Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221503 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1103 AM EDT Mon Jul 22 2019 .SYNOPSIS... A stationary front will remain off the south coast today. Two waves of low pressure will develop and move along the front, bringing showers and thunderstorms this afternoon and evening, followed by a second around Tuesday. The rain will fall heavily at times. The low and cold front move offshore Tuesday night, followed by a period of more comfortable rain free summer weather from Wednesday through the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1100 AM update... Minor adjustment to the rainfall timing this morning. Scattered rain showers will continue this morning into the afternoon, when the heavier rain and better thunderstorm chances arrive. At this time the stalled front is sitting offshore to our south and is expected to slowly move north today as a warm front, reaching generally the CT/MA border. This would put CT, RI, southeast MA in the warm sector with dewpoints reaching the upper 60s to low 70s. It brings with it increased instability on the order of 1000-2000 J/kg MUCAPE, greatest along the south coast. Shear parameters are also good for some organization and prolonged updrafts. 0-6km bulk shear values will be around 40-50kt in the vicinity with low level shear (0-1km) of 25-35kts. Hi-res guidance has progged the area of greatest updraft helicity (a good indicator of severe weather) over CT into RI, and this is where we would expect the best chance of strong to severe storms this afternoon/evening. The 12Z run has even further increased this high helicity potential, further increasing confidence that we may see rotating storms. The best chance of severe storms will likely be a window from 6pm to midnight, but can`t rule out a storm outside of that. The greatest risk looks to be from damaging winds, but given the amount of spin in the lower atmosphere, the tornado threat is non-zero. Today... Slight Risk for Severe Thunderstorms This Afternoon... Short range models were in good agreement in keeping the front pretty much stationary across the southern waters through most of the day. So will see bands of mid and upper clouds crossing the sky through around midday. Expect temps to rise through the 70s to lower 80s in the normally warmer areas, but could hold in the lower-mid 70s across the higher terrain. As an H5 long wave trough digs S across the Great Lakes during the day, winds will back to SW across the northeast U.S. Also noting a developing short wave moving into western areas between 21Z and 00Z Tue. Also noting another good influx of deep layer moisture as PWATs increase to around 2.2 inches by this evening in the mid layer SW wind flow. K indices increase to the lower to mid 30s across the entire region, as well as the TQ values rising to the upper teens to around 20 by this evening. Even with the front to the S of the region, will still see dewpoints rise through the 60s, and will reach to around 70 across N CT/RI into SE Mass during the afternoon. With the first of two low pressure areas developing along the stationary front, which will waver toward the region late in the day, will see showers and scattered thunderstorms to develop from W-E this afternoon. With these factors, along with the potential for vertical shear to develop as the short wave approaches from the W later this afternoon, could see multicells develop that may produce severe conditions. SPC has placed N CT into RI under a Slight Risk for Severe Thunderstorms this afternoon. The primary threat will be for strong to possibly damaging wind gusts, but can not rule a slight chance for hail. With increasing PWATS, could also see locally heavy rainfall develop in any convection. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Slight Risk for Severe Thunderstorms continues into This Evening... Tonight... Showers and thunderstorms will continue through around 02Z-03Z, some of which may still produce strong winds and heavy rainfall, should start to see these storms shift slowly S overnight. Winds shift to light N across inland areas as another boundary shifts across the region with the exiting short wave. The short wave will shift E, but precip will linger mainly across central and southern areas through the night, though the convection should weaken. Temperatures will bottom out in the 60s. Tuesday... Another short wave approaches during the day as PWATs remain in the 2 inch range. More instability will shift across the region, though not quite as strong as today, but could see another round of heavy rainfall as showers and scattered thunderstorms return, especially across central and southern areas. May see conditions slowly improve across western areas during the afternoon, but scattered showers may linger. Expect high temps will be in the 70s. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Big Picture... Upper ridge builds over the Western USA while a trough digs over the Eastern USA. We note the models are substantially slower with the eastern trough axis, now moving through New England on Thursday, rather than the Wednesday passage in yesterday`s runs. It is also slower to move out, lingering until Saturday morning. Strong low level jet leading the trough moves over Southern New England early in the week, then shifts well offshore Tuesday night. Heights at 500 mb over our area will be lower than seasonal normal heights Wednesday, but trend to normal Thursday and above normal by the weekend. Based on this we would expect seasonably mild temperatures Wednesday, trending above normal Friday through Sunday. Model mass and thermal fields show some differences Friday and Saturday, but are otherwise similar through Sunday. Forecast confidence is between moderate and high. Concerns... Tuesday night... Trough digging over the Great Lakes with a progressive axis and a 135 kt upper jet. Stalled cold front remains just south of New England. Right entrance region of the upper jet is over Southern New England. This generates the last of a series of waves on the front Tuesday night, and that wave moves past us by morning. If this wave is close enough, it may generate a few showers near the South Coast, especially over the Cape and Islands. The remainder of the region should be dry. Cloud-level moisture lingers during the night, so expect mainly cloudy skies. Depending on the position of the front offshore, there may be some clearing in Western MA/Western CT overnight. Wednesday through Sunday... Upper trough and a -14C to -16C cold pool lingers overhead through Friday. High pressure builds in at the surface and aloft during the period. Meanwhile, model cross-sections show a near constant moist layer between 850 mb and 700 mb through Sunday. Synoptic lift is mediocre, but convective parameters are favorable on at least two days out of Friday-Saturday-Sunday. Currently looks like a dry period, but we will need to monitor that three-day period for potential showers/thunder. Mixing reaches to about 810 mb each day. Temps at 800 mb will be 6- 8C Wednesday-Thursday, warming to 12C by Sunday. The 850-mb equiv for each will be 11-13C Wednesday-Thursday, warming to 17C Sunday. So we expect max temps upper 70s and lower 80s Wednesday, trending to 85-90 Sunday. Dew points 55 NW to 65 SE Wednesday, trending to the low to mid 60s by the weekend. Expect min temps similar to or a little higher than the dew points. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...Moderate to high confidence. Today...VFR conditions early, but clouds fill in and bases lower later this morning and afternoon. Showers with isolated thunderstorms move in from W-E. Areas of MVFR-IFR conditions in any convection. Potential for strong wind gusts and heavy rainfall in any thunderstorms from about 18Z through the remainder of the region, mainly across N CT into RI. Light E-NE winds, becoming SE along the S coast during the afternoon. Tonight...Showers and a few thunderstorms linger into early tonight with areas of MVFR-IFR conditions. Showers linger through the night across central and southern areas, but conditions may improve to locally VFR after 06Z across N Central and W Mass but could still see scattered showers. Light S-SW winds becoming N after midnight. Tuesday...Showers remain across the region, with isolated thunderstorms possibly across NE CT/RI/SE Mass through midday. Could see heavy downpours as well. Conditions slowly improve across the N central and western Mass during the afternoon. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Tuesday Night through Friday/...High confidence. Tuesday Night: Mainly VFR, with areas IFR possible. Slight chance SHRA, patchy BR. Wednesday through Thursday: VFR. Thursday Night: Mainly VFR, with local IFR in Patchy BR. Friday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. Today...Light/variable winds become E around 10 kt on the eastern waters and light SE winds on the southern waters this afternoon. Dry conditions through mid morning, then showers and isolated thunderstorms move in across the western waters around midday, filling in over remaining wates this afternoon. Visibility restrictions in patchy fog. Seas 3 ft or less. Tonight...Winds become SW up to 15-20 kt this evening on the southern waters, but remain E-NE on the eastern waters early. Seas build up to 5 ft on the outer waters as well as BI and RI sounds, so Small Crafts have been issued. Expect visibility restrictions in patchy fog and locally heavy downpours. Showers through the night, but any thunderstorms should end from N-S during the night, though may linger on the southern waters. Tuesday...Expect N-NE winds up to around 10 kt. Seas remain up to 5-6 ft on the southern waters through the day. Visibility restrictions continue in showers and patchy fog, with local thunderstorms during the morning along with locally heavy downpours. Outlook /Tuesday Night through Friday/...High confidence. Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, patchy fog. Areas of visibility 1 to 3 nm. Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Local visibility 1 to 3 nm. Thursday: Winds less than 25 kt. Thursday Night: Winds less than 25 kt. Patchy fog. Local visibility 1 to 3 nm. Friday: Winds less than 25 kt. && .CLIMATE... Temperature and Dew Point marks Tied and Set this Weekend Saturday July 20 BOS High Min Temp for Date 80, Tied 80 in 1977 BOS High Min Temp 80, Tied for 6th for All-Time Minimums BOS High Dew Point for Date 77, Surpassed 76 in 2013 BDL High Min Temp for Date 77, Broke Record of 73 in 1977 BDL High Min Temp 77, Tied for 5th for All-Time Minimums BDL High Dew Point for Date 76, Tied 76 in 2005 PVD High Min Temp for Date 77, Broke Record of 76 in 1977 PVD High Min Temp 77, Tied for 25th for All-time Minimums PVD High Dew Point for Date 79, Surpassed 76 in 2013 ORH High Min Temp for Date 75, Broke Record of 72 in 1977 ORH High Dew Point for Date 73, Tied 73 in 1991 Sunday July 21 BOS High Min Temp for Date 83, Broke Record of 81 in 1991 BOS High Min Temp 83, Tied for 1st All-Time w/ Aug 2 1975 BDL High Min Temp for Date 74, Tied 74 in 1977 BDL High Temp of 100, Last 100 was on July 18 2012 PVD High Min Temp for Date 78, Broke Record of 77 in 1980 PVD High Min Temp 78, Tied for 7th for All-Time Minimums ORH High Min Temp for Date 73, Tied 73 in 1991 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 AM to 5 PM EDT Tuesday for ANZ235- 237. Small Craft Advisory from 2 AM to 5 PM EDT Tuesday for ANZ250- 254. Small Craft Advisory from 11 PM this evening to 5 PM EDT Tuesday for ANZ255-256. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/BW SHORT TERM...EVT LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT CLIMATE... is the U.S. government's official web portal to all federal, state and local government web resources and services.