Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 180537 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 137 AM EDT Thu May 18 2017 .SYNOPSIS... Bermuda high pressure will bring unseasonably warm to hot conditions through Thursday with increasing humidity. Warm weather continues Friday as a cold front moves through ushering in more seasonable conditions for Saturday. Another cold front approaches Sunday night and moves through late Monday, with showers likely. Another system may approach by the middle of next week with unsettled conditions. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 10PM update... Still a warm evening out there in spite of the mainly SKC conditions and weak flow. The rising dwpts in response to far penetrating S sea breeze will stay mainly in the low-mid 60s, helping to keep min temps from falling too much beyond the mid- upper 60s overnight. Still monitoring for potential for marine stratus/fog development offshore with the SW flow but guidance is currently too aggressive with its formation. May be more of a case of localized radiational fog with the higher dwpts. Will continue to monitor. Otherwise, expect a mild night. Previous Discussion... Expect SW winds linger near the shore and at higher elevations, but W-SW winds become light elsewhere. With the inflow of higher humidity air, expect dew points to climb into the lower 60s overnight. Cooling temps and rising dew points may lead to areas of fog developing overnight. The best chance will be along the South Coast and Islands, where the humid air will flow over water temps that are cooler than the dew points. This will create fog and stratus that will move ashore on the Islands and South Coast. Probably not too much further, until the winds turn more from due south. Based on expected dew points, min temps will be in the 60s with some 50s on the Islands. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/... Thursday... Bermuda high pressure in place through the day with southwest flow across Southern New England. This will keep us in the humid air with dew points remaining in the 60s except a few 50s where the wind is coming directly off the ocean. Expect mixing to again reach 800 mb, where temps of 13-14C are equivalent to 18-19C at 850 mb. Expect max temps of 90-95F, but again cooler where the wind comes off the water and buffers the temps to the upper 60s and 70s. A cold front in the Great Lakes moves into western and northern New York by evening. This will be an active day for convection to our west, but questions remain as to how far east that will reach during the day. Convective parameters become favorable over northern and western Mass and northern CT. Totals reach the low 50s, CAPE reaches 1000-2000 J/Kg, LI reaches -4 to -5. But moisture remains limited and subsidence dominant over our area. The best chance for any thunderstorms will be after 5 pm over Northwest Mass. Thursday night... The chance of thunder continues during the evening but then diminishes overnight. With a continued southwest flow the night will remain mild and humid with dew points in the 60s. We will again forecast 60s inland and 55-60 along the South Coast and Islands. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Another warm day for Friday * Sunny and pleasant this weekend * Showers likely with scattered thunderstorms Monday * Unsettled conditions possible by the middle of next week Overview... H5 long wave ridge pushes E Fri as short wave and associated cold front works SE out of central Canada Friday. Looks like most of the energy with this short wave remains over northern New England, so may see only light precip late Fri/Fri evening as main front crosses. Another large ridge builds into Hudson Bay, keeping dry NW flow in place this weekend across the northeast U.S. The ridge builds E over New England and Quebec by early next week while cutoff mid level low across Ontario shifts E. This system will bring another cold front across the region around the Monday timeframe. Beyond Monday, timing issues come into play amongst the model suite which will be dependent upon position and movement of cutoff H5 low pressure as it starts to expand across eastern Canada. There may be a short wave swinging across the northern tier around the base of the low, which could lead to unsettled conditions by the middle of next week. Details... Friday... Main cold front working SE during the day. Some question as to areal coverage for any precipitation along with light QPF amounts. GFS remains the driest model, while the GGEM and ECMWF increase low level moisture across N CT/RI/E Mass. Due to these discrepancies, have kept slight chance POPs for most areas, with the best shot across the higher terrain of NW and N central Mass. Decent instability hangs across S coastal areas, with K indices in the lower 30s and surface LIs around -1 so have mentioned isolated thunder there during the afternoon and evening hours. Noting decent low level mixing as H925 jet at around 25 kt moves across with the front. Could see gusts in the 20-25 kt range across the interior during the afternoon. Any leftover showers should end as the front pushes offshore Fri evening. Dewpoint fall through the 40s overnight, down to the mid- upper 30s across N central and W Mass. N winds will stir at around 10 mph, so probably won`t see radiational cooling except for a few deeper inland valleys. Temps should bottom out in the lower-mid 40s well inland, ranging to around 50 across the coastal plain. Saturday... Will be a beautiful day with mostly sunny skies as high pressure ridge builds across New England. N winds will shift to E-SE around the base of the ridge, but winds will drop off with mostly clear skies Sat night. Highs will only reach the upper 50s to mid 60s along the immediate coast with the onshore winds, ranging to the lower 70s across the CT valley. If winds drop off, and with mostly clear skies, winds could drop off and allow radiational cooling. Expect lows in the lower-mid 40s away from the coast, ranging to near 50 along the immediate shore. Will have to watch for even chillier readings if winds become calm. Sunday... The high pressure ridge shifts slowly E during the day. Some clouds may start to push in from the W during the afternoon as winds shift to south. A warm front may start to approach late Sunday or Sunday night, so may see some scattered showers approach mainly Sun night across the interior. High will range from the lower-mid 60s along the immediate S coast to the lower 70s well inland. Monday through Wednesday... Another low and associated cold front will work across the region on Monday. Noting some decent deep layer moisture into the system from down the coast along with some instability. Have mentioned thunder along with the good chance for showers. Some model signal of another secondary low developing down the coast as the front crosses. As the front shifts E, should see drier conditions on Tuesday with seasonable temperatures. Model solution spread comes into play for the Tuesday night- Wednesday timeframe in handling another possible system as it works out of the Ohio valley. At this point, bring in the chance for showers from W-E during the day. && .AVIATION /06Z THURSDAY THROUGH MONDAY/...
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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/... Through 22Z Today...High confidence. VFR in general, outside of a brief period of MVFR/IFR fog mainly across ACK this morning. W-SW Winds 10 kt with gusts 20-25 kt late this morning and afternoon. After 22Z through Friday morning...Moderate confidence. Risk for a mix of thunderstorms and showers moving from W to E 22Z (starting in W MA/CT) through 03Z to E MA/RI. Best chance for thunderstorms mainly W of the Worcester Hills, but still a non-zero risk further E. Heavy rain/gustier winds possible with any storms. Otherwise, mainly VFR overnight outside of showers/storms. But with lingering deck of clouds with CIGS 050-100. Weaker SW winds. Low risk for more fog mainly along S coastal MA/RI. Fri...Moderate confidence. Winds shift to the W, then NW through the day. Gusts 20-25 kt once again expected. Mainly VFR outside of some low clouds/fog which may linger along S coastal MA/RI. A few morning showers and isolated thunderstorms possible mainly focused S of the Mass Pike. Otherwise, gradual improvement through the day. KBOS TAF...Generally high confidence in TAF, one issue will be risk for late day/evening showers or thunderstorms and their exact timing. KBDL TAF...Generally high confidence in TAF, one issue will be risk for late day/evening showers or thunderstorms and their exact timing. Outlook /Friday Night through Monday/... Saturday...High confidence. VFR. N wind around 10 kt, shifting to E-SE by around midday then S by evening. Sunday...Moderate to high confidence. VFR. Increasing mid level cloudiness, then CIGS lower to MVFR across central and western areas after midnight. SW winds increasing to 10-15 kt Sunday night mainly along the coast. Chance of showers across western sections, moving E Sun evening. Isolated thunderstorms after midnight central and western areas. Monday...Moderate confidence. MVFR to local IFR early Monday, improving to VFR with local MVFR possible across the interior. SW winds gusting to 20-25 kt during the afternoon along the south coast. Showers with scattered thunderstorms from N-S during the day.
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&& .MARINE...
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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/...High confidence. Tonight... SW winds gusting to 20 kt this evening, then diminishing. Seas 4 feet or less. Low visibility developing in areas of fog, mainly over the south coastal waters overnight. Thursday... Southwest winds with gusts to 20 knots. There could be a few gusts near 25 knots in Boston Harbor and Narragansett Bay, but not enough to support a Small Craft Advisory. Seas will be 4 feet or less. Low vsbys in morning fog on the southern waters. Thursday night... Southwest winds less than 20 knots. The persistent southwest winds may be enough to bring 5-6 foot seas on the southern and southeast outer waters as well as RI Sound. Outlook /Friday Night through Monday/... Friday...Moderate confidence. SW wind shifting to W-NW, gusting up to 20 kt on the southern waters. Seas up to 5-7 ft, highest on the southern outer waters. A few showers, with an isolated thunderstorm possible during Fri, improving Fri evening. Saturday...Moderate to high confidence. N around 10 kt early, shifting to E to SE during the afternoon. Seas lingering around 5 ft on the outer waters, subsiding during the afternoon. Sunday-Monday...Moderate confidence. S winds early Sunday becoming SW. Gusts up to 20-25 kt, highest on the outer waters on Monday. Seas 1-3 ft Sunday, building to 3-5 ft Monday, highest on the outer waters. Chance of showers and/or thunderstorms Monday.
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&& .CLIMATE... Higher dew point air will mean a potential for record high minimum temperatures. Temperatures early Thursday morning are expected to be in the 60s, and 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...
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CT...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for MAZ004>007-010>023-026. RI...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for RIZ001>008. MARINE...None.
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&& $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...EVT AVIATION...Doody/EVT MARINE...WTB/EVT CLIMATE...

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