fxus61 kbox 211912 

Area forecast discussion 
National Weather Service Boston/Norton Massachusetts 
312 PM EDT Tue Aug 21 2018 

a low pressure system will bring two rounds of scattered showers 
and embedded thunderstorms. The first late tonight into 
Wednesday morning, then a second round Wednesday afternoon along 
with the return of high humidity. High pressure will then 
follow, bringing sunny conditions and comfortable humidity 
Thursday and Friday. The high moves off the coast next weekend 
allowing increasing chances of showers and thunderstorms. 


Near term /through tonight/... 
light drizzle/showers still lingered around the cape and 
islands. Not expecting that to change much late this afternoon. 
Clouds will remain more prevalent across the eastern half of 
southern New England. Farther west, mostly sunny skies are more 
likely. Tweaked temperatures for the rest of this afternoon to 
reflect observed trends. 

Overnight, the main concerns will be the progress of a low 
pressure and its warm front. Have high confidence in this low 
pressure moving through the Great Lakes into Canada. Have lower 
confidence in the timing of a warm front. Many members of the 
21/12z guidance suite suggest this front could move into 
northern New England before daybreak. Thinking is this likely a 
bit too quick. Have more confidence in a warm frontal passage in 
the 8-11 am Wednesday time range. 

This could be key to weather outcomes. The higher dew points 
will be south of this warm front. These higher dew points will 
provide the fuel for local downpours, as well as lowering cloud 
bases. Some of the higher resolution short-term guidance suggest 
high values of shear in the 0-1 km layer. Thus, still need to 
be wary of rotating thunderstorms. 

A caveat to this will be the time of day. Instability should not 
be very high overnight, but would be a little higher if our 
region can tap into the higher dew points south of the warm 
front. At this point, have much more confidence in the potential 
for locally heavy rainfall. 


Short term /Wednesday through Wednesday night/... 
* a few strong to severe storms possible 
* localized street flooding possible, too 


Once our region firmly gets into the warm sector, looking at a 
strong southerly low level jet to help recharge the instability 
for Wednesday afternoon. Current timing for a cold front is 
during the afternoon and evening. The quicker sunshine develops 
in the morning, the greater the risk for strong to severe 
thunderstorms during the afternoon and evening. 

Otherwise, still expecting another day of higher heat and 
humidity. This should start to an end later Wednesday night, as 
drier air arrives behind a cold front. 


Long term /Thursday through Monday/... 
*/ highlights... 

- dry, comfortable weather Thursday into Friday 
- return warmer, humid conditions beginning over the weekend 
- accompanying chances of showers and thunderstorms 

*/ discussion... 

Warmer than average temperatures forecast towards September. Latent 
heat release per Soulik and Cimaron, becoming baroclinic, amplified 
North Pacific standing wave pattern with 500 mb Bering Sea trof, NE Pacific 
Ridge, and subsequent west Continental U.S. Trof. The latter, an associated anti- 
cyclonic rossby wave break and attendant pv-streamer influencing 
continued organized tropical convection out towards Hawaii. This and 
upstream trends as seemingly the main polar low retrogrades back N, 
500 mb ridging builds and is maintained over the southeast Continental U.S. Extending into 
New England into the end of the month per SW-NE pump. Ensemble means 
signaling anomalous 850 mb warmth, CPC 6-10 and 8-14 day temperature 
outlooks with high confidence noting warmer than average conditions. 
Can't rule out dips within aforementioned SW-NE Continental U.S. Flow, trailing 
cold fronts and chances of showers / thunderstorms, however becoming 
diffuse up against the 500 mb ridge as associated mid-level energy lifts 
N/E. It's back to north/west New England scattered wet-weather activity 
that falls apart before ever reaching the S/East Coast. 

Low confidence forecast further out in time, not noting any targets 
of opportunity. Overall, looking dry and comfortable Thursday into 
Friday, followed by the return of warmer, more humid conditions for 
the weekend into next week. Accompanying chances of showers and 
thunderstorms give increasing instability. Too early to say with any 
certainty as to strong / severe storm threats as well as any flash 
flooding potential, however such risks are being considered given 
antecedent conditions in addition to past storm trends / 


Aviation /18z 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/... 

Through 00z... 

Hold MVFR-IFR cigs over a majority of southeastern terminals. 
Perhaps interior scattered by then before dropping down again this 
evening. East winds around 10 kts. 


MVFR-IFR becoming widespread yet again. Onset -ra with isolated 
thunderstorms and rain towards morning. With any -ra, dropping vsbys down to IFR 
given forecast soupy conditions with increasing S flow. Marginal 
low level wind shear possible by morning as a warm front approaches. 


Beginning early morning, widespread -shra with embedded 
tsra/shra. Anticipating mainly MVFR-IFR conditions with isolated 
LIFR. A brief lull around noon to 2 PM, before another round of 
thunderstorms and rain/rain showers is possible with tempo IFR, if not already, before 
clearing out. 

Wednesday night... 

Mainly VFR. 

Kbos terminal...hold MVFR for today, can't rule out brief, 
tempo improvement to VFR. East winds prevailing up around 10 kts. 

Kbdl terminal...high confidence in taf. 

Outlook /Thursday through Saturday/... 

Thursday through saturday: VFR. 


forecaster confidence levels... 

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

Short term /through Wednesday night/...high confidence. 

Low risk for a brief waterspout late tonight as a warm front 
moves across the waters. Vsby lowered at times in morning fog 
and scattered thunderstorms, then improving before second round 
of thunderstorms in the afternoon. 

Some risk for small craft advisories along the southern outer 
coastal waters due to marginal 25 kt gusts and 5 ft seas. 

Outlook /Thursday through Saturday/... 

Thursday: winds less than 25 kt. Areas of seas approaching 
5 ft. 

Thursday night: winds less than 25 kt. Seas locally approaching 
5 ft. 

Friday through saturday: winds less than 25 kt. 


Box watches/warnings/advisories... 
Rhode Island...none. 



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