NEAR TERM /UNTIL 6 AM FRIDAY MORNING/

405 PM Update...

* A few showers into this evening...focused across the interior but may briefly impact the Boston to Providence corridor

* Muggy tonight with the main risk for scattered showers & perhaps an isolated t-storm or two mainly after midnight

Scattered showers have developed across southwest MA and northern CT as of mid-late afternoon. This in response to shortwave energy pushing eastward coupled with some marginal instability. We can not rule out a rumble or two of thunder across the interior into this evening...but instability and forcing is rather limited. Further east onto the coastal plain...just a few brief showers are expected into the evening. This a result of the shortwave energy weakening as it runs into subtle ridging.

Appears the better risk for some scattered showers and perhaps a thunderstorm or two will be after midnight and especially toward daybreak. This in response to a subtle surface wave that will interact with very high Pwats on the order of 2-2.25 inches and several hundred J/KG of MUCape
Therefore
any of the activity will be capable of producing brief downpours. Because the forcing is limited but the Pwat plume is very high...the uncertainty revolves around the areal coverage of the activity and the location of it.
This may need to be more of a nowcast situation...so later shifts will likely have to adjust the Pops/timing based on radar and satellite trends.

Moist southerly flow will keep overnight low temps mainly in the upper 60s to the lower 70s and it will be muggy.

SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/

Key Points...

* Very Warm & Humid Friday with highs 85-90 away from south coast * A few showers/isolated t-storms Fri with brief downpours possible

Friday...

A ridge of high pressure off the coast will result in a very warm and humid day on Friday. The majority of the day will likely feature dry weather...but a few showers/isolated t-storms will be possible at times given the tropical environment in place. We will discuss that below...but overall expect highs to reach between 85 and 90 Friday away from the south coast/Cape and Islands where highs will be in the upper 70s to the lower 80s.

Scattered showers and perhaps a few t-storms will probably be departing the region Fri morning with the subtle shortwave. A few additional showers/isolated t-storms may popup later Fri morning and especially during the afternoon given a very high Pwat plume in place and any diurnal instability. The difficulty is trying to locate/time this activity given limited forcing in a tropical-like environment with Pwats of 2-2.25 inches. Models tend to struggle in these weakly forced environments...but it will not take much for a few showers/isolated t-storms to develop. Greatest concern would be during the Fri mid-late afternoon hours into the first part of Fri evening across the interior MA and CT. This is where surface Capes may approach 1500 J/KG and CSU machine learning probs along with HRRR Neural Network/Nadocast indicate some very low severe probs in our western zones
Again
forcing is limited but given the environment can not rule out the low risk for an isolated severe t- storm/localized wet microburst or two
In addition
a very localized flood threat can not be ruled out either given Pwats 3+ standard deviations above normal. So any t-storm that is able to develop will be capable of producing torrential rainfall.

Friday night...

Initially will have to watch for a few showers/isolated t-storms early Friday evening. This activity will probably start to dissipate though with the loss of diurnal heating coupled with poor mid level lapse rates
However
we may see scattered showers/isolated t- storms redevelop overnight given a modest southerly LLJ and very high Pwats on the order of 2 to 2.5 inches! While much of this time will feature dry weather...any activity will be capable of producing brief torrential rainfall given the tropical environment in place.
The very high dewpoints will probably hold overnight low temps in the upper 60s to the middle 70s...so it will be quite muggy.

LONG TERM /SATURDAY THROUGH THURSDAY/

Key Points:

* Mid-summer heat and humidity through the period * Potential for local heavy downpours Saturday and perhaps Wednesday

Latest global ensemble model outputs don't really show any big changes from prior forecasts. The large scale weather pattern indicates a mid/upper level ridge of high pressure will remain anchored to our east, with the ridge axis from roughly Bermuda north into the Canadian Maritimes. That puts us on the western periphery of the ridge, although 500mb heights will remain above normal. That kind of pattern would keep deep southwest flow across our area, meaning an extended period of high dewpoint air. It would also be hard to get any strong upper level dynamics to really impact the region as any shortwave would be moving into the ridge.

What we will primarily experience as sensible weather for the area will be the humidity. NBM data looked quite good, so that would suggest dewpoints will rarely be below 65F through the period.
Saturday and then again Tuesday and Wednesday should be the most humid of the days, with dewpoints in the 70-75F those days. Luckily we aren't looking at extremely high temperatures (mid to upper 80s those 3 days), so heat index values will "only" be in the lower 90s.
Those most humid days are also when we will have the higher chances of showers and thunderstorms. Given precipitable water values expected to be 2-2.25" on Saturday and again mid-week, that sets us up for the potential for locally heavy rainfall. Ensemble model probabilities show about a 20-30% chance of seeing 0.5" in a 6 hour period for Saturday and again Wednesday. Although those values don't seem high, they are a respectable signal given the lower resolution of global models. With Saturday, surface CAPE values should reach 500-900 J/kg especially across western half of SNE so some convection should develop. Wind fields are not strong and mid level lapse rates are only 5-5.5C/km so not expecting any severe weather, but an isolated strong storm is not out of the question. As mentioned previously, it's the high PW values that suggest local heavy downpours as the primary issue.

Looks to be mostly dry Sunday and Monday, so those days will have the best potential to top out in the 90-92F range in many inland areas. Though it will be humid, dewpoints will be not be high enough to result in oppressive conditions. Another surge of higher moisture comes back in for Tuesday and Wednesday, thus additional showers and a few storms are expected. Much more uncertain for Thursday, so stuck with NBM which offers a climatologically normal 30-40% chance of showers and a few t-storms. However this time of year there are rarely strong synoptic signals to really latch onto, so sticking with a "climo" forecast is the best bet.

AVIATION /20Z THURSDAY THROUGH TUESDAY/

Forecast 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 into the evening hours with mainly just mid level cloudiness. We do expect MVFR-IFR ceilings to begin to develop after midnight as the boundary layer cools and low level moisture continues to increase. A few showers perhaps an isolated t-storm possible later this afternoon into this evening across the interior. Greater risk for more scattered showers and perhaps a rumble of thunder will be after midnight. Any of this activity will be capable of producing brief downpours. S-SW winds 5-15 knots becoming light tonight.

Friday and Friday night...Moderate Confidence.

MVFR-IFR conditions early Fri morning should improve to mainly VFR levels by mid afternoon
However
do expect MVFR-IFR conditions to redevelop Fri night with even some localized LIFR Cigs/Vsbys. A few showers and perhaps an isolated t-storm or two will be possible into mid-morning Fri & some re-development is possible Fri PM. Any of this activity will be capable of producing brief downpours. Timing/location of this activity is quite uncertain...but regardless thinking dry weather dominates the vast majority of the time. S winds mainly 5-10 knots.

KBOS TAF...High confidence in TAF into this evening followed by moderate confidence thereafter. Main concern is the timing of the low clouds overnight into Fri am.

KBDL TAF...High confidence in TAF into this evening followed by moderate confidence thereafter. Main concern is the timing of the low clouds overnight into Fri am.

Outlook /Saturday through Tuesday/...

Saturday: MVFR/IFR conditions possible. Breezy. Chance SHRA, slight chance TSRA.

Saturday Night: Mainly VFR, with local IFR possible. Breezy.
Chance SHRA, isolated TSRA.

Sunday through Monday Night: VFR.

Tuesday: VFR. Slight chance SHRA.

MARINE

Forecaster Confidence Levels:

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

Tonight through Friday night...High Confidence.

High pressure east of the waters will generate persistent S-SW winds of 10 to 15 knots with perhaps some 20+ knot wind gusts at times. Conditions though should generally remain below small craft thresholds...but given long southwest fetch we may see some marginal 5 foot seas develop across our southern outer- waters Fri night. We may also have to watch for an isolated t-storm or two...but widespread convection is not expected.

Outlook /Saturday through Tuesday/...

Saturday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers.

Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, isolated thunderstorms.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers.

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

Monday through Tuesday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES
CT...None.
MA...None.
RI...None.
MARINE...None.