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Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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568 FXUS61 KBOX 301404 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1004 AM EDT Sun Jun 30 2024 .SYNOPSIS... A slow-moving cold front could bring a few showers and storms to the immediate south coast this morning, but these quickly move offshore. There is a greater risk for strong to severe thunderstorms this afternoon and evening as this front is moving by. The main risk is damaging wind gusts and locally heavy downpours, but large hail and an isolated tornado cannot be ruled out. Hit or miss showers/storms are possible on Monday, although drier weather prevails most of the time. High pressure brings dry weather, seasonably warm temperatures and low levels of humidity for Tuesday and Wednesday. Turning more humid and unsettled for the Fourth of July thru the early part of the weekend with a couple of disturbances offering chances for clouds, showers and thunderstorms. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 AM update... The main concern this afternoon is severe weather. Already a pretty unstable environment this morning with MLCAPES up to 1500 J/kg across the interior. Lots of breaks in the low clouds away from the south coast will lead to further heating. This combined with rich low level moisture with dewpoints in the low- mid 70s will result in MLCAPES likely increasing to 2000-3000 J/kg which is rather robust instability for SNE. Combined with deep layer shear 40-50 kt is a very favorable environment for organized storms to develop including supercells if there are any discrete storms. CAMs indicate the most likely storm mode will be line segments or even a QLCS as linear forcing ahead of the cold front and pre-frontal trough results in storms organizing into a line. Damaging winds will be the primary threat, but large hail or an isolated tornado will be possible with any discrete supercells although this is a lesser threat. A QLCS tornado is also possible given orientation of increasing 0-3km shear. In addition to severe potential there will be a heavy rain and localized flash flood threat with any training of storms. It appears storms will initially fire along a pre-frontal trough early to mid afternoon in the interior before approaching the south coast late afternoon/early evening. Given the marine layer near the south coast would expect storms to weaken somewhat as they approach the coast but strong shear may help to sustain them longer than they otherwise would be in a weaker shear environemnt. Some of the CAMs also indicating a second line of storms with the cold front late afternoon and evening. Previous Discussion... Highlights * Main story for today will be a round of strong to severe thunderstorms this afternoon into the evening. Main threats are damaging wind gusts and heavy downpours, which could lead to flooding in some of the more flood prone spots. Cannot rule out large hail and an isolated tornado. * Any AM shower/storms along the immediate south coast push further offshore by mid/late AM. Period anticipated to start with some showers/storms along the immediate south coast today. Leaned fairly heavily on the latest NSSL WRF and ARW as they are handling the current shower activity fairly well. This uptick in showers/storms anticipated along the south coast as the 925 hPa SWly low level jet begins to slide offshore. Will see a bit of a shift of the wind direction resulting in some convergence, but think activity will mostly be focused along/south of the south coast given this is where the strongest winds are located. This first rounds activity is not anticipated to be strong/severe as despite the increasing instability the deep layer shear is disjointed until this afternoon. Confidence a bit lower than usual given guidance is all over the place, but erring on the side of caution given we are within the warm sector. Any of these showers/storms could produce heavy downpours with a 2-2.5 inch PWAT plume overhead and warm cloud layer depths of 4-4.5 km. Main concern in the forecast comes for this afternoon into the evening as a strong cold front slides in from the west and a trough digs into the eastern Great Lakes. This forcing will be more than enough to trigger activity today. As for moisture we will see the anomalous PWAT plume start sliding offshore, but still are quite high with NAEFS indicating us 2.5 to 3 STD above model climo. Should see PWATs around 1.5-2 inches, and still have fairly deep 3.5-4.5 km warm cloud layer depths. So heavy downpours still a risk, but fortunately expecting things to be fairly progressive. This should minimize the risk for flooding, but think the typical prone spots are still areas that will need to be closely monitored. As previous forecast shift mentioned and as is one of the flies in the ointment our mid level lapse rates are quite marginal. This should be made up for by the increasing heat with 925 hPa temps of 20-22 degrees Celsius and low to mid 70 degree dewpoints. This will result surface based CAPEs of 1500-2500 J/kg across the interior. Deep layer shear (0-6 km layer) will increase 40 to 50 kts and low level lapse rates will be around 6-8 degrees Celsius. Given these parameters in combination with the forcing will be very favorable for more organized storms to develop. At this point based on CAMs appears that there could be a brief window wMain theme of the forecast is still intact. Made some adjustments to account for latest observed trends. here a few discrete supercells develop before things quickly congeal into a line, given the strong linear forcing. Anticipating that risk for large hail comes in that brief window where there are supercellular storms. Overall though am anticipating more of a QLCs event as advertised by all the CAMs. This will really elevate the damaging wind gust threat. Though should note that we will have roughly 100-150 of 0-3 km SRH and low LCL heights. SPC SREF still showing moderate probs (30-50 percent) of STP >1. Think this risk will likely come with both the discrete and linear activity. Best shot for severe weather still along and NW of I-95 as the marine layer should limit near the coast. Can see this quite clearly with latest HREF 2-5 km updraft helicity moderate probabilities (30-50 percent) away from the coast where the probs quite rapidly drop. Latest ML guidance all really hitting the wind risk per the Nadocast, NCAR HRRR Neural Network Convective Hazards and CSU. Though all do also indicate that their is also a lower risk for hail and an isolated tornado. Timing wise previous shifts forecast looks quite reasonable. Still thinking will see an update 18-20Z and activity gradually moving southeastward toward the coast by the evening, 00-02Z. Uncertain how strong the activity will be along coastal areas given the stable marine layer, but the deep layer shear could sustain activity a bit. Just not sure if once it reaches the coastal areas how much of the wind will be able to get to the ground. High temps generally in the 80s for most today. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Highlights * Any lingering showers/storms along and near the south coast pushing offshore before midnight. Dry heading into Mon AM. * Widespread scattered hit or miss showers/storms on Mon. The trough over the eastern Great Lakes digs into the Mid Atlantic/Upstate NY by early Mon. The trough cuts off and lifts into southern New England by early afternoon before pushing offshore during the evening. The cold front slides offshore tonight and a surface trough digs extends into southern New England on Mon. Should see any lingering shower/storms across the region pushing offshore tonight as the cold front sweeps through. As previously mentioned there could potentially be a few strong storms given the deep layer shear. Still have questions especially SE of I-95 given the marine layer in place. Regardless, will see activity ending as it moves offshore. Drier weather anticipated after midnight in wake of the cold front. Lows will mostly be in the 60s. Main concern on Monday is that will have a roughly -15 to -20 degree cold pool moving in overhead with the cutoff low. This in combination with daytime heating will result in fairly widespread CU and hit or miss showers/storms developing. Given the environment would not be surprised if we are able to get some small hail, though do wonder if moisture will be a bit too limited. Given the focus on the immediate near term did not dive too deeply into this, but will need to be looked at in future updates. Though at the moment current ML probs don`t really hammer this risk. Expecting a much more comfortable airmass to push in as dew points fall into the 50s/low 60s. High temps a bit cooler than seasonable with readings in the 70s. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights: * Dry and warm with tolerable humidity levels for Tue and Wed. * Turning more humid and active for Independence Day, and potentially into Fri/Sat too, with clouds, showers and storms possible. Exact timing remains uncertain. 30/00Z guidance suite was in rather good agreement with the overall synoptic pattern. Broad mid level ridging to start out this portion of the forecast is expected to transition to broad troughing towards next weekend. At the surface, a large high pressure should be over southern New England Tuesday into Wednesday. This high pressure should be far enough offshore to permit a cold front to approach from the west on Independence Day, but it is a little complicated. The offshore high pressure may not permit this front to make it all the way into, or through, our region before being driven back north. Dry weather with near to slightly above normal temperatures Tuesday and Wednesday. Increasing risk for some showers and isolated thunderstorms Wednesday night into Thursday. Also expecting increased humidity with above normal temperatures Thursday into Saturday. The timing and coverage of these showers and thunderstorms remains the most uncertain aspect of this forecast. With many outdoor plans for the holiday, it would be best to monitor the forecast over the coming days. This uncertainty lingers late this week. As such, stayed close the NationalBlend solution for now, as it is ensemble-based. This meant a broad chance for showers and thunderstorms Friday into Saturday, but am not expecting it to be raining this entire time. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...Moderate confidence. Conditions improve to VFR for most by roughly 13-16Z. The only exception is for PVD-OWD where MVFR to LIFR stratus lingers. May have a few showers/storms exiting the Cape and Islands during this timeframe. Another round of showers/storms expected to materialize roughly 17-19Z across the interior. These will push southeastward toward the south coast by the evening. Strong to severe storms possible, which if impacting a terminal would bring the risk of damaging wind gusts and conditions briefly deteriorating to MVFR to LIFR. Have handled this risk with a PROB30 for all terminals, except across the south coast where am uncertain on how quickly the line weakens given the stable boundary layer as the marine layer will be in place. For now utilized a VCTS for this. Will have SW winds at 101-5 kts with gusts of 20-30 kts. Winds shifting to the W/WNW toward the evening across the interior. Tonight...Moderate confidence. MVFR to IFR with showers/t-storms in the evening along the south coast, then improving to VFR overnight. Wind shift to NW 00-06Z. Speeds of 5-10 kts with gusts diminishing toward 03-06Z, but will then increase to 15-20 kts toward daybreak. Monday...High confidence. VFR to start, but as CU develops will have borderline MVFR/VFR cloud bases. Should see hit or miss showers/storms developing, which could potentially produce some small hail. Winds out of the NNW to N at 5-10 kts with gusts of 15-20 kts. KBOS TAF...Moderate confidence in TAF. Will improve to VFR roughly 15-17Z. Scattered showers/storms developing 19-23Z and could bring strong to perhaps damaging winds and brief heavy downpours. Improving to VFR as showers/storms move toward the south coast by the evening. S to SW winds at 10-15 kts with 20-25 kt gusts. Winds shifting to the W/WNW during the evening once the showers/storms slide through. KBDL TAF...Moderate confidence in TAF. Will have increasing chances of showers/storms this afternoon/evening, some of which could bring strong to perhaps damaging wind gusts. Have handled with a PROB30 from 17-23Z. These storms moves S/SE of the terminal during the evening and will fairly quickly improve to VFR. Winds out of the S at 5-15 kts. Will shift to the W/WNW this evening in wake of shower/storm activity. Could have some 15-20 kt gusts this afternoon (not associated with the thunderstorms). Outlook /Monday Night through Thursday/... Monday Night through Tuesday Night: VFR. Wednesday: VFR. Breezy. Wednesday Night: VFR. Breezy. Slight chance SHRA. Independence Day: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Today through Tonight...High confidence. Gusty S to SW on today with gusts to 25 kt at times. SCA continued for all waters. Winds shift to NW tonight as the cold front pushes through. Fog lingers today, especially across the south coastal waters due to increasing dew points. A few t-storms possible over the waters this morning, then more showers and t-storms moving across the waters late Sun and Sun night. This later activity has the potential to still be strong to severe producing strong wind gusts. Monday...High confidence. Winds out of the N to NW. Will see speeds of 10-20 kts earlier in the day diminishing to 5-10 kts late as high pressure builds in. Seas diminishing to 2-4 ft late. Outlook /Monday Night through Thursday/... Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday through Wednesday: Winds less than 25 kt. Wednesday Night: Winds less than 25 kt. Slight chance of rain showers. Independence Day: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 AM EDT this morning for ANZ230- 236-251. Small Craft Advisory until 8 PM EDT this evening for ANZ231- 250. Small Craft Advisory until 11 PM EDT this evening for ANZ232>235-237. Small Craft Advisory until 6 AM EDT Monday for ANZ254>256. && $$ SYNOPSIS...Belk/BL NEAR TERM...KJC/BL SHORT TERM...BL LONG TERM...Belk AVIATION...Belk/BL MARINE...Belk/BL