textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes sine last forecast update.

KEY MESSAGES

- Widespread showers and thunderstorms this afternoon and evening.

- Another round of showers and thunderstorms possible Wednesday afternoon

- Seasonable temperatures and comfortable humidity returns with dry conditions for the second half of the week.

DISCUSSION

KEY MESSAGE 1... Widespread showers and thunderstorms this afternoon and evening.

Shortwave energy moving over the northeast will support a period of widespread showers and thunderstorms this afternoon and evening. Moderate instability to the tune of 750-1000 J/kg of MUPCAPE and high PWATs near 2" will support isolated to scattered areas of torrential rainfall with embedded thunder and widespread moderate showers elsewhere. There is a low-risk for some storms to be strong to severe with potentially damaging wind gusts. Relatively strong wind fields by July standards should mitigate the overall flooding threat with mid-level flow around 30 knots supporting faster moving storms. However, we'll be on the looking for any training areas of torrential rain that could lead to flooding in urban or poor drainage areas. Expect the greatest shower and thunderstorm activity across eastern MA and RI between roughly 5PM and 1AM. Shower and thunderstorm activity should wane overnight as stronger forcing shifts east of southern New England.

KEY MESSAGE 2... Another round of showers and thunderstorms possible Wednesday afternoon

Showers and thunderstorms return tomorrow afternoon with instability increasing in response to increased moisture. Dewpoints in the low to mid 70s may support SBCAPE values up to 1500 J/kg. Instability will depend greatly on how much heating we get early tomorrow. The amount of heating we get early will depend on how much clearing occurs between tonight and tomorrow morning. 12Z HREF guidance suggests enough heating to support showers and thunderstorms across eastern CT, MA, and RI during the mid-afternoon hours between 4 and 8PM. Shear will be modest with roughly 20-25 knots in the 0-6km layer. Overall severe threat is low, but a few isolated strong to severe storms will be possible as a cold front pushes through the region. A cooler/drier air mass settles over the region Wednesday night into Thursday with dewpoints falling to the upper 50s/low 60s. This should support a more comfortable day on Thursday.

KEY MESSAGE 3...Seasonable temperatures and comfortable humidity returns with dry conditions for the second half of the week.

As the cold front finally pushes offshore Thursday, the upper level trough over Quebec also pushes northeast into the Labrador Sea and a high pressure system begins to build over the Great Lakes. A drier airmass will be ushered into southern New England Thursday in the wake of the cold front, with dewpoints dropping into the mid to upper 50s, leading to much more comfortable feeling temperatures. 925mb and 850mb temperatures drop to near 16C and 10C, respectively, for the second half of the week, but intense sunlight and mostly clear skies will keep temperatures seasonable... roughly 80-85 through at least Sunday. Rain chances don't look to make a return until later Sunday evening, possible Sunday night into Monday, as south-southwesterly flow becomes more dominant and dewpoints begin to increase.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

Through 00Z...Moderate confidence.

Scattered showers from 18-21Z increasing in coverage and intensity between 21-00Z with -TSRA possible. VFR ceilings/vsbys deteriorating to MVFR. Steady south winds from 10-15 knots with gusts to 25 knots.

Tonight...Moderate confidence.

TSRA/+TSRA mainly between 00-03Z with ceilings falling to MVFR/IFR levels. Expect SHRA/TSRA to wane between 03-06Z. IFR conditions prevail thereafter with south/southwest winds around 10 knots.

Tomorrow... Moderate confidence.

IFR ceilings scatter out to VFR across the interior with some IFR/MVFR ceilings lingering over The Cape/Islands terminals into the afternoon. Showers and thunderstorms possible after 18Z, mainly across eastern MA and RI. Low confidence in intensity and coverage of showers. Southwest winds 10-15 knots with some gusts to 25 knots possible.

Tomorrow Night...High Confidence.

Any afternoon SHRA/TSRA wanes after 00Z with VFR conditions thereafter. Southwest winds shift to light out of hte northwest from 5-10 knots behind a passing cold front. Expect the wind shift around 06Z Thursday.

KBOS Terminal...Moderate confidence in TAF.

Lower confidence in shower coverage and intensity between 18-21Z. Confidence in SHRA increases around 21Z with moderate to high confidence in TSRA near the terminal between 00-03Z, though TSRA could be observed as early as 21-00Z.

KBDL Terminal...Moderate confidence in TAF.

Highest chances for TSRA between 21-00Z.

Outlook /Thursday through Sunday/...

VFR.

MARINE

Forecaster Confidence Levels...

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

Tonight through Wednesday night

Gusty south/southwest winds over the coastal waters tonight with near, but slightly below gale force gusts expected. Small Craft Advisory remains in effect for 25-30 knot wind gusts and 6-10 foot seas over the outer marine zones. Highest waves expected over the south coastal waters. Showers and thunderstorms expected this afternoon and overnight.

Strongest winds come down to 20-25 knots for tomorrow with continued steady 15-20 knot southwest flow during the day tomorrow. A cold front crosses the waters tomorrow night resulting in a wind shift to t he northwest. This will be followed by more tranquil conditions to end the week.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms.

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers.

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

Saturday through Saturday Night: Winds less than 25 kt. Slight chance of rain showers.

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Surf Advisory from 6 AM Wednesday to 8 PM EDT Thursday for MAZ020>024. RI...High Surf Advisory from 6 AM Wednesday to 8 PM EDT Thursday for RIZ006>008. MARINE...Small Craft Advisory from 8 PM this evening to 8 PM EDT Wednesday for ANZ231>235-237-250-254>256.