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fxus61 kaly 201147 
afdaly 


Area forecast discussion 
National Weather Service Albany New York 
745 am EDT sun Jul 20 2008 


Synopsis... 
a stationary front along the Canadian border early this 
morning will gradually drift south over our region today as a wave 
of low pressure moves eastward along it...passing over the area 
tonight. This will result in showers and thunderstorms becoming more 
numerous and they are expected to produce very heavy rainfall 
starting this afternoon. The front will then lift back north to the 
Canadian border late tonight with a second low pressure system 
finally dragging a cold front through our area Monday evening... 
bringing more heavy rain to our region. 


&& 


Near term /through Monday/... 
leading edge of first band of showers and embedded thunderstorms 
approaching western portion of forecast area early this morning. Much of this 
activity expected to impact northern portion of forecast area and set the 
stage for even heavier convection later on. Storm Prediction Center has forecast area outlooked 
for slight risk today and while a few storms could certainly 
become severe main concern is for flash flooding. 800 mb winds 
increase to 30-35 kts across northern portion of forecast area this 
afternoon...with sb convective available potential energy 1000-1500 j/kg for a time this 
afternoon...with the most favorable factor being 0-3 km helicity 
which briefly reach 100-250 range this afternoon. The most 
favorable factors for flash flooding today include nearly 
stationary front which by afternoon will be parked through central 
portion of forecast area as a wave of low pressure tracks east along it from 
the eastern Great Lakes...very impressive h10-800 mb Theta-E ridge 
also through central portion of forecast area with precipitable waters  mainly 1.50-1.75 
inches with a few areas over 2 inches through this evening. Hires 
WRF composite reflectivity highlights the potential magnitude of 
the convective rainfall this afternoon into this evening showing 
training of 40-60 dbz cells mainly across forecast area from Route 23 north 
starting late morning into early afternoon. Have highlighted this 
potential in the Flash Flood Watch and believe that between this 
afternoon and Monday evening that at least a few locations likely 
to experience several inches or more of rain in 1 to 2 hours time. 


Highs today are expected to be 5 to 10 degrees lower than 
yesterday as much more extensive cloud cover and more numerous 
showers and thunderstorms will keep a lid on temperatures with highs 
generally in the middle 70s to middle 80s. 


After a brief lull in the action late this evening expect more 
showers and thunderstorms to fire up late tonight through Monday 
with potential for a pre event...a heavy rainfall event that 
precedes a tropical storm...based on cstar ii research ahead of 
Tropical Storm Cristobal which is forecast to head northeast 
passing off Delaware-Maryland-Virginia early Monday. Refer to the National 
Hurricane Center products for details about Cristobal. Thus Monday 
would be the time frame for the pre to develop provided Cristobal 
gets close enough...within 1000 km...to forecast area. Still feel that the 
favored area would be from eastern Catskills to western New England. 
Precipitable waters  increase to over 2 inches for a time Monday morning across 
far southeast portion of forecast area as h10-800 mb Theta-E ridge becomes more focused 
across this region. And the southeast half of forecast area appears to be 
near the middle level jet entrance region of 500 mb wind maximum. Sb convective available potential energy 
off NAM and GFS also extremely impressive over southeast portion of forecast area in 
2500-3200 j/kg range with any heating at all on Monday excessive 
rainfall would be likely. Have thus issued a very strongly worded 
Flash Flood Watch with concerns highlighted in hydrology 
discussion below. 


&& 


Short term /Monday night through Tuesday night/... 
second wave of low pressure should finally swing cold front through 
forecast area Monday night with precipitation expected to taper off quickly around 
midnight as dry slot moves over region. More less numerous and 
less intense showers and thunderstorms possible Tuesday afternoon as 
upper tough swings into eastern Great Lakes while secondary low 
remains across eastern Canada. 


&& 


Long term /Wednesday through Saturday/... 
the main feature during this period is a deep 500 hpa trough 
over southeastern Canada and the northeastern states. Again 
the GFS and European model (ecmwf) are in fairly close agreement on Wednesday 
but then diverge as time GOES on with the GFS more progressive 
in moving out the trough. For the most part...have used a blend 
of the previous HPC and the recent gmos for maximum and min temperatures. 
A weak Canadian high builds in the Ohio Valley but some 
question remains how far east the frontal system will move 
off shore as weak disturbances move north along it. In 
addition...temperatures aloft will be fairly cool which will 
add to the instability of the air. Therefore there is a lot 
of uncertainty regarding the possibility of showers and 
midday thunderstorms. We have addressed this with low chance 
and slight chance probability of precipitation. Temperatures will be a bit cooler 
and more pleasant...highs in the upper 70s and low 80s... 
and lows and dewpoints in the 50s and low 60s...close to 
normal for this time of year and much more pleasant than 
the heat and humidity of the last few days. 


&& 


Aviation /12z Sunday through Thursday/... 
rain and embedded isolated thunderstorms are tracking across 
northern areas...just north of gfl...so putting vcsh all day 
there...as the area of rain gradually builds south through the day. 
Some areas of fog are affecting all airports this morning...and 
borderline MVFR/IFR visibilities will continue through about 14z. 
Pou and gfl just nudged out of IFR into MVFR...and alb should 
improve to MVFR between 12z and 13z. Gradually introducing vcsh a 
little later from north to south through the day...with alb late 
this morning...after 14z...and pou this afternoon...around 22z. 
Some MVFR conditions possible in showers once they do arrive. With 
great uncertainties in where and when showers and isolated 
thunderstorms will develop and track later this morning and 
afternoon...just indicating vcsh and cumulonimbus in cloud group until more 
definitive development and movement is seen. Once more definitive 
convection is observed and tracked...some brief IFR conditions will 
be indicated within convection. Chances for showers and storms will 
continue through abour 04z-06z...then areas of fog will persist 
through the early morning hours tomorrow. 


Outlook... 
Monday MVFR with periods of IFR possible in shra/tsra. 
Monday night-Wed...VFR with MVFR in scattered shra/tsra. 
Thursday...VFR...no sig weather. 


&& 


Fire weather... 
today looks like a wet flag day for virtually entire forecast area. 
A stationary front along the Canadian border early this 
morning will gradually drift south over our region today as a wave 
of low pressure moves eastward along it...passing over the area 
tonight. This will result in showers and thunderstorms becoming more 
numerous and they are expected to produce very heavy rainfall 
starting this afternoon. The front will then lift back north to the 
Canadian border late tonight with a second low pressure system 
finally dragging a cold front through our area Monday evening... 
bringing more heavy rain to our region. 


Relative humidity values will drop to 50 to 75 percent 
today...recover to near 100 percent tonight...and fall to 55 to 
80 percent on Monday. 


Light and variable winds this morning will become east to 
northeast at 5 to 10 miles per hour this afternoon with east northeast winds 
around 5 miles per hour tonight and then shifting to west to southwest and 
increasing to around 10 miles per hour on Monday. 


&& 


Hydrology... 
river levels and flows are low which is typical of middle to late 
Summer. A weak frontal boundary will become nearly stationary 
and a wave of low pressure will move along the boundary bringing 
more widespread convection to the region today into Monday. 
Precipitable water values are expected to rise to around 2 inches 
today and remain high tonight...so some of the storms will have 
torrential downpours. Terrain may also play a critical role in 
enhancing rainfall...especially in the Catskills and along The 
Spine of the Berkshire and Green Mountains. Several inches of rain 
are possible in a short time...with rapid runoff especially in 
mountainous terrain. Roads...parking lots...shopping centers...and 
built up areas area also likely to experience flooding during and 
shortly after torrential downpours. Forecast winds aloft suggest 
that cells will be slow moving today and that training and back 
building heavy precipitation cells will be a problem tonight and early 
Monday. For these regions a Flash Flood Watch has been issued 
valid from noon today until midnight Monday night. 


&& 


Aly watches/warnings/advisories... 
CT...Flash Flood Watch from noon EDT today through Monday evening 
for ctz001-013. 
New York...Flash Flood Watch from noon EDT today through Monday evening 
for nyz032-033-038>043-047>054-058>061-063>066-082>084. 
Massachusetts...Flash Flood Watch from noon EDT today through Monday evening 
for maz001-025. 
Vermont...Flash Flood Watch from noon EDT today through Monday evening 
for vtz013>015. 


&& 


$$ 
S 
aviation...NAS 

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