Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 240554 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 154 AM EDT Thu May 24 2018 .SYNOPSIS...
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Quiet and dry though morning, some spotty lower conditions associated with low clouds and fog where it previously rain and along the low coast. Thereafter, dry, warm and humid conditions return into Saturday ahead of an approaching cold front from northern New England, bringing scattered showers and isolated thunderstorms late Saturday into Saturday night. The front stalls south of the region, with patchy showers lingering through the remainder of the holiday weekend.
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&& .NEAR TERM /THROUGH TODAY/...
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130 am update... Remaining quiet. Quick glance, back door cold front slowing, stalling against W flow, convergent zone setting up over E MA. Dry air advection occurring, however over E/SE MA, higher dew- point air combined with antecedent rains, can`t rule out spotty dense fog as temperatures cool beneath mostly clear conditions. Meanwhile, several features linger offshore as discerned via GOES-16 night-time microphysics. Potential for low cloud features to expand as they progress SW, impacting Nantucket early on and potential E MA prior to sunrise. Low clouds either pushed off by mean wind or eroding into morning with daytime heating. Today... Again, back door cold front having come ashore, stalled over E MA up against W flow. With diurnal heating, convergence zone becomes well-defined, maintained by sea-breeze influences, push back into the interior. Airmass differential, likely mixing, scattered cloud decks atop the boundary layer. Gusty E winds more likely per mesoscale isallobaric wind response, whereas gusty W winds per mix-down of faster low-level winds. In both cases, potential for gusts up around 20 mph. Considerably drier W with 30-degree dewpoints, relative humidity down to 20 percent. Highs warming into the mid to upper 70s, whereas cooler, more moist E with onshore flow. Plenty of sunshine.
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&& .SHORT TERM /TONIGHT THROUGH 6 PM FRIDAY/...
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Tonight... Clear conditions, return light S flow, opportunity for radiational cooling with boundary layer decoupling early on before higher dewpoint air pushes N. Perhaps some low cloud and/or fog issues along the immediate S-shoreline of New England, otherwise leaning with coldest forecast guidance with lows upper 40s to low 50s. Friday... Mild day on tap with increasing humidity. High pressure SE, W/SW flow prevails. Warm air advection within low-mid levels, ridge builds. Up against N-stream cyclonic flow / Baffin Bay low, in the confluence of which low-level SW winds increase. Daytime heating, boundary layer mixing, mix-down of drier air, faster winds. Looking at highs around the mid to upper 80s, the warm spot over the Merrimack River Valley and Boston-metro where we`ll likely see some locations top out over 90. The deep mixing, can`t rule out SW gusts upwards of 30 mph.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Overview... 12Z model suite continues to signal the flattening of the northern stream H5 ridge across the Great Lakes into southern Ontario by this weekend. A broad, nearly zonal steering flow will take over through the holiday weekend into early next week. Short wave in the northern stream flow will shift E late Sat/Sat night, which looks to merge with another short wave working S-SW out of northern New England as a backdoor cold front moves across. SW winds ahead of this front will bring moisture up the eastern seaboard, along with the moisture associated with the southward bound cold front. The front should clear the S coast Sunday morning, bringing cooler temperatures, then stalls in the zonal flow off the S coast early next week. This may mean a continued unsettled pattern with patchy showers until later Tuesday as another front pushes SE out of central Canada. Medium range models continue to signal drier conditions with near normal temperatures by the middle of next week. Details... Friday and Friday night... High pressure off the mid Atlantic coast will bring a warm W-SW wind flow across most of the region. However, it will be cooler along the S coast with the onshore wind. Excellent mixing in place through H85 with lapse rates at 8C/km or higher, so expect temps to rise to the mid and upper 80s away from the S coast. May also see W-SW winds gusting up to 20-25 kt along the immediate S coast, Cape Cod and the islands. Dewpts slowly increase Fri night with the SW wind flow, so temps will only fall back to the upper 50s to mid 60s, coolest along the immediate S coast. Saturday and Sunday... Clouds increase from N-S as a backdoor cold front works S-SW out of northern New England. Noting dewpoint pooling, up to the lower-mid 60s by late Sat or Sat night, highest across N CT/RI/SE Mass. PWATs increase to around 1.5 to 1.7 inches Sat night, highest along the S coast. Best instability moves in as the front approaches, with total totals from 48-50, K indices up to 30-35 and TQ values in the upper teens. So, have mentioned slight chance for thunderstorms starting across NE Mass around 21Z or so Sat, then spreading S Sat night. Highs will again reach the 80s away from the S coast. The front should slowly push across the region through around 12Z Sun, then stall just off the S coast as it becomes parallel to the mid level steering flow. Spotty showers may linger through Sunday as moisture continues to feed up the coast and along the stalled front. Easterly winds set up, so cooler temps will prevail. Expect readings to only reach the lower 60s along the immediate E coast, ranging to the lower 70s across the CT valley. Monday through Wednesday... Expect continued unsettled conditions Monday into Tuesday with the stalled front remaining off the S coast. May see a weak wave move along the front during Monday, which may enhance some of the scattered showers. Then, another front pushes SE out of central Canada during Tuesday. This should push the moisture S of the region, though a few showers may still linger especially around the Route 2 area of N Mass. Medium range in fairly good agreement in pushing another large high pressure center out of southern Ontario into the northeast U.S. by next Wednesday. This should bring dry conditions and near seasonal temperatures. && .AVIATION /05Z 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/... Rest of tonight... Watching low cloud features move SW with NE flow. Concern over E MA of IFR CIGs (ACK 8-9z, E MA around 930z). E flow over E/SE MA whereas W flow elsewhere. Thursday... IFR CIGs may be an issue over E/SE MA coastal terminals early on but quickly eroding. VFR with interior W flow up against surging E sea-breeze flow. Gusts up around 20 kts for both sides of the wind shift. Will see sea-breezes push towards the interior, S flow prevailing late. Thursday night... Hold VFR but concern there could be lingering issues for S-coastal terminals of low clouds, possible fog. S/SW winds with gusts up around 20 kts for E/SE terminals towards Friday morning. Friday... VFR. S/SW winds with gusts upward of 30 kts. Possible IFR CIG conditions for S-coastal terminals early on. KBOS Terminal... E onshore flow persisting, turning S late Thursday. Some concern of lower CIG conditions overnight around 930z. KBDL Terminal... VFR. N winds much of today, gradually turning S late. Outlook /Friday Night through Monday/...Moderate confidence. Friday Night: VFR. Breezy. Saturday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA. Saturday Night: Mainly MVFR, with local IFR possible. Chance SHRA, slight chance TSRA. Sunday through Sunday Night: Mainly MVFR, with local IFR possible. Chance SHRA. Memorial Day: Chance SHRA.
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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 and Thursday... Back door cold front sweeping across the waters as of 10p will keep flow mainly NE over the waters through late Thursday. Gusts up to 20 kts possible;. Thursday night... High pressure moving off the coast will result in SW winds developing. Enough of a low level jet may result in marginal small craft conditions with southwest wind gusts of 20 to 25 knots and marginal 3 to 5 foot seas across the outer-waters. Outlook /Friday Night through Monday/...Moderate confidence. Friday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, areas fog, slight chance of thunderstorms. Local visibility 1 to 3 nm. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Sunday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers, areas fog. Local visibility 1 to 3 nm. Memorial Day: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Sipprell/EVT NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...EVT AVIATION...Sipprell/EVT MARINE...Frank/EVT

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