textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Much above normal temps today followed by a brief cooldown near the coast Monday. Then near record heat possible Tuesday and Wednesday.

- Isolated strong thunderstorms are possible in the interior Tuesday afternoon, with a better chance of scattered strong to severe storms Wednesday.

- Cooler, more seasonable temperatures late in the week.

DISCUSSION

KEY MESSAGE 1...Much above normal temps today followed by a brief cooldown near the coast Monday. Then near record heat possible Tuesday and Wednesday.

Warmup begins today as weak shortwave moves offshore this morning followed by rising heights. Warming ahead of a backdoor front to the north with 925 mb temps rising to 19-21C will support highs into the mid-upper 80s, but upper 70s along the immediate south coast and Cape Cod. The backdoor front slips south of New Eng tonight with cooler temps and light NE winds developing. However the cooler air is shallow and will mix out Monday in the CT valley where highs get back into the 80s, with mid-upper 70s elsewhere except along the immediate coast where sea-breezes may hold temps in the upper 60s to near 70.

Anomalous upper ridge builds along the east coast Tue with height anomalies 2-3SD above normal. 850 mb temps 18-19C and 925 mb temps near 25C will support highs in the low-mid 90s away from cooling effect of SW flow near the south coast. Record highs may be broken away from the south coast. Humidity levels will be on the rise with dewpoints climbing to 60-65 but heat indices should be similar to actual air temps.

The upper ridge begins to break down Wednesday as northern stream shortwave energy rides to the north suppressing the ridge to the south. However, low level temps remain rather warm through 18z suggesting another hot day, possibly a bit cooler than Tue due to increased cloud cover. Expect highs into lower 90s, cooler near the south coast, with similar dewpoints to Tue. Temps will likely be turning sharply cooler mid-late afternoon as cold front moves through with showers and t-storms developing.

Given this is the first heat event of the season it may have a greater impact on those vulnerable to heat related illnesses than it would in the middle of the summer. It is important to stay hydrated and avoid strenuous activity in the sun.

KEY MESSAGE 2...Isolated strong thunderstorms are possible Tuesday afternoon, with a better chance of scattered strong to severe storms Wednesday.

The heat and increasing humidity will result in moderate instability Tue with CAPES 1000-2000+ J/kg. However, forcing and deep layer shear is limited under the upper ridge and best moisture will be to the north and west. So while we can't rule out a few afternoon t- storms Tue in the interior we think coverage will be limited. The CSU machine learning guidance does show low probs for severe Tue which is likely a conditional risk on t-storms developing. Strong wind would be the main threat given inverted V soundings with steep low level lapse rates. But we think the greater severe weather risk will be Wed as we have stronger forcing with cold front moving into the region and stronger shear. Something to monitor going forward but at the very least expect numerous showers developing Wed afternoon along with scattered t-storms with risk for heavy downpours as PWATs increase to 1.50 to 2.00 inches which is up to 2-3SD above normal.

KEY MESSAGE 3...Cooler, more seasonable temperatures late week.

Cold front moves offshore Wed night followed by cooler and drier airmass for Thu and Fri with more seasonable temps. Dry weather expected into Fri but moisture will try to return sometime Fri night into Sat with risk of showers.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

06Z TAF Update:

VFR conditions through Monday. Increasing W winds today with gusts to 20-25 kt developing this afternoon. Winds diminishing tonight and becoming light N-NE, then SE-S 7-15 kt Monday with sea-breezes along the coast.

KBOS...High confidence in TAF.

KBDL...High confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night: VFR. Breezy.

Tuesday: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

Tuesday Night: VFR. Breezy.

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

Wednesday Night: VFR. Breezy. Slight chance SHRA.

Thursday: VFR.

MARINE

Forecaster Confidence Levels...

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

SW gusts to 25 kt will diminish by daybreak but hazardous seas around 5 ft will linger over southern waters until this afternoon so SCAs were extended here into the afternoon. Otherwise, a period of marginal W-SW SCA wind gusts to 20-25 kt will develop this afternoon over the nearshore waters, diminishing tonight then becoming light NE. E-SE wind 10-15 kt Mon.

Outlook /Monday Night through Thursday/...

Monday Night: Winds less than 25 kt.

Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft.

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 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.

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

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...Small Craft Advisory until 5 AM EDT early this morning for ANZ231>234-251. Small Craft Advisory until 8 AM EDT this morning for ANZ235- 237-250. Small Craft Advisory until 5 PM EDT this afternoon for ANZ254>256.