Area Forecast Discussion
Issued by NWS Boston, MA

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692 FXUS61 KBOX 262030 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 430 PM EDT Sat May 26 2018 .SYNOPSIS... A cold front will push south across the region this evening. A few showers or an isolated thunderstorm may occur, mainly northern areas. Much cooler weather with cloudy skies and areas of rain and fog will be the rule Sunday as low pressure along the front passes south of the region. Improving conditions by Monday. High pressure brings mainly dry and warmer weather Tuesday through Thursday. An approaching weather system brings showers Friday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 415 PM Update... This evening... Through all of the high cloudiness, satellite imagery shows a distinct band of low clouds streaming southwestward from the ME and NH coasts. Winds at Portsmouth and Manchester have both shifted to the north. All short range models show winds shifting to the N-NE across all of southern New England between 5 PM and 10 PM tonight. A secondary cold front, mainly from 950 to 850 mb, is forecast to drop southward later tonight, with 850 mb temps dropping to +9C to +10C in northeast MA by daybreak. Scattered showers and isolated thunderstorms were developing in southern NH as of 4 PM. These are forecast to head ESE and may clip far northeast MA this evening. The Storm Prediction Center has a marginal risk of severe weather for Essex County. This is predicated on strong winds aloft overcoming the lack of low level CAPE. Will keep an eye on that region, but the risk for any severe weather...or thunderstorms...is on the low side. Elsewhere, only forecasting a slight chance of a shower through the overnight hours. Isolated convection to our southwest should remain south of the region. Temperatures in the 80s this evening will fall to the 50s overnight throughout the region. Areas of fog will develop, especially in eastern MA and southern RI, with the increasing moisture on northeasterly winds. An Air Quality Alert remains in effect for RI, CT, and areas of MA adjacent to CT and RI, extending across southeast MA until 11 PM. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday and Sunday night... Low pressure will be traveling along the front on Sunday, which will be well to the south of New England. High pressure will be ridging across our region from ME to western CT. This keeps us in a moist and breezy E-NE flow. Winds of 15-20 mph are forecast with some higher gusts at the coast. Low cloudiness will persist throughout the day on Sunday and Sunday night. Rain is likely during the day across northern CT, RI, and southeastern MA with a chance of some rain farther north into interior MA, mainly south of the Mass Pike. Rain chances will diminish Sunday night as the low pulls farther out to sea to our southeast. Right now, have not yet included thunderstorms in the forecast, but cannot rule out a few thunderstorms working their way into CT from NY, as per some mesoscale models. Temperatures will be more than 30 degrees colder than today, with highs in the mid 50s east to lower 60s west in the CT Valley. Lows Sunday night will dip to the upper 40s to lower 50s. && .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * Becoming warmer and dry Tue through Thu * Return of unsettled and wet weather late week Overview... Although S New England remains on the periphery of the N stream, linked to a persistent vortex near Baffin Island, as the previous forecaster noted, low-mid lvl heights remain generally above normal through most of the long term. In fact height anomalies actually increase after Tue, as N periphery ridging continues from subtropical storm Alberto across the S central CONUS. This ridging builds into New England mid-late week, yielding positive temp anomalies as well. It is not until Alberto becomes encapsulated by a longwave trof currently along the Pacific coast, and begins to more rapidly shift E that conditions change. Remnant moisture, and subtropical energy will lead to a transition to unsettled and wet wx for the late week/weekend. Guidance is actually in fairly good agreement with this, in spite of its track record handling tropical interactions. Will use a consensus blend for this update. Daily details... Mon... Inverted ridging is somewhat relieved as return flow develops ahead of sharpening trof upstream. This should allow for warmer highs, with temps reaching back into the 70s especially across the W interior. Lingering clouds, but little in the way of precipitation expected as column is not particularly saturated. Tue... Early frontal passage is mainly dry as bufkit suggests a dry column. Mid lvl temps actually warm it`s wake however thanks to influence of the upstream ridge and warm advection. In fact, buy 00Z Wed (Tue eve) H85 temps should be approaching +14C. Therefore, expecting a return to seasonably mild temps. Mainly in the low-mid 80s for highs. Wed and Thu... Gradual airmass modification leads to H85 temps warming to near +16C through the mid week period. This should allow daytime highs to once again reach the mid 80s, potentially upper 80s where some downsloping can be realized. High pres cresting will likely allow for sea breeze development each day, leading to cooler coastlines. This is especially true for the S coast on Thu, in which the S component to the gradient winds will enhance the cooling. Dry, as high pres yields decent subsidence. Fri and Sat... Remnant moisture and upper lvl wave from Alberto transitions E late week. Noting that even ensembles support high PWATs potentially exceeding 1.5in (nearly 3 std deviations above normal). The overall trof does weaken initially but then may actually deepen again as it approaches the Atlantic coastline. Given these factors, wet and unsettled conditions are likely to return, with a risk for pockets of heavier rainfall given the high moisture content. Still some factors to be resolved, but this will be a period to watch. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/...High confidence. Tonight...A cold front will shift winds to the northeast throughout the region this evening. VFR conditions this evening will deteriorate to MVFR and IFR ceilings from east to west by midnight. The exception will be on Nantucket, where IFR already exists and will continue. Areas of fog with IFR visibilities also expected, especially across eastern MA and RI. NE winds increase to 15-20 kt along the E coastal areas overnight with a Widely scattered SHRA. An isolated thunderstorm possible this evening in far northeastern MA. Sunday... MVFR-IFR CIGS linger through most of the day. Rain developing across mainly the southern half of southern New England. Cannot rule out an isolated thunderstorm in CT. Patchy fog along S coast through the day. KBOS Terminal...High confidence in TAF. Backdoor cold front shifts winds to the northeast early this evening with a wind shift from W to NE. Wind gusts to around 20+ kt at times tonight. KBDL Terminal...High confidence in TAF. Backdoor cold front should hold off until around 02-03Z when wind shifts to NE. Outlook /Monday through Thursday/...Moderate to High Memorial Day: Mainly VFR. Monday Night: Mainly VFR, with local IFR possible. Patchy BR. Tuesday through Thursday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/...High confidence. Tonight... Small Craft Advisories are in effect for east coastal waters for winds shifting to the northeast and gusting to 20-25 knots. Seas will also build to 5 to 6 ft well to the east of Cape Ann and Boston toward Sunday morning. Isolated showers. Cannot rule out an isolated thunderstorm north of Cape Ann this evening. Areas of fog will reduce visibility to 1 to 3 miles. Areas of dense fog with near 0 visibility possible, especially southeast of Nantucket. Sunday... E-NE winds increase to 20 kt across all waters but some gusts to 25-30 kt over the southeastern waters. Seas will become rough with 5 to 8 feet over the eastern waters and 4-5 feet over the southern waters. As a result, Small Craft Advisories are in effect on Sunday for all waters except Boston Harbor and Narragansett Bay. Areas of fog will persist, especially over the southeastern waters...with reduced visibilties. Sunday night... A the low pulls away farther to our southeast, E-NE winds slowly diminish to 10-15 kt late and seas slowly subside to 4-6 feet on the eastern waters and 3-5 feet on the southern waters. Outlook /Monday through Thursday/...Moderate confidence. Memorial Day: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday Night through Thursday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert until 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert until 11 PM EDT this evening for MAZ011>013- 016>021. RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory from 7 AM Sunday to 6 AM EDT Monday for ANZ232>235-237-255-256. Small Craft Advisory from 7 PM this evening to 6 AM EDT Monday for ANZ231-250-251-254. && $$ SYNOPSIS...Field/Doody NEAR TERM...Field SHORT TERM...Field LONG TERM...Doody AVIATION...Field/Doody MARINE...Field/Doody

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