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Area forecast discussion 
National Weather Service Upton New York 
1000 am EDT sun Jul 20 2008 


Synopsis... 
high pressure over the western Atlantic will slowly give way to an 
approaching cold front over the Great Lakes states. The front is 
forecast to track slowly east over the next couple of days... 
becoming nearly stationary across the tri-state area Tuesday into 
Wednesday. This will mean a continuation of the warm and humid 
weather...but also increased chances for showers and thunderstorms. 


&& 


Near term /until 6 PM this evening/... 
the latest loop of Sat images and observations are showing 
increasing instability and moisture across the region ahead of a 
weak short wave. The highest 3 hour pressure falls are across western New York 
state...just ahead of an approaching wave along a frontal 
boundary. The lowest lifted index values of around minus 6 cover 
southeast PA and western New Jersey...which like yesterday is where a weak pre 
frontal trough will develop. Bands of cumulus clouds will develop 
within the upstream unstable air and advect east toward the area this 
afternoon. 


More stable air in the marine boundary layer behind sea breezes 
should prevent the development of thunderstorms near and east of NYC 
including Long Island and coastal CT. 


Based on this information...probability of precipitation for thunderstorms will be increased to around 30 
percent west and north of NYC and north of sea breeze fronts. Because there is a 
potential for an isolated or widely scattered severe cells...a slight 
chance of severe will be highlighted in our hazardous weather outlook. 


As for heat...expect highs to be a few degrees cooler with a more 
onshore flow at the coast...weakening cap (instability cu)...and 
gradual height falls. Heat indices remain below the 95 degree 
threshold needed for a heat advisory for NYC. This forecast is 
contingent on dew points across NYC mixing into the lower 60s 
(possibly upper 50s). MOS guidance has been notoriously too high 
with surface dew points this warm season as was evidenced in the surface 
observation on Sat. Undercut MOS dew points by about ten degrees in and 
around NYC. 


Some convective debris over the area this afternoon and instability 
cumulus will allow for partly to mostly sunny skies. Daytime highs will 
remain unseasonably warm with readings into the lower 90s across 
the interior and metropolitan New York...80s elsewhere. Highest humidity levels 
will be across Li and coastal CT where an onshore flow and shallower 
mixed layer will feature dew points around 70. 


&& 


Short term /6 PM this evening through Tuesday night/... 
convection will ramp up this evening as a short wave interacts 
with Lee trough/differential heating convection across the hills 
north and west of NYC. Moderate instability..weak shear 
environment not conducive to severe weather...but pulse severe 
storms a possibility. More importantly...precipitable water values increasing to 
around 1.75 inches supports localized heavy rainfall. Steering 
winds from the west of 15 to 20 knots should prevent any one cell 
from sitting in one spot for a long period of time. Any east-west 
oriented mesoscale scale boundaries will be a concern for training 
echoes. Good chance probability of precipitation forecast for the lower Hudson Valley and 
interior SW CT...decreasing to the southeast. Both the 00z GFS and NAM 
take convection to the coast late tonight as short wave impulse 
and upper jet dynamics lift across the NE. 


Chance for convection continues through the period as heights 
gradually fall with the amplification of the upper trough over the 
Great Lakes and NE states. Convection initiates along the Lee trough 
and higher elevations (differential heating) each after. Deep-layer 
shear gradually strengthens during the period increasing the 
severe weather threat. Heavy rainfall threat at this times looks 
to be north of the forecast area where best frontogenetic forcing 
resides. 


At this time...tropical moisture associated with T.S. Cristobal 
looks to stay to the south and east of the area. However...as the 
cold front stalls out across the area Tuesday and Wednesday...high precipitable water and 
a digging upper trough are all that needed for a heavy rainfall 
event during the warm season. Chance probability of precipitation continue through the middle 
week period...but these will likely be raised should models 
continue to trend toward a wetter scenario. 


Warm...muggy weather will continue through Tuesday...but convective 
debris and gradual cooling of the vertical temperature profile will allow 
for slightly cooler daytime highs each day. 


&& 


Long term /Wednesday through Saturday/... 
12z European model (ecmwf) has come around to the more progressive solution of the 
00z GFS. Cold front moves through Wednesday night/early Thursday with drying 
and near seasonable temperatures to follow for the end of the week. Heavy 
rainfall threat exists on Wednesday with boundary stalled out across the 
tri-state area. 


&& 


Aviation /14z Sunday through Thursday/... 
VFR conditions prevail in most places at this time...but the low stratus 
that developed along the southern New Jersey coast did make it to kisp. 
This should burn off by middle morning. Otherwise...could see some 
morning haze with MVFR conditions at kswf/kgon...which may persist 
well into the afternoon at the latter. 


SW winds should increase to 10-15 knots in most places today...perhaps 
close to 20 knots at kjfk via afternoon sea breeze. Cannot rule out an 
isolated afternoon thunderstorm...but impact on any specific terminal 
impossible to pinpoint at this time. 


Better chance for showers/thunderstorms may come to NYC area terminals 
tonight toward evening as an mesoscale convective system moves east from Northwest 
Ohio...but have only explicitly forecast sub-VFR conditions at kswf 
until evolution of this system becomes clearer later today. Coastal 
terminals may also start to see some low stratus late...along with 
low level wind shear tonight as southerly low level jet Cranks up to 35-40 knots per 
00z NAM forecast. 


Outlook for 12z Monday through Wednesday... 
sub-VFR conditions are possible via showers/thunderstorms at all terminals 
each day through Wednesday. Patchy haze also possible...and low stratus 
possible at coastal terminals into early Monday morning. 


&& 


Marine... 
seas up to 5 feet at this time at the ocean buoys may briefly come down to 4 
feet by late morning or early afternoon...but should ramp back up to 
5-6 feet tonight as SW flow increases to near 20 knots between offshore 
high pressure and an inland low pressure trough. Therefore a Small Craft Advisory for 
hazardous seas continues for the ocean from Fire Island to Montauk 
Point...and has been extended westward to include all ocean waters 
for tonight. 


A moderate risk of rip currents will continue today...as S-SW flow 
increasing to 15-20 knots this afternoon generates 4-5 feet wind waves... 
and as leftover 1-feet 9-second period swell from Bertha continues to 
impinge on the beaches. 


Tropical Storm Cristobal should pass well southeast of the waters Monday 
afternoon/night per TPC forecast...and may send enough swell 
northward to keep Small Craft Advisory conditions in place on the ocean through Tuesday. 


&& 


Hydrology... 
scattered convection the next couple of days could evolve into a more 
significant rainfall event late Tuesday and into Wednesday as a cold front 
becomes stationary across the area. 06z GFS hinting at several 
inches of rainfall across the region. Flash Flood Watch may be 
issued early in the week. 


&& 


Okx watches/warnings/advisories... 
CT...none. 
New York...none. 
New Jersey...none. 
Marine...Small Craft Advisory for hazardous seas from 6 PM this evening 
to 6 am EDT Monday for anz355. 
Small Craft Advisory for hazardous seas until 6 PM EDT Monday 
for anz350-353. 


&& 


$$ 
Synopsis...dw 
near term...gc 
short term...dw 
long term...pfm 
aviation...bg 
marine...bg 
hydrology...dw 

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