textproduct: Boston / Norton

WHAT HAS CHANGED

Increased rainfall amounts Wednesday night into Thursday. Otherwise, no other significant changes were made to the forecast.

KEY MESSAGES

- Pleasant weather continues today and tonight with the warmest temperatures inland and across the CT Valley. Much cooler conditions along the coast with highs in the upper 40s to lower 50s.

- Increasing chance for showers across coastal areas Wednesday. Better chance for a more widespread soaking rain Thursday.

- Upper level low keeps things cooler and unsettled Friday into the weekend.

DISCUSSION

KEY MESSAGE 1... Pleasant weather continues today and tonight with the warmest temperatures inland and across the CT Valley. Much cooler conditions along the coast with highs in the upper 40s to lower 50s.

Ridge of high pressure crests overhead this afternoon through the overnight. This results in a mainly clear day outside of a few mid and high level clouds ahead of an area of low pressure that will pass well to the northwest of the CWA. Of greater consequence will be a fairly stiff onshore flow that will be 5-10 mph stronger than what we saw on Monday. The stronger onshore flow will likely help knock temps a few degrees lower than we've seen in recent days. Nonetheless...still expect highs to reach into the 60s in most locations away from the immediate coast. The mild spots will again be the CT Valley where high temps of 70+ seem likely. Meanwhile...onshore flow will hold high temps mainly in the lower to middle 50s along the immediate coast, Cape and Islands.

Mainly dry and cool again tonight with another night of decent radiational cooling. The magnitude of cooling will be less than previous nights with a mid and high level cloud deck building overhead. Lows will be in the 30s in many locations with lower 40s in most areas. High res guidance shows areas of low stratus, patchy showers, and drizzle mainly across SE MA and the islands late tonight. Still somewhat low (20-35%) POPs mainly contingent upon the orientation/location of an inverted trough extending from a distant coastal low.

KEY MESSAGE 2...Increasing chance for showers across coastal areas Wednesday. Better chance for a more widespread soaking rain Thursday.

Most places remain dry for Wednesday as high pressure will be slow to move offshore. As a trough approaches from the west, Precipitable water (PWATs) increase to 0.5-1.0 inches. Therefore, expect cloudy skies for the day with continued NE flow. Highest chance of showers across eastern Mass and especially the Islands where high res guidance shows the inverted trough setting up and acting as a source of lift.

More broad synoptic-scale lift from an approaching shortwave brings an increasing chance for widespread light to perhaps moderate rainfall Wednesday night into Thursday morning. PWAT values rise to around 1.0-1.5", supportive of widespread showers/rain. Model guidance has come into better agreement with the idea of a secondary low developing offshore of the Cape Thursday. We are still monitoring the risk for coastal flooding but the probability is still quite low at this point for the eastern coastal areas. For example, the Steven's Institute coastal flooding guidance does indicate a low probability (~5%) of near flood/splashover for east- facing beaches. Thankfully, not looking at a significant chance for freshwater flooding as rainfall will be quick to move out Thursday afternoon. The 01z NBM mean 24 hour QPF has increased from a widespread 0.5-0.75" to 0.75-1.0". There is even a swath of mean QPF >1" across NE Massachusetts.

KEY MESSAGE 3...Upper level low keeps things cooler and unsettled Friday into the weekend.

Weather pattern remains unsettled Friday through the weekend as a rex blocks keeps an upper level low above northern New England. This pattern will bring a much cooler airmass with 850mb temperature anomalies falling to 6-8C below normal. Non zero POPs continue through the weekend under a slow moving cold pool aloft (500mb temps falling to -20 to -30C). This will lead to some instability, so can't rule out a graupel within any showers. There are still model/ensemble members that indicate the potential for a coastal system to give a glancing blow to southern new england over the weekend. The trend with the 00z ensembles has been a solution much further to the east. The other scenario is a closer track with rain/gusty winds later Saturday into Sunday to portions of southern New England. Again, this is still low probability scenario right now being displayed by a small percentage of model solutions. This will continue to be monitored as confidence in the upper pattern and details increases as we get closer.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

12z TAF Update:

Today...High confidence. Low/mod for low clouds on Cape/Islands.

VFR for most. A stratus deck will likely impact the immediate east coast of MA including BOS through around 16z. Confidence lower in the potential for periods of BKN low clouds near the Cape and Islands esp later in the day. E-NE winds 6-15 kts with the strongest near the coast. NE winds gust to between 20 and 25 knots across the Cape and especially Nantucket during the daylight hours.

Tonight...High confidence. Moderate for ceilings.

VFR for most. MVFR-IFR ceilings develop across Cape and Islands potentially reaching the east coast and BOS after midnight. Confidence in the inland extent of these conditions is low at this time but could extend as far inland as BED.

KBOS Terminal...Moder confidence in TAF. This is mainly attributed to the timing and persistence of the stratus deck that's approaching from the east early this morning.

KBDL Terminal...High confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday: Mainly VFR, with local MVFR possible. Chance SHRA.

Wednesday Night: Mainly VFR, with local IFR possible. Breezy. Chance SHRA.

Thursday: MVFR/IFR conditions possible. Breezy. SHRA.

Thursday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA.

Friday: VFR. Breezy.

Friday Night: VFR.

Saturday: VFR. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Today through Wednesday..High confidence.

High pressure centered over the Canadian Maritimes with distant low pressure well southeast of the Benchmark continues NE flow through much of the week. This long fetch will generate 3 to 6 foot seas across the outer-waters...so current small craft headlines will persist through at least Thursday.

Outlook /Wednesday through Saturday/...

Wednesday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, patchy fog.

Wednesday Night: Winds less than 25 kt. Areas of rough seas. Chance of rain showers.

Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Rain showers, slight chance of thunderstorms.

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers.

Friday through Friday Night: Winds less than 25 kt. Seas up to 5 ft.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Thursday for ANZ250- 254>256.