textproduct: Boston / Norton

WHAT HAS CHANGED

Small Craft Advisory extended into Wednesday night for the southern outer waters.

KEY MESSAGES

- Record heat possible Tuesday with marginal humidity, kicking off a multi-day stretch of anomalously hot temperatures (in the mid to upper 90s away from the South Coast).

- Isolated thunderstorms possible today, mainly north and west of I- 95. Better chances for strong to severe thunderstorms on Wednesday with strong damaging winds the main risk.

- Cooler and unsettled weather is possible Memorial Day Weekend, but confidence remains low.

DISCUSSION

KEY MESSAGE 1...Record heat possible Tuesday with marginal humidity, kicking off a multi-day stretch of anomalously hot temperatures (in the mid to upper 90s away from the South Coast).

A Heat Advisory remains in effect for much of southern New England on Tuesday and Wednesday, not necessarily because of the expectation for an exceptionally oppressive heat/humidity combo but due to a very early arrival of well above normal heat for mid May in southern New England. This anomalously warm airmass comes courtesy of a building mid level ridge today pushing 850 mb temps into the 18-20C range which wold be 2-3 SD above climatology. Given an expected well mixed boundary layer surface temperatures should be able to reach well into the 90s. Warmest locations will be the CT and Merrimack valleys where downslope flow will help push temps into the mid 90s with low to mid 90s elsewhere. The exception will be along the south coast where that SW wind trajectory off of the ocean keeps temps comfortably in the mid 70s. The ridge begins to break down on Wednesday with falling heights, but even so temps will remain elevated ahead of the cold front; increasing cloud cover ahead of the surface trough will help keep temps a few degrees cooler, but many locations inland still see low to mid 90s away from the water. Dewpoints in the low to mid 60s (discounting the outlier NAM) will make it feel humid but not tropical by any means.

KEY MESSAGE 2...Isolated thunderstorms possible today, mainly north and west of I-95. Better chances for strong to severe thunderstorms on Wednesday with strong damaging winds the main risk.

When it comes to thunderstorm threat each day, Wednesday looks to be the more favorable day for strong to severe storm coverage because while both days feature the necessary moisture and instability, the lift mechanism is much more robust on Wednesday (the cold front). Those elevated temps and dewpoints will contribute to instability values on the order of 1000-1500 J/kg of surface based CAPE each day while dewpoints are in the low to mid 60s as previously mentioned. 0- 6km bulk shear values, while fine (25-30 kts), are less favorable for organized severe weather than they are further north where the stronger mid level flow resides. So, the ingredients are there for some strong to severe storms. The lifting mechanism is much less robust on Tuesday, but guidance indicates a weak surface trough and mid level shortwave which will likely be enough to kick off some storms. Despite very strong low level lapse rates, the longevity and strength/height of these storms will be limited by poor mid level lapse rates. Wednesday the cold front will serve as a better lifting mechanism, timed coincident with peak diurnal heating so would expect more coverage of storms and any storms that do form have the chance to produce strong to damaging winds primarily given inverted- v soundings.

KEY MESSAGE 3...Cooler and unsettled weather is possible Memorial Day Weekend, but confidence remains low.

After high pressure delivers dry and seasonable weather to end the week, attention turns to the holiday weekend. Unfortunately it's not a cut-and-dry forecast at this point (5-7 days out) as guidance remains in two distinct camps. One would result in a cool (or downright cold for late May) and wet weekend while another keeps things dry and seasonable. Up to this point deterministic guidance has indicated a rainy Sat/Sun ahead of a low coming out of the OH Valley while the AI guidance like the AIGFS and ECMWF AIFS keep it suppressed to the south by high pressure, at least through Sunday. Obviously this would have a big impact on the weekend, with respect to both rain and temperatures. For what it's worth, the latest GFS guidance has shown that suppression to the south as well, but can't put any confidence in an individual run. Ensemble guidance continues to show a decent chance of rain for the weekend.

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 and Tonight...High confidence.

VFR. Iso'd TSRA possible by the afternoon, though confidence is increasing in impacts to BOS, BED, BDL, and ORH. S-SW wind 8 to 12 kt, gusts to 25 kt developing this morning into the afternoon. VFR overnight, with MVFR ceilings across the Cape and Islands.

Gusts drop off for terminals not on the Cape and Islands after 00z tonight.

Wednesday...High confidence.

VFR. Scattered TSRA along and ahead of a cold front in the afternoon and evening. S-SW wind 8-10kt with gusts 20-25 kts turning NW behind the cold front between 15-00Z. Strong gusts possible in any thunderstorms.

KBOS...High confidence in TAF.

KBDL...High confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday Night: VFR. Breezy.

Thursday through Friday: VFR.

Friday Night: VFR. Chance SHRA.

Saturday: Mainly VFR, with local IFR possible. SHRA likely.

MARINE

Forecaster Confidence Levels...

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

Through Wednesday: High confidence.

Small craft advisories remain in effect 12z Tuesday through Wednesday evening. SW winds will be increasing to around 15-25 kt Tuesday and remain so through Wednesday, higher over the southern waters. Seas will also be building to around 4-6 ft, with some 7 footers possible Tuesday night over the southern waters.

Mainly dry weather prevails through Tuesday night, although fog may develop both tonight and Tuesday night.

Outlook /Wednesday through Saturday/...

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

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 through Friday: Winds less than 25 kt.

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 90/2017 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...Heat Advisory from 11 AM this morning to 8 PM EDT Wednesday for CTZ002>004. Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for CTZ003-004. MA...Heat Advisory from 11 AM this morning to 8 PM EDT Wednesday for MAZ003>007-010>019-026. Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for MAZ006-007-013>019. RI...Heat Advisory from 11 AM this morning to 8 PM EDT Wednesday for RIZ001>004. Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory from 11 AM this morning to 2 AM EDT Thursday for ANZ254>256.