Area Forecast Discussion
Issued by NWS Burlington, VT

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974 FXUS61 KBTV 230819 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 419 AM EDT Sat Jul 23 2016 .SYNOPSIS... More scattered showers and thunderstorms move across the North Country today as an upper level disturbance moves through. Some thunderstorms may have gusty winds and small hail. Sunday looks mostly sunny and seasonable under hand yet another possibility of showers and thunderstorms late Monday. Warm and mostly dry summer weather looks like it will continue Tue and Wed. && .NEAR TERM /THROUGH TONIGHT/... As of 419 AM EDT Saturday...Looks like another active day today with showers and thunderstorms developing late this morning in the north and becoming more widespread this afternoon as they move southeast and diminish quickly this evening. A couple of the thunderstorms may be marginally severe with hail and gusty winds. This morning any patchy fog will burn off quickly by around 8 am followed by rapid cumulus cloud development this morning. a potent shortwave trof is expected to move across our area this afternoon with cooling at 500 mb down to -15C. This cooling aloft along with mild 850 temps of about 13C and surface temps reaching the lower 80s are expected to kick off thunderstorms late morning early afternoon in the north. Each model has a differing evolution of the convection, and is again a symptom of just how sensitive the models are to this type of weather pattern that is driven by mixed layers, elevated and low level instability and boundary layer details. Taking a blend of the models suggests that we`ll have CAPE values 750 to 1500 J/kg, highest in the St. Lawrence Valley which is decent enough to drive convection. We`ll also have 30-40kt 0-6km shear mainly over the south and west portions of our area while shear over VT less on the order of 15 to 25 kts. So we can`t totally rule out a stronger storm with gusty winds and small hail, so have included in the forecast. Looks like the highest chances 60-80% of precipitation and storms will be from mid-day until early evening. SPC has our region under "Marginal Risk" of severe thunderstorms and we are in good agreement. Scattered showers/storms (a few possibly strong early) then wane and exit southeast fairly quickly Saturday evening as upper trough swings through the region and is replaced by building high pressure. Chance of precip drop off quickly during the evening, then largely dry from midnight onward. Low temperatures near seasonal mid-summer norms in the 50s to around 60 for most locales. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 419 AM EDT Saturday...Sunday: All models agree the day will live up to it`s name and be a super spectacularly sunny and serene day. Ridging aloft and at the surface will be in place. With 925mb temperatures running between 17- 19C, we`ll see daytime highs topping out in the upper 70s (higher elevations) to lower 80s (valleys). light winds and dewpoints in the 50s will ensure comfortable conditions. Given all that model agreement, I did not stray from the guidance blend. Sunday Night: Things change as ridging moves off to the east as the next trough in the overall westerly flow starts heading our way. Warm air advection aloft will result in increasing clouds overnight. Should be some elevated instability as well, have noted Showalter Index values <0C spreading in after 06z monday in the models. That suggests showers are possible, and sure enough that is what the models show as well. Have painted in rising chances overnight. Expect a few showers, mainly across northern NY, prior to daybreak Monday. A rumble of thunder is not out of the question across the St Lawrence valley by dawn as we`ll have the nose of an 850mb jet pushing into that area to provide a little extra lift. The increasing clouds will keep it mild for most of the region. Low/mid 60s from the Champlain Valley westward. 50s will be confined to eastern VT Monday: For days, guidance has been indicating unsettled conditions (eg: showers and t-storms) to be possible. 00Z model runs are no different, and have come in more alignment with timing and overall setup of convective ingredients. Still, things are not totally clear. What I do expect is that a fairly robust shortwave will be approaching the region late in the day. This will sync up daytime heating with the best dynamics aloft. Models do indicate surface based instability of probably 1000-1500 J/kg (NAM as typical is higher) due to a warmer and more humid airmass getting into place. Just how much instability we achieve will be related to how much sun we see. 925mb temperatures are expected to become very warm -perhaps 23 to 25C. Under perfect atmospheric mixing conditions, this would result in temperatures at the valley floor pushing into the mid 90s! However, it appears we`ll have a good amount of clouds to restrict heating to a degree. Though we should still easily reach the mid 80s. The 0-6km bulk shear values are in the marginal level for getting storms to become on the strong side. SPC does have the region in a marginal risk, but they also have some concerns about the mix of ingredients lining up perfectly to develop severe t- storms. Evaluation of various forecast sounding for Monday have some "goofiness" (thats a technical term, by the way) showing in the profiles. Not sure if that is due to convection within the models, or if it`s a sign that despite the very warm air aloft, we`ll have some inversion in place to restrict mixing, which in turn means restricted convection. At this point, will play the middle ground, and keep PoPs in the 40-55% range and see how future guidance trends. Monday Night: Whatever convection we do see, will be pushing out of the region as the night goes on. Will likely be warm and muggy with lows in the 60s. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 419 AM EDT Saturday...Decent agreement with the 00z suite of models, and much of the week should be relatively quiet, but feature more or less typical summer weather. Most of the week we`ll see temperatures running a bit above normal. Looks sunny and warm for both Tuesday and Wednesday. GFS and ECMWF start to advect in some moisture for Thursday, along with having a little more troughiness develop and perhaps a weak front sliding down from Quebec. Thus it appears we`ll see some showers and t-storms around. At this point, they appear to be of the garden variety as I don`t see strong dynamics in place. Kept with a model blend and painted in 30-40% PoPs. More uncertain for Friday. 00Z runs vary from 12z guidance and keep in clouds/showers rather than having things clear out more. Really don`t have any confidence, so just went with the full blend with temperatures close to normal and low chances of showers. && .AVIATION /08Z Saturday THROUGH Wednesday/... Through 06z Sunday...Mainly VFR except some patchy fog until 12z for KSLK and KMPV. Aft 15z Sat...VFR, with scattered brief MVFR/IFR in showers and thunderstorms. Some thunderstorms may contain strong winds and hail. Showers and storms diminish between 00-03Z. Winds generally light west to northwest at 5 to 10 knots. Outlook 06z Sunday through Tuesday... Sat Ngt...VFR but some localized MVFR/IFR due to patchy fog. Sun...Mainly VFR under weak high pressure. Mon/Monday night...VFR with scattered brief MVFR/IFR in showers and thunderstorms with frontal passage. Tuesday...VFR/high pressure. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Sisson NEAR TERM...Sisson SHORT TERM...Nash LONG TERM...Nash AVIATION...Sisson

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