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Area forecast discussion 
National Weather Service Boston/Norton Massachusetts 
315 PM EDT Tue Aug 14 2018 


Synopsis... 
a warm front lifting northward across the region today will 
yield much warmer temperatures than yesterday with highs 80 to 
85 this afternoon. The heat will combine with tropical humidity 
resulting in scattered showers and thunderstorms, some with 
locally heavy rain. Very warm Wednesday and Thursday with 
slightly less humid conditions. Tropical humidity returns Friday 
into Saturday with increasing risk of showers and thunderstorms 
as a cold front approaches. Cooler weather follows for Sunday 
and Monday, then tropical weather returns yet again on Tuesday. 


&& 


Near term /until 8 PM this evening/... 
215 PM update... 


Front has lifted into northeast Massachusetts with warm sector across the 
remainder of the region with temps 80-85 except in showers/storms 
where temps have temporarily dropped into the 70s. Combination 
of warm sector airmass with ml convective available potential energy around 1500 j/kg and strong 
cyclonic flow aloft resulting in numerous showers/storms across 
northern/interior Massachusetts into Metro Boston as well. Activity becomes 
less numerous farther south into CT/Rhode Island and southeast MA, 
possibly due to depth of drier air aloft in core of dry slot. 
However this will be short lived as cold pool associated with 
mid level low approaches the region late in the day across 
western MA/CT. Thus expecting convection across eastern New York to 
traverse into western Massachusetts/CT later this afternoon. Given dew pts 
in the 70s and pwats up 1.6 inches heavy rain and gusty winds 
the main concerns with the stronger storms. 


Tricky forecast for Boston where frontal boundary remains hung 
up with cool northeast flow at Logan along with dense fog at 
times. Web cams show these low clouds touch and go. Given the 
surface low is not expected to move north of our latitude, this 
shallow cool stable air may linger until sunset and then when 
surface heating ceases, onshore flow weakens and allows surface 
winds to veer to the south and eventually south-southwest. 


&& 


Short term /8 PM this evening through Wednesday night/... 
as mentioned above, best chances of showers and thunderstorms 
will be this evening across the southern portion of the area. 
Activity will become widely scattered as the night progresses as 
the upper low becomes an open wave and moves across the region. 
It will remain warm and humid with lows from the mid 60s to 
lower 70s. 


&& 


Long term /Thursday through Tuesday/... 


Big picture... 


Zonal flow jet stream across Canada while subtropical high remains 
in control across the southern USA. The USA northern tier is in 
between, with a lighter west-east flow overhead while most of the 
jet energy stays to our north. This pattern lingers until the 
weekend. At that time high pressure builds over the western USA, 
which in turns allows a trough to dig over the Great Lakes/Midwest. 
West Atlantic high remains off our coast. 


For US, this means a couple of shortwaves moving east across New 
England Friday through Sunday. After that, our upper flow turns from 
the southwest for early next week as the Midwest trough digs. 
That would trend US back into the tropical air again on Tuesday. 


Model mass fields are similar through Sunday. Differences arise from 
the trough digging from Canada into the Midwest, although these 
scenarios all bring a southwest flow to our area. Confidence remains 
high through Friday, then moderate over the weekend and low early 
next week. 


Concerns... 


Friday-Saturday convection... 


Shortwave on the southern edge of the zonal flow approaches New 
England Friday and crosses the region Saturday. Jet support is 
mostly to our west during Friday, but moves over southern New 
England Friday evening through Saturday morning. Low level winds are 
forecast 10-20kts, while mid level forecasts top out around 35 
knots. Cape is forecast to build to 1500-2500 j/kg...the NAM even 
suggests a core of 3000 j/kg. Theta-E ridge builds over the Hudson 
Valley Friday afternoon and moves east across our area by evening. 
GFS/European model (ecmwf) show 0-1km helicity at 100-200 during the 
afternoon/evening, mostly across northern mass. Precipitable water 
builds to 2-2.25 inches by Friday. 


This suggests increased risk of showers and thunder. The high 
moisture values suggest local downpours. The winds are borderline 
for damaging convection, but combined with the stability parameters 
and helicity this suggests potential for strong wind gusts and hail 
in any convection...this will need to be monitored. 


Timing the departure of the upper trough and surface cold front 
holds less confidence, but current data supports convection during 
the first half of the day, shifting offshore during 
afternoon/evening. 


Sunday night-Monday southern offshore system... 


The next shortwave sweeps east from the Great Lakes, forming a 
surface low over/near Virginia on Sunday. This moves east-northeast 
Sunday night with the surface low passing to our south Monday. 


Weak high pressure then builds across eastern Canada. This should 
bring fair weather to US for Sunday and Monday. Main concern is the 
position of the high center, with resulting northeast low level flow 
and its effect on temps and sky cover. This has potential to bring 
showers to our area Sunday night/Monday. 


&& 


Aviation /19z Tuesday through Sunday/... 
forecaster confidence levels... 


Low - less than 30 percent. 
Moderate - 30 to 60 percent. 
High - greater than 60 percent. 


Short term /through Wednesday night/... 


230 PM update... 


Mainly VFR but MVFR at times in scattered showers/tstms. 
Exception is kbos where onshore flow continuing to result in IFR 
at times. Thinking here is after sunset winds will shift to S-SW 
supporting MVFR-VFR conditions. Modest south winds with 
occasional gusts to 20 kt. High confidence except some 
uncertainty for kbos on exacting timing of improving conditions. 


Tonight... 


Scattered showers and isolated thunder from west to east in a 
mix of VFR/MVFR. Moderate confidence given some uncertainty on 
areal coverage of convection. 


Wednesday... 


Any morning IFR/MVFR improves to VFR by mid to late morning. 
Scattered showers/storms focused over southeast Massachusetts in the 
morning then moving offshore. Can't rule out an isolated shower 
during the afternoon but VFR conditions prevail. High 
confidence. 


Wednesday night... 


VFR and dry weather. High confidence. 


Kbos terminal...very tricky forecast regarding wind direction 
and cigs/vsbys. Thinking around or after sunset once surface 
heating ceases, onshore winds will become more S-SW allowing 
cigs/vsbys to improve to MVFR/VFR. Scattered showers/storms thru 
21z then diminishing. More isolated overnight into Wed morning. 


Kbdl terminal...VFR but MVFR at times in scattered 
showers/storms. Heavy rain and gusty winds may concern. 


Outlook /Thursday through Sunday/... 


Thursday: VFR. 


Thursday night: mainly VFR, with local IFR possible. Slight 
chance shra, isolated tsra, patchy br. 


Friday: mainly VFR, with local MVFR possible. Breezy. Chance 
shra, slight chance thunderstorms and rain. 


Friday night: mainly VFR, with areas IFR possible. Breezy. 
Chance shra, slight chance tsra, patchy br. 


Saturday: mainly VFR, with local IFR possible. Chance shra, 
slight chance thunderstorms and rain. 


Saturday night: mainly VFR, with local IFR possible. Slight 
chance shra, patchy br. 


Sunday: mainly VFR, with local MVFR possible. Slight chance 
rain showers. 


&& 


Marine... 
forecaster confidence levels... 


Low - less than 30 percent. 
Moderate - 30 to 60 percent. 
High - greater than 60 percent. 


Short term /through Wednesday night/... 


415 am update... 


Today...warm front slowly lifts north through the region with 
wind shift from southeast to south-southwest as the day progresses. Areas of dense 
fog reducing vsbys this morning, then improving in the 
afternoon along with risk for a few showers and possibly a tstm 
late. 


Tonight...south-southwest winds continue along with patchy fog and 
scattered showers/tstms. Heavy rain possible in storms. Seas 
will build to 5 ft over the southern outer waters, so have 
issued a Small Craft Advisory beginning at 6 PM tonight for 
those areas. 


Outlook /Thursday through Sunday/... 


Thursday: winds less than 25 kt. 


Thursday night: winds less than 25 kt. Seas locally approaching 
5 ft. Local visibility 1 to 3 nm. 


Friday: winds less than 25 kt. Seas locally approaching 5 ft. 
Chance of rain showers, slight chance of thunderstorms. 


Friday night through saturday: winds less than 25 kt. Areas of 
seas approaching 5 ft. Chance of rain showers, slight chance of 
thunderstorms. 


Saturday night: winds less than 25 kt. Areas of seas 
approaching 5 ft. Slight chance of rain showers, isolated 
thunderstorms. 


Sunday: winds less than 25 kt. Seas up to 5 ft. Slight chance 
of rain showers. 


&& 


Box watches/warnings/advisories... 
CT...none. 
Massachusetts...none. 
Rhode Island...none. 
Marine...Small Craft Advisory until 6 PM EDT Wednesday for anz235-255- 
256. 


&& 


$$ 
Synopsis...wtb/gaf 



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