Area Forecast Discussion
Issued by NWS Burlington, VT
Issued by NWS Burlington, VT
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463 FXUS61 KBTV 080749 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 349 AM 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 348 AM EDT Monday...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 SUNDAY/... 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 /08Z MONDAY THROUGH FRIDAY/... Through 06Z Tuesday...IFR fog will continue to develop at KMPV and KSLK over the next few hours with TEMPO fog likely at KEFK from 07-10Z. After 12Z, VFR will prevail through the period with FEW-SCT fair weather cumulus developing through the afternoon, then trending to SKC-FEW after sunset. Light winds overnight will trend SSW at 4-8kts from midday through the afternoon, with the exception being KPBG where the lake breeze keeps the direction southeasterly. Outlook... Tuesday: VFR. Chance SHRA, Slight chance TSRA. Tuesday Night: VFR. Chance SHRA, Slight chance TSRA. Wednesday: VFR. Chance SHRA, Slight chance TSRA. Wednesday Night: MVFR. Likely SHRA, Slight chance TSRA. Thursday: Mainly MVFR, with local IFR possible. Chance SHRA, Chance TSRA. Thursday Night: Mainly MVFR, with local IFR possible. Chance SHRA, Slight chance TSRA. Friday: Mainly VFR, with local MVFR possible. Chance SHRA, Slight chance TSRA. && .HYDROLOGY... As of 348 AM EDT Sunday...Moisture from the remnants of Tropical Cyclone Beryl could enhance rainfall locally and lead to increased risk for flash flooding across the North Country Wednesday into Thursday given recent antecedent rainfall and saturated grounds. Much of northern NY and northern two-thirds of VT saw 125 to 200 percent of normal rainfall last month, with widespread 6 to 10 inches of monthly rainfall. Mid-range model guidance is coming into better consensus on rounds of moderate to heavy rainfall impacting these aforementioned areas. While there remains a good amount of uncertainty with regards to the heavy rain placement and potential convection outside of the Convection-Allowing Models time range, the synoptic scale dynamics are increasingly favoring a reasonable worse case scenario. With an unseasonably strong vertically stacked low pressure system over the Great Lakes as well as a strong upper level ridge centered southeast of the 70W/40N benchmark, the synoptics favor the axis of heavy rainfall over northern NY and central/northern VT in the Wednesday into Thursday time frame. With an air mass that has 2 to 2.5 inches PWATs or 3 to 4 standard deviations above normal, expect 1 to 2 inches widespread rainfall, 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. All things considered, there is increasing risk for scattered if not numerous flash flooding to occur mid to late week. 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). Terminology wise, Slight risk is 15-40 percent probability of rainfall exceeding flash flood guidance within 25 miles of a point. As for potential river flooding, the NAEFS probability of exceedance is currently only showing 10 percent probability of exceeding minor flood stage at Winooski River at Essex Junction (ESSV1), Otter Creek at Center Rutland (CENV1) and Mad River at Moretown (MOOV1) as well as a 10 percent probability of exceeding moderate flood stage at East Branch Ausable River at Ausable Forks (ASFN6). In other words, it would take a 90th percentile rainfall scenario to result in the aforementioned minor to moderate river flooding. As discussed above, the 90th percentile rainfall scenario, or the reasonable worst case scenario is becoming increasingly probable. There would likely be two peaks in the river stages, one during overnight Wednesday into early Thursday for the flashier river basins in the upper reaches (e.g. ASFN6 or MOOV1) and a second crest on Friday after the steady rain abates for the downstream mainstem rivers (e.g. ESSV1). Be sure to stay up to date with the latest forecast. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Lahiff NEAR TERM...Lahiff SHORT TERM...Chai LONG TERM...Chai AVIATION...Lahiff HYDROLOGY...Haynes