National Weather Service Boston/Norton MA
730pm EDT Wednesday Jun 12 2024
Synopsis
Sunny and mild Thursday aside from the chance for some patchy south coast fog. An approaching cold front will bring a period of scattered showers and thunderstorms Friday afternoon to the mid evening. Some of these storms could become strong to severe, as well as produce isolated instances of urban and flash flooding. Pleasant weekend with mild days, cooler nigheights and dry weather. Significant warmup towards hot and humid weather in the Tuesday to Thursday timeframe, with excessive heat and humidity possible.
Near Term - Until 6am Thursday Morning
730pm Update:
Overall a pretty tranquil late-afternoon/early evening with dewpoints falling into the lower to mid 50s in much of CT and western/central MA. Meanwhile in eastern MA and RI, satellite shows diminishing trends to diurnal cumulus fields that developed this afternoon, and the window of opportunity for a hit or miss shower is quickly diminishing too. Wouldn't rule out a shower for another hour over portions of Metrowest near the I-495 corridor but expect that to be it. Reduced Probability of Precipitation toward a dry forecast for the balance of the overnight. A few areas in northeast MA where they did get a couple showers today, as well as near some of the river valleys in CT (Quinebaug/Connecticut River valleys) could see patchy fog develop toward daybreak. Lows still look on track in the mid to upper 50s tonight.
Previous discussion:
Weak upper level disturbance/shortwave that brought renewed diurnal cumulus to the region will shift seaward this evening, allowing mid level ridging to build in overnight and into the first part of Thursday. Diurnal cumulus will dissipate quickly after 23Z this evening as well as any spotty shower activity that developed this afternoon, though most hi-res guidance overestimated the coverage of spotty showers with most of the shower activity remaining to our north and east over NH and Maine. Lows tonight will be seasonable, in the 50s.
Short Term - 6am Thursday Morning Through Thursday Night
Thursday looks like a beautiful early summer day across much of the region with mid level ridging in place and drying flow aloft as winds above 800mb shift to the NW. This wind shift will result in decreased diurnal cumulus, compared to Tuesday and Wednesday. While winds aloft shift to the NW, surface flow will remain out of the SW, which will help enhance WAA (Warm Air Advection - the movement of warm air) and increase dewpoints through the day. High temperatures should tack between 5 and 10F onto Wednesday's highs as 850mb temperatures increase from 8/9C to ~13/14C by late afternoon. Winds will increase tomorrow afternoon as weak 925mb LLJ develops along and SE of the I-95 corridor, which will mix gusts to as high as 20kt to the surface.
So, while clouds will be more sparse across much of the region, do anticipate some stratus/fog to develop along the south coast, Cape, and Islands tomorrow afternoon as dewpoints climb into the 60s.
Shortwave trough, that will bring more significant weather Friday, approaches southern New England from the Great Lakes/southern Ontario late Thursday and Thursday night. This will allow dewpoints to further increase as surface flow shifts to the south (PWATs (Precipitable Waters) increase to near 1.25" by daybreak Friday), yielding muggy, mild night with lows generally in the mid 60s.
Long Term - Friday Through Wednesday
Highlights:
* Cold front brings potential for isolated/scattered strong to severe t-storms along with isolated instances of urban and flash flooding Friday midafternoon to mid-evening.
* Beautiful weekend with mild days, cooler nights, dry weather and low humidity levels.
* Growing confidence in a period of hot and humid weather with excessive heat possible Tuesday thru Thurs, with little nighttime relief. Potential exists for heat indices in the low-100s in some spots with nighttime lows in the low to mid 70s.
Details:
Friday:
Forecast models continue to point to Fri being a active day as a SW to NE oriented surface cold front and associated weak-amplitude shortwave disturbance act on increasing moisture content and temps in the 80s through strong insolation. While the first roughly half or so of the day should be rather warm and dry, our weather turns more active during the afternoon to mid-evening hours with the risk for strong to perhaps severe t-storms. There are still some uncertainties to iron out, including the timing of greatest threat and the degree to which we can destabilize, as well as the typical sub-synoptic-scale uncertainties inherent to a Day-3 thunderstorm forecast.
On the larger-scale, have relatively weak low-level flow but quite strong mid to upper level winds leading to SWly sfc-6 km shear magnitudes around 35-50 kt, with higher values lagging the frontal boundary. CAPE values could reach into the 1000-1200 J/kg range per the Storm Prediction Center SREF mean, although those probabilities are on the lower side, greatest in interior CT/MA. Biggest potential limitation is the forecast instability being on the lower side, but it could still be sufficient for at least an isolated strong/severe t-storm risk. Hydrologically, there are some meteorological ingredients which could favor isolated to scattered instances of flash flooding even if antecedent conditions aren't necessarily favorable. For one, there is a decent pool of PWAT (Precipitable Water) values around 1.5-1.8 inches. Secondly, warm cloud depths reach into the 10,500-11,000 ft range which favor potentially efficient rains driven through warm cloud rain processes. The speed of the mid-upper level flow favors faster- moving individual storms, although the boundary- parallel orientation of the steering flow to the cold frontal boundary itself suggests re- development or backbuilding is a possibility. So besides the isolated severe weather potential in the form of localized straight line wind damage, we also appear to have a favorable meteorological setting for street flooding and some potential for flash flooding in a backbuilding, redeveloping convective situation. It appears the greatest potential for t-storms for areas north and west of I-95, perhaps into the Boston to Providence corridor in a weakening state. Added enhanced wording for heavy rain and gusty winds, meshing well with the Colorado State Machine Learning progs and Storm Prediction Center Convective Outlook and WPC Excessive Rain Outlook.
Front moves offshore later Friday night into the pre-dawn hrs, with decreasing rain showers and falling dewpoints. Lows mid 50s to low 60s.
The Weekend:
Large 1020+ mb high pressure cell builds into New England in the wake of the frontal passage from Fri/Fri night. It sets the stage for a beautiful weekend with mild days, cooler nigheights and low humidity levels. Highs mainly in the 70s, perhaps near 80 in some interior spots, with lows in the upper 40s to mid 50s. Thus a really nice weekend in store. If you haven't already done so, the weekend would be a good time as any to put in any window air-conditioning units as our weather pattern changes toward very warm and humid weather into next week.
Next Week:
There remains strong multi-model/multi-ensemble agreement on a significant warmup toward very warm to hot weather along with increasing humidity levels. This stretch of heat and humidity looks centered during the Tue-Wed-Thurs timeframe, and we could be talking about the heat and humidity turning to excessive levels on these days. By Wednesday and Thurs, high temps away from the south coast could reach well into the 90s with potential for 100 degree heat in the Merrimack and CT Valleys with little nighttime relief.
There remain details that could greatly shape the heat and humidity forecast in this time period that are unknowns, including potential for seabreezes, if we can fire off any t-storms given the warmth and humidity, as well as the potential effect of cloud cover. It looks like SWly winds are strong enough to keep the eastern MA seabreeze offshore, but we'll have to see in subsequent guidance on that aspect.
Still, what we can say is that there is quite a signal for increasing heat and humidity next week - in fact, low-level temps are approaching values which are outside model-forecast climatology for late June. Although there are still uncertainties on cloud cover and potential convection, the strong model signals elevates the potential for an anomalous heat event by mid/late June standards, and we could be talking heat indices in the lower 100s in some locations.
Marine
Forecaster Confidence Levels:
Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent.
Tonight:
Calm conditions with continued WSW winds near 10 knots at seas 1-2 feet.
Tomorrow:
Increasing SW winds gusting to 20 knots. Seas increase to 2-4 feet.
Friday:
Cold front approaches the waters Friday afternoon and early evening. Low risk for small craft conditions across the northeast waters with gusts to 25kt possible. Seas approaching but generally less than 5 ft.
Outlook /Friday through Monday/... Friday Night: Winds less than 25 kt. Chance of rain showers. Wind shift from the SW to the NW expected behind passing cold front.
Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.
Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft.
Sunday through Monday: Winds less than 25 kt.
NOAA Boston MA Office: Watches - Warnings - Advisories
CT...None. MA...None. RI...None.
Marine
None.
