textproduct: Boston / Norton

WHAT HAS CHANGED

Gale Watch converted to Small Craft Advisories due to low confidence in widespread/consistent gale-force winds tonight and Wednesday.

KEY MESSAGES

- Scattered showers and thunderstorms return this evening through Wednesday morning. Risk for a few strong to severe storms and heavy downpours. Localized flash flooding possible in poor drainage areas.

- Scattered thunderstorms possible later Wednesday, but will be dependent on degree of clearing during the day.

- Drying out Thursday into the latter part of the week with seasonable temperatures and lower humidity into the weekend.

DISCUSSION

KEY MESSAGE 1...Scattered showers and thunderstorms return this evening through Wednesday morning. Risk for a few strong to severe storms and heavy downpours. Localized flash flooding possible in poor drainage areas.

An upper level trough slowly pushes across eastern U.S./Canada today through Thursday with a several pieces of shortwave energy moving through the flow. At the surface, this will sweep a few frontal boundaries across the northeast as a surface low tracks across Quebec. This will support the return of unsettled weather with potential for strong storms and minor localized flash flooding tonight into Wednesday. A 40-50 kt LLJ with be the main player with the set-up, helping advect in richer moisture (dewpoints around 70F) with precipitable water values rising to 1.5-2.0"+ across southern New England this evening. Instability varies from marginal to moderate across high-resolution guidance with MU/SBCAPE values 800- 1500 J/kg. This is sufficient to support thunderstorm development if instability is available while aligning with an incoming shortwave. Model soundings do show hodographs supportive of stronger shear and helicity values in the 0-3 km layer. This environment will not only be support of heavy downpours, but also a strong to severe storm or two with damaging winds. Can't rule out the low risk of a brief "spin-up" given the supportive wind/shear profiles.

Anomalous PWATs combined with warm cloud depths of 11,000-13,000+ ft allow for efficient rain processes and more importantly support heavy rainfall. As far as the QPF probs go, 18z guidance backed off a little with the probs of 1"+ and 2"+ overall, but it's just one run. We are still overall seeing a good signal for 24hr amounts 1"+ shown by the moderate to high probabilities (50-80%) especially along the south coast and Cape and Islands and low to moderate probabilities for 2"+ focused along the immediate south coast and Cape/Islands.

The timeframe at which this falls will be the big factor for flash flooding potential. Given that embedded storms are possible, localized flash flooding is certainly a concern. The HREF PMM reflects this well with localized areas of 2"+ across western CT, RI and southeast MA, but also many streaks of 1-1.25" across the region. Slow storm motion will not be a concern with mid-level flow >30kts; however, there is a prominent LLJ suppling plentiful moisture upstream while steering storms consistently SW to NE. Training of heavier rain and thunderstorm may be possible as a result. The HREF probability of 1 hr precip exceeding 1 hr FFG reflects this well with higher probabilities in NJ and southern NY with low probs extending into western CT. There are a few spots of low probs across MA and RI tonight/early Wednesday. All this is pointing to is a lower probability of seeing widespread flash flood impacts. The HREF showing scattered low probabilities(< 20%) of >1.0"/hr rain rates mainly across portions of CT and moreso RI and southeast MA/Cape & Islands. So we'll have to keep an eye on localized minor flash flooding issues, especially in the more flashy areas or urban/poor drainage areas (Providence, Hartford). This is fairly consistent with the previous forecast with us seeing the risk for localized flash flooding; however, it is not widespread enough of a threat to warrant a Flood Watch for portions of southern New England.

For timing, this will primarily be and evening and overnight event into the early morning hours of Wednesday. Depending on the timing of these shortwaves showers/storms could develop may develop as early as late afternoon; however, chances increase for the evening and overnight timeframe.

KEY MESSAGE 2...Scattered thunderstorms possible later Wednesday, but will be dependent on degree of clearing during the day.

Rain continues through the early morning hours of Wednesday. There is some question on when the bulk of the rain exits and the coverage of showers. Most model guidance has the bulk of rain out by late morning; however, there is more spread on the degree of "clearing" in the afternoon. If there are lingering showers, as some guidance suggests, the suppressed heating will likely end up lowering the risk of storms later in the day. If we get enough of a window with clearing or breaks in the clouds, instability will be sufficient to support thunderstorms developing ahead of a cold front with values 1000-1800 J/kg of SBCAPE. Moisture won't be as anomalous as the previous evening through the whole column; however, the HREF has been keen on another surge of higher dewpoint air ahead of the front. Across the individual CAMs, there is plenty of spread with regards to the timing, coverage, and intensity of convection (if any). A couple hint at the potential for scattered showers and thunderstorms developing after mid-afternoon moving eastward into the evening. The western and central portions of southern New England likely have a higher risk given they would likely clear earlier and have longer to recover and heat through the day. Instability and shear values are sufficient to support a strong to severe storm or two as well. This will become higher confidence as models develop a better consensus on the extent of clearing Wednesday.

KEY MESSAGE 3...Drying out Thursday into the latter part of the week with seasonable temperatures and lower humidity into the weekend.

The cold front exits the region by Thursday morning, leaving drier and seasonable conditions in its wake. With the front, the upper level trough centered over Quebec also moves off to the northeast. High pressure starts to build in over southern New England from the Great Lakes, continuing the drier conditions into the weekend. Temperatures aloft fall slightly, with 925 mb temperatures falling to around 16C for much of the second half of the week. Notably, Wednesday night will likely be cooler as 925 mb temperatures may fall to as low as 10C over the interior. Lows there could get into the low 50s. 850 mb temperatures are expected to mostly be around 10C as well through the back half of the week. Even with these reduced temperatures, the continued high sun angle will keep temperatures from falling too low, keeping them seasonable. Conditions remain dry and seasonable until around Sunday when the flow shifts more S again, increasing humidity. A surface low may also pass just offshore, but any impacts from it -- if any at all -- are not certain at this time.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

12Z TAF Update:

Today...Moderate confidence.

Pockets of MVFR cigs this morning briefly increase to VFR for a few hours before generally deteriorating to MVFR-IFR ceilings by late afternoon. Risk of showers increased after 18z from W to E. A few embedded thunderstorms are possible and consensus has increased enough to include in PROB30s, generally 23-03z (earlier for western terminals, later for eastern terminals). Winds become SE to S and increase to 10-15 kt with gusts 20-25 kt, near 30kts for the Cape/Island terminals.

Tonight...Moderate confidence.

MVFR/IFR possible in any SHRA/TSRA, possibly even LIFR at times for south coast and Cape terminals. Rain overspreads the region this evening and tonight with chances for TSRA across the region. Winds 10-20 kt sustained particularly over SE MA, Cape & Islands. Gusts 20-25 kt, 30 kt across the Cape/Islands, remain possible through the nighttime period in these areas as well.

KBOS Terminal...Moderate confidence in TAF. Timing shower arrival and deterioration of ceilings lower confidence. Should see showers around after 20Z with ceilings falling to MVFR with their arrival by 23/00Z. Localized heavy downpours possible in heavier showers and embedded thunderstorms after 00Z. Lower confidence on potential for thunderstorms due to uncertainty in how far east the instability extends tonight.

KBDL Terminal...Moderate confidence in TAF. Showers developing mainly after 18Z with localized downpours and embedded thunderstorms possible after 20Z.

Outlook /Wednesday through Saturday/...

Tuesday Night: Mainly IFR, with areas MVFR possible. Windy with gusts up to 30 kt. Chance SHRA, chance TSRA.

Wednesday: Mainly VFR, with local IFR possible. Windy with gusts up to 35 kt. Chance SHRA, isolated TSRA.

Wednesday Night: VFR. Windy with local gusts up to 30 kt. Slight chance SHRA.

Thursday through Saturday: VFR.

MARINE

Forecaster Confidence Levels...

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

The probability for more widespread and consistent gusts at or above gale force has decreased thus leading to the cancellation of the Gale Watch and conversion to SCAs. However, some model guidance has still indicated potential for brief more patchy areas of gusts to gale force across the outer waters tonight into early Wednesday as a strong low level jet pushes in. Rough seas 10-12 ft. Near-shore waves along the southern coast, Islands, and outer Cape are likely in the 6-10 ft, and warranted High Surf Advisories starting early Wednesday and lingering late into Thursday. Strong Rip Currents and waves near 6 ft are possible, especially across the outer Cape, Thursday as the cold front stalls out near the coast, prompting the later ending to the High Surf Advisory. SCAs currently go out through Wednesday night; however, will likely need to be extended for most zones (southern waters and outer waters) as 5ft+ waves linger.

Showers and embedded thunderstorms tonight into early Wednesday bringing hazards of heavy downpours, reduced visibilities to less than 2 miles for brief periods in heavier showers/storms, and lightning.

Outlook /Wednesday through Saturday/...

Tuesday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Rain showers, chance of thunderstorms. Areas of visibility 1 to 3 nm.

Wednesday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 13 ft. Chance of rain showers, slight chance of thunderstorms.

Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 12 ft. Chance of rain showers, slight chance of thunderstorms. Local visibility 1 to 3 nm.

Thursday: Winds less than 25 kt. Rough seas up to 12 ft. Chance of rain showers, slight chance of thunderstorms.

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

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

Friday Night through Saturday: 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.