textproduct: Boston / Norton

WHAT HAS CHANGED

No changes were made to this forecast package.

KEY MESSAGES

- Lingering thunderstorm activity quickly winds down with the loss of daytime heating. Expecting very warm minimum temps tonight with lows in the 60s and low to mid 70s in more urban areas.

- Final day of record warmth Wednesday with highs in the 90s away from the immediate coast. Chance for strong to severe thunderstorms by the afternoon with a risk for damaging winds and small hail.

- Cooler temperatures through Memorial Day Weekend. Unsettled weather returns for the weekend with periods of rain possible, but confidence is low.

DISCUSSION

KEY MESSAGE 1...Lingering thunderstorm activity quickly winds down with the loss of daytime heating. Expecting very warm minimum temps tonight with lows in the 60s and low to mid 70s in more urban areas.

Showers and storms are expected to develop across western portions of our CWA between 3 and 5pm and then quickly move east. Main but guidance is showing the potential for storms to initiate over the Southern Adirondacks and Green Mountains and then gradually coalesce into a more organized line or cluster that generally moves west to east through the afternoon. Main threats with any organized storms will be strong to damaging straight-line winds given drier air in the low to mid-levels resulting in inverted-V soundings. Somewhat notably, the 12z ALB and OKX soundings featured a modified EML between 700 and 500mb with mid level lapse rates between 6.5 and 7.5 C/km. Thankfully RAP mesoanalysis indicates limited bulk shear around 25 kt this afternoon so we're not looking at a widespread severe weather episode. Still, with CAPE values between 1200 and 1500 J/kg, there could be a severe storm or two north of the Mass Pike. Activity quickly wanes with the loss of daytime heating this evening.

Bigger story tonight will be continued heat in humidly. Overnight lows remain in the upper 60s for much of the region and closer to the lower and even middle 70s across urban areas. These temperatures, paired with dewpoints in the 60s will keep apparent temperatures in the upper 70s to low 80s even well into the night. Those without adequate access to cooling and air conditioning are at greatest risk with the elevated overnight lows. There won't be much in the way of relief from winds as the boundary layer decouples and winds calm to less than 5 mph for much of the CWA.

KEY MESSAGE 2... Final day of record warmth Wednesday with highs in the 90s away from the immediate coast. Chance for strong to severe thunderstorms by the afternoon with a risk for damaging winds and small hail.

Upper level ridge begins to break down on Wednesday ahead of an approaching trough. Despite the changes to the upper pattern, 850 temps will remain above +15C S of the Mass Pike and slightly cooler to the N. Guidance has somewhat increased the speed of the cold front resulting in increased cloud cover over much of northern Massachusetts where temperatures have trended a few degrees cooler. Even in these areas, still think that valley locations see highs in the low 90s with downsloping from west winds. Further south, there is higher confidence in more widespread readings in the low to mid 90s away from the water. Greatest chance will be in the HFD PVD and BOS urban corridor. Dewpoints in the low to mid 60s (discounting the outlier NAM) will make it feel humid but not tropical by any means.

An approaching cold front provides a much more robust lifting mechanism on Wednesday. With temperatures quickly warming into the 90s, expecting surface CAPE values to rise to 1000-1500 J/kg. 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. 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. Greatest threat for any severe weather will likely be S of the Mass Pike where higher instability exists.

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

High pressure builds and mid-level ridging building in behind Wednesday's system bringing near-seasonable temperatures and dry conditions.

For Memorial Day weekend there is a signal for the pattern to become more unsettled with a few disturbances moving through bringing periods of rain. However, there is still a sufficient amount uncertainty in the details of the track and timing of the waves which will influence rain chances, timing, and amounts this holiday weekend. Across deterministic guidance, there is plenty of spread in the track and details of a system that moves in from the Ohio Valley. This has led to a range of scenarios from a drier weekend with seasonable temperatures to a weekend with periods of rain and cooler temperatures. Whats adds to the uncertainty is the lack of consistency of models from run to run as well. From a probabilistic view, ensemble means are keying in on moderate probabilities for rain across their respective members at this time, so the risk is there for periods of rain over the weekend into the holiday. We'll have to keep monitoring as well get closer and model guidance comes into better agreement.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

This Afternoon and Tonight...High confidence.

VFR. Sct'd TSRA possible by the afternoon with continued confidence in impacts between 21 and 00z from BOS, BED, BDL, and ORH. S-SW wind 10 to 15 kt gusts to 25 kt through the afternoon and evening. VFR overnight, with MVFR ceilings across the Cape and Islands. Possible IFR at ACK with areas of fog.

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 Sunday/...

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

Wednesday Night: VFR. Breezy.

Thursday through Friday: VFR.

Friday Night through Saturday Night: VFR. Chance SHRA.

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

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 through 06z Thurs. SW winds of 15-25 kt continues this afternoon and overnight Seas will also be building to around 4-6 ft, with some 7 footers possible tonight over the southern waters.

Mainly dry weather prevails through tonight with a few showers or thunderstorms possible for the northern waters., Some fog may persist and expand across the southern waters tonight on increasing southwest winds.

Outlook /Wednesday through Sunday/...

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

Thursday Night through Friday: Winds less than 25 kt.

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

Saturday through Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers.

Sunday: Winds less than 25 kt. Local rough seas. Chance of rain showers.

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