winds will continue to keep temperatures 
below normal on Sunday. Temperatures will moderate somewhat on Monday 
as surface high pressure drifts off the coast and winds shift to 
the west-southwest. 


Low pressure forecast to develop across the upper Great 
Lakes/Midwest could impact the region late Monday into Wednesday. 
There remains model uncertainty about the timing and intensity of 
this system. Model runs have been trending warmer...though precipitation 
may still start as light snow or a mix in interior locations Monday 
night. Have kept low pop for Wednesday to reflect uncertainty in model 
solutions. 


&& 


Aviation /12z Thursday through Monday/... 
localized MVFR conditions in -sn at kswf through 14z this 
morning...otherwise VFR. Northwest winds early will back to the west 
during the morning and then back to the northwest tonight remaining less 
than 10 knots (around 10 knots this morning and afternoon NYC 
terminals). Low end VFR through the afternoon...then scattered out by 
evening. Another middle and upper level disturbance approaches 
tonight...but 06z guidance has precipitation passing to our south. Ceilings 
will lower after 06z and can't rule out a few flurries late 
tonight at NYC terminals just yet. 


Outlook for 12z Friday through Monday...generally VFR Friday 
through Sunday. Brief isolated sub VFR conditions are possible Friday 
morning. More flurries and isolated MVFR conditions possible Friday 
night into Sat. Sub VFR conds possibly developing on 
Monday...depending on timing of next frontal system. 


&& 


Marine... 
winds and waves will remain below advisory levels through Friday 
morning. Low pressure will develop and intensify southeast of the 40n 70w 
benchmark on Friday causing the pressure gradient to increase 
between this storm and strong Canadian high pressure over the 
Midwest. Small Craft Advisory conditions will likely develop on the ocean waters by 
late Friday afternoon. As the high moves east and the low moves north 
while continuing to strengthen Friday night and Sat...the gradient will 
remain tight with Small Craft Advisory conditions...possibly gale force winds on the 
waters. Do not have the confidence to raise a gale watch which would 
be primarily for the 4th and 5th periods as it appears it will be a 
close call. Scenario looks very similar to Tuesday/S storm...which 
just barely reached gale force gusts on the eastern sound and ocean 
waters. So will let the day shift have another look at it and decide 
if a gale watch is necessary. However...will hoist the Small Craft Advisory for the 
ocean waters Friday afternoon and early evening. 


The pressure gradient starts to relax Sat night...but flags will 
likely be needed into Sun morning until center of the high builds 
over the eastern Seaboard. 


Small Craft Advisory conditions will likely return on Monday and Tuesday with the 
next clipper/coastal system. 


&& 


Hydrology... 
no significant precipitation expected over the next 7 days. 


&& 


Okx watches/warnings/advisories... 
CT...none. 
New York...none. 
New Jersey...none. 
Marine...Small Craft Advisory from 1 PM to 6 PM EST Friday for anz350- 
353-355. 


&& 


$$ 

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