Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 252332 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 732 PM EDT Sat May 25 2019 .SYNOPSIS... High pressure moves offshore, allowing a warm front to slip through Southern New England. This will bring mild night to our area, along with a few showers and chance of a thunderstorm. Southern New England gets a taste of summertime heat on Sunday along with isolated thunderstorms. A cold front moves through and brings moderating temperatures, but still pleasantly warm air. High pressure pushes off the coast early Tuesday. Low pressure then moves through Tuesday bringing cooler temps and rain, followed by periodic unsettled weather through Thursday night. A cold front pushes offshore late next week, with drying conditions and mild temperatures. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 730 PM update... Showers to our west over NY and PA continue to diminish as they approach eastern NY/PA and leave the area of instability (200-1000J/kg CAPE) to the west. Marginal instability isn`t expected move into southern New England until will into the overnight hours with the next round of rain. For now, some scattered light showers drifting south from NH are the only precip to speak of for the next few hours. Previous Discussion... Northern stream shortwave races into Northern New England overnight, with Southern New England at the tip of the upper jet. This places our area in a zone of lift that will be coincident with a PW max around 1.5 inches. Stability parameters are mixed in terms of being favorable for convection. For instance, totals are 46-48 and mid-level lapse rates reach 6.5C/Km while the theta-e ridge remains to our west. We will continue to mention showers and widely scattered t-storms. A 40-50 kt low level jet develops after midnight, which suggests gusty S-SW winds around 25 kt. Increasing clouds and increasing dew points due to the approaching warm front support min temps in the 50s. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday... Showers exit during the morning as the warm front moves off to the north/east. Southern New England will be in the warm sector, with dew points in the upper 50s and lower 60s. Mixing is expected to reach 800 mb, with temps at that level supporting max sfc temps in the low to mid 80s. Winds in the mixed layer have a potential for 25-30 kt. Stability parameters linger at/near unstable levels but drier air moves in at all levels above 750 mb. The dryness suggests it will be difficult to generate convection, but the parameters suggest it is not out of the question. We will mention slight chance pops in Eastern Mass for showers/tstms. Sunday night... West-northwest winds bring lower dew points move in behind the cold front. Overall a dry night with temps falling back into the 50s. && .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * Dry and mild conditions for Memorial Day * Low pressure bring showers and cooler temps Tuesday * A front lingers across the region Wed and Thu with unsettled conditions and warming temperatures * Rain ending Friday with dry and seasonable conditions toward the weekend Overview... Progressive pattern in the extended forecast, with a persistent subtropical ridge over the central/southern U.S. directing a series of shortwaves through SNE during the workweek. This will lead to periodic shower/thunderstorm chances with a bit of a roller coaster of high temperatures, warmer Monday and even moreso Thursday. By next weekend we see indications of a drier regime with high pressure in the vicinity. Details... Memorial Day... A cooler and drier post frontal airmass will be in place for Memorial Day as high pressure builds in to the north. Temperatures top out around 10 degrees cooler than Sunday, in the mid 70s (near 80 for CT valley), while dewpoints in the 40s and low 50s mean less humidity. The exception will be along the immediate coast, where high pressure and light winds will allow for light onshore flow, keeping temps in the 60s. Tuesday through Thursday... Showers return early Tuesday with cooler temperatures as a surface low pressure system approaches and crosses the region from the west. Still some uncertainty on the track and magnitude of this system. The EC has consistently brought a stronger ~1004mb low directly over SNE while the GFS keeps it weaker and further south. Given the associated greater forcing at 925 in the EC solution, it paints a far wetter pictureas well. For now, sticking with a blend of synoptic guidance brining rain on Tuesday. Instability, though, looks poor, so not expecting thunderstorms. Either way flow remains onshore Tuesday keeping high temps in the upper 50s and low 60s...mid to upper 60s inland. Rain clears out briefly early Wednesday before the next system approaches Wednesday night into Thursday. This one with several hundred J/kg of CAPE so we could see some thunderstorms overnight into early Thursday. A brief break in the rain follows, early-mid day Thursday, before the approaching cold front brings more widespread rain and potential for thunderstorms to all of southern New England late Thursday into early Friday. Wednesday and Thursday we`ll see increasing temperatures each day as a warm front slowly makes its way north on southerly flow, placing southern New England fully in the warm sector by Thursday. This is associated with a surface low over the Great Lakes moving into southeast Canada. Cloudcover will limit solar insolation Wednesday but temps manage to get into the 60s-70s. Thursday is our warmest day of the week, with 850mb temps approaching 16-18C, leading to surface highs in the 80s. Some deterministic model runs continue to predict even hotter temps Thursday, but for now keeping things in the low to mid 80s. Glancing at the EPS, only 1-2 of the 50+ members want to bring 90+ highs to PVD or BDL Thursday (even less for BOS). This of course depends on the timing of the approaching cold front which if it were to speed up could limit our warming. Friday and Saturday... The cold front passes sometime around late Thursday night/early Friday morning, bringing showers to an end as PWATs drop to <0.5" and cooler NW winds return. Under large scale troughing but generally zonal flow, things look to dry out as we head into the weekend under high pressure. Seasonable temperatures are expected, in the mid to upper 70s for most with increasingly clear skies. && .AVIATION /00Z SUNDAY 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/... Tonight...Moderate confidence. VFR, at least for the first half of the night. Potential that oncoming showers will lower cigs/vsbys to MVFR during the night. In addition to showers, expect widely scattered thunderstorms. Sunday...Moderate confidence. Showers move offshore in the morning with partial clearing taking place. Lingering instability hints at potential for midday/afternoon thunderstorms, but with the air drying out this will be difficult to accomplish. We will expect isolated shower/t-storm in Eastern MA. West winds gusting to 25 kt. Sunday night...High confidence. VFR with clearing skies and diminishing wind. KBOS Terminal...Moderate confidence in timing of lower conditions in showers tonight. Otherwise high confidence. KBDL Terminal...Moderate confidence in timing of lower conditions in showers tonight. Otherwise high confidence. Outlook... Memorial Day through Monday Night: VFR. Tuesday: VFR. Breezy. Chance SHRA. Tuesday Night: Mainly VFR, with local IFR possible. Breezy. Chance SHRA. Wednesday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, isolated TSRA. Wednesday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, isolated TSRA. Thursday: VFR. Breezy. Chance SHRA, isolated TSRA. && .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...Increasing SW winds as a low level jet moves across the waters. Gusts to 25 kt possible after midnight with building seas. A Small Craft Advisory is being issued with this forecast. Showers move through, mostly during the overnight, with reduced vsbys. Sunday...Low level jet exits SE waters early with diminishing winds. Another pulse of west gusts to 20-25 kt may develop in the afternoon over Boston Harbor and Narragansett Bay. Also some concern for a second round of showers/t-storms along the Eastern MA coast, but important conditions for shower growth are not showing in our data. We will forecast isolates showers/thunder along the North Shore and South Shore waters. Sunday night...Diminishing northwest wind and diminishing seas as high pressure builds from the west. Outlook... Memorial Day: Winds less than 25 kt. Seas locally approaching 5 ft. Monday Night: Winds less than 25 kt. Tuesday: Winds less than 25 kt. Rain showers likely. Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain showers likely. Wednesday through Wednesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, isolated thunderstorms. Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, isolated thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 2 AM to noon EDT Sunday for ANZ231>235-237. Small Craft Advisory from midnight tonight to 5 PM EDT Sunday for ANZ250. Small Craft Advisory from 2 AM to 10 PM EDT Sunday for ANZ254>256. && $$ SYNOPSIS...WTB/BW NEAR TERM...WTB/BW SHORT TERM...WTB LONG TERM...BW AVIATION...WTB/BW MARINE...WTB/BW is the U.S. government's official web portal to all federal, state and local government web resources and services.