Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 171428 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1028 AM EDT Tue Apr 17 2018 .SYNOPSIS... An upper level system will keep unsettled conditions across the region through mid week. A low pressure area will produce a period of chilly rain potentially mixing with wet snow during Thursday. Lingering rain showers Thursday night may mix with or change to wet snow across northern Massachusetts. The weekend looks to be dry but with more below normal temperatures. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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1020 AM update... Scattered rain/snow showers moving into western MA ahead of mid level low while clouds expanding east into SE New Eng. Deep moisture plume combined with steepening low level lapse rates support a few showers at times, especially interior but can`t rule out a brief shower in the coastal plain this afternoon. Mostly cloudy skies will prevail. Highs will range from mid 40s to lower 50s. Soundings support W/SW gusts to 25 mph at times. Previous discussion... Upper low will be overhead of New England today, with a -24C cold core. Cross sections show a deep moist layer reaching to at least 600 mb. Low level lapse rates reach 9C/Km, while mid level lapse rates are around 5.5 to 6C per Km. Satellite images show lots of clouds. If any breaks develop this morning, the sunshine working on the cool temps aloft should generate clouds, leaving us with mostly cloudy skies. The shortwave and instability may also generate widely scattered showers. Mixed layer is forecast to reach 850 mb. Temps at the top of the layer at -5C to -7C suggest max sfc temps in the mid 40s northwest to lower 50s coastal plain. Winds in the mixed layer reach 20 to 25 knots, so expect gusty west winds late morning and afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... Second shortwave rotates around the upper low and across Southern New England during the first half of the night. Deep moisture lingers overhead as this takes place. Expect mostly cloudy skies with widely scattered showers through midnight, then partial clearing late. We stayed close to guidance min temp forecast in the 30s. Wednesday... Another shortwave moves through during the day. This one will be supported by a 100-knot jet, but with less moisture aloft and most of that will be shallow. Expect a partly cloudy day with mixing again reaching 850 mb. Temps at that level will be a couple of degrees higher than today, about -3C to -4C. This supports max temps in the upper 40s to mid 50s. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Rain, potentially mixing with wet snow on Thu. * The coming weekend looks to be dry with gradually warming temps. Overview and model preferences... As previous forecaster noted, amplified spring pattern with several weak cutoffs linked to the N stream continue to impact the region through the mid term. However, am noting that a gradual mid and upper lvl height rises on the periphery of the modest S stream jet persist through much of the mid and long term. Therefore, following a secondary wave mid-late week (most impacts felt on Thu) the rising heights should allow enhance synoptic ridging in its wake yielding a drier and milder stretch. During this stretch, temps should generally remain at or above seasonal normal values. Synoptic differences in guidance through the mid and long term are relatively limited in magnitude. While there are track differences for deepening low pres Thu, they are close. Should the S track favored by ECMWF/CMC/UKMET and several GEFS and ECENS members, there could be a bit more cold air for mixed precip in the higher terrain. The S track is also more amplified and therefore less progressive, thanks to more residence time over the waters. Details... Wed night into Thu... Developing cutoff born of both Pacific and Nunavut energy will be moving quickly W-E across the NE, at which point it will become absorbed in the stalled cutoff near Labrador. Attending sfc low pres gradually deepens, into the 990s by the time it arrives in S new England. As mentioned above there are still differences in the track which may ultimately determine the final QPF totals. However, PWATs on both the deterministic and ensemble members peak just below 1.00in, or only about 1 std deviation above normal. LLJ energy is also modest. Therefore, regardless of track this low pres passage will not be as dynamic of the previous, with final QPF values mainly between 0.5-1.00in. Soundings are initially cold for some light wet snow, with the possibility of some minor accumulations (mainly on grassy surfaces/cars) in the higher terrain. This occurs again as precip ends Thu night. Ensemble probabilities are relatively high for at least 1.0in of accumulation before any transition. Fri into Sat... Given the reinforcement of the cutoff in the Maritimes by the wave moving through Thu, this will slow the progression of the trof E of the region. Although drier air will be shifting in from the NW, temps will remain cool as H85 temps drop to an average around -4C and H92 near +1C. By Sat, upstream ridging has full control but with little mid lvl temp changes. Mixing will make up some of the difference with temps trending back toward normal. Early next week... Continued impact from ridging, as N stream attempts to dive S again upstream allows for gradual warming throughout the column. Mainly dry with temperatures warmer than normal are expected. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/...
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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 Wednesday/... Today...High confidence. Mainly low end VFR cigs, with areas of MVFR over higher terrain. A few showers expected, especially interior but should be brief and have limited impact. W/SW gusts to 25 kt at times. Tonight and Wednesday...High confidence. VFR. Winds gusting 20 to 25 knots Wednesday. KBOS Terminal... High confidence in TAF. MVFR early morning ahead of a cold front crossing the region. Winds shift from the WSW behind the front, and ceilings improve to VFR. This happens by 10Z/6 AM. Winds gust 20-25 knots this afternoon. KBDL Terminal...High confidence in TAF. VFR. Winds gust 20-25 knots this afternoon. Outlook /Wednesday Night through Saturday/...Moderate confidence. Wednesday Night: Starting VFR trending to mix MVFR/IFR early morning. Chance RA, chance SN. Thursday: MVFR/IFR conditions expected. RA likely, SN possible. Thursday Night: Mainly MVFR, with areas IFR possible. Breezy. Chance RA, chance SN. Friday: Mainly MVFR to start, with VFR thereafter. Breezy. Slight chance SHRA. Friday Night through Saturday: VFR.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. Today... High confidence. Southerly winds turn from the west-southwest with gusts to 25 knots. Rough seas were already in place early, and with the expected winds we expect only a slow subsiding through the day. Small Craft Advisory continues for most areas. Tonight and Wednesday... High confidence. Winds diminish a little tonight, but then return to similar 25 knot gusts on Wednesday. Seas linger at or above 5 feet on the outer waters and RI waters, but subside below 5 feet on several nearshore waters including Mass Bay and Cape Cod Bay. Small Craft Advisory will continue for most waters. Outlook /Wednesday Night through Saturday/...Moderate Wednesday Night: Winds less than 25 kt. Local rough seas. Slight chance of rain. Small craft advisory for seas needed. Thursday: Winds less than 25 kt. Local rough seas. Chance of rain. Small craft advisory for seas needed. Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain showers, chance of rain. Friday: Winds less than 25 kt. Local rough seas. Slight chance of rain showers. Small craft advisory for seas needed. Friday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Saturday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 6 PM EDT Wednesday for ANZ232>235- 237-250-254>256. Small Craft Advisory until 6 PM EDT this evening for ANZ230- 236. Small Craft Advisory until 8 PM EDT this evening for ANZ231. Small Craft Advisory until 5 AM EDT Wednesday for ANZ251.
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&& $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/KJC SHORT TERM...WTB LONG TERM...Doody AVIATION...KJC/WTB/Doody MARINE...WTB/Doody

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