Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201437 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1037 AM EDT Thu Jul 20 2017 .SYNOPSIS...
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Hot and humid weather persists today. Scattered strong to severe thunderstorms are possible across the interior Northeast U.S. today while a more focused area of heavy rain and thunderstorms is expected mainly S of the Massachusetts Turnpike tonight. Dry and very warm afternoon periods expected Friday and Saturday, but with lower humidity. Unsettled weather with showers at times are expected Sunday through Tuesday with below normal temperatures.
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&& .NEAR TERM /through this afternoon/...
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1030 pm update... */ Highlights... - Afternoon strong to severe thunderstorms possible across the interior NE CONUS with threats of gusty winds, heavy rain (and potential flooding), even the possibility of hail along with frequent lightning. - Overnight thunderstorms with heavy rain and potential flooding as the main threats, more likely S of the Massachusetts Turnpike. - Heat and humidity continue. HEAT ADVISORY expanded to include N CT. */ Discussion... Quickly, another day of upper 80s and low 90s are forecast for much of S New England with heat indices rising into the low to mid 90s prior to the boundary layer mixing out and allowing the mix-down of both faster westerlies and drier air. Simply put a repeat of yesterday. Thus have expanding the HEAT ADVISORY to incorporate N CT, especially the Hartford-Metro area. Feel Springfield will be just on the cusp of mid-90 degree heat indices. This afternoon: Mid to upper level forcing associated with a mid level vortmax and LFQ of an upper level jet. Indications of diffluence and venting above a destabilizing boundary layer through which there is modest shear and lapse rates that are anywhere from 6.5-7.0 C/km within the mid levels. Have gone ahead with scattered thunderstorm wording with moderate rain. Also included gusty winds. Given the environment, can`t rule out heavy rain / flooding threats along with hail. Some indications within soundings of decent CAPE aloft. Somewhat absent is a low to mid level lifting mechanism. Feel the terrain may be of some help. There is some uncertainty but felt it warranted to increase some of the thunderstorm wording. Like outcomes advertised by the high-res 3 km NAM. Consensus of all other guidance preferred with respect to mid to upper level fields which suggest the best forcing is to the N beneath best lapse rates and forcing.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
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Overnight: Much of the region falling in wake of afternoon mid to upper level forcing mechanism. QG forcing mechanisms on the plus-side indicating suppression as well as more NVA tendencies. So the focus is on the low to mid level convergent forcing mechanisms. Along the quasi-stationary boundary, whose position is somewhat in question, there looks to be a focus of surface-h85 low-level inflow of high surface dewpoints and theta-E. But again position is in question as well as the S component of wind. The wind profile itself could be flat, more parallel to the QS-boundary lessening the convergent impact. But will lean with some sort of mesoscale convective system emerging along the boundary over the S Great Lakes region and sweeping E with the mean flow. Am leaning away from strong to severe tendencies but can`t rule them out. Am more concerned that the localized moisture convergence will lead to a period of heavy rain and potential flooding. Have highlighted heavy rain within the forecast. Could see low clouds and fog filter back in on the S coast during the overnight hours with the moist fetch.
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&& .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Fri & Sat: Dry and very warm afternoons but with lower humidity * Sun into Tue: Cooler than normal + unsettled with showers at times * Drying trend likely by the middle of next week Details... Friday and Saturday... Anomalous closed upper level low across eastern Canada will result in northwest flow aloft into southern New England Fri & Sat. The result will be dry weather into at least Saturday afternoon. Plenty of sunshine should push high temps well into the 80s on both days, and a few locations may reach 90 on Friday. However, drier air will mix down resulting in lower humidity than what we have experienced the last few days. Sunday through Tuesday... The anomalous closed low across the Canadian Maritimes will help to carve out an upper level trough across the northeast. The result will be temps averaging below normal. Specific daily high temps are uncertain and will depend on timing, cloud cover, and location of any surface boundaries. Highs will mainly be in the 70s over this time frame, to perhaps a few lower 80s. It is also possible that a some locations have a day where it struggles to break 70 as a result of a surface boundary to the south coupled with northeast winds/low clouds. Plenty of shortwave energy rotating around the upper trough will result in periods of showers and perhaps isolated thunderstorms at times. It does look rather unsettled, but the entire period will not be a washout it is just impossible to try to pin down timing this far out. Wednesday... Forecast highly uncertain in this time range, but guidance shows upper level trough finally lifting out which would result in dry and pleasant weather. && .AVIATION /14Z THURSDAY THROUGH MONDAY/...
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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 Tonight/...Moderate confidence. Today... VFR. Gusty W/SW winds up to 20 kts especially along the S coast. SCT SHRA/TSRA developing, especially N/W. Threats of heavy rain along with gusty winds. TEMPO MVFR-IFR impacts with any storms. Tonight... SCT SHRA/TSRA sweeping offshore early on, then focus is upon terminals S of the MA pike with potential +RA/TSRA threats. MVFR-IFR possible, especially along the S coast with LIFR potentially filtering in. Breezy W/SW winds. KBOS Terminal... Will continue the risk of a sea-breeze towards midday. May see some SCT SHRA/TSRA activity move through the afternoon into evening hours. KBDL Terminal... SCT SHRA/TSRA activity this afternoon is mainly anticipated N/W of the terminal, however can`t rule out the risk. Greater risk of +RA/TSRA overnight, though that may hold out more S/W. Outlook /Friday through Monday/... Friday and Saturday...High confidence in VFR conditions. Sunday and Monday...Moderate confidence. MVFR to IFR conditions likely at times in periods of showers, low clouds and fog patches. VFR conditions will also likely occur, but timing of all this is uncertain.
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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 Tonight/... Today and Tonight...High confidence. SW flow becoming W towards the morning before returning SW. During daytime periods and along the S/SE near shore, gusts up to 20 kts are expected. Seas remaining below 4 feet. Scattered showers and thunderstorms are forecast to sweep the E waters late this afternoon into evening. A more well-organized complex of showers and thunderstorms is expected across the S waters overnight. Outlook /Friday through Monday/... Friday and Saturday...High confidence. Pleasant boating weather as weak pressure gradient will keep winds/seas below small craft thresholds. Sunday and Monday...Moderate to high confidence. East to northeast winds may gust between 15 and 20 knots at times, but probably remain below small craft thresholds for the most part. Areas of fog and showers may reduce visibilities for mariners at times.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Heat Advisory until 6 PM EDT this evening for CTZ002>004. MA...Heat Advisory until 6 PM EDT this evening for MAZ017>021. RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008. Heat Advisory until 6 PM EDT this evening for RIZ001>004-006. MARINE...None.
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&& $$ SYNOPSIS...Frank/Dunten NEAR TERM...Frank/Dunten/Sipprell SHORT TERM...Dunten LONG TERM...Frank AVIATION...Frank/Dunten/Sipprell MARINE...Frank/Dunten/Sipprell

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