textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

- Frontal passage tonight into tomorrow morning brings scattered showers and gusty southwest winds, especially over far southeastern Massachusetts, Cape Cod, and the Islands.

- Dry weather, comfortable humidities and light onshore winds Friday through Saturday night. Cool nighttime low temperatures Friday night.

- Frontal boundary Sunday brings increased humidity and chances for rain Sunday afternoon and evening.

- High pressure returns for early next week, bringing a return to seasonable temperatures and low humidities.

DISCUSSION

KEY MESSAGE 1...Frontal passage tonight into tomorrow morning brings scattered showers and gusty southwest winds, especially over far southeastern Massachusetts, Cape Cod, and the Islands.

The rest of today is largely dry outside of a sct'd warm advection shower, as yesterday's cold front lifts back to the northeast, this time as a warm front. A robust LLJ advects higher moisture into southern New England as well, with PWATs climbing to near 2.0 inches after 00z. Thus, there remains a low chance for showers throughout this evening. Given the added cloud cover and winds overnight, expecting a mild night with lows ranging from the upper 60s to lower 70s.

As the cold front approaches overnight, additional showers move into the region during the predawn hours of Thursday, mainly 4-7 AM. However, dynamics are lacking as the front reaches southern New England. While there is a moisture-rich environment, the lack of ideal forcing means rainfall isn't expected to be widespread or necessarily heavy.

The wind will be more noticeable. As we've mentioned over the past day or so, there is a robust LLJ with winds aloft ranging between 45 and 55 kt. While mixing will be limited inland due to the nocturnal inversion, there should be enough mixing to allow gusts across the interior to range between 20 and 30 MPH. Along and east of I-95, gusts of 30-40 MPH remain possible, with a small area mainly near the Cape and far southeastern Massachusetts where 40+ MPH isn't out of the question prior to sunrise. Throughout the day Thursday, wind direction shifts more to the west, with gusts ranging from 20 to 30 MPH.

Currently, we feel the greatest impacts will be for the marine community. Gale Warnings are in effect around the Cape and Islands for those stronger winds. By Thursday afternoon, the surface cold front is offshore. However, temperatures aloft are slower to lower, so it will still be fairly seasonable Thursday afternoon. Then comes a reinforcement of colder air Thursday night, with 925 mb temperatures falling from around +20C Thursday to +10C Friday.

KEY MESSAGE 2...Dry weather, comfortable humidities and light onshore winds Friday through Saturday night. Cool nighttime low temperatures Friday night.

Forecast for Friday into Saturday remains consistent in showing a large high pressure area coming in from the eastern Great Lakes and southeast Ontario Friday, then settling over the Northeast Friday evening before moving offshore Saturday. This will bring dry and pleasant weather conditions to Southern New England, with seasonably mild days with full sun during the daytime hours Friday and Saturday but a cool Friday night due to strong radiational cooling. Some of the more typical sites which radiate well under clear skies and light winds could see low temperatures drop into the mid 40s, including areas around I-495, NW MA and Martha's Vineyard. High temperatures in the mid 70s, with lower 70s near the Cape and Islands. Conditions look favorable for outdoor plans Friday and Saturday.

KEY MESSAGE 3...Frontal boundary Sunday brings increased humidity and chances for rain Sunday afternoon and evening.

The high pressure moving eastward allows for an increasingly moist airmass ahead of a cold frontal boundary which arrives Sunday afternoon into Sunday evening. Most models show increasing rain chances centered around Sunday afternoon or into the evening. While there are good chances for rain in this Sunday afternoon to Sunday evening timeframe, there is more uncertainty in terms of how much rain falls. It looks as though the placement of the greatest rainfall seems confined to the South Coast, Cape Cod and the Islands. While it looks as though Sunday evening looks wet for these areas, we're not expecting rainfall to become especially heavy as the frontal boundary looks progressive.

KEY MESSAGE 4...High pressure returns for early next week, bringing a return to seasonable temperatures and low humidities.

Behind the frontal boundary Sunday night, high pressure from Canada builds back in, bringing seasonable temperatures, dry weather and low levels of humidity for the early part of next week. The potential exists for a couple of cooler nights Monday night and Tuesday night with high pressure in the vicinity.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

18z TAF Update: No significant changes.

This Afternoon and Tonight...High confidence in trends, lower in timing.

Clouds filtering in from the north, mainly VFR with ceilings 050- 080. There remains a low chance of a shower late this afternoon into early evening as a warm front lifts north, mainly between 22z-04z. Otherwise, dry overnight with low-end VFR ceilings 035-050. Winds shift to the S to SSW this afternoon and become gusty, with gusts of 20-25 kt this afternoon, then 25-30 kt overnight. Cannot rule out wind gusts between 35-40 kt overnight into early Thursday morning, mainly across far southeastern Massachusetts, Cape Cod, and the Islands.

Thursday...High confidence.

Mainly VFR with drying conditions. Winds shift from SSW to SW, while gusts ease slightly to 20-30 kt, with the strongest gusts near the coast and higher elevations.

Thursday Night...High confidence.

VFR. NW winds 5-10 kt.

KBOS Terminal...High confidence in TAF.

Sea breeze should kick out by 20z, then turning south. SHRA chances 23z-03z.

KBDL Terminal...High confidence in TAF.

Outlook /Friday through Monday/...

Friday through Saturday: VFR.

Saturday Night through Sunday: VFR. Chance SHRA.

Sunday Night through Monday: VFR.

MARINE

Forecaster Confidence Levels...

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

Through Thursday Night...High confidence.

An approaching cold front will result in a developing southwest low- level jet later today. The southwest low-level jet peaks later tonight into early Thursday morning, when 925 mb winds reach 45-55 kt. While not all of this will mix down...we expect widespread southwest wind gusts of 25-35 kt. Strong Small Craft headlines continue for most waters, but brief gale-force gusts are certainly possible. Gale Warnings continue for Buzzards Bay and Cape Cod Bay, as well as Nantucket Sound. Based on pattern recognition...a brief period of SW wind gusts of 35-40 kt appears reasonable and is supported by BUFKIT soundings.

Winds ease and become more westerly into Thursday though seas remain elevated. Any Gale Warnings likely will need to be converted to a Small Craft Advisory through Thursday afternoon.

Outlook /Friday through Monday/...

Friday through Saturday: Winds less than 25 kt.

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

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

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

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 PM this evening to 8 PM EDT Thursday for ANZ230-235>237-250-251-254>256. Gale Warning from 10 PM this evening to noon EDT Thursday for ANZ231>234.