textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Showers and thunderstorms expected to move through tonight, some of which could produce severe wind gusts.

- Drier and slightly cooler following tonight's front.

- High astronomical tides and up to one-foot of storm surge could cause minor coastal flooding for the evening high tides tonight, Monday night and Tuesday night.

- Near seasonable temperatures and dry conditions followed by more unsettled weather at the end of the work week.

DISCUSSION

KEY MESSAGE 1...Showers and thunderstorms expected to move through tonight, some of which could produce severe wind gusts.

A cold front associated with low pressure currently centered over part of MI and southern ON will continue pushing to the east and north this afternoon, moving through southern New England tonight. This will be the main driver for chances of showers and thunderstorms, primarily after 7 PM. Though, the chance for a stray shower or two ahead of the main system arriving around 6 PM at the earliest towards western MA and CT is nonzero. Storms are expected to move in between 7-11 PM tonight, and a Slight risk (level 2 out of 5) for severe weather exists over western MA into CT, primarily for an elevated severe wind risk. CAMs indicate an elevated convection setup with a strengthening wind field and good forcing from the approaching front. Despite the lack of instability overnight and forecast soundings indicating a slight inversion in the lower levels, the forcing from the front may be enough to erode that inversion and tap into the stronger wind field aloft. RAP, NAM, and NAMNEST soundings indicate a little after midnight that effective bulk shear values may increase to around 30-40 knots. With the general evolution of these storms, CAMs have come into more agreement regarding them forming along a line ahead of/along the cold front (and we are currently seeing that occur west of the region). However, the risk should die down around and after 4 AM. Weaker storms could persist then through around 7 AM.

Some spread still exists across the guidance regarding rainfall totals. Generally, the higher rainfall totals should be mostly in northern New England with part of far NW MA included, where around half an inch is possible. The majority region can expect totals ranging between 0.20-0.35" of rain and the mesoscale- influenced low pressure developing would be tracking more just off the coast of Maine. This is the feature to watch with limiting rainfall totals. PWAT values are mainly forecast around 1.5-1.8" with pockets of 2.00"+ creeping north into RI, SE MA, and the Cape and Islands. Considering the convective nature of these showers/storms, locally higher rainfall totals elsewhere are not out of the question.

KEY MESSAGE 2...Drier and slightly cooler following tonight's front.

In the wake of tonight's cold front, temperatures fall to a more seasonable level. Highs for Monday are forecast to be in the upper 70s to low 80s across southern New England with dewpoints in the 50s. 925 mb temperatures fall to around 15C from around 20-25C, with the exception of parts of eastern MA where 925 mb temps around 20C return for Monday afternoon. High pressure starts to build in as a surface ridge develops. Lows in the interior tomorrow night may fall to the low 50s with mid/upper 50s into the coastal plain. Winds go light to calm with clearer skies, lending some favorability to a possible radiational cooling setup.

KEY MESSAGE 3...High astronomical tides and up to one-foot of storm surge could cause minor coastal flooding for the evening high tides tonight, Monday night and Tuesday night.

Astronomical tides will be building to their highest level on Monday, peaking around 12.0 ft at Boston Harbor Monday evening, but remaining elevated into Tuesday night. Storm surge observations along the tidal gages shows about 0.7 to just over a foot of storm surge, and the storm surge forecast data from P-ETSS and Stevens Institute indicate storm surge values of around one half to a foot of storm surge.

With hardly any wave action, forecast total water level should be pretty similar to recent days. Coastal Flood Statements now have been extended for the southern and eastern coasts for very minor tidal flooding into Tuesday night. We may be able to get away with a Coastal Flood Statement vs an Advisory for Nantucket for the Tuesday evening high tide, but will give later shifts an opportunity to reassess that. The evening high tide cycles tonight, Monday night and Tuesday night are the periods we are most concerned for, as those high tide magnitudes are higher.

This is a low-ceiling/low-impact coastal flooding scenario, with splashover or at worst minor coastal flooding in the usual vulnerable shoreline roads (including but not limited to Easy Street in Nantucket and Morrissey Blvd in Boston). However we'll carry these headlines for more awareness, given that there are more visitors to Southern New England who may not be as familiar with coastal/tidal flooding.

KEY MESSAGE 4...Near seasonable temperatures and dry conditions followed by more unsettled weather at the end of the work week.

After the surface ridge starts to weaken its grip on the area Wednesday, southwesterly flow returns. A weak disturbance may bring some lighter precip to the west Wednesday followed by a stronger system and attendant frontal boundary Thursday. Some varied timing disagreements between the global guidance and the GFS ensembles show better chances on Wednesday than the EC but overall still pretty low.

The front expected to clear the coast Friday bringing and end to the shower and storm activity for southern New England bringing a return to dry conditions for next weekend with temperatures slightly above normal and very comfortable dewpoints in the upper 40s and 50s.

AVIATION /19Z SUNDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

18z TAF Update...

This afternoon: High confidence.

VFR. SW winds increasing to 10-15 kt, with gusts to 25 kt.

Tonight: Moderate confidence.

Areas of MVFR to IFR possible with incoming SHRA and TSRA associated with a passing cold front. Expecting bulk of the showers and storms to pass through 02-08z west of ORH and roughly 09-12z east of ORH. Winds prevailing out of the south from 10-15 kt sustained, shifting NW in the wake of the front. Gusts to 25 kt expected to continue through at least the first half of the nighttime period.

Monday: High confidence.

IFR conditions improve to VFR or high-end MVFR by 15z as the rain moves offshore. Generally NW winds decreasing to near 10 kts throughout the day.

KBOS Terminal...Moderate confidence in trends. Moderate confidence in timing. Expecting the rain to arrive by at least 07z, but could arrive as early as 05z. Forecast update has a higher chance of thunder reaching the BOS terminal but not enough for a PROB30 at this time.

KBDL Terminal...High confidence in trends. Moderate confidence in timing. Rain could arrive as early as 02z, but expecting arrival no later than 04z. Greater chance for TSRA with this update.

Outlook /Tuesday through Friday/...

Tuesday through Wednesday: VFR.

Wednesday Night: VFR. Slight chance SHRA.

Thursday: VFR. Windy with local gusts up to 30 kt. Chance SHRA, slight chance TSRA.

Thursday Night: VFR. Windy with local gusts up to 30 kt. Chance SHRA.

Juneteenth: VFR. Breezy. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Tonight...High confidence.

Seas increase to 4-5 ft across the outer waters tonight into Monday, and winds pick up slightly, sustained between 15-20 kt and gusting to 25 kt. Winds also shift more NW heading into Monday morning as the cold front crosses the waters. Some showers and thunderstorms are a possibility during the overnight period over the waters, and localized gusts associated with them over 25 kt cannot be ruled out.

Monday....High confidence.

Seas remain 4-5 ft across the southern waters, but drop to 2-4 ft across the northern waters. Likewise, wind gusts remain around 20 kts across the southern waters and drop to near 10 kts across the northern waters.

Monday Night...High confidence.

Localized parts of the southern outer waters early Monday night may see seas to 5 ft before settling to 2-4 ft overall heading into Tuesday morning. Winds remain NW, sustained between 10-15 kt with not much in the way of wind gusts.

Outlook /Tuesday through Friday/...

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Tuesday Night: Winds less than 25 kt.

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

Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Slight chance of rain showers.

Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 8 ft. Chance of rain showers.

Juneteenth: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Coastal Flood Advisory until 2 AM EDT Tuesday for MAZ024. RI...None. MARINE...Small Craft Advisory until 2 AM EDT Monday for ANZ230-236. Small Craft Advisory until 8 AM EDT Monday for ANZ231>235-237- 250-251-254>256.