Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 141331 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 931 AM EDT Mon May 14 2018 .SYNOPSIS... Weak high pressure yields dry and seasonable weather today and tonight. Some showers and thunderstorms are possible with a frontal passage Tuesday. The front pushes south Wednesday with a drying trend that develops from north to south and lingers into Thursday. However another round of showers and thunderstorms with locally heavy rain likely returns late Friday into the weekend. && .NEAR TERM /UNTIL 7 PM THIS EVENING/...
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930 AM update... Low clouds beginning to erode across interior MA. Clearing trend will continue and expand southward into the afternoon as flow becomes SW and low levels dry out. Expect increasing sunshine from north to south through the afternoon, but stratus may linger well into the afternoon over the islands. Highs expected to reach the low/mid 70s away from the south coast. Temps mainly in the 60s near the south coast where SW flow will keep it cooler along with more cloud cover. Weak sea breezes may also develop along the east coast of MA this afternoon.
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&& .SHORT TERM /7 PM THIS EVENING THROUGH 6 PM TUESDAY/... Overnight... The return flow experienced through the day continues to lead to a slow rise in lower lvl dwpt values. With continued onshore flow during the overnight hours, any offshore stratus and fog should once again shift onshore. With dwpts in the low-mid 50s by the afternoon for crossover temps, there is a risk for some patches of dense fog as this marine layer shifts onshore. Best chances are SE MA/RI. Otherwise, milder min temps, limited to the mid and upper 50s. Will need to monitor upstream convection, as early AM lapse rates will steepen, potentially allowing for early AM convection to fire to the W and move into S New England. It`s a low risk but something to watch. This supports previous forecasters thinking. Tue... Weakening cold front timing is now generally well agreed upon between deterministic guidance, bringing it into S New England during the mid day-evening time frame. Once again, early AM convection or remnants may limit this, but the timing of this front does allow for afternoon destabilization, especially as guidance supports low lvl moisture pooling, with PWATS potentially above 1.75in (nearly 3 std deviations above normal) and sfc dwpts reach the mid 60s even with mixing. With a focus for lift in the front, and mid lvl lapse rates of 6.0-6.5C/km and low lvl moisture yielding MU CAPE values between 1000-1500j/kg per latest GEFS/SREF plumes. This is coincident with 40+ kt of 0-3km bulk shear during the mid day-evening time frame. Therefore, assuming the early convection or remnants thereof do not limit the destabilization, there is a risk for thunderstorms, and given the high shear and modest CAPE. Severe weather is possible as well. Given the shear is strong but uni-directional, clustered storms are the supported mode, with a risk for strong winds. The high pwats and warm cloud layers greater thank 10kft support HPE with a risk for heavy rain. Corfidi vector magnitudes are rather high, however, so the storms should remain relatively progressive. Overall something to watch for Tuesday, with a particular focus along or S of the Mass Pike given current timing. However, portions of SE MA and RI may be limited by the onshore flow and marine layer influence. With some sunshine, temps could approach 80 across the interior as H85 temps warm to an average of +14C. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Scattered Showers and T-storms with locally heavy rain and gusty winds Tuesday evening * Drying trend develops Wed from north to south and likely continues into Thu * Showers and T-storms likely return late Fri into next weekend along with the risk for heavy rain/tropical downpours Tuesday night... Scattered showers and thunderstorms likely Tue evening and short wave and attending surface front sweep across the region. With surface dew pts in the mid 60s along with PWATs approaching 2 inches locally heavy rainfall is possible. In addition, given strong wind fields aloft and marginal instability a few strong T-storms are possible in the early evening. Wednesday... Models differ on timing of frontal passage given upstream jet energy and possible secondary wave development on the front, delaying its seaward push. Thus risk for showers Wed morning with highest probability south coast. Then the threat of rain diminishing in the afternoon from north to south. Much cooler behind the front as post frontal airmass will feature easterly flow off the chilly ocean. Thus highs only in the mid to upper 50s across eastern MA, gradually warming westward into the upper 60s in the CT river valley. Thursday... Good model agreement among ensembles and deterministic guidance that boundary will shift far enough south for deep layer dry air to overspread the area from north to south. Thus expectation is for mainly dry weather and could turn out to be a fairly nice day. Warmer than Wed as blyr winds become west ahead of next northern stream trough and associated surface front approaching from the northwest. This results in temps jumping back into the 70s to perhaps 80 degs in the CT river valley. Friday/Saturday/Sunday... Ensembles and deterministic guidance beginning to converge on Low pressure currently over the Gulf of Mexico taking on tropical characteristics and moving up the eastern Seaboard late Fri into next weekend. 850 mb and 925 mb southerly wind anomalies on the order of +1 to +2 standard deviations along the eastern seaboard, which advects a tropical airmass with PWATs increasing to +3 standard deviations above climo ontop of SNE next weekend. This is impressive for model guidance at this time range, indicating good ensemble clustering. Thus risk for at least scattered showers and thunderstorms with tropical downpours. && .AVIATION /13Z 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/... 11z update... Current IFR/MVFR conditions will lift/improve to VFR approximately 15z-18z from northwest to southeast. Thus not much change from previous TAFs. Earlier discussion below. ================================================================= Through 15Z...Moderate confidence. Mix of MVFR/IFR in some lower CIGs and fog. These burn off after 11Z, lingering through 15Z along S coastal locations. Mainly S-SE flow but light. Rest of today...High confidence. Predominantly VFR with S-SW flow. Fog and IFR CIGS begin to filter back north mainly after 21Z. Winds around 10 kt. Tonight...Moderate confidence. Low CIGS and fog with IFR or even LIFR conditions may make it as far N as the Mass Pike during the overnight. VFR elsewhere. Low risk for an early AM shower. Tuesday...Moderate confidence. Lingering IFR conditions once again gradually dissipate through morning. Risk for afternoon TSRA/SHRA especially S of the Mass Pike. Heavy rain/gusty winds possible. KBOS Terminal...Moderate confidence in TAF. Timing of lower categories somewhat uncertain this morning. Also, low risk for an afternoon sea breeze. KBDL Terminal...Moderate confidence in TAF. Outlook /Tuesday Night through Friday/...Moderate confidence. Tuesday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, patchy BR. Wednesday: Mainly VFR, with areas MVFR possible. Breezy. Slight chance SHRA. Wednesday Night: Mainly VFR, with local IFR possible. Slight chance SHRA. Thursday: VFR. Thursday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA. Friday: Mainly VFR, with areas MVFR possible. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... Through Tuesday night...High confidence. Winds remain below 20 kt today and tonight out of the S and SW, seas too remain below 5 ft. The primary issue today and tonight is the risk for some lingering fog with reduced vsbys, especially tonight. W-SW flow may approach 25 kt near shore late Tue and the persistent W-SW flow could lead to wind waves offshore 5 ft. Will need to monitor for some small craft advisory conditions. Also, afternoon/evening thunderstorms are also possible. Outlook /Tuesday Night through Friday/...Moderate confidence. Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, chance of thunderstorms, patchy fog. Local visibility 1 to 3 nm. Wednesday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers. Wednesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/Doody NEAR TERM...KJC SHORT TERM...Doody LONG TERM...Nocera AVIATION...Nocera/Doody MARINE...Nocera/Doody

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