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Area forecast discussion 
National Weather Service Taunton Massachusetts 
156 PM EST Sat Nov 21 2009 


Synopsis... 
high pressure will move from the Great Lakes into southeast Canada 
tonight...then move east of Nova Scotia Monday. A weak low pressure 
will approach southern New England from the south Monday night and 
Tuesday. Weak high pressure will probably result in a dry 
Thanksgiving...before a possible coastal storm on Friday. 


&& 


Near term /through tonight/... 
high clouds continue to stream across southern New England...with 
the thickest clouds now across Cape Cod and the islands. Farther 
west...plenty of clouds from the Great Lakes banked up against the 
west slopes of the Berkshires. With winds shifting to the north and 
remaining fairly light...do not expect these clouds to spill over the 
Berkshires into southern New England. 


Used a blend of MOS guidance for temperatures tonight. 


&& 


Short term /Sunday through Sunday night/... 
high pressure remains across northern New England and southeast 
Canada. This should mean another mostly sunny day...but this time 
with onshore winds by afternoon. These onshore winds could lead 
marine stratus late in the day into Sunday night...particularly south 
and east of I-95 in Massachusetts and Rhode Island. Given the expected timing of the wind 
shift though...thinking this may be more of a concern Sunday night. 


MOS temperatures look reasonable once again...so used a blend of the 
mav/met guidance. 


&& 


Long term /Monday through Friday/... 
confidence is below average during this period due to spread in 
model solutions as well as some run to run discontinuity. 


Seeing tendency for various medium range models to project 
measurable rain for Monday night through Tuesday...and this is supported by 
sref probabilities. Noticed that 00z European model (ecmwf) is wetter than its prior 
run. There are timing and quantitative precipitation forecast variations among the models. 
Thus...have not tried to be too precise on timing of onset and 
ending of probability of precipitation and have capped at high chance for now. Suspect most 
locations will get measurable sometime in the Monday night to Tuesday night 
time frame but cannot pin the timing down any better. The global Gem 
still looks too robust given that this system is associated with a 
dampening short wave trough moving into the long wave ridge position. 
However...as weak as system is...does look like decent overrunning 
with a subtropical moisture source. As far as timing...am suspicious 
that dry low level air associated with moderately strong high pressure 
centered over Nova Scotia at the outset may delay the spread of rain 
into southwest New Hampshire and northeast Massachusetts Monday night. 


Wednesday is a little complicated with vigorous negative tilt short wave 
trough rotating through the Great Lakes. Southern New England is 
fairly well east of significant dynamics. GFS depicts southeast flow that 
could in itself contribute to low clouds and drizzle whereas the 
European model (ecmwf) shows less relative humidity and flat gradient amidst weak surface ridge. 
Have gone with slight chance to chance probability of precipitation. Temperatures may be 
tricky for Wednesday given warm air mass aloft with 8+c 850 mb 
temperatures but inversion may prevent realizing the relative warmth 
at the surface. If get enough sun...then Wednesday maximum temperatures may 
end up considerably higher than currently forecast. 


Thanksgiving day currently looks dry with perhaps a sunny start 
before increasing clouds during the afternoon ahead of the next 
system. A vigorous short wave trough helps initiate a coastal low but 
again medium models not in good agreement in handling this system 
with regard to intensity and track. Have gone with chance probability of precipitation Thursday 
night and Friday. Air mass may cool sufficiently toward back part of 
storm to cause ptype uncertainty. Potential for rain to mix with or 
change to snow over the higher elevations of our northwest zones on Friday but 
confidence of this actually occurring is currently low. 


&& 


Aviation /19z Saturday through Thursday/... 


Tonight...VFR. Patchy fog with brief MVFR ceilings/visibilities late at night 
in the normally susceptible valleys. 


Sunday...VFR. May see patchy VFR to local MVFR ceilings along the coast 
during the late morning and afternoon. 


Sunday night...VFR most locations. Greater risk of MVFR ceilings along 
the East Coast and south and east of a line from kbos-kpvd. 
Confidence on this is not high at this time. 


Outlook /Monday through Wednesday/... 


Monday through Wednesday...VFR Monday becoming MVFR with areas IFR in rain and fog 
Monday night through Tuesday night. MVFR ceilings may persist into Wednesday. 


&& 


Marine... 
high pressure building north of the waters tonight. Winds gradually shifting 
north tonight...then NE Sunday into Sunday night. Seas below 5 feet tonight 
into early Sunday. Do expect seas to build over 5 feet across the outer 
coastal waters Sunday afternoon into Sunday night once NE winds get 
established. There is still a question about strength of these 
winds...which will be entirely dependent upon the approach of a low 
pressure from the middle Atlantic coast. 


Outlook /Monday through Wednesday/... 


Persistent east gradient with considerable fetch will likely keep seas 
near or above 5 feet exposed coastal waters Monday night through Wednesday morning and 
may be strong enough to meet Small Craft Advisory wind thresholds over most of waters. 
Respite likely late Wednesday through Thursday before risk of potent coastal low pressure 
system toward end of week. 


&& 


Box watches/warnings/advisories... 
CT...none. 
Massachusetts...none. 
New Hampshire...none. 
Rhode Island...none. 
Marine...none. 


&& 


$$ 
Synopsis...Belk/Thompson 
near term...Belk 
short term...Belk 
long term...Thompson 
aviation...Belk/Thompson 
marine...Belk/Thompson 

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