Weather

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Area forecast discussion 
National Weather Service Upton New York 
659 am EST Friday Nov 21 2008 


Synopsis... 
a deepening upper trough over the Great Lakes region will swing east 
across the area tonight into Saturday...reinforcing the cold air 
mass already in place over the tri-state area. High pressure will 
slide south of the area Saturday night into Sunday...and pass off 
the coast Sunday night while a warm front lifts north of the area. 
Low pressure will approach on Monday...and move over the area on 
Tuesday. High pressure may return by middle week. 


&& 


Near term /until 6 PM this evening/... 
have updated to bring in middle level cloud deck sooner than originally 
forecast...which should result in a mostly cloudy day for the NYC 
metropolitan area. 


Skies are starting off with a middle level cloud deck moving into the 
NYC metropolitan area approaching from the west...in association with a 
weak low pressure trough over central Pennsylvania. Strong low level 
convergence in vicinity of this trough has produced persistent snow bands 
over central PA...and these should remain well south of the area as 
the trough shifts southeast toward southern New Jersey and the Delaware-Maryland-Virginia. 


An amplifying upper trough to the west should spur cyclogenesis well 
off the middle Atlantic coast by this afternoon as it begins to take 
on a neutral/negative tilt. Precipitation with this system should also 
remain to the south/east...but enough low level moisture should 
advect eastward this afternoon to lead to a mostly cloudy afternoon 
over the lower Hudson Valley and NYC metropolitan...with perhaps a few late 
day flurries via weak low level convergence. Maximum temperatures will remain 
on the cold side...with upper 30s for NYC metropolitan and Long Island... 
and middle 30s elsewhere. 


&& 


Short term /6 PM this evening through Saturday night/... 
as the coastal low wraps up and passes well outside the 40/70 
benchmark...strong cold air advection will commence and reinforce the cold air mass 
already in place...with lows both Friday night/Sat night from the upper 
teens to middle 20s...and highs on Sat only 30-35. Meanwhile...as the 
parent upper system closes off over New England...a strong vorticity 
rotating around the developing cutoff low could bring a few flurries 
late tonight...possibly lingering over eastern CT/Long Island into 
early Sat morning. 


&& 


Long term /Sunday through Thursday/... 
high pressure building to the southern middle Atlantic coast on Sunday 
should bring a slight moderation in temperatures...a little shy of that 
predicted by GFS MOS...with highs Sunday near 40...and lows Sunday 
night in the 20s to lower 30s. A warm front passing north of the 
area Sunday night should bring some middle level clouds... but any 
associated precipitation should remain north/west of the area. 


It now appears that a developing low pressure system to the west 
should hold off long enough to warrant chance pop only as far east 
as Orange County by afternoon. This low could bring rain/snow 
showers Monday night...but temperatures should warm up enough on Tuesday to change 
precipitation to all rain. Tricky forecast beyond this point as the 00z GFS 
lingers a vertically stacked low over the northeast with unsettled 
weather through much of the remainder of next week...while the 00z European model (ecmwf) 
suggests the low should begin lifting out by Wednesday. Have sided with 
the European model (ecmwf) as it usually performs better in the long term...and also 
offers better forecast continuity...but forecast confidence is not 
very high. 


&& 


Aviation /12z Friday through Tuesday/... 
generally VFR conditions through the forecast period. Some scattered 
flurries are possible across the lower Hudson Valley late this 
afternoon...but too uncertain and chances too low to include at this 
time. Meanwhile...low pressure off the Carolina coast will track 
NE and rapidly strengthen over the Atlantic this afternoon and 
tonight. Models have been consistent in keeping precipitation 
offshore...with ceilings and visibilities remaining VFR. However...if the low 
does track farther to the west...the New York City and coastal 
terminals may see some light snow with MVFR conditions. With the 
low deepening and colder air moving in behind it...northwest winds 
increase and become gusty around 00z Saturday. 


Outlook for 12z Saturday through Tuesday...VFR through Sunday. 
Gusty northwest winds continue Sat...possibly lingering into Sat night at 
NYC metropolitan terminals...then subside. Sub-VFR conditions may 
develop on Monday and continue into Tuesday...depending on the 
timing and track of the next frontal system. Rain and snow will be 
possible inland...but currently looks to be all rain at the metropolitan 
terminals. 


&& 


Marine... 
Small Craft Advisory remains as is. Low pressure will develop and intensify southeast of the 
40/70 benchmark today...causing the pressure gradient to increase 
between the low and strong high pressure over the Midwest. 
Small Craft Advisory conditions will likely develop on the ocean waters by late this 
afternoon...and expand to the rest of the waters this evening. 
The high will build east as the intensifying low tracks north 
tonight and Sat...keeping the gradient tight with strong Small Craft Advisory 
conditions continuing on all waters. Models have been consistent 
with keeping gusts just below gale force and scenario looks very 
similar to Tuesday...where only occasional 35-knots gusts occurred on 
the eastern sound and ocean waters. So...do not anticipate gales... 
but would not be surprised to see an occasional 35-knots gust on the 
ocean east of Moriches Inlet. 


The pressure gradient starts to relax Sat night...but flags will 
likely be needed into Sun morning...possibly into Sun night on the 
ocean east of Moriches Inlet for seas...until center of the high builds 
over the eastern Seaboard. Small Craft Advisory conditions will likely return on Monday 
with the next clipper/coastal system. Gales may be possible Monday 
night into Tuesday. 


&& 


Hydrology... 
low pressure could bring significant precipitation to the area Monday 
night into Tuesday...but at this time it appears that the strongest Atlantic 
low level inflow and heaviest precipitation is more likely to set up over 
eastern New England. 


&& 


Okx watches/warnings/advisories... 
CT...none. 
New York...none. 
New Jersey...none. 
Marine...Small Craft Advisory from 6 PM this evening to 6 PM EST 
Saturday for anz330-335-338-340-345. 
Small Craft Advisory from 1 PM this afternoon to 6 PM EST 
Saturday for anz350-353-355. 


&& 


$$ 
Synopsis... 
near term... 
short term... 
long term... 
aviation... 
marine... 
hydrology... 

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