textproduct: Boston / Norton

WHAT HAS CHANGED

Made adjustments to the timing of steadier rain into this afternoon, with only intermittent showers anticipated before then in most areas. Still monitoring for the possibility of scattered snow showers Monday afternoon to evening in northeast and eastern MA, which could impact the Monday afternoon commute if they are heavy enough.

KEY MESSAGES

- Overcast with rain/snow mix north of the Mass Pike early this morning. Best chance for widespread rain is this afternoon and into tonight. Rain could mix with/end as sleet pellets and/or wet snowflakes overnight to early Monday as cold front moves offshore.

- Overcast, chilly and raw Monday as bulk of overnight precipitation moves offshore. However scattered snow showers (some briefly heavy) are possible mainly eastern and northeast MA Monday afternoon to evening.

- Splashover or very minor (shallow-depth) coastal flooding possible early Monday morning on the eastern MA coast.

- Dry and seasonable Tuesday becoming unsettled again after mid-week with a few rounds of showers. No significant precipitation expected.

DISCUSSION

KEY MESSAGE 1...Overcast with rain/snow mix north of the Mass Pike early this morning. Best chance for widespread rain is this afternoon and into tonight. Rain could mix with/end as sleet pellets and/or wet snowflakes overnight to early Monday as cold front moves offshore.

Overcast skies elsewhere as we await the frontal system's surface cold front, which will bring the best chance at widespread rainfall today as temperatures warm into the 40s to lower 50s. Latest guidance doesn't bring its leading edge into northwestern portions of Southern New England until the early afternoon; wouldn't rule out isolated or widely scattered showers prior to then, but thinking most of the morning hours ends up generally dry. This cold front will be moving into rising PWAT values given WSWly advection of a warm and moist airmass, with PWATs of around 1 inch. The front should end up moving more slowly southward through the afternoon to early evening, and there could be steadier rain or even some brief downpours. So all told, the better chance for rain in most areas is mainly focused during the afternoon.

For the evening, the cold front will continue to sag southward. At the same time, a secondary surface low will try to form along the slowing frontal zone and lead to anafrontally-enhanced precipitation, especially south of the Mass Pike. Northerly flow will eventually bring cooling sfc temperatures, however per NAM/GFS thermal profiles, it looks as though a layer of subfreezing temps between 950-925 mb advects southward overnight before sfc temps drop to near freezing. This means steady rain could become mixed with sleet pellets during the overnight hours as this shallow cold air surges south, but no accumulation or impacts are expected, other than it leading to a frankly miserable period to be outside overnight. Further north, rain/sleet pellets could end as a brief period of wet snow but here too, nothing necessarily impactful.

KEY MESSAGE 2...Overcast, chilly and raw Monday as bulk of overnight precipitation moves offshore. However scattered snow showers (some briefly heavy) are possible mainly eastern and northeast MA Monday afternoon to evening.

Residual rain/snow showers from the overnight should be pulling offshore Monday late morning. However shallow moisture is trapped underneath a dry layer of air aloft, with enhanced NNE winds along the eastern coast. The net result is a pretty dreary, chilly and raw day with lots of overcast. Leaned on cooler side of guidance with highs only in the 30s, with optimism for spot 40 degree readings in the CT Valley but it will feel pretty chilly and raw with northerly winds.

However a number of convection allowing models and even the coarser GFS are showing a band of scattered snow showers in eastern/east- central MA northward into NH as soon as the midafternoon hours, aligning along a leftover surface inverted trough. These snow showers could be infused by the passage of a shortwave trough aloft, and models also show some shallow instability associated within these narrow bands. Late-season snow accumulation scenarios which tend to be time-of-day and snow-rate driven. And it's possible some greasy coatings of snow on pavement could occur in eastern and northeast MA, and possibly impact the Monday afternoon to evening commute. This would hinge on if we can get snow showers to occur in brief bursts of higher intensity. This activity should move offshore before midnight.

KEY MESSAGE 3...Splashover or very minor (shallow-depth) coastal flooding possible early Monday morning on the eastern MA coast.

As the cold front sags southward, it brings with it enhanced NE winds. We're also headed into a period of higher astronomical tides, with Boston Harbor's high tide around 3 AM Monday morning is 11.04 ft MLLW. As per P-ETSS and Stevens Institute datasets, the northeast winds seem progged to generate a storm surge of around 1 to 1.2 ft. The timing of the windshift to NE is important here, as a more- delayed windshift after the high tide would lead to little if any coastal flooding risk at all. Most probabilistic forecast hydrographs show total water levels in the near-flood stage. A coastal flood statement could address this potential. Storm surge values should be on the higher end of the above range for the Monday afternoon high tide, although that astro tide is quite a bit lower (9.63 ft MLLW) and not of concern.

KEY MESSAGE 4... Dry and seasonable Tuesday becoming unsettled again after mid-week with a few rounds of showers. No significant precipitation expected.

Conditions will continue to dry out Tuesday morning as surface high pressure builds into southern New England. This will yield a brief period of fair weather feature sunshine and light NW winds. High temperatures will range in the 40s, which is right around normal for late March.

These conditions will be short-lived as the upper level pattern transitions back to an active/unsettled regime by mid-week. Ensemble guidance shows general agreement on quasi-zonal flow aloft, bringing a series of shortwave troughs through the region from Wednesday PM through at least late week. This setup will support a few rounds of showers; however, subtle discrepencies in the timing of individual waves among model guidance have resulted in uncertainty regarding exactly when each round of showers will occur. Generally, we will see a round of showers sometime Wed night-Thu followed by another round sometime in the Thu night-Fri timeframe. Currently, guidance does not highlight a strong signal for sigificant precipitation with any of these systems. A plume of above- normal moisture advects into the region Wednesday night ahead of the initial wave, supporting scattered to widespread showers. Temperatures lean near to slightly above normal in the mid to late- week period. Overnight lows drop into the upper 20s and low 30s across the interior higher terrain, and mid to upper 30s elsewhere.

If a round of showers were to move through during the overnight hours, particularly Thu Night-Fri, can't rule out the lower probability scenario of snow mixing in/light wet snow across portions of northern MA. Models are varied with the strength of the WAA Thu night, which is why the mixed precip is lower probability right now. If it did occur, it would likely be low impact.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Through Sunday Night: Moderate confidence.

VFR generally prevails through 09z Sun, then risk for rain/snow showers mainly northern MA away from TAFs. Ceilings steadily lowering to MVFR levels this morning in all areas. More widespread rains then develop along a cold front which moves in between 16-18z, then slowly sags southward toward the southern coast through tonight. Visbys in rain in the 3-6 SM range, with areas of steady moderate rain possible. Ceilings then start to lower into the IFR (perhaps LIFR?) range into the evening as winds shift to northeast.

Into the evening, steady rain continues mainly south of the MA Pike but could mix with sleet pellets and/or wet snow overnight as colder air works southward. Any wintry precip is not expected to accumulate.

SE winds around 5-10 kt, trend to light southerly in steadier rain. As the cold front trudges southward, expect a windshift to N/NE and increasing to around 10-15 kt with gusts 20-25 kt, strongest near the eastern coast.

Monday and Monday Night: Moderate confidence.

Widespread precip from overnight tapers off Monday mid to late morning...but IFR/MVFR ceilings hold stubborn. However, a period of snow showers is possible late Mon into Mon night across central MA/RI and especially eastern MA. This may lower conditions back to MVFR levels with perhaps even localized brief IFR conditions. North winds 5-15 knots with some 20-25 knot gusts expected at times at least along the coastal plain.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night: Mainly VFR, with local MVFR possible. Breezy.

Tuesday through Tuesday Night: VFR.

Wednesday: VFR. Breezy.

Wednesday Night: VFR. Slight chance RA.

Thursday: Mainly VFR, with local IFR possible. Breezy. Chance RA.

MARINE

Forecaster Confidence Levels...

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

Small craft advisories have been posted on all waters for tonight into Monday.

SE to S winds will increase today into the 15-20 kt range, and could at times gust to near small craft advisory levels along/ahead of a cold front that will be moving southward today. This will also bring with it reduced visibilities from a band of steady light to moderate rain. The period of greatest concern though is an abrupt windshift as the cold front move southward to northerly, with winds increasing to around 25-30 kt in gusts. Onset of these northeast winds to begin soonest in the northeast MA waters, then build steadily southward tonight and into Monday. SCA level gusts are expected to continue into at least Monday afternoon, but could linger into Monday evening and overnight. Seas will build and become rough as the northerlies pick up, in the 4-8 ft range.

Outlook /Monday Night through Thursday/...

Monday Night: Winds less than 25 kt. Areas of rough seas.

Tuesday: Winds less than 25 kt. Local rough seas.

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

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

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 PM this evening to 8 PM EDT Monday for ANZ230>237-250-251-254>256.