textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- A cold front brings the return of heavy rain and gusty winds Sunday night and Monday. Localized flash flooding possible Monday.

- Dry and less humid weather returns Tuesday

- Weather pattern quickly becomes unsettled again by later Wednesday, continuing late week and into the weekend, with periods of showers and storms, noticeably more humid as well.

DISCUSSION

KEY MESSAGE 1...A cold front brings the return of heavy rain and gusty winds Sunday night and Monday. Localized flash flooding possible Monday.

A potent shortwave trough and anomalous low track across the Ohio Valley toward the northeast. A strong 30-45 kt LLJ pushes northward into the region tonight helping facilitate lift and advection of an anomalous plume of moisture (2-2.5 SD above normal). Positioning under the upper level jet, an incoming cold front, and marginal instability will provide ample support for widespread heavy rainfall Monday.

Tonight, expect a scattered showers perhaps a weak thunderstorm as synoptic ascent/moisture increases ahead of the approaching system. Majority of the rain will fall Monday morning through Monday evening. This can be divided into two "rounds", categorized by timing and prominent hazard. The first "round" will be showers and embedded thunderstorms pushing across southern New England in the morning through mid-afternoon timeframe. The heaviest rain rates will be driven by the location of embedded thunderstorms; therefore, the location of the heaviest rainfall will be less certain. The HREF shows some isolated low probabilities for rain rates 1" or greater during the day Wednesday. The HREF PMM 3 hour amounts range from 0.75 to 1.75". With deep moisture/lift efficient rainfall is supported. This is where localized to scattered flash flooding will be a concern, especially in our areas that are still fairly saturated from last week's rain event and the usual poor drainage areas. The second "round" will be the potential for a round of scattered thunderstorms pushing from west to east mid afternoon through the evening. There is lower confidence in this as it will depend on how much clearing we get after the first round of showers and thunderstorms. If we can get a period of clearing with breaks of sun this will allow the atmosphere to destabilize. The unstable/moisture environment combined with the incoming cold front and sufficient shear from the LLJ would provide an environment support of even a few strong to severe thunderstorms if the atmosphere can recover from the first round of rain. The primary hazards within any stronger storms will be strong downdrafts/gusts and brief heavy downpours.

In terms of rain totals, the specific locations of the heaviest rainfall will highly depend on where the heavier rates/thunderstorms track. Based on global and high-resolution ensemble guidance, many areas will see a range of 0.75-2.0" with potential for localized 3". HREF ensemble maxes even show potential for close to 4" in localized spots. These higher values will be quite localized and will be driven by the location of embedded thunderstorms.

The last thing to address is the gusty winds. With strong winds not far from the surface within the strong 30-45 kt LLJ, gusty SW winds are likely Monday. Ensemble means show gusts in the 20-35 mph range. With the core of the stronger winds positioned across east and especially southeast MA, these areas will have the greater risk of seeing the higher end of that range. However, there is an increasing risk for higher gusts in recent guidance. A big factor will be if we clear enough from the first push of rain to promote mixing, especially in the east/southeast areas under the strong core of winds aloft. If winds mix efficiently, 40 mph gusts are possible in the afternoon in areas (E/SE MA).

KEY MESSAGE 2...Dry and less humid weather returns Tuesday

Post frontal airmass overspreads the region Tue with good column drying, as PWATs plummet to less than an inch. Dry weather, plenty of sunshine, dew pts 55-60 and seasonably warm highs of 80-85 expected. Easily the pick of the week, enjoy!

KEY MESSAGE 3...Weather pattern quickly becomes unsettled again by later Wednesday, continuing late week and into the weekend, with periods of showers and storms, noticeably more humid as well.

Remnant mid level closed low and sub-tropical moisture over the southeast states, begins to advect up the eastern seaboard into SNE beginning late Wed/Wed night. This moist flow into SNE may persist into late week and the weekend, as subtropical ridge develops across the western Atlantic, in concert with upstream trough across the western Great Lakes. This will translate to an unsettled weather pattern for SNE, in the form of periodic showers and thunderstorms chances through the period, probably greatest during the afternoon/early evening, in response to max heating.

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:

Today: High confidence.

VFR conditions with SCT to BKN cloud decks this afternoon. SSW winds around 8-12 kts with occasional gusts to 20 kt.

Sunday Night: Moderate confidence.

Ceilings fall to IFR from south to north after 00Z with LIFR possible in areas after 03Z. Scattered -SHRA develop 00-02z with potential for a band of more moderate showers to push from south to north between 06-10Z. Embedded thunder is possible, but too low confidence to include in TAF at this time. LLJ pushes in from the south after 06Z, so added wind shear to Cape terminals, BOS, and PVD.

Monday: Moderate confidence.

IFR/LIFR in RA/+RA through the day. Embedded storms possible within the first few rounds of rain 12-21Z. Included in TAFs as PROB30s until we have higher confidence in the timing/probability of storms. Another round of scattered thunderstorms likely pushes through from west to east 19Z-02Z ahead of the approaching cold front. Wind shear likely continues in the east/southeast terminals during the day. SW winds 10-20 kts with gusts 20-30 kts. Could see period in the afternoon that winds gust 25-35 kts for the east and especially SE terminals if we clear out before the early evening round of storms.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Monday Night through Friday/...

Tuesday through Tuesday Night: VFR.

Wednesday through Wednesday Night: VFR. Slight chance SHRA.

Thursday through Thursday Night: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

Friday: VFR. Breezy. Chance SHRA, chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Winds and seas increase late tonight into early Monday morning across the southern and eastern waters eventually increasing across the northern waters. Seas 5-9 ft for the outer coastal waters, especially south. 5-7 ft seas stretch into portions of the inner souther waters. SW winds 15-20 kts with gusts around 30 kts. Can't rule out a few isolated gusts near Gale-force late morning-afternoon if winds mix down from the low level jet overhead. SCAs are in effect for all the coastal marine zones. Elevated seas will keep SCAs around through Tuesday for the southern waters.

A high surf advisory has been issued for the south coast and islands Monday. A strong 35-45 kt southerly low level jet will help build waves 5-9 ft across the waters of RI and south of MVY-ACK Monday. Rough surf and strong rips are likely to linger into Tuesday. Will have to watch this as Tuesday is shaping up to be an ideal beach day.

Outlook /Monday Night through Friday/...

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

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

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

Wednesday Night: Winds less than 25 kt.

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

Thursday Night through Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Surf Advisory from 7 AM to 9 PM EDT Monday for MAZ023-024. RI...High Surf Advisory from 7 AM to 9 PM EDT Monday for RIZ006>008. MARINE...Small Craft Advisory from 5 AM to 5 PM EDT Monday for ANZ230- 236. Small Craft Advisory from 8 AM to 5 PM EDT Monday for ANZ231- 251. Small Craft Advisory from 8 AM Monday to 2 AM EDT Tuesday for ANZ232>234. Small Craft Advisory from 5 AM Monday to 2 PM EDT Tuesday for ANZ235-237. Small Craft Advisory from 8 AM Monday to 2 PM EDT Tuesday for ANZ250-254. Small Craft Advisory from 5 AM Monday to 8 PM EDT Tuesday for ANZ255-256.