textproduct: Boston / Norton

WHAT HAS CHANGED

High Wind Watches issues for eastern Massachusetts and Rhode Island Monday night. Storm Watches issued all waters for Monday night. Flood Watches may need to be issued in the next 12 to 24 hours for the potential of flooding of rivers, small streams as well as urban poor drainage areas.

KEY MESSAGES

- Gusty westerly winds weaken overnight before a seasonable Sunday with highs in the mid to upper 40s. Deteriorating conditions Sunday night into Monday as a warm front lifts through the region ahead of a powerful low pressure system.

- Periods of heavy rain with embedded t-storms Mon and especially Mon night may lead to some river and small stream flooding and potentially some urban poor drainage areas too. A Flood Watch may be needed. In addition...strong to damaging wind gusts possible Mon night. There is even a low risk for severe weather.

- Mainly dry and trending colder Tue into Wed then moderating toward the end of next week.

DISCUSSION

KEY MESSAGE 1...Gusty westerly winds weaken overnight before a seasonable Sunday with highs in the mid to upper 40s. Deteriorating conditions Sunday night into Monday as a warm front lifts through the region ahead of a powerful low pressure system.

Efficient mixing behind a cold front that passed through the region this morning will continue to support gusty westerly winds through sunset. A mid-level ridge quickly builds in from the west overnight and will help relax the pressure gradient. This will result in today's gusty westerly winds gradually diminishing overnight. Winds should weaken enough to support fairly efficient radiational cooling which will allow surface temps to fall into the mid to upper 20s for much of southern New England tonight into tomorrow morning. Expect a quiet and seasonable Sunday thereafter with a mix of sun and clouds and highs in the mid to upper 40s. Conditions deteriorate Sunday night into Monday as a warm front lifts north over the region. This will be accompanied by a few showers and strengthening south/southeasterly winds. Conditions will continue to worsen on Monday as a powerful low pressure system approaches from the west. See KEY MESSAGE 2 for additional details.

KEY MESSAGE 2...Periods of heavy rain with embedded t-storms Mon and especially Mon night will likely lead to some river and small stream flooding and potentially some urban poor drainage areas too. A Flood Watch will likely be needed. In addition...strong to damaging wind gusts possible Mon night. There is even a low risk for severe weather.

An anomalously strong low pressure system near 980 mb will be tracking from the Great Lakes into Quebec Mon into Mon night. Given meridional upper level flow out ahead of this system an abundance of low level moisture will continue to stream northward during the day Monday...thanks to a southerly 850 mb jet of 50 to 70 knots. This will result in periods of showers with locally heavy rainfall during the on Monday. A few embedded t-storms are possible too given several hundred J/KG of MUCape. So pockets of typical nuisance street flooding are possible during the daylight hours Monday.

The main concern with this forecast revolves around Monday night as a strong cold front approaches from the west. This will induce a very powerful southerly LLJ on the order of 4-5 standard deviations above normal. We tend to focus on the high resolution guidance on the magnitude of the low level jet. In this case...the NAM is indicating an 850 mb LLJ on the order of 80-90+ knots across the region. In fact...the 925 mb LLJ across eastern MA/RI is 75 to 85 knots which is quite high. These inversion cases are always difficult to forecast especially this time of year given the rather cold ocean which can limit mix down potential. Nonetheless..the potential is there for strong to damaging wind gusts given the anomalous nature of this system. The risk for some of these stronger winds to mix down will increase if we reach or break 60 degrees...which is certainly possible. Any convective elements may also aid in bringing the strongest winds down...which will be determined on the mesoscale.

Based on the above...have gone ahead and issued a High Wind Watch for eastern MA and RI. Certainly the potential for 60+ mph wind gusts and power outages if the inversion can weaken enough. If the inversion holds on stronger...thinking we still will at least see Wind Advisory criteria so felt a watch was worth it given the need for just 50 percent confidence. FWIW...the CIPS analogs are indicating modest probs of 50+ knots of wind across CT/RI and eastern MA. And the potential is there for the stronger winds to reach further back into the interior...but we will let the later shifts evaluate. In addition...the RRFS actually shows some Updraft helicity swaths make it into southern New England. So we can not rule out a low probability of severe weather if a squall is able to survive into southern New England. The CSU machine learning probs do indicate some modest severe wind probabilities into our region... but that might also be more synoptically driven. Either way it is certainly an indicator for a period of potentially strong to damaging wind gusts.

The other concern is periods of heavy rain and embedded t-storms which will likely result in some river and small stream flooding and potentially some urban/poor drainage flooding too. Flood Watches will likely need to be issued in the next 12 to 24 hours. The ensemble situational awareness tables are showing the Pwat plume 3-4 standard deviations above normal for this time of year. This coupled with a powerful LLJ/strong forcing should result in widespread 1-2" of rain with localized 3"+ possible in any convective elements. Also...our stream levels are running above to much above normal from our recent rain and rapid snow melt we experienced over the past week. Therefore... thinking we will likely need to issue a Flood Watch in the next 12-24 hours.

KEY MESSAGE 3...Mainly dry and trending much colder Tue into Wed then moderating toward the end of next week.

The cold front will have cleared the coast by early Tue morning...allowing much drier and chillier air to work back into the region. Highs Tue will be held in the 40s with gusty west winds making it feel even colder. The cold will peak Tue night and Wed as 850T drop to around -16C. Lows Tue night will mainly be in the teens and 20 to 25 in the urban centers. Highs on Wed will mainly be in the 30s to perhaps near 40 in a few spots. Temps then begin to moderate for the latter half of next week with the majority of the time featuring dry weather...outside a few brief showers possible Fri and/or Sat with a cold front. No significant storms or precipitation events though after the Mon into Mon night system.

AVIATION /19Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

Rest of Today and Tonight... High Confidence

VFR. 30+ knot westerly winds remain possible through about 00Z. Thereafter expect winds to gradually weaken with gusts below 20 knots by 06Z. Winds become northwest around 5 knots after 06Z.

Sunday... High Confidence

VFR. Winds becoming east/southeast between 5 and 10 knots after 15Z.

Sunday Night...High Confidence

Deteriorating conditions Sunday night into Monday as a warm front lifts north over the region. MVFR/IFR ceilings and visibility developing after 06Z as light to moderate rain overspreads southern New England.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence

Outlook /Monday through Thursday/...

Monday: IFR. Strong winds with gusts up to 45 kt. SHRA, isolated TSRA.

Monday Night: Strong winds with gusts up to 50 kt. Chance SHRA, isolated TSRA.

Tuesday: Strong winds with local gusts up to 40 kt.

Tuesday Night: Windy with local gusts up to 30 kt.

Wednesday through Thursday:

MARINE

Forecaster Confidence Levels...

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

Rest of Today and through Sunday night

Subsiding seas and diminishing winds this evening and overnight as high pressure quickly builds over the region. Expect a period of quiet conditions tomorrow with 1-3 foot seas and modest east/southeast winds. Conditions deteriorate again tomorrow night into Monday as powerful low-pressure system approaches from the west. Seas increase to 6 to 8 feet by 12Z Monday with 25 to 35 knot sustained winds out of the southeast. Conditions continue to worsen throughout the day Monday.

Outlook /Monday through Thursday/...

Monday: Strong winds with gusts up to 45 kt. Rough seas up to 16 ft. Rain showers, isolated thunderstorms. Local visibility 1 to 3 nm.

Monday Night: Moderate risk for storm force winds with gusts up to 50 kt. Rough seas up to 19 ft. Rain showers, isolated thunderstorms.

Tuesday: Strong winds with gusts up to 35 kt. Rough seas up to 15 ft.

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 12 ft.

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

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

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Wind Advisory until 7 PM EDT this evening for MAZ002-004-008- 009-022>024. High Wind Watch from Monday afternoon through late Monday night for MAZ005>007-013>024. RI...High Wind Watch from Monday afternoon through late Monday night for RIZ001>008. MARINE...Gale Warning until 11 PM EDT this evening for ANZ230>237-250- 251-254>256. Storm Watch from Monday afternoon through Tuesday morning for ANZ230>237-250-251-254>256.