textproduct: Boston / Norton

WHAT HAS CHANGED

Small craft advisories hoisted on most waters for this evening into Thursday. River Flood Warning in effect for the Taunton River at Bridgewater due to snowmelt-induced river rise to minor flood stage. Cold front moving in Thurs has slowed down and could allow for hit or miss showers or even a rumble of thunder tonight.

KEY MESSAGES

- Backdoor cold front brings cooler temps and northeast winds to northern/eastern MA today.

- Mild tonight with hit or miss showers, with thunder also possible.

- Cold front moves thru Thurs, bringing rain and falling temps through the day. Even a risk for minor snow accumulation in interior Southern New England Thurs depending on the timing.

- Minor snow accumulations possible late Fri-Fri night over the higher elevations.

- Potential for widespread showers with locally heavy rainfall Sunday night into Monday which may lead to minor river flooding.

DISCUSSION

KEY MESSAGE 1...Backdoor cold front brings cooler temps and northeast winds to northern/eastern MA today.

A backdoor cool front is taking shape early this morning in northern ME and is expected to build southward thru ME/NH thru early overnight and into eastern and central MA by the early morning hours and then eventually stalling into CT/RI by late in the morning. North of the boundary, expect an overcast and raw day with NE winds and temps either hovering or slowly falling for a time (some areas today in northern MA may struggle to get out of the mid 40s), with warmer temps south and west of the frontal boundary (highs in the lower 60s for CT and southern RI). Other than hit or miss showers on the cool side of the frontal boundary, most of the daytime period should be largely dry. In response to strong southerly flow and warm advection, this boundary returns back northward as a warm front during the afternoon to early evening, which then brings with it a surge in temps and dewpoints. So for areas in northern and northeast MA on the north side of the frontal boundary, calendar-day highs are more likely to be met after sundown.

KEY MESSAGE 2...Mild tonight with hit or miss showers, with thunder also possible.

Ahead of a powerful but slowing cold front, a strong burst of warm/moist advection takes place tonight on 850 mb winds around 45- 55 kt. This brings rising dewpoints (in the 50s) and PWAT values of up to 1.5 inches, as well as decreasing the static-stability above the strong frontal inversion. Most models show showalter indices dropping below 0 indicative of elevated instability...but the degree to which they do and what may force any shower activity is in some question. Similar to yesterday the 00z NAM shows a very-probably- overdone look with lapse rates above 700 mb around 7 C/km, which generates an exorbitant amount of MUCAPE around 1500 J/kg. RAP and GFS MUCAPEs seem more realistic at around 200-400 J/kg. This could be enough to produce some thunder and have included a mention of thunder in with the chance-level PoP during the evening. It also looks like the forcing is slower to arrive, which lends itself to questions on what may get any showers going. And it could be from remnant convective activity in the mid-Atlantic given more active thunderstorms anticipated there today, as offered by some high- res solutions. With the front now slower to lurch into the Berkshires, low temps look to stay in the low to mid 50s in most areas given the southerly winds and overcast.

KEY MESSAGE 3...Cold front moves thru Thurs, bringing rain and falling temps through the day. Even a risk for minor snow accumulation in interior Southern New England Thurs depending on the timing.

Cold front then moves into SNE Thurs, which is when the best chance for widespread precipitation sets in. Much of this will initially fall as rain, but strong shallow cold advection should allow for falling temps through the day. One thing to watch is the potential for anafrontally-forced precipitation Thurs aftn as secondary shortwave trough in the central Appalachians draws moisture northward from the mid-Atlantic on the cold side of the frontal boundary, into an airmass which would be cold enough for a minor accumulation of wet snow. If that were to happen, minor accumulating snow could develop with better chance in interior Southern New England. While snow in March is not uncommmon, considering the mid to upper 70s some areas saw on Tuesday it would be quite a whiplash.

Mild temps from the overnight (mid 50s) should serve as Thurs's highs, with hourly temps then falling steadily through the day.

KEY MESSAGE 4...Minor snow accumulations possible late Fri-Fri night over the higher elevations.

Another robust northern stream shortwave moves across New Eng Fri night before lifting to the north Sat. Modest low level jet develops ahead of the shortwave which will bring increasing moisture into SNE while also enhancing forcing for ascent from warm advection. A period of rain and snow showers are likely sometime Fri afternoon into Fri night. Snow showers will be favored over the Worcester Hills and Berkshires where up to an inch accum is possible. Mainly rain showers in the coastal plain. System lifts to the north by Sat with clipper low tracking to the north and pushing a cold front through by Sat morning leading to a dry and seasonable Sat but with blustery W winds.

KEY MESSAGE 5...Potential for widespread showers with locally heavy rainfall Sunday night into Monday which may lead to minor river flooding.

More amplified upper trough approaches from the west Monday as strong cold front moves into the region. Robust pre-frontal low level jet develops with anomalous PWATs advecting northward into the region. Wind and PWAT anomalies along with elevated instability suggest potential for locally heavy rainfall with convective showers and possible a few t-storms, centered around the Sunday night into Monday period. Probs for 1"+ of rain are 30-50 percent but this is a decent signal at this range. The 90th percentile which represents a reasonable worst case scenario indicates 1-2 inches rainfall which could lead to some river flooding concerns given the rising water levels from snowmelt. MMEFS ensemble guidance from the NAEFS show 40- 60 percent probs for minor flooding early next week along the Pawtuxet, Pawcatuck, and Wood Rivers in RI and the Taunton River at Bridgewater. Low probs (20-30%) for minor flooding noted along the lower reaches of the mainstem Connecticut River from Hartford through Middle Haddam.

Still too early to know how much wind we'll have with the LLJ as this will depend on how mild it gets, but expect a period of gusty S winds. After a cool day Sunday, rising temps likely Sun night and a mild day Monday.

Cold front moves through by Mon evening with another surge of modified arctic air on Tue resulting in temps well below normal.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

12z TAF Update:

Moderate confidence.

Primarily VFR across the area currently, but that is expected to change as we go through the morning into afternoon. Low clouds and BR are expected to develop especially across the eastern half of southern New England. We have a weak "back door" front draped across the area, with NE surface winds north of that front, and light S winds south of the front. Satellite shows an area of low clouds about 50 miles east of BOS headed westward, so thinking this is a signal that for IFR ceilings to occur by sometime early this afternoon. Timing is difficult, so generally stayed close to projections in the 06z TAFs. How far west the IFR conditions are able to push this evening is also in question. Best guess at this point is not much further west than an ORH-IJD line, with some MVFR into the CT River Valley.

That front will start lifting northward overnight, resulting in surface winds transitioning to S/SW direction, and gusting over 20kt in some areas. However MVFR/IFR conditions will continue.

Expect scattered SHRA and perhaps and isolated TSRA to move across the region overnight. Again, confidence is moderate at best with regard to timing at any location. Probability of TSRA at this point is 15-25% during the 02-06z timeframe, so have left explicit mention of TS out of the TAFs at this point.

Lastly, strong winds just above the surface (1000-3000ft AGL) are expected to increase to 45-55kt late this afternoon through a good part of the overnight hours. Thus have maintained mention of LLWS.

As we get into Thursday morning, a front will swing through the region, and VFR conditions are expected to develop after the front along with surface winds turning more W, with gusts over 20kt at times.

KBOS Terminal...Moderate confidence in TAF. IFR ceilings are expected to develop, but timing is in question. 18z is current best guess, but it could be anywhere in the 16z-21z timeframe. Small probability for brief TS in the 02-07z period, but not enough confidence to add to TAF.

KBDL Terminal...Moderate confidence in TAF. Expect MVFR conditions to develop by 01z, but it could be earlier. Uncertainty in the specific timing of SHRA and perhaps a brief TSRA late today/this evening. Confidence of TS is too low to include in TAF at this point, but could occur between 00-05z time.

Outlook /Thursday through Sunday/...

Thursday: Mainly VFR, with local MVFR possible. Windy with areas of gusts up to 30 kt. RA likely, chance SN.

Thursday Night: VFR. Breezy.

Friday: VFR. Breezy. Slight chance RA.

Friday Night: Windy with areas of gusts up to 30 kt. Chance RA, chance SN.

Saturday: Windy with areas of gusts up to 30 kt.

Saturday Night: Breezy.

Sunday: Breezy. Slight chance RA.

MARINE

Forecaster Confidence Levels...

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

A backdoor cold front will be working southward from the coastal ME waters which will bring an increased NE wind to the waters today at around 10-15 kt which then becomes ESE/SE late in the day. Elsewhere winds mainly southerly today around 10-15 kt, increasing to around 20 kt southern waters late.

SCAs hoisted on most waters starting early this evening and through midafternoon Thurs as southerly winds increase to around 25-30 kt in gusts. Seas will also be building to 5-10 ft thru Thurs. Winds will eventually shift to NW late Thurs afternoon to SCA criterion.

Outlook /Thursday through Sunday/...

Thursday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of rough seas. Rain likely.

Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas.

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

Friday Night: Strong winds with gusts up to 35 kt. Areas of rough seas. Chance of rain.

Saturday: Strong winds with gusts up to 35 kt. Rough seas up to 11 ft.

Saturday Night: Winds less than 25 kt. Local rough seas.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 PM this evening to 2 PM EDT Thursday for ANZ232>235-237-250-254>256.