Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 190214 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1014 PM EDT Sun Jun 18 2017 .SYNOPSIS... Humid conditions continue through Monday night. There is a risk for some severe thunderstorms, as well as flash flooding for some during Monday afternoon and evening. A cold front over Southern New England moves offshore Tuesday. Weak high pressure then builds over the region Wednesday and Thursday with seasonable temperatures and lower humidity. Warmer temperatures return late in the week as the high moves off to the east and another cold front approaches from the west. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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1015 pm update... Areas of fog were impacting the south coast, Cape and Islands this evening. However, stronger surface wind than last night was preventing widespread/dense fog in this region, so no advisory planned at this time. Temps along the south coast will hold in the middle to upper 60s overnight. Away from the marine influence of the south coast, low temps will not drop below 70 given non-decoupling boundary layer and high dewpoints in place. Not much fog if any expected across this region given enough boundary layer wind. As for precipitation chances, we are watching a line of showers and thunderstorms across Central NY into portions of northeast PA. Bulk of this activity will weaken/fall apart as it lifts northeast and out runs the stronger synoptic scale forcing. However, a few showers and perhaps an isolated t-storm may survive into western MA after 6z. May also see a few showers and perhaps an isolated t-storm popup toward daybreak across southeast New England where a 30 to 40 knot low level jet interacts with some elevated instability. Overall much of the region will remain dry through daybreak, but a few spot showers and perhaps an isolated t-storm will be possible.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... */ Highlights... * Localized Flash Flooding along with the potential for severe thunderstorms exists across western and central MA and portions of northern CT tomorrow afternoon and night */ Discussion... Strong upper level jet and attendant mid lvl shortwave pivot into NY/PA through the morning tomorrow. This allows the equatorward entrance region of the mean upper jet to line up such that best venting is focused on E NY/PA/NJ into western SNE by mid-late afternoon. Lapse rates in this airmass are not the best, mainly 5.5-6.0C/km given this is not an EML type situation. If anything, the high PWATS promote HPE type storm development. The cold front remains W through 00Z, shifting into New England only by about 06Z, however noting enough slackening in mass fields to suggest modest pre-frontal trof as a focus for convection beginning across E NY/PA/NJ first, then shifting E as remnant low stratus dissipates mid day-afternoon. Given these facts, best chance for severe will remain within area enhanced area highlighted by both SWODY3 (yesterday) and SWODY2 (today), mainly across E NY into portions of W New England. Timing... Prefrontal trof development, combined with a consensus look at available mesoscale guidance, suggest that best activity falls across W MA/CT between 2PM and 8PM from W-E. While linear structure to the convection is expected moving E with mid lvl flow, individual cells/line will actually be shifting SW-NE with time, slowing the overall progression, hence the slow timing. Further E, late evening and overnight timing expected for E MA/RI, as activity may not start until near or after sunset for those furthest E. Storms could even be dissipating with time due to loss of diurnal support. Will continue to highlight a slight risk for severe however, as upper lvl CAPE profiles are maintained well into the overnight hours and shear values change little. Severe threat... 0-6 km shear values peak during the afternoon within a somewhat linear wind profile, 30-40 kt. This is within an airmass holding 1500-2000j/kg sfc based and ML CAPE values. Noting that hodographs within this shear suggest multicell clusters potential forming a more organized lines of convection which would enhance the risk for damaging winds. This will be the primary severe threat. 400-500j/kg of CAPE within the hail growth region support some small hail as well. Noting EHI values 1-3 and low LCLs, which also suggest a low risk for an isolated tornado as well, which will need to be watched especially where enhanced valley motions are able to shift the low lvl flow more SE enhancing the low lvl shear. Especially true if some more discrete cells can form ahead of the lines. Will continue to highlight winds as the primary threat given the convective mode supports this. Regarding how far the severe risk extends, the best chance will be mainly W of the Worcester hills/W Connecticut through the afternoon and evening. The threat is non-zero further E however, but it will be later (mainly later evening and overnight), with less SB convection and more elevated convection. Probabilities of precipitation will reflect this thinking as well. Flash flooding threat... As mentioned before an east propagating line of storms will see individual cell structures actually move SW-NE, this suggests training within an airmass primed for HPE storms. CAPE profiles are relatively skinny given the modest lapse rates. PWATS exceed 2.00 inches widespread and warm cloud layers 10-13kft. This could easily lead to rainfall rates near or exceeding 2.00inches/hour especially across W MA and CT where highest theta-e plume is noted. This plume has shifted slightly E with latest data updated, and given that at least elevated convection will hold later into the evening/overnight will be expanding the Flash Flood Watch to include Tolland CO in CT as well as Worcester and NW Middlesex COs in MA. Overall, will highlight strongest threat for severe weather and flash flooding late day tomorrow into the early hours of the overnight tomorrow night. The severe threat will diminish as the storms move east into E MA and RI late evening and overnight, but these will still need to be watched even as some storms continue. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Big Picture... The early week upper pattern shows closed lows over the Great Lakes and Gulf of Alaska with a ridge in between over the Western USA. The Great Lakes low ejects northeast into Canada, and sweeps a broad trough across New England Tuesday and Wednesday. The western ridge flattens, leaving a zonal flow across the northern tier of the USA. The Gulf of Alaska low ejects eastward into that zonal flow, approaching New England toward next weekend. Contour and thermal fields are near normal midweek and above normal late week and weekend. Expect near normal temperatures midweek trending warmer than normal Friday-Saturday. Model mass fields are in agreement through Thursday, then diverge. The main effect for us would be on timing of the system that reaches us next weekend. Model thermal fields over the Northeast are similar through Friday and then diverge next weekend. Run-to-run consistency through midweek, with more notable differences Friday to Sunday. This means confidence on the high side of moderate midweek, diminishing to low confidence Friday to Sunday due to model-to-model and run-to-run changes. Details... Tuesday...Moderate confidence. One shortwave moves through the cyclonic flow Tuesday morning, primarily crossing from NY into VT/NH/ME. The associated upper jet lingers to our west over upstate NY. With the main upper low so far to the north, shear across the cold front is low and makes it difficult to locate on forecast charts. Low level thickness field suggest it is centered over Srn New England and lingers through the day. This is closely aligned with the southwest-northeast upper flow, suggesting it is either stalled or sluggish. High precipitable water values are depicted over SE Mass, especially Cape Cod and Islands, and moving offshore in the afternoon. Cross sections of moisture show drier air moving in aloft by mid Outlook /Monday through Thursday/... Moderate confidence. Tuesday... Showers and a few thunderstorms linger Tuesday morning, bringing reduced visibility. Areas of fog as well. This will move off to the east as a cold front slowly crosses the waters. Southwest flow with gusts reaching 25-30 knots during the day. Seas 5-7 feet, mainly on the waters exposed to a southwest wind. Seas will subside Tuesday night. A small craft advisory may be needed. Wednesday-Thursday... Winds southwest to west during this time with gusts to 20 knots. A few gusts may approach 25 knots. Showers will bubble up over land Wednesday and may affect nearshore waters. Lingering 5-6 foot seas on Wednesday, mainly in the exposed southern waters. Seas less than 5 feet on Thursday. Friday... Winds 20 knots or less. Seas 4 feet or less. An area of scattered showers may move across the waters bringing briefly lower visibility. morning west and by midday in the east. Expect lingering showers, mostly in RI/SE Mass, with a trend to clearing west-to-east from mid morning to mid afternoon. Expect mixing to 800 mb, which would support max sfc temps in the low to mid 80s. Winds aloft in the mixed layer reach 25-30 knots, so a gusty afternoon is expected. Wednesday-Thursday...Moderate confidence. Upper trough and its cold pool move overhead Wednesday and off to the east Thursday. Meanwhile, surface high pressure builds in. A reflection of the upper trough swings through on Wednesday, which could work with the cold temps aloft to destabilize the airmass and bring scattered showers. Dry weather follows Wednesday night and Thursday. With the cold pool in place, expect mixing to 750 mb Wednesday, followed by mixing Thursday a little above 850 mb. This suggests max sfc temps upper 70s and lower 80s. Winds in the mixed layer will be around 15 knots. Thursday night through Sunday...Low confidence. As noted, differences in timing among the models, which lowers confidence in the details. But somewhat higher confidence in the overall trends. The forecast will use a blend of the data. Timing of these features will likely change over the next few days. High pressure moves off to the east. This brings increasing southwest flow toward our area. The resulting warm front will bring increasing clouds along with a chance of showers and possibly a thunderstorm. Upper low pressure over the Plains...which was earlier in the Gulf of Alaska...drives east moving surface low pressure and cold front through the Great Lakes and toward New England. As the cold front approaches, expect showers and scattered thunder to move through our area. After the front moves through, expect a short period of dry weather. Another shortwave then races out of the Plains to bring another chance of showers. Here is our current timing for the late week and weekend. The warm front moves through our area during Thursday night or Friday morning, followed by the cold front Friday night. Much of Saturday would be dry. The next shortwave arrives during Sunday. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Tonight...Moderate confidence. VFR most areas north of CT-RI-SE Mass this evening, with IFR across RI and SE Mass as fog and stratus move ashore for another night. Expect the lower cigs to spread across much of Southern New England during the night. Low vsbys are showing on parts of the South Coast. Expect IFR or LIFR vsbys in fog along the coast overnight. Otherwise, S-SW winds continue, with gusts 20-25 kt possible at times through much of the overnight hours. This will also help limit the fog threat. Monday and Monday night...Moderate confidence. IFR/MVFR conditions gradually dissipate from N-S through the morning, lingering possibly along S coastal areas through mid day. VFR thereafter. Afternoon/evening risk for thunderstorms moving from W-E, beginning in W MA/CT after 18Z. These could slowly move through at least midnight, with a low risk for some lasting even later still. Strong winds and very heavy rainfall are most likely issues with any severe storms that develop. The highest risk for severe storms will be mainly W MA/CT, but is non-zero everywhere. KBOS Terminal...Moderate confidence. VFR continues through about 00-02Z, after which another round of MVFR/upper IFR CIGS are expected. Winds remain elevated overnight. KBDL Terminal...Moderate confidence. VFR to start, then a return to lower MVFR/IFR CIGS again overnight. Outlook /Tuesday through Friday/... Moderate confidence. Tuesday... Lingering MVFR/IFR conditions especially over Eastern MA and RI during the morning. Otherwise conditions improve to VFR all areas during the day. Expect gusty SW winds by afternoon with gusts of 25- 30 knots possible. Wednesday-Thursday... VFR. Developing daytime clouds and scattered showers Wednesday, which then diminish Wednesday evening. Gusty W-SW winds near 20 knots. Fair weather Thursday. Patches of IFR in fog each night. Thursday night-Friday... Increasing clouds with scattered or widely scattered showers. A few thunderstorms also possible on Friday. Generally VFR, but with patchy MVFR in showers. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Moderate confidence. S to SW flow continues well into Monday night over the waters. Although they may diminish somewhat overnight, gusts to at least 25-30 kt cannot be ruled out at times. The winds peak again during the afternoon/evening tomorrow, especially near shore where gusts 30-34 kt are likely. Seas will continually build during this time, such that they reach 6-8f on the SE open ocean waters by tomorrow evening. Areas of fog likely again overnight mainly over the S and SE waters with very low visibilities expected. A low risk for some late night thunderstorms over the waters tomorrow night along with another round of fog. Outlook /Tuesday through Friday/... Moderate confidence. Tuesday... Showers and a few thunderstorms linger Tuesday morning, bringing reduced visibility. Areas of fog as well. This will move off to the east as a cold front slowly crosses the waters. Southwest flow with gusts reaching 25-30 knots during the day. Seas 5-7 feet, mainly on the waters exposed to a southwest wind. Seas will subside Tuesday night. A small craft advisory may be needed. Wednesday-Thursday... Winds southwest to west during this time with gusts to 20 knots. A few gusts may approach 25 knots. Showers will bubble up over land Wednesday and may affect nearshore waters. Lingering 5-6 foot seas on Wednesday, mainly in the exposed southern waters. Seas less than 5 feet on Thursday. Friday... Winds 20 knots or less. Seas 4 feet or less. An area of scattered showers may move across the waters bringing briefly lower visibility. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Flash Flood Watch from Monday afternoon through late Monday night for CTZ002-003. MA...Flash Flood Watch from Monday afternoon through late Monday night for MAZ002>004-008>012-026. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Tuesday for ANZ231>234-251. Small Craft Advisory until 1 AM EDT Tuesday for ANZ230-236. Small Craft Advisory until 5 PM EDT Tuesday for ANZ235-237-250- 254>256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...Frank SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody

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