Area Forecast Discussion
Issued by NWS Burlington, VT
Issued by NWS Burlington, VT
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732 FXUS61 KBTV 191921 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 321 PM EDT Fri Jul 19 2024 .SYNOPSIS... Seasonable temperatures will occur this weekend with a weak front bringing some sprinkles or a very light shower Saturday night. Mostly dry conditions will persist into next week before the pattern shifts to daily potential for wetting rains and afternoon thunderstorms as early as Tuesday. Some periods of rainfall may be heavy in localized areas, but details are few this far out. && .NEAR TERM /THROUGH SATURDAY/... As of 309 PM EDT Friday...High pressure will hold sway through tonight into Saturday keeping temperatures right around seasonal averages with another relatively cool night in the 50s to 60 degrees and highs tomorrow in the upper 70s to mid 80s. A weak front will begin dropping southward out of Canada tomorrow helping to tighten the pressure gradient and supporting area southerly wind gusts 10-20 mph for Saturday. The front`s parent low will be pulling well away into he Canadian maritimes causing forcing along the boundary to fall apart with only sprinkles to a few hundredths of an inch of rain possible. Tomorrow night`s lows will be marginally warmer in the upper 50s/mid 60s due to increased cloud cover. && .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... As of 309 PM EDT Friday...A surface cold front will continue to drop southward Sunday morning, with significant advection of cool, dry air on its north side due to light northerly winds. The passage of the front probably will be marked by a brief period of sprinkles with some embedded light showers, mainly in northern areas. Model guidance often has a wet bias with light precipitation, so the idea of most locations seeing no measurable rain seems reasonable. In addition, drying low level air will make it more difficult to have sufficient moisture for rain as the day wears on. Highest chances of seeing measurable rain Sunday morning will be in the northern portions of the Adirondacks and high terrain more generally. The axis of a strong upper level trough will be centered to our north and east, which will ensure a beautiful summer day once the clouds clear out. The very dry air will limit even fair weather cumulus during the afternoon as dew points fall into the upper 40s to low 50s. Temperatures will be a bit below normal. The more anomalous 850 millibar air mass appears to stay just north of our area, which will ensure that high temperatures still manage to be comfortably warm in the 75 to 83 range for most locations. Weak high pressure will be across the area with a front to our south; I do not expect enough southerly flow to produce stratus, so areas of patchy dense fog should develop late Sunday night in the typical river valleys. Given how low afternoon dew points will be, it may be difficult to see more widespread coverage of fog in the context of crossover temperatures. In any event, it should be a cool night with temperature dropping back into the 45 to 55 range for most spots. && .LONG TERM /MONDAY THROUGH FRIDAY/... As of 309 PM EDT Friday...Monday will mark the fifth and final day of lower humidity and primarily dry weather for our region. Then the pattern will abruptly flip to a wet and humid one for the remainder of the week. Through Friday, temperatures will stay seasonably warm without any signals for hot weather. The best chance of seeing uncomfortable heat is on Tuesday when temperatures are most likely to be very warm with full sunshine and light winds; forecast WBGT values creep into the extreme values during the afternoon in some valley locations. Humidity will become high, with dew points almost certainly running from the mid 60s into the mid 70s Tuesday onward, with low spread in model guidance until late Friday. By the weekend there are some indications that low level flow will become westerly rather than southerly/easterly, which could end this string of humid days. With the latest NBM run, when there is the most spread in temperatures is on Tuesday, especially in Vermont from the Lamoille River Valley and points south. The potentially warmer conditions southwards generally corresponds to a higher thunderstorm risk, which is pretty high at this time range at about 40% to 50% from 2 PM Tuesday to 2 AM Wednesday. Unsurprisingly, higher probability of showers exists as one goes southward as well during the period, with 6 hourly PoPs ranging roughly from 40 to 65% in this corridor between 2 and 8 PM. Three of the four ensemble clusters show a combination of at least modest instability coinciding with about 30 knots of 0-6 km shear. Given potential for a warm front in the vicinity, Tuesday could be an active severe weather day if the ingredients come together in our region. Generally the highest likelihood of rain is on Wednesday afternoon/evening, although rain will be possible at times late Tuesday through at least Thursday night. NBM 6 hourly Precipitation Potential Index suggests relatively high confidence of rain compared to Tuesday in a similar timeframe despite similar PoPs. However, the greatest magnitude of rain currently is indicated for the pre-dawn hours on Wednesday, which falls well outside of the probabilistic forecast data. What this tells us is that both the timing and magnitude of rain is highly uncertain at this time. Outside of a quasi-stationary front expected to be draped from west to east near or over the North Country/northern Vermont, we don`t have an obvious trigger for heavy rain. We will continue to monitor if the ingredients may come together for a shorter period of intense rain that would suggest a flood risk. Looking at the ensemble clusters from the latest (12Z) guidance, there is broadly good agreement in the upper air pattern with southwesterly flow, but subtle differences lead to deeper moisture with greater 700 millibar moisture in most scenarios where mid-level winds are southwesterly rather than westerly. These wetter scenarios currently show a pretty big divide, especially on Wednesday, between most GEFS and most EPS/GEPS members. Therefore, the signal for heavy rain and thunderstorms is rather mixed at the moment, with generally a solution that has a stronger upstream trough more likely to lead to more dynamic, impactful weather late Tuesday through Thursday. Stay tuned! && .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/... Through 18Z Saturday...High pressure will keep most locations VFR through the periods with the exception of SLK/MPV/VSF where localized fog is probable between 06-11Z. VIS reductions could briefly be VLIFR with more likely IFR period from 08-10Z. Fog will briefly lift into an IFR CIG through 12Z before dissipating and transitioning into a SCT-BKN fair weather cumulus deck. Light winds will generally prevail varying northwesterly this afternoon before becoming light overnight. Weak pressure gradient begins to tighten marginally after 12Z Saturday with southerly flow expected. Channeling down the St Lawrence River could induce some gusts to 20kts at MSS while other terminals remain 5-10kts. Outlook... Saturday: VFR. NO SIG WX. Saturday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA. Sunday: Mainly VFR, with local MVFR possible. NO SIG WX. Sunday Night: VFR. NO SIG WX. Monday: VFR. NO SIG WX. Monday Night: VFR. NO SIG WX. Tuesday: VFR. Slight chance SHRA, Slight chance TSRA. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Boyd NEAR TERM...Boyd SHORT TERM...Kutikoff LONG TERM...Kutikoff AVIATION...Boyd