textproduct: Boston / Norton

WHAT HAS CHANGED

Forecast remains on track.

KEY MESSAGES

- Diurnal shower and thunderstorm chances remain a threat today. Heat and Humidity remain elevated, but slowly trending down compared to the last few days.

- Very warm and humid weather continues through Wednesday. Daily shower and storm chances each day too, but lesser of a chance Sunday.

- Turning much less humid with cooler temperatures late this week.

DISCUSSION

KEY MESSAGE 1...Diurnal shower and thunderstorm chances remain a threat today. Heat and Humidity remain elevated, but slowly trending down compared to the last few days.

The overall upper-level pattern becomes more zonal this weekend; however, another quick moving shortwave will be the catalyst for another round of showers and thunderstorms this afternoon and evening. Much like the last few days, CAM guidance isn't handling the environment well. Uncertainty remains high in where and when storms could initiate this afternoon as well as how far east they'll travel. However, the 00z runs of both CSU's ML guidance and the HRRR Neural Network indicate higher confidence that some severe storms could pop over interior southern New England. Given dewpoints will once again be in the low to mid 70s, PWATs will likely be in the 1.75"-2.00" range. Latest guidance suite also indicates that the mid- level steering flow could be a little faster than we've seen over the last few days. This indicates that while there could be heavy downpours/torrential rainfall within storms, they will likely be moving a little faster than we've seen previously and less likely to train. Latest model soundings also indicate higher DCAPE values (ranging from 600-1,000 depending on the model) across portions of western MA and CT. Both CSU's ML probabilities and the HRRR's Neural Network also show 15%-30% probs of severe winds draped across the Hartford-Worcester- Lowell corridor.

Regardless, any thunderstorms that do pop both today have the potential to produce frequent lightning, gusty winds, and torrential downpours, and we can't rule out localized flash flooding if a storm or two were to push over a more vulnerable area (urban/poor drainage, area of recent heavy rainfall).

KEY MESSAGE 2...Very warm and humid weather continues through Wednesday. Daily shower and storm chances each day too, but lesser of a chance Sunday.

Overall pretty persistent summer weather pattern really right through Wednesday, with enhanced quasi-zonal flow in midlevels interacting with summerlike temperatures and humidity levels.

Warm temperatures and elevated but not really oppressive humidity levels (dewpoints in the mid to upper 60s) can be expected to continue through Wednesday. General sameness to 925-850 mb temps each day, only slightly cooling by a degree or two from Sunday through Wednesday, which supports highs in the mid/upper 80s and heat indices in the lower to middle 90s. While not necessarily a hazardous combo of heat and humidity and most are already acclimatized to the summer warmth, thus not looking at headline- level heat, it is still an extended spell of heat indices in the low to mid 90s, so still continue to adequately hydrate and take breaks in cooler spaces.

With westerly mostly non-convergent low-level flow and modest height rises Sunday, Sunday should end up being a generally dry day as will be tough to get enough lift to spark storms. Otherwise, daily shower and thunderstorm chances again resume for early to mid next week, and with somewhat stronger flow, a couple of those storms could become strong or severe in this timeframe. The issue is timing the storms out, as has been the case most of this week; this is very challenging as any ripple in the faster mid-level flow could focus better storm chances given the warm and humid weather. At this forecast time horizon timing specific day(s) have more storm chances than others is a tough ask. Won't be raining and thundering all the time but there will be storms to dodge here-and-there.

Tough to find many areas to improve upon NBM so stayed fairly close to outputted NBM forecast values during this period.

KEY MESSAGE 3...Turning much less humid with cooler temperatures late this week.

The true "break of the heat and humidity" takes place late in the week, as we become more entrenched in cyclonic flow aloft with northwest winds bringing both temps and dewpoints down to more seasonable levels. Highs closer to the upper 70s to lower 80s; wouldn't rule out a spot shower or two under the cyclonic flow but nothing that looks necessarily significant.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

12Z TAF Update:

Today through Tonight: Overall high confidence, but lower on TSRA timing.

Patchy MVFR/IFR stratus/fog along the south coast/Cape/Islands/CT River Valley improves by 14z to VFR. Generally WSW winds today, with some gusts near 20 kts along the Cape/Islands. Still some uncertainty in timing of TSRA, which again varies between 18-20z at earliest to as late as 01z, then moving eastward. PROB30s remain, but pushed 1-2 hours later than originally anticipated. TSRA dissipates late this evening, with south-coastal fog/stratus returning with VFR elsewhere.

KBOS Terminal...Moderate confidence.

SW winds around 8-12 kt, instances of gusts up to 18 kt. Seabreeze not likely today. Monitoring for TSRA this aftn/evening but not likely any sooner than 21z if they get close.

KBDL Terminal...Moderate confidence. Generally VFR. Another risk for TSRA this aftn/evening, but exact timing is still uncertain.

Outlook /Sunday through Wednesday/...

Sunday: VFR. Breezy.

Sunday Night through Monday: VFR.

Monday Night: VFR. Slight chance SHRA, slight chance TSRA.

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

Tuesday Night: VFR. Slight chance SHRA.

Wednesday: VFR. Slight 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 Sunday... High Confidence.

High pressure remains positioned off of the east coast. This will continue to produce a SW flow across the entire region into the weekend. Gusts to 20 kts possible over the southern waters today, but should remain under SCA levels.

Outlook /Sunday through Wednesday/...

Sunday through Sunday Night: Winds less than 25 kt. Areas of seas approaching 5 ft.

Monday: Winds less than 25 kt.

Monday Night through Tuesday: Winds less than 25 kt. Slight chance of rain showers.

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

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

BOX WATCHES/WARNINGS/ADVISORIES

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