textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Mix of clouds and sun with off-and-on showers through the afternoon, with decreasing clouds and a cool evening tonight. Full sun, seasonable temperatures and low humidity for the Labor Day Monday holiday.

- Dry conditions Tuesday through much of Wednesday. Pattern becomes more unsettled later Wednesday into early Friday with periods of showers possible.

DISCUSSION

KEY MESSAGE 1...Mix of clouds and sun with off-and-on showers through the afternoon, with decreasing clouds and a cool evening tonight. Full sun, seasonable temperatures and low humidity for the Labor Day Monday holiday.

We're still governed by broad cyclonic flow aloft around a closed mid/upper low over Nova Scotia, a feature which has moved very little over the last 24 hours. This has led to a mix of clouds and sun, and some heating where clouds have scattered has led to "destructive sunshine" which has blossomed additional cloudiness with scattered pop-up showers moving southward. These clouds and scattered showers will continue through sundown. An additional shortwave and mid-level moisture move southward from coastal ME could sustain cloud cover across eastern New England for a few hours after sundown, but we should trend mostly clear with falling dewpoints overnight supporting good radiational cooling. There could be some patchy mist or fog developing around sunrise, especially in those areas which received rain. We also have one final evening with nuisance splashover around the early-evening high tide at Nantucket and the Cape and Islands with coastal flood advisories still in effect. We fell just short of Advisory criterion last night at Nantucket at 4.91 ft, and tonight should end up being similar with a higher high-tide but weaker northerly winds. Lows tonight should dip into the mid to upper 40s in the favored radiational cooling locations, and in the low to mid 50s elsewhere, warmest lows Cape and Islands.

Rising 500 mb heights on Labor Day Monday support full sun and temperatures warming back into the mid to upper 70s, with low humidity levels (dewpoints in the 40s to low 50s!) with some northwesterly winds around 10 mph. Overall about as ideal weather conditions for outdoor activities as one could ask for. By Monday evening, clear skies early favoring strong radiational cooling then give way to increasing midlevel cloud cover from the northwest by the pre-dawn hours as a strong shortwave disturbance aloft passes to our northeast. Lows end up slightly warmer, in the to mid 50s for most, with mid/upper 50s for the eastern and southern coastlines.

KEY MESSAGE 2...Dry conditions Tuesday through much of Wednesday. Pattern becomes more unsettled later Wednesday into early Friday with periods of showers possible.

On Tuesday, the deeper mid-level trough continues to shift further eastward as a mid-level ridge continues to build in from the west. 925mb temperatures warm to +17-18C yielding high temperatures in the upper 70s with locations potentially hitting the 80 degree mark in the valleys. High pressure in place will support mainly dry conditions.

Ensemble guidance shows a signal for a transition to a more unsettled pattern mid to late week. The mid-level ridge begins to break down Wednesday, becoming more zonal by Thursday. Ensemble guidance shows a consensus with a series of shortwave troughs moving through the flow later Wednesday through Friday. This should be accompanied by a plume of above normal moisture, rising to 160-190% of normal by early Thursday yielding PWATS of 1.5" approaching 2.0". The positioning of an upper jet should supply ascent for periods of showers. There is spread across guidance in the finer details of the timing of the waves which will influence timing of the showers. A shortwave pushes through sometime Wednesday evening/night triggering the first round of showers which should taper off through the morning Thursday (at latest). The next wave moves through late Thursday/Thursday Night bringing another round of showers. With the timing of the waves still variable across guidance this leaves some uncertainty in thunderstorm/downpour potential. There will be plenty of moisture to support downpours; however, the timing of the wave would need be early enough to tap into the marginal instability build through the day for convection. Something to keep monitoring. For now, we are not seeing a strong signal for severe or flash flooding, just periods of showers with generally light accumulations (< 0.60").

An 850mb 35-45kt LLJ shifts across southern New England into Thursday. There does raise a flag for potentially gustier winds; however, this will come down to how well we can mix down from 850mb or even 925mb. If things align, ensembles show potential for gusts in the 20-35 mph range on Thursday across the land and coastal waters. Still plenty of details to work out in the pattern before we come into better agreement about the gusts, but something to watch for now.

Temperatures Wednesday/Thursday lean near to slightly above normal with temperatures in the mid to upper 70s. A cold front pushes through early Friday bringing a cooler airmass this will support highs in the upper 60s low 70s.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

15z TAF Update:

Today: High confidence.

SCT-BKN VFR/MVFR ceilings (020-040 bases) continue as of mid- morning, with widely SCT light -SHRA around pushing southward. While showers come to an end shortly, anticipate some lifting of cloud bases closer to VFR levels by afternoon. Another surge of moisture comes down now over central Maine could bring an increase in clouds toward BKN later this afternoon for eastern terminals. North to northeast winds around 5-10 kt.

Tonight: High confidence.

Mostly VFR as cloud cover decreases. Some risk for patchy fog in the river valleys prior to sunrise (08-12z) but its development is uncertain. Winds decrease to light northerly.

Monday and Monday Night: High confidence.

VFR. NW winds increase to around 8-12 kt during the daytime, easing and becoming WNW with an increase in VFR clouds after 06z Tue.

KBOS Terminal...High confidence in TAF. Mix of SCT-BKN VFR/MVFR ceilings, but should trend VFR by early afternoon. NE winds around 5-10 kt. Seabreeze possible but could be late in the day.

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Friday/...

Tuesday through Tuesday Night: VFR.

Wednesday: VFR. Breezy.

Wednesday Night: Mainly VFR, with local IFR possible. Windy with local gusts up to 30 kt. Chance SHRA.

Thursday: VFR. Windy with areas of gusts up to 30 kt. Slight chance SHRA.

Thursday Night: VFR. Breezy. Slight chance SHRA.

Friday: VFR.

MARINE

Forecaster Confidence Levels...

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

Through Monday Night: High confidence.

SCAs continue through early this evening to allow for residual 5 ft seas to subside.

Northerly winds around 15-20 kt this afternoon decrease to around 10-15 kt overnight tonight. Essentially a repeat of those wind conditions for Monday. Northerly winds pick back up again Monday to around 15-20 kt, then subside overnight. Seas will continue to decrease to 3 ft or less all waters by tonight and remain in that range through Monday night.

Off-and-on light showers will continue through late this afternoon. Otherwise dry weather with good visibility then predominates.

Outlook /Monday Night through Friday/...

Tuesday through Wednesday: Winds less than 25 kt.

Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of rain showers.

Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft. Slight chance of rain showers.

Thursday Night: Winds less than 25 kt. Areas of rough seas. Chance of rain showers.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Risk for Rip Currents until 8 PM EDT this evening for MAZ007- 014>016-019-022>024. Coastal Flood Advisory until 8 PM EDT this evening for MAZ022>024. RI...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ250- 254-255.