Area Forecast Discussion
Issued by NWS Boston, MA

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568 FXUS61 KBOX 210748 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 348 AM EDT THU JUL 21 2016 .SYNOPSIS...
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Heat and humidity return Friday along with the risk for scattered showers and thunderstorms during the afternoon and evening, some of which may be severe. Hot weather is expected to continue this weekend. An approaching cold front may bring us some relief from the heat and another round of scattered showers and thunderstorms sometime Monday and/or Tuesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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SKC conditions again this morning have still allowed for several locations to drop back into the low-mid 50s. With these same clear conditions, temps will be able to rise quickly this morning, mixing to H85 where temps hover near +18C. Therefore, will likely see a few 90s today for the first time in a few days. 80s outside of the spots where downsloping will help. Dwpts will begin to increase thanks to return flow with high pres to the E, although the good mixing should keep them mainly in the upper 50s to low 60s, still comfortable in spite of the increased temps.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
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Tonight... Another quiet night wx wise thanks to last minute influence of high pres to the E. However in this case, in spite of the clear skies and weak pres gradient, rising low-lvl moisture suggests a more mild night across the region. Expect temps mainly in the low to mid 60s. Some patchy fog possible too thanks to this increase in moisture, but likely only in the typically prone spots. Fri... The primary issue for Fri remains the risk for late-day severe wx. Although heat/humidity will be a factor. H85 temps will be approaching +18C while sfc dwpts increase into the mid 60s to near 70, which given enough sunshine is enough for highs reaching the upper 80s to low 90s and a low risk for heat advisory criteria. Note that most of the following discussion will actually be moot for the Friday (daylight) time frame if the the continued slow- down in the guidance continues, suggesting the pre-frontal trof doesn`t arrive until late evening, potential limiting sfc based instability as we experience diurnal heat loss. Also, early Fri may be marred by upstream convective cloud debris limiting the destabilization. Still plenty of factors pointing in the right direction, remnant EML (peaks overnight Fri) with 6.0-7.0C/km lapse rates arrives in time for the return flow and moistening low levels. Yielding ML CAPE values in the afternoon near 1500j/kg (per ensemble probabilities). Profiles are relatively wide, with hail-CAPE values +300j/kg. Shear reaches 30kt at 0-3 and 0-6km with a nearly 40 kt H9 LLJ. Therefore, should be enough for organization of any storms that form, with hail an issue and some rotation in updrafts. Winds still look to be the primary issue, thanks to inverted-V profiles and LLJ, and as such will be upgrading wording to strong winds with this morning`s wx update. While winds/hail remain the primary risks, especially since shear/CAPE profiles support MCS organization which would peak overnight (at the point of EML peaking), there is a low end tornado risk given the lowest lvl shear values and EHI between 1.0-2.0. However, not a pure S New England tornado break case per internal study and the very dry mixed layer (high LCLs). For now given the combination of timing uncertainty and mitigating factors mentioned above, will be slowing timing and upgrading gusty winds to strong winds, especially with the recent slow-down and factors suggesting an MCS could be the primary convective mode. Will also be adding T+ (severe type) wording given continued agreement with SPC SWOs, which continues to highlight most of S New England away from the ocean. Stay tuned to updates as timing/mesoscale issues remain somewhat uncertain.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights.. * Heat and humidity return Friday through the weekend * Risk of showers/thunderstorms, some severe possible Friday * Another cold front may bring showers and storms again Monday Overview...The focus of the long term forecast will be on the thunderstorm potential for Friday, therefore, won`t go into that much here. Aside from that, hot and humid weather continues into early next week. A cold front late Monday into Tuesday will likely bring a little relief from the heat as well as another chance of showers and thunderstorms Monday afternoon and night. High pressure then builds over southern New England Tuesday and Wednesday. Overall, the models are in fairly good agreement on the synoptic pattern for this period. As always, there are slight timing differences and certainly mesoscale patterns that have yet to be resolved, however, for the bulk of the long term, see no problem using a blend of the models. For temperatures, Friday into the weekend, will likely raise the blend by a few degrees to retain the heat a bit better than the blend. Friday...Main concern for the long term is the thunderstorm potential. There are many things that are seemingly coming together for thunderstorms and even a few severe thunderstorms to be possible. However, there are still some potential kinks in the system that could limit the instability, etc. Other than the potential for storms during the afternoon and evening, much of the day will be dry but hot and humid with temperatures climbing into the low to mid 90s for much of the region. Positives for thunderstorm development and severe potential: - a shortwave moving across northern New England - low pressure in Quebec pulling first a pre-frontal trough and then a cold front through southern New England - decent shear (about 30 kts from 0-6km) - increasing dewpoints - strong low level jet (35-45 kts at 925mb) Negatives or questionable elements: - surface CAPE increasing to over 1000 J/kg during the afternoon (This will be dependent on sunny skies during the morning and early afternoon, which is of question. There is potential for lingering overnight convection over the Great Lakes to move over southern New England during the morning hours, with cloud debris limiting the instability potential.) - remnant EML moving over southern New England (The GFS specifically shows this better than either the NAM or ECMWF. While there is some evidence of a mixed layer on the NAM and ECMWF, it`s not as defined or strong as it is on the GFS, and is focused more across northern New England than southern New England.) All that to say, there is certainly potential for severe thunder- storms Friday afternoon and evening, as indicated in the Day 3 SPC outlook. However, as indicated yesterday in this discussion, so much of severe weather comes down to the mesoscale which is rarely clear this far out. Timing of all these features and the development of the instability during the day Friday will all be factors in determining how widespread any severe weather is, should it develop. Those with outdoor plans on Friday should keep a close eye on the weather over the next day or so. Saturday and Sunday...Could see some showers lingering into Saturday if the cold frontal passage is a bit slower than currently forecast. Otherwise, much of the weekend should be dry, though hot and humid. Temperatures will soar well into the 90s for much of the area with dewpoints in the mid 60s under mostly sunny to sunny skies. Monday...Low pressure moves through Quebec bringing another cold front through southern New England late Monday into early Tuesday. This will likely bring a bit of relief from the heat and humidity of the weekend. In addition, another round of showers and thunderstorms is possible. At this point, this looks like your garden variety diurnal showers and thunderstorms, but will need to keep an eye on it through the weekend. Tuesday and Wednesday...Behind the cold front, high pressure builds into southern New England. This will result in quiet, dry weather. Temperatures/dewpoints will be a bit cooler than the weekend but will remain in the normal range for late July. && .AVIATION /07Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...High Confidence. VFR into Fri. Fri afternoon/evening occasional T-storms and showers possible with strong winds and localized IFR possible. Winds mainly W-SW, nearing 20 kt this afternoon around the S coast. KBOS TAF...High confidence. KBDL TAF...High confidence. Outlook /Friday Night through Monday/... Friday...Moderate confidence. Mainly VFR conditions expected. Brief periods of MVFR/IFR conditions possible in SCT SHRA/TSRA. A few of these storms may become severe, producing strong, gusty winds and large hail. Saturday...Moderate confidence. Mainly VFR. Could see some lingering MVFR conditions in lingering -SHRA or areas of fog in the morning hours. Sunday...High confidence. VFR. Monday...Moderate confidence. Mainly VFR conditions expected. Brief periods of MVFR conditions in SCT SHRA/TSRA possible. && .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 Friday/...High confidence. Today and tonight... Winds shift mainly to W-SW. A few gusts around 20 kt possible through the afternoon, and a very low risk of a few gusts reaching 25 kt, but these will likely be few and far between. Otherwise quiet boating wx. Winds drop off tonight. Sat... Increasing wind gusts ahead of approaching cold front. SW gusts 25-30 kt at times expected, will be issuing small craft advisory. This will also lead to some wind driven seas near 5-6ft. Outlook /Friday Night through Monday/... Seas increase Friday and Friday night as a cold front approaches and then moves over the coastal waters Friday night into Saturday morning. Seas could reach 5 to 7 feet on the outer waters during this time. They then gradually subside through the weekend before increasing again slightly in response to another approaching front late Monday night. Small craft advisory conditions are expected Friday and Friday night with southwesterly winds increasing to 15 to 20 kts with gusts in the 25 to 30 kt range. Otherwise, winds will remain below 25 kts through the weekend and into Monday. Showers and thunderstorms are likely possible over the waters Friday evening and night and again on Monday afternoon. Storms Friday may become strong, producing dangerous boating conditions at times. In addition, areas of fog may develop on Friday night, limiting visibilities.
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&& .FIRE WEATHER...
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Massachusetts state fire officials have advised that given the forecast warm temperatures into the upper 80s to low 90s, low humidity values, and lack of rainfall as of late that there is a heightened fire danger. Such conditions likely exist for other parts of southern New England, namely CT and RI.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from noon to 8 PM EDT Friday for ANZ231>234-236. Small Craft Advisory from 11 AM to 8 PM EDT Friday for ANZ235- 237-255-256. Small Craft Advisory from 2 PM to 8 PM EDT Friday for ANZ250- 254.
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&& $$ SYNOPSIS...Belk/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Belk AVIATION...Belk/Doody MARINE...Belk/Doody FIRE WEATHER...staff

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