Area Forecast Discussion
Issued by NWS Boston, MA

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786 FXUS61 KBOX 180552 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 152 AM EDT Tue Jun 18 2024 .SYNOPSIS...
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Dangerous heat and humidity arrive Tuesday away from the south coast and then peaks in the Wednesday through Friday time frame. Near to record breaking high temperatures are possible. The main threat for scattered showers and thunderstorms will be Friday into the weekend, but specific timing remains uncertain. We do expect the excessive heat to break this weekend, but it will remain very warm and humid.
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&& .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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2 AM update... Convection/showers to our west across central PA/NY continue to weaken with cloud tops warming, in response to mid/upper level ridge building across the area and boundary layer stabilizing. Thus, not expecting this activity to impact SNE early this morning. A warmer night ahead than previous nights, with lows in the 60s. Although, humidity not oppressive with dew pts only in the upper 50s and lower 60s. Could be some patchy fog along the south coast, where surface dew pts and cooler SSTs are within a few degs of each other. Otherwise, a tranquil, warm June overnight.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... Tuesday... * Dangerous heat and humidity ramps up away from the south coast * Low risk for an isolated t-storm in the interior Very anomalous upper level ridge builds over New Eng. In fact, 500 mb heights are maxed out for this time of year relative to CFSR climatology. 925 mb temps increase to 24-26C in the interior which will translate to highs low-mid 90s away from the south coast with hottest temps in the CT and Merrimack valleys. SW flow will keep it cooler across RI and SE MA with highs mainly mid-upper 80s while the immediate south coast and Cape/Islands will be upper 70s to lower 80s. Dewpoints will be climbing into the mid-upper 60s with some 70 degree dewpoints in the CT and Merrimack valleys where heat indices are expected to reach the low 100s. Elsewhere, away from the south coast heat indices will reach mid-upper 90s. We converted the Excessive Heat Watches for Tue to heat advisories and we trimmed the heat advisory across much of RI and SE MA where heat advisories will fall short of criteria. The heat and humidity will result in moderate instability developing with SBCAPES 1000-2000 J/kg. However, forcing is rather weak with ridging in place and there may be a cap in place. HREF max updraft forecast is suggesting isolated to widely scattered convection developing in the afternoon/evening across interior MA so can`t rule out a few showers or a t-storm but areal coverage will be limited and any storms will be short lived as deep layer shear is quite low. Continued slight chc PoPs across interior MA. Tuesday night... Any convection will dissipate in the evening, otherwise looking at mainly clear to partly cloudy skies. Lows will be mostly in the mid- upper 60s. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Key Points... * Dangerous Heat & Humidity continues through Fri * Near Record Highs with Heat Indices from 98 to 106 degrees * Main threat for scattered showers/t-storms is Fri into the weekend * Excessive Heat breaks this weekend...but it still will be humid Details... We continue to beat the drum regarding an extended stretch of dangerous heat and humidity which will stretch through the end of the week. Guidance continues to be in excellent agreement that the peak of the heat occurs Wednesday and Thursday. Relief won`t come quickly, though, as it will be a more gradual easing of high temperatures from Friday to the weekend. The culprit is a very anomalous ridge of high pressure (sinking air which heats the airmass) overhead. In fact, as the previous forecaster mentioned, the magnitude of this ridging (600 dm) is rarely seen in southern New England which signals the potential for record breaking heat. The forecast hasn`t changed much in the last 24 hours, with highs projected to peak on Thursday at or just over 100F in the warmest locations like the CT Valley and Merrimack Valley. Making the situation more dangerous is the moist airmass in place with dewpoints in the upper 60s and low 70s making it feel over 105F in spots. Thus, an Excessive Heat Watch continues for these locations, with a Heat Advisory elsewhere. Don`t focus on the headline, however, as it will be dangerously hot regardless. Something that will exacerbate the hazard is that we`ll get little relief at night, only cooling off to the upper 60s/low 70s. Expect mostly dry weather for Wed and Thu but a few scattered t-storms are possible. Friday into the weekend we get a little relief, but the real airmass change won`t come until a cold front arrives around Monday night/early Tuesday. That`s when dewpoints will finally drop back toward the 50s. In the meantime, ahead of this cold front and its associated shortwave the airmass will remain largely unchanged Sat- Mon with highs in the 80s each day. The approaching shortwave will also increase the chance of showers and thunderstorms each day starting on Friday. Too soon to nail down timing details but unsettled weather looks more likely through Monday. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 06z TAF Update... Through 12z...High confidence for VFR, light SSW winds 5-10 kt and dry weather. After 12z...High confidence for VFR, SSW winds 10-15 kt, gusts up to 20 kt at times along the south coast and Islands. Low risk for an isolated shower or T-storm this afternoon across western-central MA. Low to moderate risk of brief seabreeze 15z-19z along eastern MA coast, including KBOS, with winds become SE. Tuesday night...High confidence for VFR, S-SW winds 5-10 kt along with dry weather. Wednesday...almost a rinse and repeat forecast, VFR, mainly dry weather along with SW winds 10-15 kt with gusts up to 20 kt at times. KBOS TAF...High confidence in TAF. Only wrinkle is duration of seabreeze today. Winds remain below 10 kt thru the column until about 18z, then increase steadily after 18z. Hence, a southeast seabreeze likely 15z-19z approximately, then shifting back to SSW thereafter. KBDL TAF...High confidence in TAF. Outlook /Wednesday Night through Saturday/... Wednesday Night through Thursday Night: VFR. Breezy. Friday through Friday Night: VFR. Chance SHRA, isolated TSRA. Saturday: VFR. Slight chance SHRA, isolated TSRA.
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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. Today: Continued clear conditions with increasing southerly winds at 10-15 knots, gusting up to 25 knots across the near shore waters along the south coast. Opted to hoist a short-fused small craft advisory for these waters and seas may be choppy. Seas 2-4 ft Tonight: Clear conditions with slowly decreasing southwest winds 10-15 kts. Seas 2-4 ft. Tomorrow: Clear conditions with increasing southerly winds gusting 20-25 knots once again. Seas 2-4 ft Outlook /Wednesday Night through Saturday/... Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Juneteenth: Winds less than 25 kt. Wednesday Night through Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers. Friday through Saturday: Winds less than 25 kt. Slight chance of rain showers, isolated thunderstorms.
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&& .CLIMATE... All Time June High Temperature Record (Daily) BOS - 100 F (06/06/1925, 06/26/1952 and 06/30/2021) BDL - 100 F (06/26/1952, 06/30/1964) PVD - 98 F (06/25/1943, 06/15/1945 and 06/30/1945) ORH - 98 F (06/26/1952) Most recent day of 100 F (or greater) BOS - 100 F (07/24/2022) BDL - 100 F (07/21/2019) PVD - 100 F (07/28/2020) ORH - 102 F (07/04/1911)* * For ORH this is the only 100+ degree day in its period of record. Daily High Temperature Records... June 18th BOS - 94 F (1907 and 1929) BDL - 95 F (1957 and 1994) PVD - 94 F (1929) ORH - 93 F (1907 and 1929) June 19th BOS - 96 F (1923) BDL - 95 F (1995) PVD - 94 F (1923) ORH - 93 F (1923) June 20th BOS - 98 F (1953) BDL - 97 F (2012) PVD - 95 F (1941) ORH - 93 F (1953) June 21st BOS - 96 F (2012) BDL - 96 F (1953 and 2012) PVD - 96 F (1941) ORH - 91 F (Multi-Years) Daily Warmest Low Temperatures Records... June 18th BOS - 72 F (1929) BDL - 70 F (1905) PVD - 72 F (1929) ORH - 69 F (1905 and 1994) June 19th BOS - 73 F (1995) BDL - 72 F (1929) PVD - 70 F (1995) ORH - 72 F (1929) June 20th BOS - 78 F (1931) BDL - 74 F (1931) PVD - 74 F (1931) ORH - 72 F (1931) June 21st BOS - 80 F (2012) BDL - 73 F (1923 and 2012) PVD - 75 F (2012) ORH - 74 F (1923 and 2012) && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for CTZ002. Excessive Heat Watch from noon EDT today through Friday evening for CTZ002-003. Heat Advisory from noon today to 7 PM EDT this evening for CTZ002-003. Heat Advisory from noon today to 7 PM EDT Friday for CTZ004. MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for MAZ005>007-010>012. Excessive Heat Watch from noon EDT today through Friday evening for MAZ003-005-006-010-011-013-014. Heat Advisory from noon today to 7 PM EDT this evening for MAZ003-005-006-010-011-013-014. Heat Advisory from noon today to 7 PM EDT Friday for MAZ002- 004-007>009-012-015-016-026. Heat Advisory from noon Wednesday to 7 PM EDT Friday for MAZ017>019. RI...Heat Advisory from noon today to 7 PM EDT Friday for RIZ001. Heat Advisory from noon Wednesday to 7 PM EDT Friday for RIZ002>005. MARINE...None.
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&& $$ SYNOPSIS...KJC/BW NEAR TERM...Nocera SHORT TERM...KJC LONG TERM...BW AVIATION...Nocera/BW MARINE...KJC/BW CLIMATE...BL