Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 181735 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 135 PM EDT Thu May 18 2017 .SYNOPSIS... The hottest day of the week is expected today with temperatures well into 90s away from the south coast. A thundershowers are possible late this evening across western and northern Massachusetts, then again Friday but farther south across Connecticut...Rhode Island and southeast Massachusetts. Not nearly as hot Friday but still warmer than normal. More seasonable pattern for the weekend into early next week. Dry initially then turning wet late Monday into Monday night ahead of a sweeping cold front. A brief reprieve Tuesday prior to more rain expected mid to late week along with cooler conditions. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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135 PM Update... Temps continue to soar this afternoon with record highs already set in Boston, Providence, and Hartford. Worcester should make it before too long. With plenty of sunshine, deep mixing, and westerly flow we should see temps rise another 3-4 degrees in most locations through late afternoon. Sea breeze has taken hold along South Coast, and even into Providence which has dropped back into upper 80s. This should erode later this afternoon and SW flow takes over, so Providence temps may be able to briefly rise again later today. Regarding storms tonight, starting to see good consensus among high-res models for activity reaching western New England 8-9 pm and weakening in RI/SE MA closer to 1-2 am. Still assessing latest guidance and will have more details by 4 pm.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Update: More of the forecast guidance is indicating a potential line of convection for the evening period. In coordination with the Storm Prediction Center there is a potential threat with any storms of wind gusts up to 40 mph. This may result in downed limbs, trees, and potentially power lines. Previous discussion... Will need to monitor convective threat as remnant convection firing in NY/QC, where higher moisture/CAPE/shear rest much of the day begins to slide well ahead of approaching cold front, within a slight pre-frontal trof apparent in meso-scale mass fields. CAPE profiles of 1000-1500j/kg sfc based don`t become apparent until after 22Z or 6PM local this evening mainly across the W as a plume of higher theta-e air and K-indices reach 25-30. This will be accompanied by convection already moving into portions of W-MA/CT, shifting E through the remainder of the evening. Meso-scale guidance has latched on to this thinking with overnight runs, so will increase to at least chance POPs from W-E 22Z-04Z, highlighting thunderstorms moving W-E, until the loss of the remaining sfc instability. Noting modest shear, mainly 30-40 kt 0-6km, however the remaining well mixed BL and inverted-V sounding suggests that with any storms that do move through late this evening there is a risk for stronger winds as the primary impact. Tornadic threat is minimal due to the mixed lower atmosphere/high LCLs. Low risk for some hail as CAPE profiles support 200-400j/kg in the classic -10C to -30C layer. This is supported by latest SPC SWODY1, which highlights general thunder category further E than previous outlooks, but keeps slight/marginal risk further W where highest shear/best daytime instability reside. Otherwise overnight, mild again as temps will struggle with enhanced cloudiness and cold front just entering the area during the early AM to dip lower than the mid-upper 60s across much of the region. Friday... Indications suggest cold front should be draped through central MA/CT and pushing into E MA/RI through the morning. Lingering moisture/instability, though much weaker than today suggest another round of showers/low risk for TS until the front fully clears the region by mid day. GFS continues to be on the dry side with this fropa, but would rather lean more heavily on ECMWF and mesoscale guidance which indicates at least a few showers/storms remaining, although much weakened compared to evening/overnight Thu. Gradual improvement however expected from NW-SE as the front clears. This will allow cold/dry advection aloft through the late morning and afternoon. H85 temps drop to between +6C and +10C by 00Z Sat, so will likely see highs limited to low-mid 80s even where afternoon sunshine is observed, but cooler elsewhere where clouds may struggle to fully clear. Still mild, but definitely not as warm as previous days. Another breezy day as the combination of mixing and the frontal passage will allow wind gusts to reach 25-30 mph at times as they shift from the SW to NW by late afternoon. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... */ Highlights... - Seasonable and dry for the weekend - Rain with embedded heavier showers for Monday - Dry on Tuesday - Wet for the mid to late week period, turning cooler */ Overview... Persistence of high pressure across the NE Pacific into the W CONUS promotes downstream troughing over the Central and E CONUS as energy is sheared equatorward. Indicated by -EPO trends, during the month of May into June, climatologically, below-average temperatures are observed across the N-Central and NE CONUS. Unclear is this longwave pattern alteration. Relatively weak MJO signals albeit transitioning across the Indian Ocean, perhaps supporting high pressure more over the Central Pacific. +WPO signals prior to -EPO signals signaling the enhancement of high pressure into the Gulf of Alaska. Maybe some of the influences to the cooler, long-term pattern. Some things, how- ever, are just unclear and will have to simply take the consensus trend at face value, that being, an active wet weather pattern with temperatures near- or below- seasonable. Details below. */ Details... Saturday... Pick of the weekend. High pressure building across the region behind the cold front now well offshore. Cooler airmass building in aloft with H85 temperatures around +4-6C with light winds at the surface allowing for sea-breezes along the shores. Mostly clear conditions, thinking seasonable temperatures with highs around the upper 60s to low 70s. Perhaps staying mild overnight. Light winds beginning to turn S as high clouds filter out of the S/W, lowering and thickening through the overnight hours. Lows in the 40s. Sunday into Monday... Anomalous yet efficient sub-tropical warm-rainfall setup, especially late Monday. Thinking Sunday remains dry only with increasing clouds over the region, lowering and thickening with time. Near-seasonable temperatures. Overnight, surge of warmer, moist air. Lows likely in the evening, then a non-diurnal trend into Monday. Low clouds, fog and/or drizzle as 60-degree dewpoint air surges N over cooler waters (especially along the S coast). Thinking dreary conditions linger Monday prior to +2 standard deviation of low level moisture influx into S New England ahead of a sweeping cold front late Monday into Monday evening. Efficient warm-rain process with deep layer moisture beneath roughly an 11 kft freezing level. Deep-layer ascent above convergent forcing plus negative showalters, in all progressive, expect widespread rain with embedded heavier showers dousing the region with around half to an inch of rainfall. Have to closely watch S/SW flow aloft as to its potential for mixing down to the surface. This in hand with boundary layer destabilization prior to a sweeping cold front. Thinking best thunder chances if any would be S/W CT, however low confidence. Will forego any mention of thunder in the forecast grids. Keep conditions near-seasonable given the anomalous S-moist inflow. Drying out after midnight into Tuesday morning with NW winds. Tuesday... A lull in the weather as agreed upon by ensemble means and consensus of operational guidance. Brief period of cooler air prior to warmer air surging back N prior to a follow-up midweek disturbance. Given proximity, thinking a mix of sun and clouds as temperatures hold to near-seasonable levels for late-May, upper 60s to low 70s. We should definitely see some sunshine, however thinking it will not be mostly clear. Wednesday into Thursday... Periods of wet weather and near-seasonable conditions as a series of mid-level disturbances rotate across the area subsequent of upstream enhanced ridging shearing energy equatorward. Low confidence with respect to outcome specificity. However, am confident that with time we`ll see the beginnings of cooler air aloft surging across the area and later maintaining thus prevailing below-average temperatures at the surface for the remainder of the week. The possible return of dreary, cool, damp conditions like that we saw during early May. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... 18z TAF Update...High confidence. W winds gusting to 20-25kt away from coast gradually back to SW and diminish late this afternoon. VFR through early tonight, then localized MVFR/IFR in scattered showers/storms starting in western New England 00z-02z, which should weaken as they reach RI/eastern MA 04-06z. Main concern is strong wind gusts (40kt) with storms in western New England. Also expecting patchy fog near South Coast, Cape, and Islands with local MVFR/IFR visibilities, which rapidly improve after sunrise Fri. Winds shift to W and then NW during day Fri. VFR outside of isolated showers/storm mainly focused south of Mass Pike thru mid afternoon. VFR Fri night. KBOS TAF...High confidence, including timing of winds to SW around 20z and timing of VCTS later tonight. KBDL TAF...High confidence, including timing of VCTS tonight. Outlook /Friday Night through Monday/... Friday night through Sunday night... VFR. N winds becoming light into Saturday, sea-breezes expected. Turning S into Sunday as CIGs lower and thicken. Possible MVFR by Monday morning. Monday into Monday night... MVFR-LIFR. Lowest conditions likely for the S-coast with potential fog impacts. Increasing S winds ahead of a line of widespread -RA with embedded RA/+RA late and into evening. Gusts up to 25 kts. Low risk TSRA. Possible LLWS impacts with S/SW winds 2 kft AGL 40-50 kts.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... 7 am update...No major changes to the forecast. Today...high confidence. Although winds near shore may gust out of the S-SW 20-25 kt, over the waters winds should remain generally 15-20 kt and with seas only 3-4 ft at their peak this morning, expect mainly quiet weather across the waters through the day today. Tonight into tomorrow...moderate confidence. The persistent S-SW winds continue into the overnight hours, still gusting 20-25 kt at times, while these remain below small craft thresholds, the persistence may allow seas to reach around 4-5 ft on the S ocean waters by late tonight. Am not fully confident in this, but a late small craft advisory may have to be issued for these seas. Winds will shift to the NW by late tomorrow afternoon over all the waters, with gusts closer to 25 kt possible. However, these may be short lived and given the uncertainty in whether seas will reach 5 ft by tonight, the need for a small craft advisory remains somewhat uncertain, so stay tuned to later updates. Outlook /Friday Night through Monday/... High pressure building across the waters beginning Friday night allowing seas to subside. Great boating weather over the weekend with light winds on Saturday with near-shore sea-breezes, while light S for Sunday. Waves 1-2 feet. Going into Monday, S winds on the increase with gusts up to 25 kts ahead of a cold front along with a widespread line of rain with embedded heavier showers late Monday into Monday evening. A low risk of thunderstorms. Clearing out after midnight into Tuesday morning though waves build 5 to 6 feet and are slow to diminish under NW winds. && .FIRE WEATHER... Although dewpoints early this morning are running in the low-mid 60s, late morning/afternoon drying could once again lower the dewpoints back into the 50s in some areas, yielding min RH values as low as 25-35 percent in some areas. W-SW winds this afternoon will also be gusty at times mainly as high as 25 mph. Temperatures will be very warm for mid may, mainly in the upper 80s to upper 90s away from the south coast. Therefore, marginally enhanced fire weather is possible through the late morning and afternoon period. With greenup, and a prolonged moist period last week into early this week, this will limit the risk somewhat. Note, there is a risk for thundershowers late this evening and early overnight, along with general rain showers which will limit the risk going into tomorrow. && .CLIMATE... Higher dew point air will mean a potential for record high minimum temperatures early this morning. Early Thursday night temperatures are expected to be in the 70s inland/60s coast. Dew points will not be at their extremes for May 18th. The historical extremes for the date are 67-69, and forecast values are 60 to 65. Record high minimum temps for Thursday May 18th: Boston.......63/1896 Hartford.....64/1906 Providence...62/1943 Worcester....63/1943 Record high temps for Thursday, May 18th: Boston.......91/1936 Hartford.....90/1936 Providence...92/1936 Worcester....90/1906 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert until 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert until 11 PM EDT this evening for MAZ004>007- 010>023-026. RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008. MARINE...None. && $$ SYNOPSIS...Doody/Sipprell NEAR TERM...Doody/JWD SHORT TERM...Doody LONG TERM...Sipprell AVIATION...JWD MARINE...Doody/Sipprell FIRE WEATHER... CLIMATE...

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