textproduct: Boston / Norton

WHAT HAS CHANGED

Trends have shifted the main rain axis father south, so widespread rainfall Sunday night into Monday now looks less likely than before.

KEY MESSAGES

- Mothers Day weekend looks split, with widespread showers and cooler temperatures today followed by somewhat warmer and mainly drier conditions Sunday.

- Notable changes to the rainfall forecast Sunday night into Monday, widespread rainfall is looking less likely.

- Briefly dry Tuesday, perhaps turning unsettled Wednesday into Thursday.

DISCUSSION

KEY MESSAGE 1...Mothers Day weekend looks split, with widespread showers and cooler temperatures today followed by somewhat warmer and mainly drier conditions Sunday.

As an upper trough digs into the Great Lakes, a warm front lifts north of southern New England this morning. Increasing large-scale forcing combined with a strengthening southerly low-level jet will support scattered showers developing during the morning hours, becoming widespread from west to east during the afternoon and early evening as the main shortwave and pre-frontal trough approach the region. This is captured well by the 00z suite of high-res CAMs and aligns well with NBM PoPs for this morning and evening.

As a moistening airmass overspreads the region, a strengthening southerly low-level jet develops this afternoon into Saturday evening, enhancing lift and focusing moisture transport across southern New England. Near the nose of the LLJ, mainly across RI and southeast MA, modest elevated instability may support brief heavy downpours and an isolated embedded thunderstorm; however, widespread thunder is not expected. PWATs increase to around 0.81.0 inches per the 00z HREF, near climatological normals for early May. Ensemble guidance continues to show a moderate chance of a beneficial wetting rain, with GEFS probabilities around 4050 percent for amounts over 0.50 inches and ECMWF ensemble probabilities of 7090 percent across coastal RI and southeast MA. This supports rainfall amounts of 0.300.60 inches, with locally higher amounts possible in heavier showers or any embedded convection. Flooding concerns remain low given recent dry conditions. Clouds, onshore flow, and rainfall will keep temperatures cool, with highs mainly in the mid 50s to lower 60s. Mild and somewhat humid conditions persist tonight with lows only falling into the upper 40s to lower 50s amid continued low clouds and areas of fog.

Conditions improve heading into Sunday as subtle mid-level height rises support quieter and milder weather for Mothers Day. While southwest to west-southwest flow persists, a notably drier airmass is not expected to move into southern New England. In fact, dewpoints climb into the low to mid 50s, making it feel somewhat more humid compared to the much drier conditions observed late this week. Given lingering moisture, embedded shortwave energy within the flow, and an approaching cold front, cannot completely rule out a passing afternoon shower or thunderstorm, as a few hundred J/kg of CAPE may develop. That said, Sunday does not appear to be a washout, though those with outdoor plans should remain weather aware. Afternoon highs have trended a few degrees lower, mainly due to the amount of cloud cover; nevertheless, highs remain comfortable in the upper 60s to lower 70s.

KEY MESSAGE 2...Notable changes to the rainfall forecast Sunday night into Monday, widespread rainfall is looking less likely.

There is a notable change to the forecast as the previously advertised second round of beneficial rainfall late Sunday night into Monday is looking less likely. A mid-level trough and embedded shortwave move into the Northeast, with the best forcing now shifting south of southern New England. A surface low is expected to develop southeast of the Benchmark (40N, 70W) before tracking into the North Atlantic.

Model guidance over the past 24 hours has shown a notable southward shift in the axis of precipitation, mainly offshore of southern New England, though areas from New Haven to Woonsocket to Boston and points southeast have the greatest potential for rainfall, albeit less than previously expected. GEFS and ECMWF ensemble guidance are in fairly good agreement, with probabilities of 24-hour rainfall greater than 0.50 inches now below 30 percent. Nantucket and outer Cape Cod maintain higher probabilities in the 5060 percent range. One outlier remains the NAM 12 km, which continues to depict a more widespread rainfall solution.

That said, there remain low chances of precipitation across interior southern New England, with any activity more likely in the form of diurnally driven showers rather than a widespread stratiform rain event. Temperatures Monday afternoon trend 812 degrees cooler than Sunday, resulting in a noticeably cooler feel.

KEY MESSAGE 3...Briefly dry Tuesday, perhaps turning unsettled Wednesday into Thursday.

The mid-level trough shifts offshore on Tuesday, with mid-level ridging developing in its wake. Flow aloft turns north-northwest, advecting lower humidity and cooler temperatures into the region, leading to a cool but sunny day.

This is short-lived, as unsettled conditions may return by Wednesday and continue into Thursday as another mid-level trough and embedded shortwave move through the Northeast, though confidence in timing and impacts remains low given the longer forecast range. High temperatures remain near to slightly below climatological normals for mid-May, moderating into the upper 50s to lower 60s Tuesday through Thursday.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

12z TAF Update:

Through Tonight: Moderate confidence.

VFR ceilings/visbys with initial -SHRA 13-15z Sat, which tends to lift N/NE. Period of relatively dry weather (wouldn't rule out a spot -SHRA especially north of BED, but generally dry) thru 17z. Then a period of widespread steady rains with MVFR- IFR ceilings/visbys spreads eastward between 17z-22z. TEMPO'd in potential for some heavier downpours or a rumble/two of thunder from BDL-PVD south and east. Steadier rain comes to and end 00-03z west to east, but trapped moisture supports widespread IFR ceilings with possible mist/fog into the evening/overnight.

Winds start off light ESE to S winds, but speeds will be picking up to around 8-12 kt by this afternoon, with S/SW winds under 10 kt by tonight. Possible areas of low-level wind shear along the South Coast/Cape and Islands once rain ends and southerly windspeeds ease some.

Sunday...Moderate confidence.

Low-end VFR. Chance of a pop-up shower during the afternoon. SW winds 8-12 kt with some gust to 20 kt.

KBOS Terminal...Moderate confidence in TAF. VFR unrestricted light -SHRA thru 14z, then generally dry thru at least 19z. Then a period of MVFR SHRA 19-20z thru 02z, rumble of thunder possible but not as likely as southeast MA. Ceilings then drop to IFR levels once rain ends. Winds start off light ESE but speeds increase to around 10-12 kt this afternoon, then becoming light south as rain ends.

KBDL Terminal...Moderate confidence in TAF. VFR unrestricted light -SHRA thru 13z, then generally dry thru at least 17z. Then a period of MVFR SHRA 17-18z thru 00z, rumble of thunder possible. Ceilings then drop to IFR levels once rain ends. Winds start off light northerly, then flip to SE by mid- morning with increase to around 10-12 kt this afternoon, then becoming light south as rain ends.

Outlook /Sunday Night through Wednesday/...

Sunday Night through Monday: VFR. Chance SHRA.

Monday Night through Tuesday Night: VFR.

Wednesday: VFR. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Sunday...High confidence.

An area of low pressure tracks well inland on Saturday, a warm front lifting north during the day and periods of rain and embedded thunderstorms. A wind shift to the S-SSE and increasing wind speeds 15 to 20 kt and gust 25+ kt. Seas increasing from S to N as well throughout the day 4 to 7 ft leading to a Small Craft Advisory today into early Sunday. SW to S wind Sunday with gusts less than 20 kt and seas below 5 ft.

Outlook /Sunday Night through Wednesday/...

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

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

Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers.

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

Wednesday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from noon today to 2 AM EDT Sunday for ANZ231>235-237-251. Small Craft Advisory from noon today to 8 AM EDT Sunday for ANZ250-254>256.