Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221845 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 245 PM 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 6 AM SATURDAY MORNING/... 2 pm Update... Getting heat index values in the upper 90s and just shy of heat index criteria. Did add a short paragraph in the updated SPS for convection. WSW wind is quite favorable for high temperatures along the eastern MA coastal plain given some downsloping in addition to an air mass with 850 mb temperatures pushing 20C. There remains below average confidence on convection for later this afternoon and evening. Looks like enough instability to initiate convection after about 3 or 4 PM in the western zones. Vertical wind shear is marginal and still think convection is most likely to take the form of individual pulse cells or small clusters, although some guidance hinted at the possibility of a mini-MCS trying to form near NW CT and SW MA. Not really buying into the mini-MCS solution at this point but does look like SW MA and CT may be a somewhat more favored region for scattered thunderstorms mid to late afternoon and then possibly shifting to RI and SE MA during the evening. Confidence is quite low how this might evolve. There are signals of drier air attempting to work its way into northern MA and have lowered POPs some in that region but not confidence enough to go below chance. Low level jet across SE MA resulting in significant surface wind gusts, especially along portions of the Cape. This will likely continue into early evening and then gradually diminish. 1015 AM Update... Heat indices are expected to reach mid to upper 90s across much of the area for a few hours this afternoon. This remains below Heat Advisory criteria but still high enough to have some impact. May make a mention in the SPS for convection that will be updating shortly. Most of the forecast area remains in a slight risk from SPC. Vertical shear looks to be marginally high enough to support a strong storm or two combined with sufficient instability. NAM may be a little overdone in its convective robustness but still looks like enough ingredients in place for pulse storms and perhaps small clusters with the potential for localized damaging winds and large hail. Precipitable waters of 1.5+ inches suggest heavy downpours could also be an issue. It is difficult to pinpoint exactly where these storms will fire. There is a suggestion of a weak boundary in eastern NY along the I-90 corridor and some activity may initially fire over the higher terrain. Then, may see outflow boundaries from one storm or cluster be the initiating mechanism for future convection. The surface instability looks to remain high well into the evening and think may see individual storms or small clusters develop off and on through at least mid evening. Believe we may start to see thunderstorm development initiate around 3 PM. Have adjusted temperatures a little upwards in portions of the area based on current trends and consistent model 850 mb temperatures of 19C to 20C mid to late afternoon across eastern half of area. 7 AM Discussion... 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 for 3 PM to 10 PM time frame... ***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. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Updated at 2 PM... * Risk of strong to severe thunderstorms Saturday afternoon into early evening across northern and eastern Massachusetts... Increasing confidence of a severe thunderstorm threat across northern and eastern Massachusetts Saturday mid afternoon to early evening. Main threats look to be locally damaging wind gusts and hail. The 12Z GFS and NAM continue to paint an area of instability across northern MA and eastern MA Saturday afternoon/early evening. This being helped by an impressive 500 mb cold pool of temperatures lowering to -12C to -13C in the 21Z to 00Z time frame. In addition, the vertical wind shear looks more impressive Saturday afternoon/evening than this afternoon with 500 mb winds near 40 knots. Other considerations include low level moisture convergence signal from what is left of the surface frontal boundary and the presence of the left front quad of a 90+ KT upper jet over NE MA. Have updated the HWO to communicate this increaseing concern of the risk for severe thunderstorms during Saturday afternoon/evening, mainly northern and eastern MA. 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 /SUNDAY THROUGH FRIDAY/... Big Picture... High pressure covers the Southern and Central USA much of the week. A zonal flow extends across Canada and the USA Northern Tier. Shortwaves in that zonal flow will move through New England Monday night and again on Friday. In between, parts of the large high will push up over our area. Forecast contour height fields remain higher/warmer than normal levels through the period, so expect temperatures to run warmer than normal most of the time. Details... Sunday... High pressure in place through the day. Some signs on the GFS and ECMWF that a twist may develop in the flow in Eastern MA that could generate an eastern coast sea breeze during the afternoon, but low confidence at this time. Solar mixing to near 850 mb with temps at 850 mb forecast at 16-17C. This would support max sfc temps inland at 87-91F, with ocean buffering keeping levels near 80 Cape and Islands. Northwest flow will bring lower/drier dew points of 55 to 60 inland and lower 60s along the South Coast. Light wind Sunday night. Some increase in high clouds and slowly increasing dew points should keep min sfc temps in the 60s, although upper 50s not out of the question in some normal cool spots. Monday... Shortwave drives low pressure across Eastern Canada with a cold front sweeping across NY during the day and New England Monday night. Most of the day should be rain-free. Some signs of marginal instability crossing during the morning which may be a warm front with a couple of light showers. Better instability moves in from the west during the afternoon and evening. Totals forecast in the upper 40s and mid-level lapse rates 6.2 to 6.5 C/Km. Precip Water values climb to near 2 inches. This would support thunderstorms, some with heavy downpours. Winds of 20-25 kt at 850 mb and 35 kt at 500 mb suggest potential for strong gusts in convection. Expect scattered afternoon/early night convection with local downpours. Temps of 18C at 850 mb supports another day of max temps in the low 90s. Tuesday-Wednesday... High pressure takes control and brings fair weather. Temps aloft will again be 16-18C which will support upper 80s and lower 90s. Thursday-Friday... Next upper shortwave moves through during this time. Both GFS and ECMWF seem inconsistant with their surface features but indicate at least one cold front moving through during this time. We will show chance pops in western sections Thursday afternoon and all areas Friday. Low confidence in details at this time. && .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Rest of this afternoon...High confidence except for placement of scattered late afternoon/evening thunderstorms. VFR conditions expected to prevail except briefly lower in vicinity of scattered late afternoon and evening thunderstorms. Some hints that the thunderstorms would be more likely across SW MA and CT late this afternoon but confidence no more than moderate on this. A few thunderstorms may contain strong wind gusts and hail. Surface wind gusts to 35 KT across Cape terminals due to presence of low level jet aloft. Tonight...Moderate confidence. A short period of low level wind shear is also possible around mid/late evening as surface winds diminish. Areas of MVFR cigs/vsbys and even pockets of IFR tonight in areas of stratus and fog, mainly along the south coast and a few interior valley locations that receive rain. Winds shift to the NW overnight. Saturday...Moderate confidence. After any early fog dissipates. VFR dominates again most of the day. Afternoon/early evening thunderstorms are possible, mainly across northern and eastern MA. A few of these thunderstorms may contain strong wind gusts and hail. Winds mainly W. KBOS TAF...High confidence. WSW flow should dominate during the afternoon and think a sea breeze is unlikely. There remains a chance of an evening thunderstorm but thinking it is a low chance at this point. There looks to be a better chance of thunderstorms in the vicinity of the terminal Saturday late afternoon to early evening. A few of those thunderstorms could contain strong wind gusts. KBDL TAF...High confidence of VFR prevailing. There is a risk of thunderstorms in the vicinity of the terminal late this afternoon and early evening. Any thunderstorms could produce a brief strong wind gust. If it rains over or near the airfield, there would a possibility of MVFR visibilities in overnight fog patches. Outlook /Sunday through Wednesday/... Moderate-high confidence. Sunday...VFR. Possible areas of IFR in early morning fog. Monday... VFR. Possible IFR in early morning fog. Brief MVFR/IFR cigs/vsbys in afternoon and early night showers/tstms. Tuesday-Wednesday... VFR. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...High confidence. Today and tonight... 2 PM update... Have continued with the SCA in place. Seeing some gusts of 35+ knots over land but generally in the 25 to 30 knots category over the adjacent coastal waters. Prior discussion... 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 /Sunday through Wednesday/... Moderate confidence. Winds less than 20 knots and seas less than 5 feet through the period. A cold front will bring scattered showers/thunderstorms Monday afternoon and night, with potential for strong wind gusts and brief poor vsbys in heavy downpours. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert until 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert until 11 PM EDT this evening for MAZ005>007- 012>021. RI...Air Quality Alert until 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...WTB/Doody/Thompson NEAR TERM...Doody/Thompson SHORT TERM...Doody/Thompson LONG TERM...WTB AVIATION...WTB/Doody/Thompson MARINE...WTB/Doody/Thompson is the U.S. government's official web portal to all federal, state and local government web resources and services.