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Area forecast discussion 
National Weather Service Taunton Massachusetts 
711 PM EST Wednesday Mar 10 2010 


Synopsis... 
weak high pressure will maintain dry weather tonight and most of 
Thursday along with cooler temperatures as clouds and onshore flow 
gradually increase Thursday. A large intensifying low pressure system 
developing slowly east from the Central Plains will reach the middle 
Atlantic coast this weekend and then push east of New England early next 
week. This system will probably deliver a number of hazardous 
conditions to the region this weekend into next Monday. 


&& 


Near term /until 6 am Thursday morning/... 


Overall previous forecast on track with current observations. As of 
630 PM marine layer has pentrated well inland across all of Rhode Island and 
the i495 corridor of eastern Massachusetts. Farther inland temperatures will also fall 
fairly quickly this evening given low dew points...light winds and clear 
skies. Although this trend will likely level off later tonight as middle 
and high clouds overspread the region from the southwest. So updated 
hourly temperatures and sky grids to reflect these trends. Otherwise 
previous forecast on track. Previous discussion below... 


Another quiet night expected tonight with chilly temperatures 
and dry weather as high pressure slides slowly off the coast. Expect 
increasing high clouds through the night. Coupled with higher 
dewpoints /ranging from the single digits to the middle 20s this 
afternoon/...this may result in slightly warmer temperatures 
overnight. Lows will still be in the upper 20s to middle 30s. Used a 
blend of the lamp/mav guidance for temperatures. 


&& 


Short term /6 am Thursday morning through Thursday night/... 
the middle level ridge just to the west of southern New England will 
continue to influence our weather through Thursday. Thursday should 
be another pleasant day. Expecting increasing clouds...depending on 
how quickly this occurs temperatures may be a couple of degrees 
cooler than currently forecast. With models slowing down the 
approach of low pressure...keeping precipitation out of southern New 
England through the day. Have chance probability of precipitation /10/15z sref probability of precipitation/ moving 
into the area from the southwest after midnight Thursday night. 
Thickening cloud cover will keep overnight temperatures mainly in 
the 30s. 


&& 


Long term /Friday through Wednesday/... 
forecast confidence remains above average on large coastal low 
impacting the region this weekend into early next week. However 
forecast confidence is below average on the details revolving around 
onset of precipitation...magnitude of quantitative precipitation forecast and ptype early this weekend and 
then again early next week. 


Potential hazards remain the same...heavy quantitative precipitation forecast and associated 
flooding...strong winds...coastal flooding and beach erosion along 
with some icing northwest Massachusetts and southwest New Hampshire. Here are the details 
regarding each hazard... 


Quantitative precipitation forecast and flood potential... 
unfortunately models continue to display large scale differences on 
track of middle level circulation and corresponding surface low. 12z 
European model (ecmwf) is farthest north with vertically stacked low moving into 
south coastal New England. This shifts the strongest wind anomalies 
and precipitable water plume into from southern New England late Sat/Sat night to northern 
New England sun and Monday. This results in only a brief 12 hour period of 
heavy quantitative precipitation forecast here from late Sat into early Sun morning...then dry 
slotting thereafter with heavy quantitative precipitation forecast moving into northern New England. 
Obviously this would reduce the flood threat here. However the 12z 
European model (ecmwf) is on the northern edge of the 12z guidance with the 12z GFS on the 
southern edge and the 12z ggem and UKMET in the middle of the pack. 
The 12z gefs is closer to the 12z European model (ecmwf). So in a nutshell there is 
enough model spread here to play things conservative for the moment. 


In addition...even the northern ec solution doesn/T bring warm air 
into the interior where a modest snow pack exist. Thus it appears a 
low probability that much of the snow pack would be released given 
cool surface temperatures. Therefore the flood threat may hinge solely on 
heavy quantitative precipitation forecast without any contribution from snow melt. Furthermore the 
snow pack across the Berkshires/northern Worcester Hills into 
southern New Hampshire is below normal for this time of year and will likely 
continue to erode over the next few days before the heaviest quantitative precipitation forecast 
arrives late Sat into early sun. Therefore not expecting much 
contribution from the snow pack. 


As mentioned above another limiting factor for flooding is the 12z 
European model (ecmwf) and to a lesser extent the 12z gefs with wind anomalies and 
precipitable water plume being progressive and traversing from southern New 
England into northern New England Sunday...shutting off the heavy 
quantitative precipitation forecast across southern New England. 


While we shouldn/T dismiss the flood potential threat for this 
weekend we need to consider these possible limiting factors which 
would suggest a greater risk for minor flooding. In 
addition...typically flood events here in southern New England 
require 2 heavy quantitative precipitation forecast events in a short period of time with the first 
event being the primer for the second event. Our last large quantitative precipitation forecast event 
is now nearly 2 weeks ago. Thus an excessive long duration quantitative precipitation forecast event 
will be required this weekend for significant flooding. It remains 
possible but all elements will have to come together...especially 
without any or very little contribution from snow melt. 


Regarding probability of precipitation...models continue to trend slower with the onset of 
quantitative precipitation forecast. This seems reasonable given persistent mean trough over southeast 
Canada which stalls middle level ridge over New England. In 
addition...models in good agreement on holding off heavy quantitative precipitation forecast here 
until late Sat...especially eastern Massachusetts. So while there may be some 
light spotty light rain here Friday/Friday night into early Sat...outdoor 
plans may be a go Friday into early Sat given a washout is not expected. 


Strong wind potential... 
high likelihood of Wind Advisory criteria of east winds 30 to 40 miles per hour 
with gusts to 50 miles per hour along the coast. As the event nears will have to 
watch potential for high wind criteria /gusts 60 miles per hour and higher/ as 
some guidance indicating low level jet up to 70 knots. 


Ptype... 
low risk of freezing rain continues across the higher terrain of 
northwest Massachusetts into southwest New Hampshire. Risk is greatest late Sat and Sat 
night. By Sunday shallow cold air should erode/warm. Then by Sun night 
into early next week as middle level low approaches southern New 
England columns begins to cool. This may support some wet snow 
across the higher terrain. 


Coastal flooding/beach erosion... 
while astro tides are relatively low and the GFS surge guidance only 
offers 1 feet surge...if we get a 2.5 feet surge along with seas near 20 
feet the risk of minor coastal flooding exist. Given the strength of 
the low level jet and duration of onshore flow...think a 2-2.5 feet 
surge is not out of the question along with seas near 20 feet. In 
addition if water levels become higher than predicted by the GFS 
storm surge model combined with large seas pounding the 
shoreline...there may be a greater risk of significant beach 
erosion. 


&& 


Aviation /00z Thursday through Monday/... 
today...high confidence VFR. Local seabreezes develop after 18z. 


Tonight...VFR ceilings spreading across the area from 6000-12000ft 
lower west. 


Thursday...VFR ceilings 3000-5000ft. Spotty -shra in the afternoon 
mainly northwest mass and SW New Hampshire. 


Thursday night...VFR to start. MVFR conditions may develop in scattered 
showers late. 


Outlook... 


Friday...high confidence for MVFR with a low probability of IFR in periods of 
rain/drizzle and fog. 


Sat...MVFR lowering to IFR from west to east. East g30-40 kts mass 
coast with asstd low level wind shear. Ice possible northwest Massachusetts and southwest New Hampshire. 
Low confidence on timing of MVFR to IFR. 


Sat night...high confidence on IFR in wind driven rain. Strong east 
winds 30-40 knots eastern Massachusetts along with possible low level wind shear. 


Sun...high confidence IFR/LIFR in rain/drizzle fog...possibly 
changing to snow at night orh and other high terrain terminals. 


Monday...MVFR/IFR in periods of rain possibly mixed with snow high 
terrain terminals. 


&& 


Marine... 
a quiet period over the marine zones through Wednesday night. Seas 
will remain well below 5 feet and variable winds 10 to 15 kts will 
prevail. 


Thursday...winds start increasing Thursday afternoon with isolated 
wind gusts to 25kts. Seas increase to near 5 feet late Thursday 
night. 


Outlook... 
Friday...Small Craft Advisory develops late with increasing east-southeast winds and seas. 


Weekend-Mon...east gales likely from time to time becoming NE Monday. 
Seas eventually build to 16 to 22 feet. Low probability storm force gusts Sat 
night or Sun morning. 


&& 


Fire weather... 
the continued dry pattern will allow the minimum relative humidity 
to drop to between 25 and 35 percent Thursday. Winds/gusts will 
remain below 15 miles per hour so no fire weather headlines anticipated. 


&& 


Equipment... 
the National oceanic and atmospheric administration weather radio transmitter at Blue Hill will be off the air 
through most of this week due to a broken antenna. Needed parts have 
been ordered and the transmitter may return to service the week of 
March 15th. 


&& 


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


&& 


$$ 
Synopsis...nocera/rlg 
near term...nocera/rlg - updated 711 PM 
short term...rlg 
long term...nocera - updated 711 PM 
aviation...nocera/rlg 
marine...nocera/rlg 
fire weather...rlg 
equipment...staff 

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