Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 180057 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 857 PM EDT Sat Jun 17 2017 .SYNOPSIS...
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Warm and humid days and foggy nights linger through Monday. A cold front approaches from the west Monday, bringing a risk of thunderstorms later Monday and Monday night. The front crosses Southern New England on Tuesday. Seasonable temperatures and lower humidity follow on Wednesday. Warmer temperatures return late in the week.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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855 pm update... ***Dense Fog Advisory In Effect For The South Coast, Cape, And Islands Into Early Sunday Morning*** ***Also watching the potential for a round of scattered showers with brief heavy rainfall impacting the southeast New England coast toward daybreak*** Light southerly flow with high dewpoint air moving over the relatively cool ocean is a good recipe for dense fog development along the south coast, Cape and Islands. Therefore, have gone ahead and hoisted a Dense Fog Advisory through 8 am Sunday morning in these locations. We may need to expand it further northwest, but felt this was a good starting point and will continue to monitor. Other than the concern for dense fog, there appears to be a decent shot at a round of scattered showers developing toward daybreak along the southeast New England coast. Some brief heavy showers and even an isolated t-storm can not be ruled out. Despite upper level ridging, developing 30+ knot low level jet coupled with a few hundred J/KG of elevated instability have us more concerned about this potential. Some of the high resolution guidance is hinting at this potential as well so have increased pops. As for low temps they should not fall much with most locales only bottoming out in the middle to upper 60s. Surface warm front still jammed up across northeast MA and not sure how much northward progress it will make overnight, but either way will only impact temps by a few degrees.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday... Another round of fog/low stratus early should burn off more quickly tomorrow during the AM hours thanks to continued rising heights (ridging) and faster BL mixing developing away from the immediate S coast. As fog/clouds burn off expect increased risk for S-SW wind gusts 20-30 kt (roughly 25 to 35 mph) at times, so will need to watch for the breaks of sun. Also, the better mixing expected as breaks of sun occur, should tap the H85 airmass where temps will be +15C to +17C, yielding afternoon highs in sunshine into the mid 80s. Onshore flow will keep south coastal RI/MA much cooler, holding in the 70s (potentially cooler if stratus hangs longer than current forecast). With high PWATS good BL development will struggle to mix out the very high dwpts associated with the warm sector airmass. Upper 60s to low 70s dwpts will make it feel quite oppressive and allow heat indicies to inch into the upper 80s to near 90 in the warmest spots such as the upper CT valley and Merrimack Valley. Risk for shower/thunderstorm continues, however they are likely to continue to be few and far between. Rising heights and overall limited forcing will mitigate development across S New England. Noting SB CAPE values 500-1000j/kg, so should something pop a risk for thunder/heavy rain exists. These will have to be monitored, but given the lack of any trigger and some capping represented by the rising heights, the general thunder in latest SWODY2 from SPC make sense. Better forcing is upstream. Tomorrow night... Very moist airmass remains with dwpts likely to stay in the upper 60s and low 70s even into the overnight hours. This will be the floor for overnight mins as well, so a hot sticky June night expected. Overnight low risk for SHRA/thunderstorms linger given little change in the airmass. However, the lack of any apparent forcing mechanism will once again limit coverage. Best chance will be for any lingering convective activity that develops upstream across NY/PA which will need to be monitored. Will continue to focus POPS mainly W of the Worcester Hills and higher terrain of central CT. Winds may remain somewhat elevated overnight, though not as strong as during the daylight hours. Little change in the LLJ and warm overnight mins support at least some of these winds mixing to the sfc. && .LONG TERM /MONDAY THROUGH SATURDAY/... Big Picture... Closed low over the Great Lakes early in the week. This lifts north through Canada midweek, leaving zonal flow across the northern tier of the USA the remainder of the week. Contour heights range from near normal midweek to above normal late week, suggesting a period of near normal temps midweek followed by a warm-up late week. Model mass fields are similar through Wednesday, but show increasing differences Thursday through Saturday. Thermal fields show a similar variation. This leaves a moderate confidence Monday to Wednesday, and low confidence Thursday to Saturday. Details... Monday... Closed low north of the Great Lakes with trough axis across the lakes. Ascending node of the supporting upper jet covers the Eastern Great Lakes with the right entrance region across Western PA and Central NY. Surface pattern shows a cold front aligning with this jet. It also shows a lee trough extending south from the Hudson Valley. Although the surface cold front is so far west, the lee trough is in position to supply low level convergence while the upper jet approaches late in the day to provide upper venting. Forecast divergence fields aloft suggest this support may approach us late in the day. Stability parameters, while not unanimous, suggest sufficient instability. SBCAPE for example is forecast at 1000-1500 J/Kg in CT and Western MA and K index is in the 30s. Also of note is a strong theta-e ridge across Western MA and NW CT late in the day. All models show winds at 850 mb forecast to reach 40-50 knots, while winds at 500 mb are forecast at 40-50 knots late Monday and increase to 70-80 knots Monday night. Wet Bulb Zero values are forecast at 13500 to 14500 feet. Model timing of the onset of precip seems a little fast given the location of the features. But once we reach the afternoon/evening the pattern seems favorable for showers/tstms across at least our western areas...spreading east over Monday night. Convection and the strong winds aloft suggest the potential for strong and possibly damaging wind gusts from late day/early night thunderstorms. We note the Day 3 outlook from SPC early this morning showed a slight risk of severe storms for areas west of I-95 and an enhanced risk west of the CT River. We will be monitoring this potential. Tuesday... The actual cold front moves through on Tuesday, bringing additional chance of showers for part of the day followed by drying out after cold fropa. Moisture profiles show a diminishing trend by Tuesday afternoon, suggesting at least partial clearing. At the same time, cold advection aloft will destabilize the airmass and may allow for clouds to regenerate as well as a few scattered showers. Best chance for this would be in Western areas. Wednesday... The upper trough and its cold pool will be overhead Wednesday. This will bring an increased chance of diurnal clouds and scattered showers. The cold temps aloft will also support deep mixing, with temps aloft favoring surface temps in the mid 70s to lower 80s. Thursday through Saturday... High pressure builds over the region by Thursday bringing dry weather but then moves off just as fast. Less confidence late week due to difficulty in timing shortwaves in a zonal flow, especially this many days out. At least one shortwave is noted in the Canadian Prairies midweek. We are showing a chance of showers later Friday into early Saturday, but this timing will likely change. Temps aloft climb from around 10C midweek to the mid teens...so max temps late week should be more generally in the 80s. && .AVIATION /01Z SUNDAY THROUGH THURSDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 855 pm update... Tonight...Moderate confidence. Increasing low level moisture will result in IFR to LIFR conditions overspreading the region in low clouds and fog overnight. Worst of the fog should be near the south coast, but it is a bit uncertain on how far northwest the dense fog will spread overnight. Will continue to monitor. Sunday...Moderate confidence. Widespread IFR conditions early morning should improve to mainly MVFR to even VFR by afternoon as the fog/low CIGS burn off. SW wind gusts of 20 to 30 knots are expected to develop by afternoon with the strongest of those winds along the coastal plain. Sunday night...Moderate confidence. Another round of low clouds and fog develop with the potential for a return to IFR conditions across much of S New England, mainly due to low CIGS as winds will limit fog development away from the S coast. S winds will continue to gust, mainly 20-25 kt overnight. KBOS Terminal...Moderate confidence. Cigs and Vsbys should lower to IFR thresholds over the next few hours. Uncertain if dense fog impacts the terminal overnight resulting in LIFR conditions, but something will have to monitor. KBDL Terminal...Moderate confidence. Cigs and Vsbys should lower to IFR thresholds over the next few hours. Uncertain if dense fog impacts the terminal overnight resulting in LIFR conditions, but something will have to monitor. Outlook /Monday through Thursday/... Moderate confidence. Monday-Tuesday... MVFR cigs and vsbys with areas of IFR in fog and developing showers. Southwest winds will increase with gusts 25 to 30 knots. Potential for strong thunderstorms Monday afternoon and early night especially in Western Mass and adjacent CT. Strong straight-line winds would be the largest concern. A cold front sweeps through on Tuesday, ending the showers and allowing drier air to move in during the afternoon. Conditions improve to VFR at this time. Wednesday-Thursday... VFR. Developing daytime clouds and scattered showers Wednesday, which then diminish Wednesday evening. Fair weather Thursday.
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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 Sunday Night/... Tonight...High confidence. Lingering swell continues with 5-6 ft seas mainly on the outer ocean waters. Otherwise, a few showers but more areas of fog/drizzle over the waters again, yielding very low visibility at times. Sunday and Sunday night...High confidence. Increasing southerly flow will reach 25-30 kt near shore (potentially a few Gale force gusts possible) and hold around 25 kt on the outer waters. These remain persistent after sunrise, so wind driven waves also increase through the day and overnight hours. Seas peak around 6 ft on the SE ocean waters, 5-6 ft elsewhere outside of sounds and bays. However Narragansett bay may see some seas near 3 ft the further up the bay one travels thanks to persistent S flow. Little change in the overnight conditions. Therefore Small Craft Advisories will linger well into the overnight hours. Will need to watch for a second round of fog over the waters, especially the waters S of New England. Outlook /Monday through Thursday/... Moderate confidence. Monday-Tuesday... A cold front over New York Monday crosses the waters on Tuesday. Persistent SW winds Monday with gusts 25-30 knots. Winds diminish a little Tuesday once the front moves through, but still gusts near 20 knots. Expect rough seas on the outer waters and the exposed southern waters, such as RI and Block Island Sounds, with heights 5 to 8 feet. Strong thunderstorms will be possible over land, and some may affect the nearshore waters during late Monday afternoon and early night. A Small Craft Advisory will be needed. Wednesday-Thursday... Winds southwest to west during this time with gusts to 20 knots. Showers will again bubble up over land Wednesday and may affect nearshore waters. Quiet weather Thursday. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Dense Fog Advisory until 8 AM EDT Sunday for MAZ020>024. RI...Dense Fog Advisory until 8 AM EDT Sunday for RIZ004>008. MARINE...Small Craft Advisory from 11 AM Sunday to 5 AM EDT Monday for ANZ230>234-236-251. Small Craft Advisory until 8 AM EDT Monday for ANZ235-237-250- 254>256.
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&& $$ SYNOPSIS...WTB/Doody NEAR TERM...Frank SHORT TERM...Doody LONG TERM...WTB AVIATION...Frank/Doody/WTB MARINE...WTB/Doody

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