textproduct: Boston / Norton

WHAT HAS CHANGED

No changes.

KEY MESSAGES

- Dry and cool weather through at least Tuesday night. Breezy, especially across Cape Cod and the Islands.

- Dry and cool conditions persist through mid-week. Monitoring potential late-week coastal low that has potential to bring rain, strong winds, and coastal impacts depending on the track.

DISCUSSION

KEY MESSAGE 1...Dry and cool weather through Tuesday night. Breezy, especially across Cape Cod and the Islands.

Anomalous 1030+ mb high pressure over southeast Ontario Monday, builds eastward into Quebec Tuesday, providing dry and cool weather for Southern New England through at least Tue night. Northeast gradient will maintain breezy conditions, especially across southeast MA, including Cape Cod and Islands. Model soundings here support NE winds 15-20 mph with gusts up to 25 mph at times, less wind away from the coastline.

Abundant sunshine Monday afternoon, except post frontal strato-cu over southeast MA from departing frontal wave, combined with cool northeast flow over the relatively mild ocean. Mid/high clouds will preclude radiational cooling tonight, but a very dry airmass (dew pts in the 40s and 50s) will be sufficient for min temps to fall into 40s, except low to mid 50s along the coast, including metro Boston, Providence and Hartford.

Tuesday, short wave over the OH valley today spreads mid/high clouds into SNE tonight and Tue. However mid level confluent flow over the region will provide sufficient subsidence for dry weather to prevail. These mid/high clouds will yield filtered sunshine Tue. Cooler temps aloft (+7C to +9C) combined with subsidence inversion from 1030+ mb high, will yield another day of cooler than normal highs Tue, with highs only in the low to mid 60s, about 10 degs below normal.

KEY MESSAGE 2...Mainly dry and cool conditions persist through mid- week. Monitoring potential late-week coastal low that has potential to bring rain, strong winds, and coastal impacts depending on the track.

Ensemble guidance remains in good agreement that an anomalously strong surface high will set up over Quebec and persist through late week. This will advect in a below normal airmass, yielding high temperatures in the upper 50s and 60s alongside a prolonged period of northeasterly flow through the week. A weak low pressure system moves through the Mid-Atlantic on Tuesday. There remains uncertainty regarding its northern extent; however, if it tracks north enough, a few light showers are possible. The higher probabilities for any showers will be for CT, south coast, and Cape/Islands. Otherwise, expect mainly dry conditions through mid-week.

Attention turns to a developing coastal low late in the week. Deterministic and ensemble guidance continues to highlight a slow-moving low developing and tracking northward toward the northeast coast by late-week. However, considerable spread remains in the exact track and strength of the low. Ensemble means have largely settled on a middle-ground solutions of the low developing off the Mid-Atlantic coast Wednesday, followed by a northward track into closer proximity of southern New England Thursday and Friday. Looking at individual members, tracks range from as far west as Long Island to near Cape Cod, to further offshore southeast or east of southern New England. Recent ECMWF ensemble runs have exhibited subtle shifts east/west in the cluster of low tracks, while the GEFS has show a subtle but consistent trend west. Regardless of the exact track, ensemble means bring the low close enough to bring strong gusts, rain, and coastal impacts(erosion/coastal flooding). It is important to note that the exact track and low strength will strongly influence the magnitude of gusts, QPF, and how far inland the rain extends.

While the center of the low likely holds off until late week, marine and coastal impacts may begin as early as Wednesday. As the low deepens in the Mid-Atlantic, the robust high to the north will aid in tightening the regional pressure gradient, particularly across the southern coastal waters. This will support development of strong winds by Wednesday AM, well ahead of the primary low. The exact positioning and strength the associated areas of high/low pressure will dictate how strong the winds could be. There is growing confidence in Gale force(40mph+) for the southern coastal waters which has been heavily supported by the ECMWF and its ensemble, though the GEFS/GFS remain slightly weaker. A Gale Watch has been issued in these areas. As the system lifts northward expect strong gusts to continue if not strengthen and expand across portions of the waters. The ECMWF ensemble even shows low probabilities (10-25%) for greater than 45 mph gusts Thursday into Friday. Inland winds will also be gusty, though lighter than over the waters with gusts 25-35 mph, strongest along the immediate coast.

This prolonged period of gusty northeasterly winds will build rough seas and elevate the threat for coastal erosion and flooding depending on the track. Given the aforementioned spread in track and intensity across models, rain amounts have also remained uncertain. Most guidance suggests the rain shield will clip at least the far eastern portions of MA with the 50th and upper percentiles shifting the shield further west. Some deterministic run and individual ensemble members indicate amounts > 1.0" which will require monitoring given this is a slow moving system.

In summary, confidence is steadily increasing that a coastal will at least impact the coastal waters and coastal areas late this week. However, this is still far enough out that the details (track, strength, upper level pattern), so the exact impacts and magnitude are still uncertain at this time. Stay tuned.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

Today...High confidence forecast.

VFR conditions throughout the region at 17z, except VFR cigs over southeast MA, including Cape and Islands. Trend is for VFR to overspread all regions later today and tonight, but MVFR cigs may return Tue across southeast MA, as a reinforcing surge of cool air aloft overspreads this area on NE flow. NE winds 10-15 kt with gusts up to 20-25 kt at times across southeast MA terminals.

KBOS Terminal...high confidence in TAF.

VFR and dry weather, with NE winds 10-15G20 kt at times.

KBDL Terminal...High confidence in TAF. VFR and dry weather with a modest NE wind 10-15 kt, gusting up to 20 kt at times.

Outlook /Wednesday through Saturday/...

Wednesday through Wednesday Night: VFR. Windy with gusts up to 30 kt.

Thursday: VFR. Windy with gusts up to 35 kt.

Thursday Night: VFR. Strong winds with local gusts up to 40 kt. Slight chance SHRA.

Friday: VFR. Strong winds with gusts up to 45 kt. Chance SHRA.

Friday Night through Saturday: Mainly VFR, with local MVFR possible. Strong winds with gusts up to 45 kt. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Tuesday Night...High Confidence.

NE winds 15-20 kt with gusts up to 25 kt at times will continue, as strong 1030+ mb high pressure over southeast Ontario on Monday, advects eastward into Quebec. This northeast fetch will yield rough seas on the ocean waters. Dry weather and good vsby prevail. Potential for gale force gusts appear to hold off until Wed and beyond.

Outlook /Wednesday through Saturday/...

Wednesday: Strong winds with gusts up to 35 kt. Rough seas up to 13 ft.

Wednesday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 14 ft.

Thursday: Strong winds with gusts up to 45 kt. Rough seas up to 15 ft. Slight chance of rain showers.

Thursday Night: Moderate risk for gale force winds with areas of gusts up to 45 kt. Rough seas up to 16 ft. Chance of rain showers, slight chance of thunderstorms.

Friday: gale force winds with gusts up to 45 kt. Rough seas up to 18 ft. Rain showers likely, slight chance of thunderstorms. Local visibility 1 to 3 nm.

Friday Night: gale force winds with gusts up to 45 kt. Rough seas up to 18 ft. Rain showers likely.

Saturday: gale force winds with gusts up to 45 kt. Rough seas up to 12 ft. Rain showers likely, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 5 PM EDT this afternoon for ANZ231>235-237. Gale Watch from Tuesday afternoon through Thursday afternoon for ANZ235-237-255-256-282-283. Small Craft Advisory until 3 AM EDT Tuesday for ANZ254>256.