textproduct: Boston / Norton

WHAT HAS CHANGED

Increasing confidence of record heat Tuesday for parts of southern New England

KEY MESSAGES

- Cooler temperatures near the coast today but continued very warm in the interior.

- Record heat likely Tuesday for portions of SNE with hot conditions lingering into Wednesday. Isolated thunderstorms possible Tuesday afternoon with better chance for scattered strong to severe storms on Wednesday.

- Cooler, more seasonable temperatures late in the week. Increasing risk for showers next weekend.

DISCUSSION

KEY MESSAGE 1...Cooler temperatures near the coast today but continued very warm in the interior.

Weak backdoor front slides south of the coast early this morning. This shallow boundary is expected to lift back to the north this afternoon or mix out as winds become southerly. Highs in the interior will soar into the 80s again, warmest in the CT valley where upper 80s to near 90 is possible as 925 mb temps increase to 22-23C. Sea-breezes will keep coastal locations mostly in the 70s. Bulk of the instability this afternoon will be to the west but some instability is forecast to get into western MA/CT where a spot shower is possible. Otherwise dry weather.

KEY MESSAGE 2...Record heat likely Tuesday for portions of SNE with hot conditions lingering into Wednesday. Isolated thunderstorms possible Tuesday afternoon with better chance for scattered strong to severe storms on Wednesday.

An anomalous upper level ridge builds over the region Tue with rather warm low level temps. 850 mb temps are 2-3SD above normal which is close to the maximum relative to CFSR climatology for this time of year suggesting very anomalous heat. Actual 850 mb temps forecast to be 18-20C with 925 mb temps 24-26C. This suggests highs in the mid 90s away from the south coast with potential for some upper 90s in the CT valley. SW flow will keep temps in the upper 70s and low 80s along the immediate south coast. Dewpoints are forecast to reach lower 60s but with mixing could drop into the 50s in the interior which would keep heat indices below advisory criteria. This is our first heat of the season, so be sure to use caution and hydrate/cool accordingly.

Despite upper ridge breaking down and becoming suppressed to the south, another hot day Wed as thermal ridge ahead of cold front lingers across SNE. Low level temps cool slightly and more cloud cover expected but still hot with highs 90-95 away from the south coast.

Moderate instability is expected to develop Tue afternoon with CAPES 1000-2000+ J/kg. While large scale forcing is limited under the ridge there is a weak shortwave on the periphery of the ridge that approaches so can't rule out a few storms Tue afternoon in unstable environment. Deep layer shear is relatively weak but localized strong gusts are possible in any storms given inverted V profile. The greater risk for t-storms will be Wed afternoon as cold front moves into the region which is favorable timing to take advantage of diurnal max in instability along with deeper moisture profile. Deep layer shear values are more favorable for storm organization so can't rule out scattered strong to severe storms with damaging wind the primary risk although high PWAT airmass will support localized downpours and heavy rainfall.

KEY MESSAGE 3...Cooler, more seasonable temperatures late in the week. Increasing risk for showers next weekend.

Cooler airmass behind the cold front for Thu and Fri with near seasonable or slightly below temps. High pres builds in for the end of the week bringing dry conditions. Then the high retreats next weekend with a frontal boundary approaching from the south and ensemble guidance suggest increasing risk for showers next weekend but details are uncertain.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

Mainly VFR conditions through Wed. However, stratus and fog will likely develop over the ocean as SW flow brings higher dewpoints over cooler ocean and this may impact portions of the south coast and Cape/Islands tonight. LLWS risk for all terminals tonight heading into early Tue morning. Isolated t-storms possible Tue afternoon, but uncertain on if they will impact any terminals. E-NE wind becoming S 10-20 kt this afternoon. S-SW wind 5-10 kt tonight increasing Tue with gusts to 25 kt developing.

KBOS...High confidence in TAF. Seabreeze today with wind shift to S late today or early evening.

KBDL...High confidence in TAF.

Outlook /Tuesday Night through Saturday/...

Wednesday: VFR. Breezy. SHRA likely, isolated TSRA.

Wednesday Night: VFR. Breezy.

Thursday through Friday: VFR.

Friday Night: Mainly VFR, with local IFR possible. Chance SHRA.

Saturday: VFR. SHRA likely.

MARINE

Forecaster Confidence Levels...

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

Today through Tuesday...High confidence.

Easterly winds this morning becoming SE-S this afternoon with speeds below 20 kt. S-SW wind 10-20 kt tonight increasing to 15-25 kt Tue. Seas building to 4-6 ft.

Outlook /Tuesday Night through Saturday/...

Wednesday: Low 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.

Thursday Night: Winds less than 25 kt.

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

Friday Night: Winds less than 25 kt. Chance of rain showers.

Saturday: Winds less than 25 kt. Rain showers likely.

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...Air Quality Alert until 11 PM EDT this evening for CTZ002. Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for CTZ002>004. MA...Air Quality Alert until 11 PM EDT this evening for MAZ008>011. Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for MAZ003>007-010>019-026. RI...Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for RIZ001>004. Air Quality Alert from 11 AM to 11 PM EDT Tuesday for RIZ001>008. MARINE...None.