Area Forecast Discussion
Issued by NWS Boston, MA

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537 FXUS61 KBOX 232338 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 738 PM EDT Wed May 23 2018 .SYNOPSIS... Other than a few brief showers possible this evening across eastern MA, dry weather will continue for the remainder of the work week with very warm temperatures for Friday afternoon. Warm and humid conditions continue on 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. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 725 PM Update... Areas of quick moving showers ahead of an approaching cold front moving across the Route 2 area of N central and NE Mass will push offshore and weaken through around 02Z or so, clipping along E coastal Mass and outer Cape Cod. Should see brief precip as these showers pass. Noting the remnants of the southerly sea breeze across N CT into interior SE Mass, which may enhance some of the showers as they pass. Elsewhere, dry conditions in place. Will see wind shift to W-NW as the front passes, and may see brief gusts up to 15-20 kt. Clouds will linger across E and central Mass into E RI, while skies will average out mostly clear further W. Have updated near term to reflect current conditions and incorporated into the evening forecast. Previous Discussion... A secondary cold front across northern New England will drop southward into our region this evening. Limited moisture in any showers and diminishing instability will keep any of this activity short lived and isolated. From the late evening onward, expect dry weather with overnight lows bottoming out in the upper 40s to the middle 50s. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thursday... Large high pressure in control will result in plenty of sunshine and a beautiful day. Somewhat cooler mid level temps behind tonight/s cold front will keep high temps mainly in the 70s. However...weak pressure gradient will allow for sea breezes holding high temperatures in the 60s along the coast. Thursday night... High pressure will move off the coast Thu night allowing for a return south to southwest flow of milder air. Initially boundary layer may decouple enough to allow for overnight lows to drop into the upper 40s to the lower 50s near or just after midnight in some locations. However...some boundary layer mixing may occur resulting in rising temps toward daybreak in locations that are able to decouple for a time. && .LONG TERM /FRIDAY 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 /00Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Tonight through Thursday night...High confidence in mainly VFR conditions right through Thursday night. A quick moving, few brief showers this evening across eastern Mass which should move off the outer Cape by 03Z. Otherwise, the main issue will be timing the arrival/departure of diurnal sea breezes along portions of the coast during Thursday. KBOS Terminal...High confidence in VFR conditions. Sea breeze should develop around 15Z-17Z Thursday. KBDL Terminal...High confidence in VFR conditions. Outlook /Friday through Monday/...Moderate confidence. Friday through 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. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Tonight and Thursday...High confidence. A weak pressure gradient with high pressure in control will keep winds and seas below small craft advisory thresholds. Thursday night...Moderate to high confidence. High pressure moving off the coast will result in southwest 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 through Monday/...Moderate confidence. Friday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of seas approaching 5 ft. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/EVT NEAR TERM...Frank/EVT SHORT TERM...Frank LONG TERM...EVT AVIATION...Frank/EVT MARINE...Frank/EVT

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