949 
fxus61 kbox 240107 
afdbox 


Area forecast discussion 
National Weather Service Taunton Massachusetts 
907 PM EDT Wednesday Jul 23 2008 


Synopsis... 
a slow moving cold front will bring heavy rainfall and possible 
flooding to portions of southern New England through Thursday 
evening. A second cold front will approach late this weekend with 
another round of showers and thunderstorms possible. Unsettled 
weather will remain across the region through early next week. 


&& 


Near term /until 4 am Thursday morning/... 
threat of severe thunderstorms seems to be diminishing slightly. 
Current plan is to keep the thunderstorm watch in effect until 10 PM. 
Will reevaluate this watch in the next 30 minutes. 


Very heavy rainfall continues with these showers and thunderstorms... 
with localized urban or flash flooding likely. 


&& 


Short term /4 am Thursday morning through 6 PM Thursday/... 
anomalous late July pattern with upper low amplifying across Great 
Lakes and acquiring some negative tilt. Slow moving cold front will 
be focus for very heavy rainfall...but locally heavy rain also 
possible east the boundary as we are witnessing currently in high 
precipitable water airmass. 


GFS/NAM/ECMWF and all gefs ensemble members in reasonable agreement 
on axis of heaviest rain setting up across our west zones...especially 
CT valley and Berkshires. Amounts differ but general location is 
similar so we have enough confidence to issue Flood Watch for west 
zones and extending it eastward to S New Hampshire...orh County and Rhode Island/CT border. 
Highest confidence of flooding rains will be over CT valley and 
Berkshires. 


NAM indicating up to 10 inches across west zones while GFS/European model (ecmwf) show 
up to 5 inches. Gefs ensembles Show Low level jet +4sd for 24h 
period which suggest persistent inflow and intense low level 
convergence along frontal boundary. Given slow movement of 
front...we cant completely rule out localized 8" with widespread 
2-5" amounts in watch area...highest CT valley and Berkshires. 
Further east in the coastal plain...our best estimate is for 1-2". 


We focused highest probability of precipitation in west zones...then decreasing to the east. 
Given high amplitude pattern much of the rain may remain west of 
cape/islands through Thursday and we have just chance probability of precipitation here. 


Severe weather also remains a concern late tonight and Thursday given 
very strong low level wind field producing a highly sheared 
environment. Instability will be limited...but any storms will have 
to monitored very closely. Any storms stay develop will have 
potential to become supercells. 


Surf zone...we issued high surf advisory for Thursday/Thursday night due to 
increasing south wind wave. Seas will build to 8 feet offshore 
resulting in a high risk of rips on south facing beaches. 


&& 


Long term /Thursday night through Wednesday/... 
models showing less continuity this afternoon...lending to less 
confidence to this portion of the forecast...especially from Monday 
Onward. Leaned toward the GFS and European model (ecmwf) ensembles for this portion 
of the forecast. 


One thing that appears to show some continuity is the long wave 
upper level trough that is expected to remain along the eastern U.S. 
Through at least Tuesday. 


Thursday night...front moves slowly across the region with likely to 
categorical shwrs/tstms. Models having difficulty with the timing of 
this system...though appears that the front will work into eastern 
Massachusetts by 12z. With high precipitable water values continuing...will also see more heavy 
rainfall as well. 


Friday...cold front swings offshore...then stalls S of the region in 
the general SW upper flow as high pressure remains stalled over the 
western Atlantic. Some leftover showers will end over eastern Massachusetts in the 
morning. Dewpoints will drop a bit...but remain in the Lower-Middle 60s as 
highs head to the lower to middle 80s. Will also remain dry Friday night 


Saturday...next front begins to approach around the cutoff upper low 
over Quebec. Timing is suspect as models showing some spread at this 
point...but have included chance probability of precipitation from west-east during the day. 
Dewpoints head back up with the continued SW flow. Current timing 
suggests best shot of showers/thunderstorms will occur during the nighttime 
hours...but not a whole lot of confidence. 


Sunday through Tuesday...at this point...appears that the front 
slows to a crawl as it crosses the region over this weekend...then 
will push just off the coast on Monday. Have carried chance probability of precipitation 
through Monday...though still have not a lot of confidence with the 
timing of this system. 


Wednesday...appears that it will be a dry day with high pressure trying 
to push east out of the Great Lakes. However...low confidence due to 
the very amplified upper pattern in place. 


&& 


Aviation /01z Thursday through Monday/... 
tonight... 
a mixture of MVFR/IFR conditions were observed across southern New 
England this evening. Conditions will deteriorate to IFR/LIFR across 
the region overnight. In addition...periods of heavy showers and 
embedded thunderstorms will continue...especially across the 
interior. Some of the stronger storms will produce brief gusty 
surface winds. 


Thursday... 
MVFR/IFR continue with periods of heavy showers and embedded 
thunderstorms. The heavy showers will result in brief vlifr/LIFR 
conditions along with gusty winds. 


Outlook...Thursday night through Monday... 


Thursday night...expect IFR for all locations with rain showers/scattered thunderstorms and rain. Some 
rain showers will have heavy rainfall. May see some slow improvement over 
the CT valley after midnight. 


Friday...conditions gradually improving to VFR...lingering MVFR-IFR 
conditions possible cape and islands. 


Sat...VFR early. Conditions lowering to MVFR-IFR in scattered rain showers/thunderstorms and rain 
from west-east during the day. 


Sun and Monday...IFR am/MVFR PM scattered rain showers/thunderstorms and rain and fog patches S coast 
each day. 


&& 


Marine... 
southerly winds will increase late tonight and Thursday as low level jet 
develops. Low level inversion will limit mixing but 40-50 kts just 
above the surface should allow for g25 kts. Undercut wnawave 
guidance as it is exhibiting its high bias in south flow. Swan 
output looked much better with seas building to 8 feet Thursday on outer 
waters. 


Outlook...Thursday night through Monday... 


Thursday night and Friday...expect Small Craft Advisory for hazardous seas expected 
with building southerly swell. Wind gusts to 25 knots possible Thursday 
night. Undercut wna guidance slightly given low level inversion and 
an overforecast bias on the S swells. 


Saturday and Sunday...Small Craft Advisory for hazardous seas will continue with 
building S-SW swell expected ahead of the next approaching cold 
front...mainly on the southern waters. 


Monday...winds shift to west and diminish. Leftover seas will remain 
close to small craft criteria on the southern outer waters. 


&& 


Hydrology... 
quantitative precipitation forecast amounts will vary across our region. Maximum amounts expected over 
CT valley and Berkshires with 2-5 inches and locally 8 inches possible 
through Thursday night. Amounts decrease to the east with generally 
1-2 inches across the coastal plain...and less across the cape and 
islands. Flood Watch has been issued due to this threat. 


Urban and poor drainage flooding likely along with river and small 
stream flooding...highest risk CT valley and Berkshires. 


&& 


Box watches/warnings/advisories... 
CT...Flood Watch through late Thursday night for ctz002>004. 
Massachusetts...Flood Watch through late Thursday night for maz002>004-008>012- 
026. 
High surf advisory from 8 am Thursday to 8 am EDT Friday for 
maz020-023-024. 
New Hampshire...Flood Watch through late Thursday night for nhz011-015. 
Rhode Island...high surf advisory from 8 am Thursday to 8 am EDT Friday for 
riz006>008. 
Marine...Small Craft Advisory from 8 am to 6 PM EDT Thursday for 
anz231>234-250. 
Small Craft Advisory from 10 am to 6 PM EDT Thursday for 
anz236. 
Small Craft Advisory from 1 am to 6 PM EDT Thursday for anz235- 
237. 
Small Craft Advisory until 6 PM EDT Thursday for anz254-255. 


&& 


$$ 
Synopsis...kjc/evt 
near term...Belk 
short term...kjc 
long term...evt 
aviation...evt/Frank 
marine...kjc/evt 
hydrology...Belk/kjc 

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