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Area forecast discussion 
National Weather Service New York New York 
111 PM EST Sat Nov 21 2009 


Synopsis... 
high pressure slowly builds into the northeastern United States 
through Sunday...then slowly retreats into the Canadian maritime 
provinces through early next week. Meanwhile...an area of low 
pressure will move up the southeast coast Monday...then continue 
northeast to a position east of Cape Cod by Wednesday morning. 
Weak high pressure builds in from the north Wednesday. Another 
storm system will then impact the tri-state to close the upcoming 
work week. 


&& 


Near term /until 6 PM this evening/... 
high clouds thinning early this afternoon. Low St/SC over central 
New York/PA is drying up nicely. This is resulting in mostly cloudy 
skies. The forecast remains generally unchanged except for a minor 
tweak to cloud cover (generally downward). Temperatures approaching 
afternoon highs at this time...with no further updates planned. 


&& 


Short term /6 PM this evening through Monday night/... 
ridging building in from 700 hpa to the surface should create 
enough subsidence for mainly clear sky conditions tonight...and 
allow for decoupling of the boundary layer to produce light winds 
at the surface. As a result should have decent radiational 
cooling tonight...with lows in the 30s (around 40-middle 40s NYC 
metro). These values are near normal...except 3-5 degrees above 
normal in the NYC metropolitan and followed cooler met guidance (mav too 
warm given expected radiational cooling and relatively shallow 
nature of cold air moving in). 


Deep layer ridging builds in Sunday...limiting mixing to around 
950 hpa and minimizing any cloud cover. So even with abundant 
sunshine...will only have highs around 50 - or near normal for 
this time of year). 


Ridge axis slides offshore Sunday night...allowing for onshore 
flow to set up...but damming surface high will continue low level 
cold advection...as a result will end up seeing lows a couple of 
degrees cooler than tonight (once again used cooler met 
guidance). 


GFS and European model (ecmwf) have a flatter base to the surface ridge than do 
the NAM/CMC ggem Monday-Monday night. This difference allows for 
low level moisture to get further inland in the GFS/European model (ecmwf) than in 
the NAM/ggem. The sref supports the structure of the GFS/European model (ecmwf) so 
have opted for a GFS/European model (ecmwf) blend for the remainder of the short 
term. 


With over running setting up across the entire tri-state Monday 
afternoon bring chance probability of precipitation to all areas by late afternoon and 
continue Monday night. Better chance of rain Monday night with 
500 and 700 hpa shortwaves crossing the area as well. For 
temperatures used met/mav blend Monday and mex/mex ensemble mean 
blend Monday night...with highs near normal on Monday and 4-8 
degrees above normal Monday night (cloud cover/rain limiting 
diurnal range). 


&& 


Long term /Tuesday through Friday/... 
low confidence forecast in the extended with all the models 
showing something different. Ggem is most progressive with 
coastal low for Tuesday...gefs slowest and most suppressed to the 
south (like yesterday/S 12z ecmwf) and GFS and European model (ecmwf) in between 
(with European model (ecmwf) more progressive than the gfs). 


For the next system moving through the Great Lakes into the 
northeast during the middle to late part of next week. The GFS 
maintains two distinct cutoff lows (actually linking the first 
one which it has passing to our northwest Wednesday with the departing 
storm system early this week)...while the European model (ecmwf) absorbs the 
second chunk of upper level energy into the first cutoff low with 
this low slowly crossing the NE late next week. The ggem 
maintains two distinct circulations without any phasing...but 
does suggest by looking at the individual systems that the European model (ecmwf) 
solution is plausible. For now have opted for a GFS/European model (ecmwf) blend 
in the extended...but as noted in the first sentence of the first 
paragraph...this is a low confidence forecast...so expect 
changes. 


Appears for now that rain will linger into Tuesday...then taper 
off Tuesday evening as coastal low departs (chance probability of precipitation Tuesday 
and slight chance probability of precipitation Tuesday night). While there is some 
suggestion Wednesday will end up dry with high pressure building 
in...if GFS ends up being right....could see some light showers 
Wednesday...so slight chance probability of precipitation. On the flip side...GFS would 
have things dry Wednesday night/Thursday...but European model (ecmwf) brings in 
precipitation ahead of consolidated cutoff...so have slight 
chance probability of precipitation both periods. Both models suggest rain from cutoff 
low Thursday night/Friday (possibly mixed with/changing to snow 
north/west of NYC depending on exact track of cutoff low and how much 
low level cold air actually ends up moving in with it)...so 
chance probability of precipitation. 


For temperatures used a mex/mex ensemble mean blend with the high 
member of the mex ensemble mean blended in Wednesday night- 
Thursday night to reflect warmer European model (ecmwf) solution at those times. 


&& 


Aviation /18z Saturday through Thursday/... 
northwest flow under 10 knots should veer north tonight...and then NE with speeds 
increasing to around 10 knots...higher at NYC/coastal terminals... 
toward or just after daybreak Sunday morning as high pressure over 
the the Ohio Valley builds NE into New England. Radiation fog with 
MVFR conditions may is likely late tonight at kswf and kgon...and 
will have to monitor other terminals outside the NYC urban heat 
island for this possibility close to/just after sunrise. 


Outlook 18z Sunday through Thursday... 
Sunday afternoon/night...VFR. 
Monday...sub-VFR possible. 
Monday night-Tue...sub-VFR likely. 
Tuesday night-Wed...becoming VFR. 
Thursday...sub-VFR possible. 


&& 


Marine... 
high pressure building into the area waters today results in west 
to northwest winds. Winds and seas will remain below Small Craft 
Advisory levels. The high builds into southeastern Canada and 
northern New England tonight and Sunday and strengthens. Winds 
shift to the north the to the northeast. Sunday night and Monday 
the high continues to strengthen...there is also the potential 
for a coastal low to move into the high. This will increase the 
winds...gusts...and seas across the area with small craft 
conditions becoming likely by Sunday night. Winds could drop 
below small craft levels Tuesday as the forecast waters are 
between two low pressure systems. Another low may approach from 
the west during Tuesday and into Wednesday. There is some 
uncertainty to the track and timing of the low at this time. Seas 
on the ocean will likely remain at small craft levels into 
Wednesday. 


&& 


Hydrology... 
dry through Monday morning. A quarter to a half an inch of 
rainfall is possible Monday evening through Tuesday with 
locally higher amounts within the reasonable realm of 
possibilities. More precipitation is expected towards the end of 
next week. 


&& 


Okx watches/warnings/advisories... 
CT...none. 
New York...none. 
New Jersey...none. 
Marine...none. 


&& 


$$ 
Synopsis...bs 
near term...bs 
short term...pfm 
long term...pfm 
aviation...met 
marine...bg/pfm 
hydrology...bg/pfm 

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