textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

- Warm and more humid today with isolated showers and thunderstorms possible in the afternoon, mainly across south coast and southeast MA.

- Trending cooler late-week. Scattered showers possible Thursday with next period of rain arriving as early as Friday afternoon.

- Becoming unsettled by the weekend with below normal temperatures and shower chances.

DISCUSSION

KEY MESSAGE 1...Warm and more humid today with isolated showers and thunderstorms possible in the afternoon, mainly across south coast and southeast MA.

For today, surface high pressure will be positioned to the south of the region while our next system and cold front approach from the west. This will advect in more moisture at the surface along with weak warm advection bringing up 925mb temperatures a degree or two. This will support high temperatures in the mid to upper 80s with southwest to westerly winds. Dewpoints rise into the low 60s, making it feel a touch humid.

The added heat and moisture ahead of the approaching front will build marginal instability through the day (MLCAPE 200-500 J/kg). This is enough to support thunderstorms as long as the cold front does not come in too quickly (before instability builds up enough). The moisture is fairly limited, especially evident in the mid-levels which will favor more isolated to scattered showers or thunderstorms this afternoon. 0-6 km bulk shear values range 25-35 kts to support organized convection if it develops. However, the weaker mid-level lapse rates may act to suppress thunderstorm development and longevity.

The 00Z high-resolution guidance has stayed split on potential for showers and thunderstorms. The NAMnest/FV3, and RRFS show isolated showers and storms developing across the southern portion of the region, mainly across the south coast. The ARW remains dry. The HRRR is mainly dry, with an isolated shower or two. This supports the lower probability (< 30%) for isolated showers or thunderstorms in the afternoon. Highest chances for showers/storms will be in the 2-7 PM timeframe near the south coast into southeast MA. The cold front fully pushes through by tomorrow night diminishing any lingering showers.

KEY MESSAGE 2...Trending cooler late-week. Scattered showers possible Thursday with next period of rain arriving as early as Friday afternoon.

A cooler post-frontal airmass will be in place Thursday and Friday which will bring temperatures closer to normal in the upper 60s near the coast and low-mid 70s further inland/valleys. A shortwave trough drops southward across the region on Thursday which may bring a period of scattered light showers.

Confidence in the upper level pattern decreases Friday, mainly with regards to the speed and evolution of a cutoff low that tracks south from Canada. The GFS and AI GFS remain the quicker solutions, bringing the system in quicker with rain arriving as early as mid- afternoon Friday. Remaining deterministic guidance has the low and accompanying rain arriving in the late Friday night-early Saturday AM timeframe. Despite the different timing in the deterministic GFS, the GEFS shows the highest probabilities for rain overnight Friday- Saturday with more moderate values for Friday. There is a little more confidence in the widespread rain arriving late Friday night; however, a few showers are possible Friday at this time.

KEY MESSAGE 3...Becoming unsettled by the weekend with below normal temperatures and periodic rain chances through early next week.

Previously noted, there are timing differences for the cutoff low amongst guidance that will dictate the timeframe of the greater shower/rain chances over the weekend. A quicker arrival of the system could taper off the more widespread showers Saturday afternoon. Regardless of the timing, Saturday will be unsettled with at least scattered showers for the first half of the day if not a large portion of the day. Sunday will be the drier day. The cutoff low will bring in a cold pool aloft southward into the region favoring below normal temperatures for Saturday in the 60s and closer to seasonable for Sunday. The pattern stays fairly steady through early next week due to a blocky pattern aloft. This will keep a troughy pattern over the region keeping temperatures seasonably cool. A few embedded shortwaves move through the flow bringing a period or two of scattered showers with a greater chance on Monday.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

12Z TAF Update:

Today...High Confidence overall. Low-moderate for timing of ceilings at Cape terminals.

Mainly VFR. IFR stratus/fog at ACK with LIFR possible later this afternoon/evening. Stratus may expand across other portions of Cape Cod by late morning and through the afternoon. Low risk for afternoon SHRA/TSRA mainly over CT, RI, and Cape/Islands terminals with highest chances after 21Z. Confidence is not high enough to include PROB30 group in TAFs, especially given the isolated nature of any showers/storms. SW-W winds 10-15 kts with gusts 20-25 kts, Winds shift to NW after 21Z, eventually making the shift to NW for the south coast by 03Z.

Wednesday Night...High Confidence.

Winds shift NW/N behind the cold front. This will push out any fog/low stratus over The Islands terminals. VFR elsewhere with north/northwest wind 5-10 knots.

Thursday...High Confidence.

VFR. Light NE winds across eastern MA, the Cape and Islands in the afternoon, and NW around 10-15 kt across western MA and north central CT. Gusts in those NW winds around 20 kt possible. Chance SHRA across much of the region in the afternoon as well.

KBOS...High confidence in TAF. Low chance (< 25%) of an isolated shower or storm after 21Z. Greater risk more likely to be south of BOS around the south coast.

KBDL...High confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night: VFR.

Friday: VFR. Chance SHRA.

Friday Night through Saturday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA.

Saturday Night: VFR. Breezy. Slight chance SHRA.

Sunday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Today:

Southwest winds associated with high pressure to the south will persist over the coastal waters today around 10-15 kts with a few gusts up to 25 kts possible. Seas remain sub- SCA level with significant wave heights between 1-3 feet.

This evening-Tonight:

Cold front pushes through the waters shifting winds to the NW in the evening (after 6 PM). This may be accompanied by a few showers and possibly a thunderstorm near the south coast. Winds NW around 10 kts and seas 1-3 feet.

Outlook /Thursday Night through Sunday/...

Thursday: Winds less than 25 kt. Chance of rain showers.

Thursday Night: Winds less than 25 kt.

Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

Friday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.

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

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

Sunday: Winds less than 25 kt. Seas up to 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

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