Area Forecast Discussion
Issued by NWS Boston, MA

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379 FXUS61 KBOX 221130 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 730 AM EDT FRI JUL 22 2016 .SYNOPSIS... Heat and humidity increase today along with the risk for scattered showers and thunderstorms during the afternoon and evening, some of which may be severe. There may be a few leftover thunderstorms Saturday. Hot weather is expected to continue this weekend. An approaching cold front will bring another round of scattered showers and thunderstorms Monday night and Tuesday. More warm weather follows midweek. && .NEAR TERM /UNTIL 3 PM THIS AFTERNOON/... No major changes to the ongoing forecast this morning. A few showers developing in the Berkshires already. All varieties of CAPE were less than 300 J/kg, so not expecting thunderstorms just yet. Tweaked the forecast to bring conditions back in line with observed trends. Previous discussion... Pre-frontal trof is already igniting low-topped convection across N NY but this continues to slide mainly into Canada, closer to where the developing poleward region of the mean jet resides. The lack of influencing clouds, most of the upstream clouds continue to dissipate, will allow good mixing this morning which will yield very hot wx. H85 temps are already approaching +20C and with morning lows only dropping into the upper 60s there is plenty of room for heating into the mid 90s across much of the region. Dwpts in the mid 60s will also yield heat-indices between 95-100, just shy of heat advisory criteria, but this will have to be monitored. Bulk of convective potential after 3PM, so see below for severe risk. && .SHORT TERM /3 PM THIS AFTERNOON THROUGH 6 PM SATURDAY/... ***Risk for strong to severe thunderstorms late today into the early overnight hours*** Late today and tonight... Convective overview... At this time it appears the marine layer under SW flow will be the primary mitigating factor for destabilization as overnight cloud cover remains mostly CI which will do little to impact the diabatic heating. All guidance, including meso-scale guidance continues to indicate ML CAPE values reaching nearly 1500j/kg by 21Z-00Z thanks to remnant EML with mid lvl lapse rates 6.0-7.0C/km peaking late evening. Therefore, still plenty of instability above the marine layer to work with, supporting CAPE values near the -20C layer around 300. At odds is timing and how widespread the convection will be, initial pre-frontal trof looks to trigger scattered convection earlier in the day mainly across N New England, closer to the poleward exit region of the mean upper lvl jet streak. Later, with convection firing upstream in NY state (less of a marine influence and therefore better sfc based instability). What this means for S New England will take some time to fully flesh out, but it appears that the upstream convection will need to be watched as it will most likely define the overall convective mode, should an MCS form there it could track mostly SW of the region with more sct pulse storms across NE or much less activity altogether, but any shift would bring it through our region instead. WRF/NAM support this possibility. Either case it/s later, more with the core of the remnant EML and upper lvl shortwave passage (mainly after 21Z and continuing through about 04Z or midnight local). The risk remains, mainly later in the day, so this morning`s update will reflect this timing/thinking: Winds... Primary risk with 0-3 and 0-6km shear reaching about 35 kt with modest turning in the hodograph. Plenty of CIN supporting high downdraft CAPE as well. Therefore, regardless of the mode, straight-line damaging winds remain the highest threat, especially if an MCS or bowing reflectivity patterns are observed. Hail... Secondary risk due to relatively high hail-CAPE as mentioned above. The shear mentioned above also supports some rotation in the updrafts of organizes storm which will enhance hail-growth. Mitigation is the very high WBZ temps, mainly at/or above 10kft. Will continue the use of small hail with this update. Heavy rain... Warm clouds layers exceeding 10kft combined with PWATs nearly 1.75 (1 std deviation above normal) will allow any storm to produce localized heavy rain. Corfidi vectors suggest some back- building potential, so will need to monitor for localized urban flooding in particular. Tornado... Low risk, but non-zero. Very high LCLs is not overly supportive. However should an isolated supercell structured storm develop especially in the vicinity of the southern sea breeze or other localized low-lvl vorticity source, this will need to be watched. EHI values already progged to be between 1.0-2.0 and near sfc lapse rates are nearly dry-adiabatic thanks to mixing. As mentioned above, this lingers into the early overnight hours until cold front itself stalls near the south coast. Will continue to monitor the risk, although it`s likely to gradually dissipate during the morning as drier air begins to move in. Saturday... Another hot day as H85 temps will struggle to drop lower than +18C, so expect another round in the low 90s. Dwpts will be slightly lower however thanks to influence of low-lvl drier air, so expect heat indices to still remain below advisory thresholds. Otherwise, am concerned about two factors. The first is the cold front itself which may remained stalled very close to the S coast and provide a secondary focal point for afternoon convective activity. The other is a secondary shortwave which could ignite convection across Quebec which may slide south into New England late day as the northeast CONUS remains along the outer edge of the ring-of-fire. Convective mode from today and final frontal placement will all need to be watched as a secondary risk for thunderstorms looms for Saturday, which could once again contain strong winds. Should conditions not be favorable, the highest risk would remain in northern New England. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Big Picture... 23/00z guidance has come into better agreement with the overall synoptic pattern for this portion of the forecast. Still some detail differences, but that is not unusual for this time range. Will favor a consensus approach as the basis for this forecast. Mean mid level trough shifts east late this weekend, leaving behind a flattening ridge for early next week. This leads to a nearly zonal flow through the mid week. This pattern will have an impact on our weather by slowing the passage of a cold front, and keeping it nearby. Details... Saturday night and Sunday... A cold front will continue to slowly move offshore. Moisture fields show some post-frontal drying across our area. Max temperatures should be slightly lower Sunday, despite abundant sunshine. Monday and Tuesday... It still appears our region will see a period of wet weather sometime during these two days. The most probable period would be late Monday into Monday night. A warm frontal passage Monday will lead to a return of heat and high humidity. These conditions should linger until a cold front passes by Tuesday. Anticipating enough instability to mention both showers and thunderstorms in the forecast. Wednesday and Thursday... Weak high pressure builds over the region. This should result in a quiet weather period. Temperatures are still expected to be above normal. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... Through today...Moderate confidence. VFR through most of the day. Afternoon, expect showers and thunderstorms to develop first north and west of southern New England, then across southern New England mainly 21Z-03Z. Some of these storms could have localized strong winds/heavy rain and hail, as well as localized IFR conditions. Winds mainly SW, with gusts 20-30 kt mainly across SE MA. Tonight...Moderate confidence. Storms will gradually dissipate and move offshore through about 03Z-06Z. While VFR may follow, periods of IFR/MVFR fog and lower clouds may follow, especially at airports the received rain. Winds shift to the NW. Saturday...Moderate confidence. After any early fog dissipates. VFR dominates again most of the day. Afternoon shra/t-storms possible, although mainly across northern MA at this time. Winds mainly W. KBOS TAF...Sea-breeze may hang just off the door-step of the terminal around 19-20z. W/SW winds prevailing. KBDL TAF...VFR prevailing. Outlook /Saturday Night through Tuesday/... Sunday...High confidence. VFR. Monday into Tuesday...Moderate confidence. Mainly VFR conditions expected. Brief periods of MVFR conditions in SCT SHRA/TSRA possible. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. Today and tonight... SW winds continue and increase especially near the southeast coastline today. Gusts 25-30 kt expected. This will also allow winds to increase to near 7 ft by the overnight hours. Small craft advisories will continue. Will also need to watch for strong thunderstorms with localized higher winds, hail and heavy rainfall. Saturday... Winds and seas generally slacken such that small craft advisories may be dropped through the early half of the day. Will still need to monitor for some isolated showers and thunderstorms. Outlook /Saturday Night through Tuesday/... Winds and seas briefly diminish Sunday behind a cold front. Showers and thunderstorms are possible over the waters from late Monday afternoon into Tuesday. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for MAZ005>007-012>021. RI...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory until 8 AM EDT Saturday for ANZ231>234- 236-250-254. Small Craft Advisory until 2 PM EDT Saturday for ANZ235-237- 255-256. && $$ SYNOPSIS...Belk/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Belk AVIATION...Belk/Doody MARINE...Belk/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.