Area Forecast Discussion
Issued by NWS Burlington, VT
Issued by NWS Burlington, VT
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169 FXUS61 KBTV 081749 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 149 PM EDT Mon Jul 8 2024 .SYNOPSIS... Surface high pressure will provide the North Country and Vermont with dry weather again today, and a weak disturbance passing through aloft will increase the chance for isolated showers, thunderstorms, and heavy rain on Tuesday. Thereafter, our attention turns to an increasing potential for heavy rain and flash flooding Wednesday Night into Thursday associated with the remnants of Hurricane Beryl. && .NEAR TERM /THROUGH TUESDAY/... As of 145 PM EDT Monday...Still no major changes to the forecast for this afternoon. Some fair weather cumulus has formed and temperatures are on track to reach the upper 80s to around 90 this afternoon. Dewpoints are running a bit higher than forecasted, so made some tweaks there. Previous discussion follows. We`ll enjoy another day with warm temperatures and somewhat comfortable humidity again today as surface high pressure crests over the region. Morning fog will rapidly lift into fair weather cumulus through the midday to afternoon hours, and 925mb temps just 1-2C warmer than yesterday will support highs once again in the mid- 80s to low 90s. As it occurred the past 2 days, an isolated shower can`t be ruled out, likely developing along the lake breeze over the Adirondack Coast this afternoon. Confidence is low on this occurring so will highlight just some 5-10% PoPs. For tonight, mid/high clouds will shift into the region after midnight which should limit the fog threat, and any evening showers will dissipate. Lows will be a little milder than the previous night in the 60s to around 70. Finally for Tuesday, the picture is murky on the potential for convective development as there`s little forcing available, but some weak embedded shortwave energy aloft interacting with rising humidity, dewpoints in the mid/upper 60s, and some weak surface instability may spark some isolated to widely scattered storms. The main threat from any of these would be heavy rain as PWATs look to rise towards 1.5" again. Highs will be similar to Monday in the mid-80s to low 90s, and combined with the increased humidity some areas could flirt with heat advisory criteria. Right now there doesn`t appear to be enough coverage to warrant headlines, but the experimental heat risk does highlight a moderate risk for parts of southern Vermont, mainly due to consecutive days of heat. && .SHORT TERM /TUESDAY NIGHT/... As of 348 AM EDT Monday...This is the calm before the storm, i.e. the increased flooding risk associated with the enhanced moisture plume from Tropical Cyclone Beryl. Models have trended drier for overnight Tuesday across our region. WPC has our area in the Marginal (level 1 of 4) category for their Day 2 Excessive Rainfall Outlook but any isolated flash flooding risk will be tied to convection. It will be a muggy night with dew points remaining mainly in the 60s to near 70 in the Champlain Valley. Overnight lows should generally be in the mid 60s to low 70s. For the discussion from Wednesday and beyond, including the entirety of the heavy rain and flooding threat, refer to the long term discussion. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... As of 348 AM EDT Monday...BLUF: The potential for a significant hydrologic event continues to increase for mid to late this week. We remain concerned about the potential for scattered to perhaps numerous flash flooding given the risk for rounds of heavy rain falling on saturated grounds. At this time, the most probable time frame for flash flooding threat is from Wednesday into Thursday time frame. Unfortunately, it coincides with the 1 year anniversary of the July 9-11, 2023 Vermont floods. To be clear, there are important differences between July 2023 and July 2024. Last year`s flooding was not tropical in nature. But there are also many similarities. According to the Northeast Regional Climate Center data, much of our forecast area with the exception of Windsor county saw 125-200 percent of normal monthly rainfall in June. And July has been wet so far. Grounds are saturated and dry stretches have been hard to come by. The ICON model, which has pretty much nailed the track of Tropical Cyclone Beryl so far, has the heftiest rainfall over northern NY and northern VT, which corresponds to the stronger low pressure system over the Great Lakes and a comparatively strong ridge over the Atlantic Ocean. While the highest PWATs remain just to our south, it does look like the best dynamics for rounds of moderate to heavy rainfall is increasingly likely over our CWA, particularly the northern two-thirds. So that would include most of northern NY, as well as central and northern VT, roughly from I-89 corridor and point north. A word of caution is that past performance is not indicative of future results. Warm season convection remains rather challenging to pin down, especially outside of the Convection- Allowing Models time frame. If we were to wager a preliminary rainfall forecast, expect widespread 1 to 2 inches of rain, with a more focused swath of 2 to 4 inches to fall in the 48 hours between Wednesday morning and Friday morning. Remember, that is not taking into account convection. There are increasing indications that we could be seeing scattered if not numerous 4 to 6 inches totals, where convection or training of the moderate to heavy rainfall occurs, with isolated 6 to 8 inches amounts. Those numbers, if they do materialize, would be comparable to the flooding event we experienced almost exactly a year ago. In coordination with the WPC and neighboring offices, we have continued with the Slight risk (level 2 of 4) Excessive Rainfall Outlook for Day 3 to 4 (Wednesday 8 AM to Friday 8 AM). A reminder that Slight risk is 15-40 percent probability of rainfall exceeding flash flood guidance within 25 miles of a point. PWATs, which are already AOA 90th percentile, will be enhanced even further past maximum of SPC sounding climatology due to the moisture from Tropical Cyclone Beryl. PWATs of 2.3 to 2.5 inches are 3 to 4 standard deviations above normal. With slow-moving surface boundaries and generally a lack of atmospheric forcing to push out the tropical air mass, rounds of heavy rainfall and embedded thunderstorms will greatly the risk of flash flooding given the grounds are becoming saturated. The greatest sources of uncertainty right now are the areal placement of the richest PWAT plumes and timing of convection. It is entirely plausible that our region still escapes this week relatively unscathed. After all, warm season convection remains extremely challenging to forecast. But it is important to be prepared for the reasonable worst case scenario, which is for scattered to numerous flash flooding if we do get repeated rounds of heavy rainfall and storms to tap into the abnormally moist atmosphere. Please stay up to date with the latest forecast. For more details on the flooding threat, refer to the hydrology section of the AFD. As we head into Friday and next weekend, a cold front should push the richest PWAT plume (200+ percent of normal) into southern New England and the mid Atlantic states which will allow for slightly drier albeit still seasonably moist air mass to potentially give the North Country a break from the rainfall. Some of the more pessimistic guidance shows potential for the cold front to lift back north as a warm front and bring the risk for more heavy rainfall into the North Country next weekend. So removed the heavy rain wording from the forecast from Friday onwards but remember that if we do get a break from the heavy rainfall, we might still be dealing with main stem river flooding, especially further downstream. But that is a low probability scenario at this time so generally went with a blend of guidance for this portion of the forecast. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... Through 18Z Tuesday...VFR will prevail across the region through the period with FEW-SCT fair weather cumulus developing through the afternoon, then trending to SKC-FEW after sunset. Winds SSW at 4-8kts from midday through the afternoon, with the exception being KPBG where the lake breeze keeps the direction southeasterly. Winds return to light and variable tonight, with lingering mid level clouds around which should help to inhibit widespread fog formation. Winds pick up out of the south southwest during the daytime on Tuesday. Outlook... Tuesday Night: VFR. Chance SHRA, Slight chance TSRA. Wednesday: Mainly VFR, with areas MVFR possible. Chance SHRA, Slight chance TSRA. Wednesday Night: MVFR. Likely SHRA, Slight chance TSRA. Thursday: Mainly MVFR, with areas VFR possible. Likely SHRA, Slight chance TSRA. Thursday Night: MVFR/IFR conditions possible. Chance SHRA, Slight chance TSRA. Friday: Mainly MVFR, with local IFR possible. Chance SHRA. Friday Night: VFR. Chance SHRA. Saturday: VFR. Chance SHRA. && .HYDROLOGY... As of 1040 AM EDT Monday...Confidence is moderate to high on heavy rainfall Wednesday evening into Thursday will drive the potential for flash flooding. On Wednesday afternoon, scattered showers and thunderstorms will develop, followed by a period of moderate to heavy rainfall during the overnight hours. The heaviest rain will lift northeast Thursday morning, and then scattered showers and thunderstorms will develop again Thursday afternoon. The axis where the highest rainfall totals are likely remains uncertain, but the spread is beginning to decrease. For Wednesday night, increasing the potential for flash flooding is relatively high soil moisture content across the Adirondacks and across parts of northern and Central Vermont. The experimental SPoRT model indicates values in the 80th to 90th percentile, and are primed to produce this potential. Although we lie outside the range of convective allowing models, lower resolution models are indicating the potential for 6 hour rainfall rates to approach 1.5 to 2 inches nearing Flash Flood Guidance thresholds. As such, a Slight Risk (Level 2 of 4) Excessive Rainfall Outlook has been issued for our region. At this time, the probabilities of exceeding 2 inches is greatest south of the international border at about 20 to 30 percent. Unlike last year`s flood event, this will not be a long duration scenario. The heaviest rainfall is likely to fall within a 6 to 12 hour window Wednesday night. With this rainfall totals forecast by the Weather Prediction Center suggest 1 to 2 inches and this would likely involve an embedded swath where values of 3 to 4 inches could take place. If values in excess of 3 inches occur, then there could be additional flooding across low lying plains and roads in addition to flash flood concerns. This forecast will continue to evolve as we become more certain on the track and development of convection. Users are encouraged to prepare for scattered flash flooding, and consider the possibility of a swath where embedded numerous flooding could take place. On Thursday, runoff from the rain will begin to bring river levels up. At this time, river levels are above normal, mainly in north- central Vermont. Although the latest NAEFS only depicts the Mad River at Moretown reaching minor flood stage, the reasonable worst case scenario, or 10th percentile), would also support the potential at the Winooski River at Essex Junction (ESSV1), Otter Creek at Center Rutland (CENV1), and East Branch Ausable River at Ausable Forks (ASFN6). Redevelopment of scattered showers and thunderstorms will take place Thursday afternoon, although the extent and intensity may be limited due some dry intrusion. Still, any storms produce locally heavy rain and with the wet grounds, this could produce additional flash flood concerns on Thursday. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Lahiff NEAR TERM...Lahiff/Neiles SHORT TERM...Chai LONG TERM...Chai AVIATION...Neiles HYDROLOGY...Haynes