Area Forecast Discussion
Issued by NWS Burlington, VT
Issued by NWS Burlington, VT
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224 FXUS61 KBTV 072358 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 758 PM EDT Sun Jul 7 2024 .SYNOPSIS... Few impacts are expected over the next 48 hours, with temperatures remaining near 90 for most locations. Tuesday afternoon will bring a threat of thunderstorms, with the potential for locally heavy rainfall. The bigger story is Wednesday and Thursday, where considerable uncertainty exists. There is a potential for flooding as a warm front taps into the moisture associated with Tropical Storm Beryl. && .NEAR TERM /THROUGH MONDAY/... As of 756 PM EDT Sunday...Cumulus clouds are diminishing as expected this evening, and we are setting up for a tranquil overnight period with the exception of some patchy dense fog in the favored valley locations in the 06-12Z Monday time frame. Forecast is on track with minimal changes with the early evening update. Previous discussion...a widespread fair weather cumulus field will linger through the evening hours, with some chance for precipitation to fall mainly along the international border. The clouds will dissipate in the evening when the daytime heating shuts off. Overnight, skies will clear and the winds will become light and variable. We will see some fog formation along low lying areas. Monday will be another warm day, with temperatures reaching into the 90s in some locations. No heat headlines are anticipated, but the experimental Heat Risk product shows widespread moderate impacts for Vermont. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 308 PM EDT Sunday...On Tuesday, another warm and humid day is anticipated. An upper trough will enter a pocket of channeled flow between an upper low in the Labrador Sea and a strengthening Bermuda High. This will act to stretch the feature apart. There`s plenty of shear present on the order of 45 to 55 knots due to the faster flow aloft. In the low levels, there is some 850mb divergence and the upper jet is not very favorably set up at this point, though. So thunderstorm activity will mainly be driven by micro to mesoscale features and propagate from there with only a minor amount of frontal forcing available across southern and eastern Vermont mid- afternoon before scooting east. Conditions start out relatively dry followed by the increasing moisture and instability towards evening although the front comes through earlier in the afternoon. So the timing of features does not appear to align. But if anything can get going in the evening, PWATs are expected to surge up to 2 inches. So any storm will be capable of locally heavy rainfall for which a marginal risk Excessive Rainfall Outlook (Level 1 of 4) remains and and perhaps gusty winds due to strong winds aloft. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 308 PM EDT Sunday...Attention turns to Wednesday and tracking the remnant moisture of Tropical Storm Beryl (at time of writing). Before reaching our area, the tropical cyclone will have undergone a couple interactions with two northern stream shortwaves, and the track of moisture will also be influenced from the placement of that stretched out frontal boundary that will have crossed through on Tuesday. So there are some track complexities to consider, along with some model differences with the speed this all takes place. However, there is potential that these features line up in such a way that we could receive a lot of rain between Wednesday evening into Thursday. The timing on last year`s flood anniversary is unfortunate, but like severe weather, July is climatologically a common month for dealing with Flash Flood concerns. The 90th percentile (or a 10% chance) based on the trained NBM data yields about 2 to 3 inches of rain, and with any convective element would likely include locally higher amounts. However, given the uncertainty in exact track, there is a similar 10% chance of receiving a tenth to quarter inch. Little has changed with the picture of climatological statistics, PWATs running 2-2.25" and even 2.5" possible, which is at least 3 standard deviations above normal. The GFS rainfall forecast exceeds 90 percent of its model climatology and the extreme forecast index from the ECMWF is on the upper end, but not nearly so stark as last year`s flood event (for which the ECMWF EFI will have in its reforecast system). There is a large amount of model spread, and so users are encouraged to monitor, but average NHC low tracks in the 4 to 5 day time range are 150-200 miles. The forecast for this impacting our region has only begun to emerge in the last 24 hours, and the axis of heavier rain has been in flux with no clear trend at this time. More will be known tomorrow. Forecast models diverge even further beyond Friday. The pattern looks warm and humid, but the chances for precipitation will depend on the moisture plume around a Bermuda High that models are quick to weaken. This seems unlikely given the strength of it Thursday into Friday where the 500hPa heights are above 600 dam. So would think we might be more along the ring of fire than not. && .AVIATION /00Z MONDAY THROUGH FRIDAY/... Through 00Z Tuesday...VFR conditions will prevail through the evening at all terminals. Patchy fog will develop later in the night and continue until a couple hours after daybreak. Fog is likely to develop at the climatologically favored locations of SLK and MPV soon after midnight while it will probably take another hour or two at EFK. The development of fog or low clouds is possible at MSS but the confidence is lower than at the aforementioned terminals. While unlikely, fog cannot be completely ruled out at other terminals. After the fog lifts in the morning, VFR conditions will prevail for the rest of the day. Winds will be light during this period, generally switching from calm or terrain driven tonight to southeasterly tomorrow. Outlook... Monday Night: VFR. NO SIG WX. Tuesday: VFR. Chance SHRA, Slight chance TSRA. Tuesday Night: VFR. Chance SHRA, Slight chance TSRA. Wednesday: VFR. Chance SHRA, Slight chance TSRA. Wednesday Night: Mainly VFR, with local MVFR possible. Likely SHRA, Slight chance TSRA. Thursday: Mainly MVFR, with areas IFR possible. Chance SHRA, Chance TSRA. Thursday Night: Mainly MVFR, with local IFR possible. Chance SHRA, Slight chance TSRA. Friday: Mainly MVFR, with local IFR possible. Chance SHRA, Slight chance TSRA. && .HYDROLOGY... The potential for heavy rainfall on Wednesday evening into Thursday will drive the potential for flash flooding. There could be two phases to this with initially more widespread rainfall Wednesday night and early Thursday, and then scattered to numerous precipitation Thursday afternoon with locally heavy rainfall. The forecast range for precipitation is currently high, and multiple run model trends are not showing much consistency in placement. In short, confidence is high that somewhere will get very heavy rainfall amounts with totals in excess of 3 inches. Exactly where that will be will need some more time. For now, early forecast estimates between Wednesday evening and Thursday indicate averaged totals across the region of roughly 1 to 2 inches. The 90th percentile (or 10 percent of model forecast probability) suggests 3 inches, and we are not yet within the range of high resolution guidance that can depict heavy rainfall processes better. On the other end, the reasonable best case scenario suggests about a tenth to quarter inch of rain. This is mainly due to track uncertainty impacting rainfall totals. At this time, ensemble forecasts of rivers do not indicate much of a river response. However, streamflows are currently running on the 90th percentile of observations, mainly in central Vermont. So we may have to monitor the evolution of river forecasts as well. Additionally, if rainfall in excess of 3 inches is realized, we may need to consider low level, farm field and road type flooding. The Day 4 Excessive Rainfall Outlook covering this event indicates a slight chance (15 to 40 percent chance of rain exceeding Flash Flood Guidance). && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Langbauer NEAR TERM...Banacos/Langbauer SHORT TERM...Haynes LONG TERM...Haynes AVIATION...Myskowski HYDROLOGY...Haynes