textproduct: Boston / Norton

WHAT HAS CHANGED

No major changes with expectation of a soggy Sunday across much of Southern New England. A narrow swath of heavier rain is possible near the South Coast, and if heavier rain develops in this corridor, the potential exists for ponding or localized street flooding in more urban areas.

KEY MESSAGES

- Rain gradually moves eastward late this evening. Soggy and cool Sunday, steady moderate rain in all areas and localized downpours possible, especially the RI/MA South Coast. Rain pulls offshore Sunday night with slow warming, but with areas of fog and drizzle into Monday morning.

- Wet to start Memorial Day, drying out through the day. Above normal temperatures for the middle of next week before trending closer to normal late next week. A couple periods of showers possible.

DISCUSSION

KEY MESSAGE 1...Rain gradually moves eastward late this evening. Soggy and cool Sunday with steady moderate rain in all areas and localized downpours possible, especially the RI/MA South Coast. Rain pulls offshore Sunday night with slow warming, but with areas of fog and drizzle into Monday morning.

Sfc ridge of high pressure is still maintaining itself for most of Southern New England, with regional radar mosaic as of Saturday afternoon showing a distinct NW to SE cutoff in light rain trailing from roughly Watertown NY through Westerly RI south and west. Only measurable rain so far per METARs has been near Hartford. This precip shield will be slowly advancing eastward, making its greatest eastern inroads after midnight. Onset of light rains into eastern MA and RI should take until the early pre-dawn hours. Nighttime cooling with be slow under overcast and east flow but some wet-bulb cooling once rain arrives should bring lows in the 40s to low 50s.

Sunday is a soggy, cool day with steady rain bringing a welcomed and much needed rainfall to Southern New England. A southeast low-level jet of 40-45 kt should help lead to favorable moisture convergence (PWATs rising to around 1.4-1.7", approx 2 standard deviations above late-May PWAT climatology); there are indications in the higher- resolution solutions showing a swath of potentially heavy downpours in southern RI into the RI/MA South Coast, Cape and Islands during the afternoon. Could be some ponding in some of the more urbanized locations such as Newport, Fall River and New Bedford. Left low probs for a rumble or two of thunder in the forecast but I'm not really bullish on thunder prospects as lapse rates are poor, CAPE is minimal, and K indices are around 30 units. Winds will increase especially in eastern/southeast MA to around 20-25 mph in gusts, but will become NE late in the day into the evening and lighten up as rain begins to move offshore late in the day to early in the evening. With steadier rain all day and onshore winds, daytime temperatures Sunday aren't likely to go very far, in the upper 40s to mid 50s.

Temperatures slowly rise through low-level warm advection Sunday evening as steadier rain pulls offshore. Even though temperatures will be slowly warming into the 50s overnight, a light northeast drainage flow wedges southwestward from interior ME/NH Sunday night. BUFKIT profiles show a layer of low-level RH about 3000-5000 ft thick from the near-surface with drier air aloft across interior Southern New England into the coastal plain. So while steadier rain ends, this is a profile that is rife for development of mist, fog and areas of drizzle for Sunday overnight into the early morning of Memorial Day. PoPs decrease into the Chance range, reflective of more intermittent/showery activity or drizzle.

Storm total rainfall through early Monday ranges from 0.75" to 1" north of the Mass Pike, to 1-2" of rain for CT-RI-SE MA, with a stripe of locally higher amounts up to 2.5" from Newport to New Bedford where I've added a mention of heavy rain at times wording in the zone forecast. Of note is the the 12z SPC HREF localized probability-matched-mean QPF, often considered a reasonable worst- case, which shows amounts in that RI/MA South Coast corridor up to 4.5 inches. If values that high were to materialize, then we could have to monitor for street flooding and river/stream/creek rises, but it would take more convective- enhanced rates to see values that high, which is not a favored outcome.

KEY MESSAGE 2...Wet to start Memorial Day, drying out through the day. Above normal temperatures for the middle of next week before trending closer to normal late next week. A couple periods of showers possible.

The mid level synoptic pattern next week will be more of a little dance between the Bermuda High and a cutoff over southeast Canada and the Maritimes. The Bermuda High is projected to shift west and south over the course of next week, permitting the mid level cutoff to move towards the Maritimes from southeast Canada. There remains considerable uncertainty with how quickly this cutoff will move east after Wednesday, leading to below normal confidence in the details late next week. For this forecast, thinking we'll see above normal temperatures Tuesday into Wednesday, which looks to be our warmest day. Temperatures should then trend towards near to slightly below normal temperatures for late next week.

Rainfall-wise, we will still be dealing with lingering showers for Memorial Day. Expecting drier conditions to overspread southern New England from NW to SE during the day. By afternoon, the risk for showers should be confined to the Cape and Islands. Even there, dry weather should arrive by evening. Thinking there will be two more periods of concern for precipitation. The first of these should be with a cold front Wednesday, which could lead to the possibility for mainly afternoon showers and thunderstorms. A pool of colder air aloft could lead to another round of showers for Friday. This is part of the uncertain timing of the colder air late next week, so confidence is low in this outcome.

AVIATION /19Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

18z TAF Update:

Through 00z Sunday: High confidence.

BKN/OVC VFR high clouds for most of the TAFs; exception is BDL with OVC VFR ceilings to go along with light -RA. E/SE winds 5-10 kt.

Tonight: High confidence in TAF/trends but moderate on timing.

Mainly VFR through midnight/05z, with VFR -RA at BDL continuing. Rain shield then begins to spread slowly eastward, and as steadier rains develop, expect categories to deteriorate to sub-VFR levels. MVFR visby RA with MVFR- IFR ceilings at BDL, and deteriorating eastward with onset of light rain 07-10z, becoming steadier shortly thereafter. SE winds increasing to around 8-12 kt.

Sunday: Moderate confidence.

MVFR to IFR ceilings, steady light to moderate rain with visibilities3-5 SM. Localized downpours mainly for TAFs S/E of I-95 in the afternoon. SE winds around 10-15 kt with gusts 20-25 kt, becoming E/ENE for the eastern TAFs in afternoon.

Sunday Night: Moderate confidence.

MVFR-IFR in steady rain moving off the east coast around 03-06z. Clearing of steadier rain then brings widespread IFR-LIFR with drizzle, mist and fog with light NE winds.

KBOS...High confidence in TAF. VFR, east winds become SE around 10 kt by 00z Sunday. Onset of RA not likely until at least 08z, becoming a steadier light rain with MVFR more likely after 10z.

KBDL...High confidence in TAF. VFR light -RA prevails thru 03-05z, then becoming steadier with MVFR visibilities and ceilings becoming MVFR-IFR. SE winds around 5-10 kt.

Outlook /Sunday Night through Thursday/...

Memorial Day: Mainly VFR, with local IFR possible. Chance SHRA.

Monday Night through Tuesday Night: VFR.

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

Wednesday Night through Thursday: VFR. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Far from ideal boating conditions expected this weekend. SCAs take effect tonight on most waters with SE winds becoming 15-25 kt tonight, then increasing to 25-30 kt on Sunday. Seas will be building to 4 to 8 ft (higher southern waters) tonight, then build to the 6 to 10 ft range by Sunday into Sunday night. Areas of rain will reduce visibility to around 3 miles.

Rain moves offshore early Sunday night, with winds easing and becoming ESE to S around 10-15 kt. Seas will still remain elevated at 5-7 ft through Sunday night, thus SCAs continue into most of Monday to allow for seas to subside. Although rain comes to an end, fog and drizzle is likely to develop Sunday night on the waters, with locally dense fog possible.

Outlook /Sunday Night through Thursday/...

Memorial Day: Winds less than 25 kt. Rough seas up to 9 ft. Chance of rain showers.

Monday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers.

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

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

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

Wednesday Night through Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 5 AM Sunday to 2 AM EDT Monday for ANZ231. Small Craft Advisory from 2 AM Sunday to 5 AM EDT Monday for ANZ232>235-237. Small Craft Advisory from 5 PM Sunday to 8 PM EDT Monday for ANZ250. Small Craft Advisory from 8 PM this evening to 8 PM EDT Monday for ANZ254>256.