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fxus61 kokx 182059 
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Area forecast discussion 
National Weather Service Upton New York 
459 PM EDT Friday Jul 18 2008 


Synopsis... 
high pressure remains entrenched over the middle Atlantic tonight and 
Saturday before drifting farther offshore Saturday night and 
Sunday. A slow moving cold front will approach from the northwest 
on Sunday...becoming nearly stationary over the area into Monday. 
The front will push just east Monday night. The front then 
remains until the end of the week. 


&& 


Near term /until 6 am Saturday morning/... 
with high pressure anchored off the middle Atlantic coast the warm 
and humid airmass will remain in place. Although the heat and 
humidity remains there was enough mixing of drier air aloft across 
northeastern New Jersey to keep heat index values below 100...so 
the advisory was cancelled. Across New York City heat index 
values just reached the middle 90s and the advisory continues. 


With the short wave moving to the north scattered thunderstorms 
did develop and some of the storms could drift south...so will 
continue with slight chance into this evening across the northern 
tier of zones. 


For lows used a blend of the higher NAM and FWC numbers. 


&& 


Short term /6 am Saturday morning through Monday/... 
Saturday the heat and humidity continues with similar conditions 
to Friday. Again there will be some mixing of drier air in the 
afternoon with dewpoints lowering into the lower 60s. Heat index 
values in some isolated areas may approach 100 for an hour or so. 
Not widespread enough to issue an advisory. However New York City 
will see widespread heat index values in the afternoon between 95 
and 100. A weak shortwave will approach with a slight chance of 
showers and thunderstorms in the late afternoon into the evening. 


During Sunday cold front begins to sag into the area with lowering 
heights aloft. So peak of the heat will be shifted to the south 
and east. Also will be more clouds so combo will keep temperatures 
several degrees lower. With pooling of dewpoints ahead of the 
front Sunday afternoon and limited mixing the higher dewpoints 
with the lower temperatures could result in heat index values in 
New York City approaching 95. With the advisory currently up 
through Saturday evening will allow middle shift to re evaluate. 


With the upper trough digging into the region by late Sunday into 
Sunday night the chances for showers and thunderstorms will 
increase and continue into Monday. Will keep chance probability of precipitation through 
the period with the better chances Sunday night into Monday. 


&& 


Long term /Monday night through Friday/... 
models have continued the progressive trend with the upper level 
pattern. Main feature is currently over the Canadian rockies. After 
precipitation chances end Monday night...have lowered probability of precipitation for Tuesday to the isolated 
category. Mesoscale convective system prognosticated to develop over the northern plains then track 
across the country and bring thunderstorms and rain to the County Warning Area on Wednesday. Wednesday/Wednesday night 
therefore with the highest probability of precipitation. Heights remain suppressed across 
the northestern Continental U.S. So low probability of precipitation through the extended. Dry air at the 
middle/upper levels with the deep moisture remaining offshore. Temperatures warm 
through the extended but not extreme. Best chance for a relatively cool 
day is Wednesday at this time if the timing of the northern plains mesoscale convective system spreads enough 
cloud cover across the County Warning Area. If the mesoscale convective system ends up being a bit slower 
allowing for surface heating...strong afternoon thunderstorms will be possible. 


Low pressure off the Georgia/SC coast still prognosticated to track east of the County Warning Area as 
the upper level trough reaches the NE. Will continue to follow the 
trends of this source of deep moisture. 


&& 


Aviation /21z Friday through Wednesday/... 
VFR at most sites except kgon...which although back to VFR at the 
moment...should drop to MVFR visibility tonight and possibly tempo IFR 
late tonight. Otherwise...westerly flow flow surface and aloft has held 
the sea breeze at Bay at klga...but based on tjfk radar it now 
appears it should arrive by 22z or 23z...with winds shifting SW. 


Sea breezes should die down early this evening...with SW flow under 
10 knots tonight into Sat morning...and then return between 15z-16z at 
coastal terminals at speeds slightly higher than those of today... 
10-15 knots and highest at kjfk. 


Outlook for 18z Saturday through Wednesday...could see an isolated 
late day thunderstorm late Sat afternoon with localized sub-VFR and gusty 
winds...especially inland. Sub-VFR conditions in haze possible 
Sunday...especially in the morning...as dewpoints increase to near 
70. A slow moving frontal system should then bring chance of thunderstorms 
throughout from late day Sunday through Monday...which could linger 
into Tuesday as its parent upper level trough moves overhead. 


&& 


Marine... 
long fetch SW flow at or below 15 knots in combination with leftover 2-3 feet 
swells from Bertha should result in seas no higher than 4 feet through 
Sunday....so have dropped Small Craft Advisory for eastern ocean waters that was in 
effect for tonight. Seas could eventually build to 5 feet by Sun night 
into Monday as wind fetch backs to a more favorable southerly 
direction...but this still remains uncertain. 


Cold front for the beginning of the week has been trending 
faster...clearing the coastal waters Monday night. This results in an 
end to the southerly flow and therefore have decreases seas. A 
convective complex bringing a round of thunderstorms and locally higher 
seas is possible on Wednesday/Wednesday night. The exact timing is still 
uncertain. Weak high pressure looks to build into the region for the 
remainder of the week. Isolated to widely scattered thunderstorms possible due to the 
nature of the upper air pattern. 


&& 


Hydrology... 
mainly dry through Saturday night. Just isolated thunderstorms 
possible across the norther tier into this evening and across the 
region Saturday and Saturday night. Local amounts of a quarter to 
one half inch possible in any convection. The next chance of any 
significant rainfall will be Sun night through Thursday with 
convection ahead of a slow moving front. 


&& 


Okx watches/warnings/advisories... 
CT...none. 
New York...heat advisory until 6 PM EDT Saturday for nyz072>076. 
New Jersey...none. 
Marine...none. 


&& 


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

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