518 
fxus61 kokx 121205 
afdokx 


Area forecast discussion 
National Weather Service New York New York 
705 am EST Friday Mar 12 2010 


Synopsis... 
high pressure slowly retreats to the northeast over southeast 
Canada through the weekend...as low pressure approaches from the 
west through Saturday night. This low passes just south of Long 
Island Sunday and Sunday night...then slowly pulls away to the 
east early next week. A dry cold front will cross the tri-state 
Tuesday...followed by high pressure building in through the 
middle of next week. 


&& 


Near term /until 6 PM this evening/... 
an area of light to moderate rain currently stretching from north 
central PA...southeast to the Delaware-Maryland-Virginia peninsula will slowly lift 
northeast today...as the low to middle level ridge axis slowly lifts 
to our northeast. Before the ridge axis passes by...moist low 
levels with dry air above will promote mainly patchy drizzle 
across the tri-state. Also acting to limit the spread of the rain 
to the northeast...is relatively low dewpoint air (low to middle 30s 
coming in from the east-northeast on low level east-northeast 
flow which will keep low level temperature-dewpoint spreads 
fairly large. Also a vorticity minimum at both 700 and 500 hpa 
will approach ahead of the area of rain...adding to low to middle 
level subsidence and drying. 


As a result have limited probability of precipitation this morning to slight chance east 
and chance west (with a small sliver of liklies far sw)...that 
increase to categorical west to slight chance far northeast by 
late afternoon (ridge axis still over far NE areas late this 
afternoon). 


With light precipitation (much of it evaporating in low levels 
causing evaporative cooling)...overcast sky conditions...and 
strengthening onshore flow...blended cooler met guidance with NAM 
2-meter temperatures for highs today - mainly middle 40s. 


&& 


Short term /6 PM this evening through Sunday/... 
GFS remains a fast and deep outlier with the cutoff low...mainly 
due to convective feedback issues ultimately leading to it 
eroding the ridge ahead of the cutoff low to quickly. The NAM and 
European model (ecmwf) were fairly similar so followed a blend through the short 
term. 


Expect precipitation to gradually become steadier Friday 
night...and then gradually increase in intensity 
Saturday...becoming heavy at times Saturday afternoon in response 
to an easterly low level jet increasing to 50-60kt...increasing 
low to middle level frontogenetic forcing...increasing quasi- 
geostrophic forcing (as indicated by q-vector convergence)...and 
increasing divergence aloft in response to the region becoming in 
the left front quadrant of a 85 knots jet 250 hpa streak. The rain 
will continue to be heavy at times Saturday night...with the low 
level easterly jet increasing in strength to 70-80kt. Based on 
this...have continued the Flood Watch for the entire County Warning Area Saturday 
and Saturday night - see the hydrology section of the afd for 
details on quantitative precipitation forecast amounts and flood threat. 


As the low tracks to the south Sunday/Sunday night...the pressure 
gradient relaxes due to the proximity of the low...and the low 
level jet weakens to 30-40 knots by Sunday evening. The low level 
flow becomes sheared Sunday night with flow at 850 hpa remaining 
east northeast...while from 950 hpa to the surface the flow backs 
to the north-northeast...reducing the low level moisture feed 
even further. As a result taper probability of precipitation down from categorical Sunday 
morning...to chance far west and likely remainder by late Sunday 
night. In addition with the cut off low tracking just to the 
south Sunday night...will have enough of the cold pool over northwest 
areas...that there should be some snow mixing in with the rain 
across the northern interior late Sunday night/Monday 
morning...especially across higher elevations. 


In addition to the precipitation...the storm will produce gusty 
easterly winds across the region from Saturday into Sunday night. 
The strongest winds will likely be from Saturday afternoon into 
Sunday morning corresponding with the period of the strongest low 
level jet. With the jet being 70-80 knots from 950-850 hpa Saturday 
night...there is a solid potential for Wind Advisory sustained 
winds...especially over coastal areas and higher elevations. In 
addition...depending on how strong the low level inversion ends 
up being...and how much stronger wind mix down with heavier 
rainfall...there is some potential for gusts to High Wind Warning 
strength (at least 58 mph). Will mention this threat in the 
severe weather potential statement...but with confidence in this at less than 50 percent...did 
not go with a high wind watch at this time. Most likely a Wind 
Advisory will end up being issued for at least coastal portions 
of the tri-state for some if not all of the time frame from 
Saturday afternoon into Sunday morning. 


With the low cloud cover and precipitation there will be little 
in the way of diurnal range through the short term...until the 
area gets on the backside of the cutoff low Sunday night and 
colder air comes in from the north and is also mixed down from 
aloft. 


Followed a blend of met/mav temperatures for lows tonight...a 
blend of cooler mav and NAM-2 meter temperatures on Saturday...a 
blend of mav/met/NAM 2-meter temperatures Saturday night and 
Sunday and a blend of mex/mex ensemble mean and NAM 2-meter 
temperatures Sunday night. Basically lows from the middle 30s to 
around 40 and highs in the lower to middle 40s..except across the 
northern interior areas Sunday night where lows in the lower 30s 
are currently forecast. 


&& 


Long term /Sunday night through Thursday/... 
used the more consistent European model (ecmwf) for the first part of the 
extended...and continued chance of rain (rain/snow northern 
interior Monday morning) - with likely probability of precipitation east Monday 
morning...tapering down to chance probability of precipitation Monday afternoon then to 
dry weather by late Monday night. 


A dry cold front should pass through Tuesday...followed by high 
pressure building into the middle of the week as split flow sets 
up over the eastern United States...with the a resultant dry 
northwest flow aloft over the region. 


Followed a blend of mex/mex ensemble mean/low mex ensemble member 
for highs Monday (reflecting impact of wetter solution than GFS 
based mex is based on)...then a mex/mex ensemble mean blend for 
the remainder of the extended. After another below normal day on 
Monday...expect temperatures for the remainder of the extended to 
run slightly above normal. 


&& 


Aviation /12z Friday through Tuesday/... 
MVFR ceilings and VFR visibilities this morning. Ceilings and 
visibilities lower to IFR as light rain develops after 16z...then 
remains through the end of the forecast. Some uncertainty with 
the timing of the lowering ceilings. In the metropolitan area ceilings 
could fall to high end IFR for a short period early this morning. 


Easterly winds from 060 to 090 at around 10 knots increase and become 
gusty this afternoon...with gusts 20 to 25 knots. Late tonight winds 
become sustained at 20 to 25 knots with gusts around 30. A developing 
low level jet will increase chances of low level wind shear after 
05z with winds between 1000 and 2000 feet running 40 knots to 50 knots. 


Kjfk afternoon hourly wind forecast in support of runway 
construction all wind directions are true...add 13 degrees for 
magnetic 
12/19z 07015g24kt 
12/20z 07015g26kt 
12/21z 07016g27kt 
12/22z 07017g28kt 
12/23z 08017g28kt 
13/00z 08018g28kt 
13/01z 08018g28kt 
13/02z 07018g28kt 
13/03z 07018g29kt 


Outlook 12z Saturday through Tuesday... 
Saturday and Saturday night...IFR in rain and fog. Strong gusty 
winds. Low level wind shear likely. 
Sunday through Monday...IFR likely into Sunday night...improving to VFR by 
Monday morning. 
Tuesday...VFR. 


&& 


Marine... 
with the tightening pressure gradient between low pressure over 
the central United States to the southeast coast...and high 
pressure over southeastern Canada expected to increase today and 
into Saturday will continue with the Small Craft Advisory and 
upgrade the gale watch to a warning. Winds aloft will be in the 
40 to 50 knots range but not sure how much of this will mix to the 
water surface. Winds aloft increase Saturday into Saturday 
night...70 to 80 knots for a time Saturday night. So will extend the 
Gale Warning into Saturday night. Will have to consider if these 
higher winds do mix down to Post a storm watch for Saturday 
night. 


Winds are expected to diminish Sunday as surface low pressure passes 
south of the waters and moves slowly east into Monday. 


With the powerful low approaching this weekend...seas of 14 to 18 
feet quite possible over the open ocean waters Saturday. 


Small Craft Advisory conditions should continue on most waters into Sunday night- 
Monday as the coastal storm slowly pulls away from the area. Ocean 
seas will be rather slow to subside thereafter...with long period 
swells over 5 feet lingering through at least Tuesday morning and 
possibly through Wednesday morning. 


&& 


Hydrology... 
spotty light rain/patchy drizzle today...will gradually give way 
to a steady rain during tonight across the entire tri-state. The 
rain will then continue through the weekend...with the heaviest 
rainfall occurring Saturday afternoon and night...and then to come 
to an end by late Monday night. Currently expect 2.5-4 inches over 
the lower Hudson Valley/NE NJ/NYC...and 2.5 to 3 inches further 
east with locally higher amounts. With precipitable waters around 
1 inch Saturday afternoon and night...there is the potential for 
rainfall rates of 1/2 to 1 inch per hour at times (though very 
likely not for an entire hour). With this prolonged period of 
heavy rainfall...with occasionally strong rainfall rates...there 
is a high potential for urban/poor drainage and river/stream 
flooding throughout the area Saturday and Saturday night. Residual 
river flooding could linger Sunday if heavy rainfall materializes 
as anticipated. If as suggested by some guidance...the heavier 
rainfall lingers into Sunday morning...the Flood Watch could end 
up being extended by later shifts. 


For NE New Jersey and the lower Hudson Valley...flash flood guidance is 
lower (especially for NE nj)...with wet antecedent conditions. In 
addition higher elevations in these regions still have some snow 
pack. Further east...saturated grounds across Long Island and 
southern CT have resulted in high water tables. These saturated 
grounds will be unable to absorb the rainfall...and poor 
drainage is quite likely/and significant basement flooding is 
also possible. 


&& 


Tides/coastal flooding... 
a persistent easterly flow...steadily increasing through 
Saturday...and increasing astronomical tides due to an 
approaching new moon...may result in widespread minor coastal 
flooding Saturday during the times of high tide with departures 
of 2 to 3 feet. Then Saturday night widespread minor coastal 
flooding during the high tide cycles is likely with locally 
moderate possible in the sound. Departures of 3 to 4 feet are 
possible. These levels will put parts of western Li sound over 
moderate benchmarks. This area will need to be monitored closely 
for a potential coastal Flood Watch. On the Atlantic side...any 
moderate coastal flooding will likely be confined to the South 
Shore back bays of western Li. Again...during the Sunday morning 
high tide cycle moderate flooding will be possible in the western 
Li sound with widespread minor along the South Shore. Locally 
moderate flooding will again be possibly especially in the South 
Shore back bays. 


In addition...beach erosion is likely for the South Shore beaches 
with high surf from easterly swells. Seas on the ocean are 
forecast to range from 15 to 18 feet Saturday night into Sunday. 


&& 


Okx watches/warnings/advisories... 
CT...Flood Watch from Saturday morning through Sunday morning for 
ctz005>012. 
New York...Flood Watch from Saturday morning through Sunday morning for 
nyz067>081. 
New Jersey...Flood Watch from Saturday morning through Sunday morning for 
njz002>006-011. 
Marine...Small Craft Advisory until 6 PM EST this evening for anz330- 
335-338-340-345-350-353-355. 
Gale Warning from 6 PM EST this evening to 6 am EDT Sunday for 
anz330-335-338-340-345-350-353-355. 


&& 


$$ 
Synopsis...pfm 
near term...pfm 
short term...pfm 
long term...pfm 
aviation...met 
marine...met 
hydrology...pfm 
tides/coastal flooding...met 

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