Area Forecast Discussion
Issued by NWS Boston, MA

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669 FXUS61 KBOX 261154 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 754 AM EDT Sun May 26 2024 .SYNOPSIS... Potential for stronger to marginally severe thunderstorms this afternoon, especially north of the I-90 corridor. Cloudy and somewhat humid during the morning hours of Memorial Day, although a frontal system brings increasing chances for rain with embedded thunderstorms Monday afternoon into early Tuesday. Localized downpours are possible. Generally drier mid to late in the week with cooler temperatures. Cannot rule out hit or miss showers/storms on Wednesday and Thursday. Shower activity more spotty as we head into the weekend. && .NEAR TERM /THROUGH TONIGHT/...
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730 AM Update... Haven`t made any major adjustments to the current forecast. Though did want to highlight that there were a few thunderstorms this AM as there was a few hundred J/kg of MUCAPE in place. Should see this early AM activity taper off as highlighted by latest HREF. Big question will be if skies clear enough that we can destabilize, which we are already seeing breaks across northern New England. Per the latest deterministic guidance we do have 30-40 kts of deep layer shear along the MA/VT/NH border this afternoon and latest HREF is indicating there are some low probs (10-20 percent) of 2-5 km updraft helicity greater than 75. The latest NADOcast guidance also highlighting some probs for hail/wind. So will be something we keep an eye on this afternoon. Best shot for activity to fire is roughly 18-20Z. Previous discussion... A mid level shortwave moving across northern New England early this morning will trigger some showers, mainly north of the Mass Pike. Still not overly concerned about thunderstorms, at least for this morning. Mid level lapse rates were still steeper than moist adiabatic, but it is unlikely that updrafts will be strong or deep enough to produce thunderstorms. Quite a different story heading into this afternoon. A weak low pressure is expected to move from the lower Great Lakes into northern New England, leaving our region on the warmer and more humid side. This should result in most unstable CAPE values on 1,200-1,700 J/kg. Model soundings are also more supportive of thunderstorms compared to yesterday, having minimized a mid level inversion between 700-500 mb. However, there is still not expected to be much shear to organize this energy. Thus, we are more likely to see typical rather than severe thunderstorms today. That said, a few storms could get strong enough to produce some hail or gusty straight-line winds. The more sunshine we get this morning, which is not expected to be much overall, the greater the risk for stronger storms this afternoon. Precipitable water values are also expected to be in excess of 1.25 inches, meaning locally heavy rainfall is also possible. Will need to be wary of how fast any thunderstorms move this afternoon, and where exactly the rains fall. While most locations should see rainfall less than one half inch, where it rains at all, there is a chance for a quick inch of rainfall in a short time. Another day of above normal temperatures.
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&& .SHORT TERM /MONDAY/... Instability drops off quickly tonight as surface flow shifts to the east. So, while the thunderstorm threat diminishes after about 11 PM, scattered rain showers are possible overnight. Higher dew points should keep low temperatures between the mid 50s and lower 60s. The attention turns to Memorial Day. A low pressure over the Great lakes is expected to push another warm front our way. With a lack of sunshine and an easterly flow to start, expecting high temperatures to be 5-10 degrees lower when compared today. Outdoor plans are likely to be impacted. Thinking scattered showers during the morning hours, with more widespread showers and isolated thunderstorms arriving from the west during the afternoon. Thinking the heaviest rainfall holds off until Monday Night. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights * Widespread showers with embedded thunderstorms Mon Night into early Tue. Heavy downpours anticipated at times. * Turning drier and mild on Tue. * Generally dry for much of the week. Though there could be some hit- or-miss showers and thunderstorms Wed/Thu. Spotty activity possible late in the week and heading into the weekend. Monday Night into early Tuesday... Will have a potent shortwave trough initially over the eastern Great Lakes into the Mid Atlantic Mon evening. This will lift through much of New England by early Tue. Will likely be within the warm sector of a frontal system through much of this period. The trailing cold front may begin sweeping through parts of western New England early on Tue. This is the next opportunity for impactful weather across southern New England. Have an anomalously robust PWAT plume and low level jet impinging on our area. Main concern at this point is locally heavy downpours, which could potentially lead to scattered hydrologic issues. Could also see some stronger storms given the strong low level jet. Heavy rain threat... The latest NAEFS and EPS guidance show that our PWATs will be roughly 2-3 STD above model climatology. Deterministic and ensemble guidance show values ranging from generally 1.5 to 1.75 inches, though could get as high a 2 inches. The V component of the wind is highly anomalous at 3-4 STD above model climo. Should have a few hundred J/kg of MUCAPE in place with a skinny profile and warm cloud layer depths of roughly 3-4 km. This should result in efficient warm rain processes. The latest WPC Excessive Rainfall Outlook of Marginal still suffices given we`ve been pretty dry. Though if the heavier rain can fall over an urban area then there will likely be hydrologic issues. Ensemble guidance continues to show an interesting spread with the probs of 1+ and 2+ inches of rainfall. The GEFS still much more muted than its international counterparts (EPS/GEPS) with essentially nil probs for southern New England. The GEPS is the most hot to trot with low to mod probs (10-60+ percent) of 24 hr QPF AOA 1 inch with the risk greatest for CT, RI and into western MA. The EPS shows a similar pattern as the GEPS, but is not as broad. The GEPS even indicating some low probs (10 percent) of 2+ inches of precip. As have previously mentioned am wondering if the lack of higher probs for heavier precip is due to the progressive nature of the system and the dry slot punching through. The dry slot tends to not be well modeled and slides through quicker than guidance usually indicates. Severe thunderstorm risk... There are still some indications that there could be some strong to perhaps severe storms. The low level jet impinging on the region is 2.5-3 STD above model climo at the 850 hPa level. Though should note that the speeds at 925 hPa are not to be scoffed at either. Should see a 925 hPa jet of roughly 20-50+ kts. Both GFS/NAM Bufkit soundings are indicating that we are pretty inverted and our instability is somewhat paltry. Despite this deep layer shear will be right around 40 kts and hodographs do show some curvature to them as the low level jet is sliding through overnight. This results in roughly 100-300 units of 0-3 SRH. Concerning signals given we`ve got a closed upper low setting up over the eastern Great Lakes and feeling somewhat like a fall pattern setup and we`re solidly in the warm sector. CIPS and CSU ML guidance somewhat split on the severe risk with the CIPs analogs not highlighting SNE at all. The CSU ML guidance does indicate some low probs across western portions of New England. Something we will need to keep a close eye on. Rest of Tuesday... Caught under cyclonic flow through this timeframe. Will have a broad trough/cutoff setting up over the eastern Great Lakes/Quebec. Cold front crosses much of southern New England by late in the day. Though there is some discrepancy on the exact timing. Should see the dry slot continuing to work its way through our region. Thinking the bulk of shower/storm activity will be tapering off in the morning before drying out in the afternoon. Though wondering if the dry slot pushes in/through a bit faster than am currently indicating. Regardless, not anticipating much precip wise during this period. Should see a decent SW to W low level jet setting up with 925 hPa winds of 15-30 kts. Should be somewhat mild given our 925 hPa temps range from 16-19 degrees Celsius. Bumped up our highs toward the 75th percentile of guidance given some uncertainty on cloud cover. Will be mild with highs generally in the 70s for most, but there could be some low 80 degree readings across the Merrimack and CT River Valley. Wednesday through Saturday... Blocky cyclonic flow pattern in place through much if not this entire timeframe. There is some uncertainty on how things will evolve especially as we head late into the week. The question will be on where the cutoff is located. Overall think much of this timeframe will feature dry weather for much of southern New England. Think that the best opportunities for more scattered showers/storms comes on Wed/Thu with the trough/cutoff overhead. When it comes to Fri/Sat should be drier especially as a ridge builds into the Great Lakes, but cannot rule out some spotty activity at this point. The NBM handled this period pretty well, so have not deviated from it at this point. Temperatures will be turning near to cooler than seasonable. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/...
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Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...High confidence in trends, moderate in timing. VFR across the interior today, but will need to keep an eye out for hit or miss showers/storms this afternoon. Timing wise am thinking things initiate at roughly 18-21Z. If a shower/storm can move over a terminal then will have risk of gusty winds and and small hail. IFR in low clouds across the south coast. Though think we improve for PVD-FMH-HYA with some heating around roughly 14-16Z. Winds out of the S/SE at 5-10 kts. Will have localized sea breezes developing roughly 14-17Z. Tonight...Moderate confidence. Winds become east at 5-10 kts, which will generate reduced cigs/vsby as IFR/LIFR stratus and fog advects in. Memorial Day...Moderate confidence. IFR with localized LIFR conditions with E to SE winds. Should see speeds increasing to around 10-15 kts during the afternoon along with gusts. Should see spotty showers becoming more widespread during the afternoon. Could have some embedded thunder during the second half of the day. KBOS TAF...High confidence in TAF trends, moderate in timing. VFR for much of the forecast today. Will have the shot for showers/storms this afternoon generally 21Z through 01Z. Given the risk have added a PROB30 for thunder. In this window if a storm passes over a terminal could see MVFR to perhaps IFR conditions along with gusty winds and small hail. Should have sea breeze developing roughly 14-15Z. Confidence lower for tonight as onshore flow pushes in IFR/LIFR stratus and fog. KBDL TAF...High confidence in TAF trends, moderate in timing. VFR for much of the day. Risk of hit or miss shower/storms 18-23Z. If storm moves over the terminal could see MVFR to IFR conditions. Winds turning more SE tonight, which may spread IFR stratus/fog in. Outlook /Monday Night through Thursday/... Monday Night: Mainly IFR, with local MVFR possible. Breezy. SHRA, chance TSRA. Tuesday: Mainly VFR, with local IFR possible. Breezy. Slight chance SHRA. Tuesday Night: VFR. Wednesday: VFR. Chance SHRA. Wednesday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA. Thursday: VFR. Breezy. Slight chance SHRA.
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&& .MARINE...
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Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Through Memorial Day... Moderate Confidence in fog. Low risk for showers and a few thunderstorms this afternoon, mainly across the eastern coastal waters. Greater risk for showers and a few thunderstorms during the afternoon on Memorial Day. Fog less likely tonight as winds shift to the E/NE. Seas remain between 1-3ft through this portion of the forecast. Outlook /Monday Night through Thursday/... Memorial Day: Winds less than 25 kt. Chance of rain showers, patchy fog. Local visibility 1 nm or less. Monday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Rain showers, slight chance of thunderstorms. Tuesday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, isolated thunderstorms. Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Wednesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Wednesday Night through Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Belk/BL NEAR TERM...Belk/BL SHORT TERM...Belk LONG TERM...BL AVIATION...Belk/BL MARINE...Belk/BL