textproduct: Boston / Norton

WHAT HAS CHANGED

Shower chances today now look more focused during the late morning to afternoon hours, but rain amounts won't be significant. Potential for a couple of 90 degree days Tuesday and Wednesday.

KEY MESSAGES

- Dreary, gloomy Friday with cooler than normal temperatures. Better chance at light rain showers late this morning into the afternoon but not a washout.

- Warmer temperatures for the weekend into with mostly dry weather.

- Well above normal temperatures Tuesday and Wednesday, with some areas reaching the low 90s. Possible strong thunderstorms Wednesday.

DISCUSSION

KEY MESSAGE 1...Dreary, gloomy Friday with cooler than normal temperatures. Better chance at light rain showers late this morning into the afternoon but not a washout.

Upper level low located near the Southern Tier of NY is beginning to move slowly southeastward. On its far eastern periphery is a stream of subtropical moisture which trails from well east of Cape Cod and Nantucket then back northwestward through northern ME/NH and northeast VT, quite a ways further north than prior expectations. Early this morning most of the eastern third of Southern New England was socked in with fog and low clouds, but with more breaks in cloud cover over interior Southern New England.

As the upper level low continues to gradually pull southeastward, it will draw that ribbon of enhanced moisture over northern New England and an inverted sfc trough southward as surface winds become northeasterly. Exactly when this occurs is still a little unclear but it doesn't look to occur any sooner than the mid to late morning hours for the North Shore, and then gradually advance southward into the afternoon. Although mostly cloudy to overcast conditions with periods of drizzle at times continue into the morning, it's by-and- large dry until that inverted trough and moisture build back southward during the late morning to mid-afternoon hours. The one area that we could have to watch for some steadier showers is along the South Coast in the afternoon once the upper low settles in/passes to the southeast. All in all, not a washout and not as soggy as it may have looked previously, but nonetheless a pretty dreary, gloomy Friday. Temps probably won't go very far in northern MA today with highs in the low 50s, and only a narrow window for some modest warming in RI/CT/SE MA before northerly flow kicks in, with highs perhaps getting to 60 before cooling off.

More significant clearing takes place late this afternoon to the north but then delayed until early to mid evening elsewhere. With damp ground and likely narrow temperature/dewpoint spreads once clouds clear out, it's a potential target for patchy fog to develop overnight, and it could be a little more widespread than just in the river valleys. Lows in the mid 40s to near 50.

KEY MESSAGE 2...Warmer temperatures for the weekend into with mostly dry weather.

The weekend continues to offer a significant pattern change toward warmer temperatures. Troughing/closed low will be exiting eastward and in its place is a broad quasi-zonal flow, which will draw in warmer air on WSW winds. Saturday looks mostly sunny with SWly breezes with temps running in the mid to upper 70s (cooler 60s/lower 70s South Coast and the Cape). There is a weak impulse which passes to our north Saturday night to early Sunday, but it should just bring some enhanced cloudiness for the evening. Even warmer temps for Sunday after a dry surface trough passage takes place. Westerly breezes to around 20-25 mph but full sun will bring highs in the low to mid 80s, with still tolerable levels of humidity. So a late spring/early-summer feel for the weekend.

KEY MESSAGE 3...Well above normal temperatures Tuesday and Wednesday, with some areas reaching the low 90s. Possible strong thunderstorms Wednesday.

While significant warm advection takes place Monday (e.g. 850 mb temps rise in most areas into the 14 to 16C range), at least eastern MA will be kept several degrees cooler than the CT Valley as a backdoor cool front/onshore flow develops at least for a brief period of time. Highs could reach the upper 80s with an outside shot at 90 degrees in the CT Valley, but in upper 60s to mid 70s for eastern MA.

Tuesday and Wednesday look to be the warmest days as 500 mb ridging begins to weaken/shift east. Of the two days, Tuesday projects as the warmest with full sun and many areas seeing the mercury rise into the upper 80s to low 90s! Southwest flow increases some as a cold front works its way ever-so-slowly east from the eastern Gt Lakes/NY State, so both days will also offer SW breezes too. These warm temps will help drive at least a modest degree of instability, so some threat for showers/thunderstorms could develop, as reflected in machine-learning convective progs. At least for Tuesday, deep- layer flow is pretty weak with the stronger belt of SWlys stil well to the northwest. Wednesday could be a day to watch for stronger storms to develop as we get more into the belt of stronger southwest flow aloft and better convergence from the approaching cold front, and this has some support from the machine learning convective hazard progs.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Through 12z Friday: Moderate confidence.

Lowest categories (IFR-LIFR) due to stratus and mist/fog for eastern/northern MA and RI. Low-end VFR to periods MVFR ceilings for ORH and BDL with no visby restrictions. These general trends should continue through 12z Friday, although fog coverage should expand a bit further north and east. However better moisture supportive of -RA is confined well to the north so should be largely dry. Light mainly WNW winds but could turn calm for a while.

Today: Moderate confidence.

IFR-LIFR stratus/fog for eastern/northeast MA and RI remains, although visbys could improve toward 3-6 SM range. MVFR ceilings to eventually fill back into BDL and ORH thru this morning. Winds eventually turn NE 6-10 kt, more likely around ~13-15z in far northeast MA and then gradually building southward through ~17-19z. That probably brings the best chance at drizzle or light rain showers too; one secondary area to watch for pop-up showers is along the South Coast in the afternoon. Gradual clearing from north to south toward MVFR/perhaps spot VFR levels after 22z, as winds turn light N/NW.

Tonight: High confidence in trends, but moderate on fog development.

Period of clearing takes place 00-03z Sat with at least brief VFR. However MVFR- IFR patchy fog could develop upon clearing, given recent damp ground, light winds and a likelihood of narrow temperature/dewpoint spreads. It is uncertain if fog develops and its areal coverage, but most TAFs seem prone to fog development besides the usual fog trouble spots. Light northerly winds, which shift to SW 5 kt or less by daybreak.

Saturday: High confidence.

Any mist or fog from overnight dissipates rapidly after sunrise to VFR conditions. SW winds increase to around 10-13 kt with gusts in the 20-25 kt range.

KBOS...Moderate confidence in TAF. IFR ceilings with NW winds around 5-8 kt. Winds to flip to NE around 14-16z which brings better chance at either -DZ or light -SHRA thru 22z. Winds then flip to light northerly after 22-00z. Possible mist/fog for Friday night.

KBDL...Moderate confidence in TAF. Low end VFR ceilings to trend to MVFR 09-13z, with north/northeast winds 5-10 kt. Ceilings then scatter to VFR levels late this afternoon, but clearing could bring a risk for possible mist/fog Friday night.

Outlook /Saturday through Tuesday/...

Saturday through Sunday: VFR. Breezy.

Sunday Night through Monday Night: VFR.

Tuesday: VFR. Breezy. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

SCAs remain in effect on most waters as easterly swell brings elevated seas in the 4-7 ft range today and tonight. Light winds today but will trend NE around 10-15 kt by late morning nothern waters to this afternoon for the southern waters. Fog could be locally dense on the waters today, but better chance for light rain from mid morning northward to later this afternoon southern waters. Fog is also possible again tonight on the waters.

SW winds increase on Saturday to around 20-25 kt in gusts, which could warrant extending SCAs into Saturday. Seas around 4-6 ft.

Outlook /Saturday through Tuesday/...

Saturday through Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft.

Sunday through Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft.

Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ235-237- 251. Small Craft Advisory until 8 AM EDT Saturday for ANZ250- 254>256.