textproduct: Boston / Norton

WHAT HAS CHANGED

SPC has placed much of southern New England in a general risk for thunderstorms this afternoon. There is a low chance for a couple of stray showers or thunderstorms this afternoon, mainly across cenral and eastern MA. Otherwise, no changes beyond this afternoon's forecast.

KEY MESSAGES

- An isolated shower or thunderstorm possible this afternoon

- Warm/dry weather continues tonight through tomorrow night

- Above normal heat Tuesday. Isolated strong thunderstorms possible for interior Tuesday afternoon. Better chance for scattered strong to severe storms Wednesday.

- Cooler, more seasonable temperatures late in the week.

DISCUSSION

KEY MESSAGE 1...An isolated shower or thunderstorm is possible this afternoon

Very subtle short-wave energy aloft and 1000+ J/kg of SBCAPE may be enough to support a couple of stray convective showers or thunderstorms this afternoon. Hi-Res model guidance began picking up on this solution with the 12Z model runs. The consensus is for some showers to develop over southern NH and progress southeast over northeastern MA this afternoon. Confidence in a shower or storm is still fairly low given a fairly strong cap in place and substantial height rises this afternoon. Nonetheless, we'll be keeping an eye on the radar for any isolated storms that may develop. Any thunderstorms that do develop would likely be sub-severe and limited to a brief downpour and some thunder and lightning.

KEY MESSAGE 2...- Warm/dry weather continues tonight through tomorrow night

High pressure builds east of southern New England on Monday. This will support onshore flow for most of the region which will result in a fairly sharp temperature gradient between the interior locations and the coastal plain. A warmer than normal air mass will remain over the region with 925 hPa temps between 15 and 20C. This will supprt high temps in the low to mid 80s across the interior and low to mid 70s near the coast. Winds gradually shift from east to south throughout the day tomorrow. Dry conditions persist through Monday night.

KEY MESSAGE 3...Well-above normal heat Tuesday. Isolated strong thunderstorms possible for interior Tuesday afternoon. Better chance for scattered strong to severe storms Wednesday.

An anomalous upper level ridge builds over the region Tuesday with 850mb temperatures 2-3 standard deviations above normal. 925mb temperatures 22-25C will likely support well above normal highs in the upper 80s and 90s. Areas along the south coast and Cape will stay in the 70s-80s from the cooler SW flow. There is still uncertainty in the potential for 90F+ and the coverage of this across inland southern New England. The GEFS has higher probabilities for 90+ while the ECMWF has leaned cooler. Regardless, the higher range of probabilities is highlighted across the valleys, especially the CT Valley. This is where we could see the potential for 90-95 degree readings on Tuesday. Dewpoints 60-64F may add some humidity to the air; however, won't nudge the heat index (feels like) temperatures up more than a few degrees. It will be a hot day, especially in the valleys. This is our first heat of the season, so be sure to use caution and hydrate/cool accordingly.

The heat and increased humidity levels will build in moderate instability Tuesday with CAPE 1000-2000 J/kg. However, the forcing is lacking. Shear is also on the limited side. While this is not generally supportive of widespread severe thunderstorms, can't rule out some isolated storms in the afternoon with a few stronger storms possible. Model soundings show a classic inverted-V profile, typically pointing to a threat for strong winds/downdrafts.

The greater thunderstorm risk looks to be Wednesday with the arrival of a cold front. This will bring in stronger shear values to support more organized thunderstorms, with potential to be strong to severe. Something to keep an eye on. There is a better signal for moisture as well which will support heavy downpours in any storms. For now, we see a good shot at scattered showers and thunderstorms with the risk for some being strong to severe.

KEY MESSAGE 4...Cooler, more seasonable temperatures late week.

A cold front will swing through by Thursday bringing in a cooler airmass. This will bring in more seasonable/seasonably cool temperatures late-week with mainly dry conditions and northerly component flow.

AVIATION /00Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

00Z TAF Update:

Tonight and Monday...High confidence in TAF

VFR. Winds becoming easterly overnight less than 10 kts. For Monday, ESE winds becoming SE/SSE in the afternoon 8-12 kts. Occasional gust up to 18 kts possible.

Monday Night...High confidence in TAF. VFR. Winds shift S early evening becoming SW Monday night into Tuesday. Potential for BR again at ACK developing Monday evening.

KBOS...Moderate to High confidence in TAF.

Boundary arrives from south to BOS 02-03Z. If WNW winds stay on the lighter side (10 kts or less), we may see a brief shift to a southerly component wind (S/SSE). Winds should shift to NW/northerly by 06Z then shifting over to easterly early Monday morning.

KBDL...High confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday through Tuesday Night: VFR. Breezy.

Wednesday: VFR. Breezy. Chance SHRA, slight chance TSRA.

Wednesday Night: VFR. Breezy. Chance SHRA.

Thursday through Thursday Night: VFR.

Friday: VFR. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Tonight through Tomorrow Night

High pressure moves east over the coastal waters tonight through tomorrow bringing relatively calm conditions to the coastal waters. Generally expect seas with significant wave heights 3 feet or less and modest wind speeds around 10 knots with some gusts to 15 knots possible. Winds shift from west/northwest to easterly overnight with a gradual shift to the south as the day progresses tomorrow.

Outlook /Tuesday through Friday/...

Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft.

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas.

Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of rain showers.

Wednesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, slight chance of thunderstorms.

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

Thursday Night: Winds less than 25 kt.

Friday: Winds less than 25 kt. Slight chance of rain showers.

CLIMATE

Record Highs for Tue May 19...

BOS 90/1949 BDL 94/1962 PVD 91/2017 ORH 92/1962

Record Highs for Wed May 20...

BOS 91/1996 BDL 99/1996 PVD 95/1996 ORH 91/1903

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.