806 
fxus61 kokx 281940 
afdokx 


Area forecast discussion 
National Weather Service Upton New York 
340 PM EDT Thursday Aug 28 2008 


Synopsis... 
high pressure centered over the Canadian Maritimes will head farther 
east tonight. A cold front thins approaches from the west during 
Friday and Saturday. The front will move through the area Saturday 
night. High pressure returns Sunday and remains over the area 
through middle week. 


&& 


Near term /tonight/... 
500 mb ridge over the area remains as we're in 
between a closed low south of Nova Scotia and an approaching 
longwave trough to the west. Okx sounding from this morning showed 
plenty of dry air in the low to middle levels...and moisture to the 
west over PA and western New York has had a difficult time moving 
eastward into the high pressure ridge. The trough to the west will 
continue to draw nearer...however...the low in the western 
Atlantic is prognosticated to move just about due north. I do think there 
will be an eastward shift of the rainfall during the night...but 
not far enough east to affect our County Warning Area. This thinking is along the 
lines of the NAM. Will therefore keep rain out of the forecast. 


&& 


Short term /6 am Friday morning through Sunday/... 
low pressure to the east continues to drift north towards Nova 
Scotia while cold front associated with the 500 mb trough to the west 
reaches the eastern Great Lakes by Friday evening. The ridge 
protecting US from rainfall weakens through the day. GFS now brings 
quantitative precipitation forecast across our northern zones during the day...but given the 
remaining strength of the ridge still in place...think that amount 
and eastward progression of the quantitative precipitation forecast is too much. NAM brings quantitative precipitation forecast 
right to our western border by Friday evening...which seems more 
realistic. Will therefore go with a slight chance of showers Friday 
afternoon for the westernmost zones only. Feel that afternoon hours 
would be the most probable time as both NAM and GFS show weak middle 
level lift moving in. There's considerable spread among the MOS 
guidance regarding tomorrow's highs. FWC MOS looks too high as it is 
too far west with moisture...and mav appears to be too low as it is 
the deepest with moisture over the area. Therefore sided with the 
met MOS numbers. 


Areal coverage of rain approaching from our south and west expected 
to diminish Friday night...and with no apparent source of 
significant lift...have decided to keep the area dry for the night. 


Cold front continues its approach from the northwest on 
Saturday...slight chance of showers by noon northwest...then chance probability of precipitation for 
showers/T-storm (better convective available potential energy for convection) by Saturday evening 
for all spots except Long Island...where slight chance probability of precipitation were kept. 
NAM is a little faster than GFS with frontal passage...but it does appear to 
occur sometime around midnight...so probability of precipitation tapering off during the 
overnight hours. 


High pressure builds back in behind the front on Sunday. Deep northwest 
flow will bring plenty of sunshine. 850 mb temperatures forecast to be 12 
to 13c...and with decent downsloping...expecting highs in the middle 
80s for the normally warmest spots. 


&& 


Long term /Sunday night through Thursday/... 
a large middle and upper level ridge will develop and remain in control 
over most of the eastern half of the country Monday through 
Wednesday. This ridge will then start to flatten out on Thursday. 
Meanwhile...at the surface...strong high pressure will slowly drift 
from the Great Lakes to the middle Atlantic coast by Thursday. As a 
result...expect no threat of precipitation with little or no clouds 
until Wednesday night or Thursday as the ridge aloft begins to weaken. 


Temperatures will run above normal between the strong ridge aloft 
and full sunshine each day. Expect the typical end of Summer trend 
of warm days and cool nights as the days get shorter and nights 
longer. 


&& 


Aviation /20z Thursday through Tuesday/... 
VFR weather conditions will persist until 06z to 09z tonight when 
MVFR ceilings might develop. 


A ridge axis extending from NE of New England SW across the NYC 
metropolitan area will move very slowly east through 18z Friday. 


Moisture from the remnants of Fay extended north along the back side 
of this retreating ridge and just ahead of a weak short wave. Broken 
middle and upper level clouds will slowly advect east across the 
region...mainly after 00z this evening. 


More importantly...forecast model soundings show the development 
of low level moisture in the form of MVFR stratus developing 
toward 09z. Because current dew point depressions are more than 30 
degrees in some areas...I decided to tempo MVFR stratus mainly 
from 09z to 03z Friday. Low end VFR ceilings are likely as low level 
moisture continues to increase during Friday. 


Winds will be onshore and generally less than 10 knots during the next 24 
hours...except around 10 knots from 160 degrees at kjfk until 23z. 


Outlook for 18z Friday through Monday... 
as high pressure moves slowly east and further offshore...a weak cold 
front will approach and pass across the region Sat night. This 
will be followed by high pressure Sunday through Monday. MVFR ceilings are 
possible with the approach of this front Friday night into Sat along 
with scattered showers and a few thunderstorms. Otherwise...VFR weather 
conditions are forecast. 


&& 


Marine... 
winds and seas expected to remain below advisory levels through the 
period. 


High pressure over the Canadian Maritimes will drift farther east 
tonight. Onshore winds of 10 knots or less tonight combined with a 
little southeast swell will have seas on the ocean at 2 to 3 feet tonight. 


A cold front will move across the waters Sat night with winds 
becoming northerly by sun. Expect gusts 15 to 20 knots on the waters 
behind the front on Sunday as high pressure builds in. 


High pressure builds over the northeast Monday and Tuesday...with an 
offshore northerly wind expected. For now kept seas below Small Craft Advisory...but 
will have to watch the progress of Tropical Storm Hanna which is 
forecast to be a hurricane northeast of the Bahamas early next week. 
Swells from this system will have the potential to affect the local 
ocean waters. 




&& 


Hydrology... 
dry weather expected through at least Friday for most of the 
County Warning Area...with only a slight chance of light rain amounts across the 
western fringe. Scattered rainfall of one quarter to one half inch 
possible Saturday into Saturday night. Dry again Sunday into middle 
week. No hydrologic problems are expected. 


&& 


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


&& 


$$ 
Synopsis...jc 
near term...jc 
short term...jc 
long term... 
aviation...gc 
marine...jc/ 
hydrology...jc 

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