Area Forecast Discussion
Issued by NWS Boston, MA

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393 FXUS61 KBOX 270639 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 239 AM EDT Sun May 27 2018 .SYNOPSIS...
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Scattered showers continue overnight as a back door cold front continues to push S. Much cooler weather with cloudy conditions and areas of rain today as low pressure along the front passes south of the region. Scattered showers or areas of drizzle over- night prior to possible improvement into Monday. High pressure brings mainly dry and warmer weather Tuesday through Thursday. An approaching weather system brings showers Friday.
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&& .NEAR TERM /THROUGH TODAY/...
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2 am update... Scattered shower activity persisting rearward of the back door cold front which has pushed S of New England as high pressure continues to build over SE Canada. Stretched mid-level short- wave energy NW-SE across NE New England up against energy ejecting out of the OH River Valley along with a continental- tropical airmass, observing mid-level lift and isentropic ascent along the cold frontal slope over S NH and NE MA, as thunder- storms have begun to erupt across the Mid-Atlantic an offshore, consequential of tightening gradients lending to a H3 jet max aloft of S New England, the right entrance region providing favorable venting above mid-level energy ejecting out of the OH River Valley. Likely PoPs for N/NE MA continue with chance PoPs elsewhere. A bigger challenge overnight has become low clouds and fog. Some confidence gained that with cooler, drier air pushing in from N/E beneath warmer air aloft, that beneath the inversion we should see cooling towards condensation overnight lending to low clouds, a bit less on fog. The further depth of cool, dry air S/W, the higher the cloud base. Figuring most locations will be at or below 1000 feet. Patchy fog issues retained for areas of rain and along the S/SE-coast. Preference to the NSSL WRF member within HREF members which has been doing well with respect to model reflectivity output. Other- wise, no specific forecast model has been outperforming the rest of the others with respect to winds, ceilings, and visibility. A preference to consensus of short-term, high-res guidance along with the NWS NBM. Today... Rain forecast. S-stream energy caught up in Pacific-vortmax, weak mid-level trof axis out of the OH River Valley with an associated continental-tropical airmass. Up against a cool, dry airmass brought S/W behind a N-stream vortmax / surface high pressure over SE Canada. Expecting isentropic upslope along 295-300K surfaces with SW H9-7 flow above undercutting surface-H9 E/NE winds. Ascent along the front up against the surface ridge over Canada, at its peak during the morning, lingering impacts late as ejecting energy / weak surface wave low along the now stalled frontal boundary sweeps E. However continued onshore E flow with high pressure NE, the low slipping off S/E, weak, onshore upslope component, expecting a transition from widespread rain to drizzle over much of S New England. Low clouds hanging through much of the day. Likely PoPs for locations along and S of the MA-pike. Prevailing with rain. Holding ceilings at or below 1000 feet.
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&& .SHORT TERM /TONIGHT THROUGH 6 PM MONDAY/...
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Tonight... A mix of scattered showers OR areas of drizzle. Abundance of low-level moisture, however crux of mid-level energy and trof axis is offshore. Beginning to see subtle height rises with drier air starting to work in within mid-levels. However, continued onshore NE flow as temperatures remain cool, steady around the low 50s. Onshore, upslope ascent beneath cyclonic flow aloft, some mid-level energy lingering, and stout dry-inversion pressing down through the column, leaned with scattered showers OR areas of drizzle. Undoubtedly continued low clouds, perhaps some fog issues for the high terrain with lower clouds, maybe along the SE coast. Monday... Clearing out? Weak ridging ahead of another potent N-stream vort- max over S Canada, kicking earlier synoptics E, out to sea, surface winds begin to revert S. Some improvement is possible. However it remains to be seen whether the combination of daytime heating and boundary layer mixing is able to meet with the suppressing dry-inversion such that the cloud layer between is mixed out. Am somewhat hesitant on forecast model clearing given such weak advective patterns, leaning that clouds hold in tight. However, if dry-air subsidence prevails, lowering down through the column, mechanical mixing could lead to low cloud erosion. Low confidence forecast at this time, but kept with earlier forecast trends of milder conditions W while cooler E remaining socked in longer.
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&& .LONG TERM /MONDAY NIGHT 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 /06Z 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. Short Term /through Monday/...Moderate confidence. 6z update... Remainder of tonight... Lowering MVFR-LIFR CIGs if not so already. NE winds persisting. Sustained around 10 kts, coastal gusts up around 20 kts. Low confidence concerning VSBYs, trending lower for high terrain terminals and along the S-coast. Today... Continued MVFR-LIFR CIGs with -RA/RA, especially along and S of the MA-pike. NE winds persist, strongest along coastal terminals. A threat of LLWS over the Cape and Islands. Tonight... No let up in CIGs nor winds. -RA/RA replaced by SCT SHRA and DZ. Monday... Possible gradual improvement W to E as winds shift S. CIGs lifting MVFR to low-end VFR, lingering longest for SE coastal terminals. KBOS Terminal... Confident in MVFR-IFR CIGs. Possible today that CIGs could lift to low-end VFR mid to late in the day, lowering back into evening with DZ. Majority of -RA/RA S of terminal today. KBDL Terminal... Mainly IFR today into tonight. N winds prevailing through the CT River Valley. Outlook /Monday Night through Thursday/...Moderate confidence. Monday Night: Mainly VFR, with local IFR possible. Patchy BR. Tuesday through Thursday: VFR.
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&& .MARINE...
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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 Monday/...High confidence. Through tonight... High pressure situated over SE Canada with low pressure sweeping E along a stalled frontal boundary S of New England, NE winds persist with potential gusts up to 25 kts, especially through tonight. Waves building 6 to 8 feet over the E/SE waters, trickling into the S waters later tonight, beginning to dissipate after midnight tonight. Small craft advisories continue. Potential visibility restrictions today as rain is forecast, especially for the S/SE waters, while drizzle will be an issue overnight for a majority of waters. Patchy fog at times. Monday... Weak high pressure setting in, winds relax and begin to revert S. Will continue to see wave action dissipate, as well as the conclusion of small craft advisories. Outlook /Monday Night through Thursday/...Moderate confidence. 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.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 7 AM this morning to 6 AM EDT Monday for ANZ232>235-237-255-256. Small Craft Advisory until 6 AM EDT Monday for ANZ231-250-251- 254.
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&& $$ SYNOPSIS...Doody/Sipprell NEAR TERM...Doody/Sipprell SHORT TERM...Sipprell LONG TERM...Doody AVIATION...Doody/Sipprell MARINE...Doody/Sipprell

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