088 FOUS30 KWBC 050139 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 939 PM EDT Tue Aug 04 2020 Day 1 Valid 0132Z Wed Aug 05 2020 - 12Z Wed Aug 05 2020 ...A MARGINAL RISK OF EXCESSIVE RAINFALL REMAINS ACROSS INTERIOR NEW ENGLAND, THE CENTRAL HIGH PLAINS, AND SOUTHERN PLAINS... ...Interior New England... Showers and thunderstorms continue to move northward from NH into western ME which led to the maintenance of the Marginal Risk area, while the heavy rain north of the center of Isaias across northeast NY and northern VT has ejected into Quebec. The decline in the coverage and intensity of the heavy rainfall led to the elimination of the Slight Risk area here. Heavy rains across western ME should move northward while fading overnight as Isaias moves into eastern Canada. ...Plains... Convection has attempted to organize over the High Plains of CO and has been moving southeastward. There remain indications that convection could grow upscale/organize overnight over TX/KS/OK while remaining generally progressive. The 12z Hi-Res CAMs are presenting a greater potential further south into S CO moving into the OK/TX panhandles and lesser influence further north, which is also in line with the 12z GFS. There is some indication in the guidance that activity could hang up across the TX Panhandle generally in the 04-08z time frame, though this area has had below average two week precipitation. There doesn't appear to be a mid-level capping inversion to argue for the more northerly 12z ECMWF and 12z Canadian solutions, so they were deemed lower probability alternate scenarios. The rates, totals and still lingering uncertainty still falls below criteria for introduction of a Slight Risk, so kept the risk level Marginal. Roth/Gallina/Chenard Day 2 Valid 12Z Wed Aug 05 2020 - 12Z Thu Aug 06 2020 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE CAROLINAS AND INTO THE MID-ATLANTIC... ...Southern Appalachians eastward to the coast and north to Maryland... A slow moving cold front will serve as the impetus for thunderstorms on Wednesday as it drifts eastward across the region. As it moves east, forcing for ascent will be provided by low-level convergence, but also within a modest RRQ of a departing upper jet streak, and weak vorticity maxima rotating northeastward embedded within the mid-level flow. Instability will climb through the day due to steepened lapse rates beneath an upper trough and surface heating, with MUCape reaching 2000 J/kg across the eastern Carolinas/VA, decreasing to 500-1000 J/kg into the mountains and north into Maryland/Delaware. At the same time, modest 850mb moist flow will drive PWs above 1.5", approaching 2" near the coast, which is 0.5 to 1 standard deviations above the climo mean. In this environment, thunderstorms are likely to develop and become scattered to widespread during the aftn/eve. While storm motions may be quick at 15 kts to the northeast, training along the front due to boundary parallel flow, and storm interactions further east, could lead to prolonged periods of heavy rainfall exceeding 1"/hr at times. Much of this region has seen anomalous rainfall in the past 7 days, mostly from recently departed Tropical Storm Isaias, and soils are fully saturated. This has led to extremely compromised FFG which is as low as 0.5"/1hr or 1-1.5"/3hrs across parts of the region. The inherited MRGL risk was expanded to include a larger area where rainfall rates on top of saturated soils could lead to isolated instances of flash flooding. Note that some guidance produces an additional 2-3+" of rainfall in a short period across parts of VA/MD. This is not out of the question as the weak impulse aloft tapping the favorable thermodynamics and potential for training could lead to prolonged or repeated rounds of heavy rainfall. If this should occur, it may necessitate an upgrade to SLGT risk especially over areas which just received excessive rainfall from Isaias. However, there is enough uncertainty in placement and coverage at this time to keep the risk at MRGL, but should be re-evaluated with the next model cycle and with the addition of the high-res guidance. Weiss Day 3 Valid 12Z Thu Aug 06 2020 - 12Z Fri Aug 07 2020 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR NORTH CAROLINA NORTHWARD TO NEW YORK CITY, AND PARTS OF THE CENTRAL PLAINS... ...North Carolina northward to New York City... Another day of scattered to widespread thunderstorms is predicted as a slow moving cold front remains in the vicinity beneath a mid-level trough. Although jet-dynamics will be pulling off to the northeast during day 3, instability which may reach 2000 J/kg, focused in the eastern parts of the risk area, will combine with PWs of 1.5-2", 1-1.5 standard deviations above the climo mean, to produce an environment favorable for heavy rain rates. Although there remains some uncertainty into the coverage of convection on D3, rainfall rates of 1-1.5"/hr anywhere across the region could lead to flash flooding as soils are saturated from 7-day rainfall that is 300-600% of normal. Additionally, note a widespread signal for NWM streamflow anomalies ranging from much above normal, to high, flow across the region, indicating that any heavy rainfall could lead to stream rises and flash flooding. FFG across the area is generally less than 1.5"/3hrs, slightly higher in NC, so these rain rates could produce additional flash flooding Thursday afternoon into Thursday night. ...Southeast Kansas, Northeast Oklahoma, Southwest Missouri, Northwest Arkansas... Continued a small MRGL risk area but with slightly lower QPF than inherited as an MCS is likely to dive southeastward Thursday night into Friday morning across this area. A shortwave cresting the ridge will dive into a region characterized by extreme instability of >3000 J/kg and PWs exceeding 1.25", about 1 standard deviation above the climo mean. At the same time, an 850mb LLJ will be emerging from the the S/SW transporting moisture and resupplying instability, likely leading to a more southward diving convective cluster than shown by some of the guidance which progresses the feature too far to the east. This is echoed by a local 700mb omega max beneath a region of enhanced divergence. ECENS and GEFS probabilities show low-end chances for 2" of rainfall, and much of this region has seen 7-day rainfall of more than 300% of normal. Any heavy rain could quickly lead to runoff and flash flooding, and with some training potential along the W/SW side of the MCS, flash flooding will be possible. Weiss Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$
