Area Forecast Discussion
Issued by NWS Boston, MA

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700 FXUS61 KBOX 231750 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 150 PM EDT Mon Apr 23 2018 .SYNOPSIS... High pressure remains in control through Tuesday bringing dry weather with mild daytime temperatures. Low pressure will pass through New England late Wednesday and bring showers to the area. Another weak low pressure center may bring another round of showers to the area Friday night. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 145 PM Update... Dry high pressure overhead, with abundant sunshine, light winds and local sea breezes ongoing. Forecast highs look on track, mainly in the 60s except cooler highs in the 50s near the shoreline. Early morning discussion follows... High pressure remains in place over Southern New England. Expect lots of sunshine and light wind. Mixing again should reach to at least 850 mb and possibly higher than 800 mb. Note that inland mixing yesterday reached above 750 mb. Temps at 850 mb support max sfc temps of 62-64F, while 800 mb temps support 65-68F. We tend toward the higher values inland, especially in the traditional hot spots... Hartford-Springfield and parts of the Boston Metrowest. The light flow will support sea breezes along the coast, which will keep temps cooler along the coast. Will maintain max temps in the 60s inland and 50s coast. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... The high pressure remains overhead tonight, then shifts off to the east Tuesday. This should maintain clear skies tonight, and sunshine through high clouds Tuesday. Radiational cooling tonight should again allow temperatures to fall into the 30s, with the usual cold spots around freezing or a little below. With the surface high moving offshore, Tuesday will feature a developing south to southwest wind which will allow a sea breeze only along the south-facing coasts. The ridge will maintain dry air through the day. High level moisture fields do indicate increasing clouds during the mid to late afternoon, but late enough to allow sunshine through the day. Mixing will be a little shallower, but temps at all levels will be increasing. So temperatures Tuesday should again climb into the 60s except 50s along the South Coast. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Showers Wednesday into Wednesday night * Another round of showers possible Friday night and late Saturday Discussion... Multiple short wave energy systems will be affecting our area during this forecast period. A southern stream short wave trough and surface low will affect southern New England late Tuesday night into Wednesday night. This system begins to phase with a northern stream short wave as it passes through our region. Still another southern stream short wave and an associated weak surface low pressure center may affect the area around Friday night, but consistency in handling this feature among the medium range models is low. This system currently looks as though it will will run a little ahead of an approaching northern stream trough which then becomes a dominant feature with possibly cooler temperatures for the rest of the weekend. There is a signal for a long wave pattern change leading to higher heights and warmer temperatures across our region after this forecast period. Tuesday night to Wednesday night... The 00Z operational model runs have converged some on timing for the southern stream short wave energy during this time period. As the mid tropospheric southern stream short wave begins to phase with the northern stream short wave, the surface weather pattern may take on the look of a surface low that moves north up the mid Atlantic coastal plain and ends up with the look of a triple point low passing over or near central New England. Suspect that cooler air will be somewhat resistant to an approaching warm front due to onshore maritime polar air being fed on E to SE winds. Anticipate rainfall will be showery but confident that all areas will measure. PWATs look to crest about 1.2 to 1.4 inches, and a 50 to 60+ knot low level SE jet at 925 mb will help focus that moisture over southern New England. There also looks to be some elevated instability Wednesday evening with embedded convection a possibility. This is reflected on the latest GFS run with mid level lapse rates of 6.5 to 7 degrees C/km as well as other typical instability indices such as Showalters a little below 0, total totals of 50+ and K indices above 30. As far as QPF, may see some upslope enhancement given the nose of the SE low level jet across the Worcester hills and/or east slopes of the Berkshires depending upon the exact track of the low level features. Suspect that rainfall amounts could end up higher than model output due to subtropical moisture source, strength of the low level jet, and some convective enhancement but variable due to the showery nature. A preliminary estimate would be a fairly widespread .5 to 1 inch amounts with locally higher spot amounts of 1.5 to possibly 2 inches. Thursday through Friday... Except for possibly widely scattered late Thursday afternoon or early evening instability showers over the NW higher terrain, it looks dry Thursday afternoon through at least most of Friday. Confidence has increased of at least most of daytime Friday remaining dry. Friday night system... The second southern stream short wave upper trough with a weak surface low passing along or off the coast may produce showers across at least some of southern New England, but confidence is low due to poor model consensus on this feature. The new 00Z ECMWF keeps this system far offshore, although some of its ensemble members have it passing very near SE New England. Thus, given the uncertainty, we will just indicate low chance POPs for now. There is also some uncertainty as to how much interaction, if any, with an approaching northern short wave trough. Rest of Weekend... A cold front associated with the northern stream trough may result in scattered showers late Saturday, but there`s no real model consensus on that now so confidence is low. That northern stream short wave trough brings cooler temperatures to southern New England for the latter half of the weekend into Monday of next week. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High confidence. This afternoon...VFR with clear skies and excellent visibility. Local sea breezes persist thru the afternoon. Tonight...VFR with clear skies and excellent visibility. Tuesday...VFR. Prevailing wind out of the SSW, initially light enough where a sea breeeze may develop along east coastal MA for a time. Then as winds aloft strengthen somewhat during the afternoon, the winds along east coastal MA should shift back to the SSW. Tuesday night...VFR to start. Becoming mainly MVFR after midnight, with local IFR possible towards daybreak. SHRA spreading into the area mainly after midnight. KBOS Terminal...High confidence in TAF. Sea breeze this afternoon, with a sea breeze probable for late morning into early afternoon Tue. KBDL Terminal...High confidence in TAF. Outlook /Wednesday through Friday/...High confidence. Wednesday: Mainly IFR with SHRA and areas BR. Areas of MVFR conditions possible during the afternoon. Potential for LLWS. Becoming windy along the coast with local gusts to 30 kt during the afternoon. SHRA and areas BR. Wednesday Night: Mainly MVFR, with local IFR possible. Windy with gusts to 30 kt in the evening. SHRA diminishing overnight. Isolated tstms during the evening. Thursday: Mainly VFR, with local MVFR possible over higher terrain. Breezy. Slight chance SHRA over NW higher terrain. Thursday Night and Friday: Mainly 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. High pressure over the waters today with light winds, flat seas, and good visibility. The high moves off to the east tonight and Tuesday, with winds turning from the south by Tuesday, but remaining light. Only concern for recreational boaters is for sea breezes along the coastline today with 15-20kt gusts and choppy seas near shore, especially when opposing outgoing tide. Outlook /Tuesday Night through Friday/...High confidence. Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Local visibility 1 to 3 nm. Wednesday: Small Craft Advisory winds possible with gusts up to 30 kt possible. Low risk of gale force gusts, especially south and east of Nantucket. Areas of rough seas above 5 feet, especially over the exposed south coastal waters. Rain showers, areas fog. Visibility 1 to 3 nm. Wednesday Night: Small Craft Advisory winds possible with gusts up to 30 kt. Low risk of gale force gusts east of Cape Cod. Rough seas up to 13 ft over the outer coastal waters. Rain showers. Thursday: Winds less than 25 kt, but areas of seas at or above 5 feet. Thursday Night: Winds less than 25 kt. Local rough seas around 5 feet. Friday: Winds less than 25 kt. Areas of seas approaching 5 feet over the outer south coastal waters. Slight chance of rain showers late. && .EQUIPMENT... The NOAA Weather Radio transmitter serving Providence is out of service due to phone line problems. The phone company has prioritized the issues and is working to get the weather broadcast back on the air as soon as possible. The transmitter serving Hyannis is back in service. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Thompson NEAR TERM...WTB/NMB/Thompson SHORT TERM...WTB LONG TERM...Thompson AVIATION...WTB/NMB/Thompson MARINE...WTB/Thompson EQUIPMENT...Staff

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