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Area Forecast Discussion
Issued by NWS Burlington, VT
Issued by NWS Burlington, VT
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308 FXUS61 KBTV 091701 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 101 PM EDT Tue Jul 9 2024 .SYNOPSIS... A shortwave passing through aloft will increase the chance for scattered showers, thunderstorms, and heavy rain today. Thereafter, our attention turns to an increasing potential for heavy rain and flash flooding tomorrow night into Thursday associated with the remnants of Tropical Cyclone Beryl. && .NEAR TERM /THROUGH TONIGHT/... As of 100 PM EDT Tuesday...A few thunderstorms have now managed to fire. Given the mid-level dry air, but plenty of surface heating and instability, they are quickly going up and then back down. We should see them expand in coverage as a weak trough shifts east. Only minimal changes here and there were needed with the rest on track. Have a great day! Previous discussion below: Previous discussion...Tuesday will be another hot day with 925mb temperatures peaking at around 25-26 degrees C in some areas. This will result in surface high temperatures in the mid 80s to lower 90s F, with the core of heat in the Champlain and Connecticut River valleys. In addition to the pure heat, it will be quite muggy with dew points reaching into the lower 70s for the aforementioned valleys and 60s elsewhere. This will result in heat indices climbing into the upper 80s to upper 90s. The most intense apparent temperatures will be located in eastern Essex County, New York, western Addison and western Rutland counties of Vermont, and we have issued a Heat Advisory for these three zones from 3 PM to 6 PM today. Those working or recreating outdoors during these hours may want to consider taking extra breaks from the heat and being extra vigilant about good hydration. Upper level shortwave energy and deep tropical moisture passing through the forecast area and the significantly warm air mass today will encourage the development of some scattered showers and thunderstorms this afternoon and evening. Modeled anomalous precipitable water values in the 1.75-1.90" inch range will allow any thunderstorm to potentially be heavy. Most of the rain will fall east of the Adirondacks, and this area is under the WPC`s marginal level of excessive rainfall. This means there is at least a 5% chance of flash flooding. These storms continue to look very scattered in coverage, so we are not expecting widespread issues. This will, however, also be a primer for Wednesday`s more widespread rainfall, which will be directly associated with Tropical Cyclone Beryl`s remnants and tropical air mass. More on that in the short term discussion. Due to strong heating, high dew points, 40-50 knot flow at the 500mb level, modest CAPE, and up to 50 knots of 0-6km wind shear, there is also the potential for some of the storms, particularly in southern and eastern zones, to produce strong winds approaching 50 knots. However, the threat for severe weather is of lower likelihood and concern than localized flash flooding. Tonight will be another mild night with lows in the mid-60s to lower 70s as clouds increase ahead of the remnants of Tropical Cyclone Beryl. Chances of precipitation will increase towards sunrise from west to east. && .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/... As of 453 AM EDT Tuesday...Those looking for more clarity in the forecast for expected impacts from post Tropical Cyclone Beryl won`t be happy as there still remains considerable differences amongst the latest NWP guidance on where the heaviest rain axis will set up. The large scale synoptic setup remains unchanged with Beryl still expected to phase with a digging northern stream trough Tuesday night before slowing it`s northward movement over the Great Lakes Wednesday into Thursday due to blocking high pressure across eastern Ontario and Quebec. A warm front lifting through the Northeast Wednesday continues to be the focus for convective development driven by an seasonally strong upper level jet which combined with tropical PWATs rising to near 2.5" will enhance convective rainfall rates and increase the risk for flash flooding over the forecast area. Where the heavy rain sets up remains very much uncertain with 2 different model camps in the 00Z guidance. The ECMWF/GFS/NAM continue to focus the heaviest rain over northern zones along the Canadian border while the CAM consensus (HREF) probability match mean QPF is much further south from the Tug Hill eastward through the southern Adirondacks and southern Vermont. Not wanting to discount either of these camps, our official forecast of regionally averaged QPF features 1-3", but a swath of 2-4" with isolated amounts up to 5" remains very likely as the CAMs do highlight pockets of hourly rainfall rates up to 2". In addition to the flash flood threat is the potential for strong to severe thunderstorms. The threat is conditional to the position of the warm front Wednesday afternoon which consensus supports draped through central portions of the forecast area, leaving central/southern Vermont as the potential zone for convective development. North of the front 0-3km shear is off the charts nearing 60kts along the Canadian border which would hamper updraft development, but in the warm sector it`s closer to 30-40kts and coincides with MUBCAPE`s upwards of 1500 J/kg. Adding to the threat is strong rotational shear/helicity with soundings indicating big hodographs and 0-1km SRH of 200-300 m2/s2. This supports a non-zero threat for tornados though the risk is low and more likely we could see some strongly rotating mesos capable of producing damaging winds. && .LONG TERM /THURSDAY THROUGH TUESDAY/... As of 453 AM EDT Tuesday...Heading into Thursday there is strong consensus among global models that the aforementioned warm front lifts north of the Canadian border and is replaced by a strong mid- level dry slot from early Thursday morning through the afternoon. This should allow for rain to taper to isolated to scattered showers, but also opens the door for additional convective development as well as a moderate heat risk as 925mb temps support highs across VT in the mid/upper 80s and combined with dewpoints around 70 we`ll see heat indices up into the low/mid 90s. It won`t be until Thursday night into Friday that the system lifts far enough north to drag a cold front through the region, lowering PWATs below 1.5" and dewpoints come back down to more reasonable vales in the upper 50s to low 60s. Friday into the weekend guidance continues to favor dry conditions, though the Bermuda high appears to strengthen again and some increased moisture work back into the region supporting the chance for showers. There`s not much consensus on this so we`ll offer a dry forecast for now until Monday when a weak northern stream trough potentially brings a chance for showers. && .AVIATION /17Z TUESDAY THROUGH SUNDAY/... Through 12Z Wednesday...VFR conditions are expected to prevail for much of the next 24 hours as fog burns off quickly at MPV. Winds this morning will be generally out of the southwest, with some gusts possible 18Z-22Z Tuesday about 15-20 knots. There will likely be some scattered showers and thunderstorms out across the forecast area this afternoon and evening, which will have the potential to be heavy and restrict visibilities. After 00Z Wednesday, winds will become much more calm and variable, and the potential for patchy fog returns to the forecast area overnight Tuesday night. Outlook... Wednesday: Mainly VFR, with local MVFR possible. Chance SHRA, Slight chance TSRA. Wednesday Night: Mainly MVFR, with local VFR possible. Definite SHRA, Slight chance TSRA. Thursday: MVFR/IFR conditions possible. Likely SHRA, Slight chance TSRA. Thursday Night: Mainly VFR, with local MVFR possible. Chance SHRA, Slight chance TSRA. Friday: Mainly VFR, with local MVFR possible. Chance SHRA, Slight chance TSRA. Friday Night: VFR. Chance SHRA, Slight chance TSRA. Saturday: VFR. Chance SHRA, Slight chance TSRA. && .HYDROLOGY... As of 453 AM EDT Tuesday...Confidence continue to be moderate to high on the potential for heavy rainfall Wednesday afternoon into early Thursday morning increasing the risk for flash flooding. Uncertainty still exists in where the axis of heaviest rainfall will be but periods of scattered to numerous showers and thunderstorms are likely through the day with rainfall rates within the strongest cells upwards of 2" per hour. Over the past 40 days portions of the Adirondacks and central/northern Vermont have seen rainfall in excess of 8", some areas over 10" which ranks 150-200% above normal. Streamflows in these areas have responded and are currently running 70-85% above normal though river stages are relatively low but still above normal. These antecedent conditions only increase the threat for flash flooding with both low and high resolution guidance depicting the potential for rainfall of 1.5 to 2.5 inches in 6 hours, which would be enough to exceed Flash Flood Guidance. In coordination with neighboring offices and WPC, we`ve maintained a Slight Risk (Level 2 of 4) Excessive Rainfall Outlook for our region, mainly due to the uncertainty in where the heaviest rains will occur, but if greater consensus is found in the next forecast cycle we could see a portion of our area upgraded to a moderate risk. 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 late Wednesday afternoon into Wednesday night. With this, widespread rainfall totals are forecast to be 1 to 3 inches and this would likely involve an embedded swath where values of 2 to 4 inches could take place. Within that 3 to 4 inch swath, there could be a few spots that approach 5 inches. 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. 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, and a reasonable worst case scenario would support the potential for minor flooding at the Mad River at Moretown, Winooski River at Essex Junction (ESSV1), Otter Creek at Center Rutland (CENV1), and East Branch Ausable River at Ausable Forks (ASFN6). At this time, it appears that redevelopment of convection could be limited Thursday afternoon, but we will continue to monitor the position of dry, stable air that could limit this potential. For details on specific area rivers and lakes, including observed and forecast river stages and lake elevations, please visit the Advanced Hydrologic Prediction Service /AHPS/ graphs on our website. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...Heat Advisory until 6 PM EDT this evening for VTZ009-011. NY...Heat Advisory until 6 PM EDT this evening for NYZ035. && $$ SYNOPSIS...Storm NEAR TERM...Haynes/Storm SHORT TERM...Lahiff LONG TERM...Lahiff AVIATION...Storm HYDROLOGY...Haynes