Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 354 PM EST Thu Feb 25 2021 Day 1 Valid 16Z Thu Feb 25 2021 - 12Z Fri Feb 26 2021 The probability of rainfall exceeding flash flood guidance is less than 5 percent. However, a swath of moderate to heavy rainfall is expected around the Arklatex, where amounts of ~2" are expected. Good coupling of low level moisture convergence and upper level divergence within the right entrance region of the 250mb jet, supports the expanding coverage of rain into tonight. A forecast of 2" areal averaged rain is often enough to consider at least a Marginal risk. However with only weak instability in place, the rates just do not seem to be there with this event. Localized swaths of 1" in an hour are likely...but overall it looks like 1hr and 3hr rainfall should peak at only ~50% of FFG. Also, current streamflows and soil saturation over the region are around typical levels for the time of year...thus not giving any reason to suggest flooding is likely below FFG levels. Thus in general it looks like this heavy rain later today into tonight will help saturate soil conditions and result in some ponding of water in spots, but overall is not expected to result in much of a flash flood impact...thus will carry no risk area at this time. Chenard Day 2 Valid 12Z Fri Feb 26 2021 - 12Z Sat Feb 27 2021 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE LOWER MISSISSIPPI VALLEY...TENNESSEE VALLEY...CUMBERLAND PLATEAU...SOUTHERN APPALACHIAN MOUNTAINS...AND THE CAROLINA PIEDMONT... 2030 UTC Update -- Have hoisted a Marginal Risk over the area, based on the latest guidance trends, particularly late in the period (00-12Z Sat) as the warm front lifts slowly northward Fri-Sat. PWs Friday night climb to 1.25+ inches, which is about 2 standard deviations above normal. This as southwesterly low-level inflow increases to 50-55 knots, becoming nearly parallel while approaching the magnitude of the mean 850-300 mb low -- setting the stage for upwind propagation and cell training with elevated convective bands north of the surface warm front Friday night. Instability looks to be on the marginal side, with MUCAPEs generally averaging between 250-500 j/kg, which for the most part will cap 3 hourly rainfall rates between 1-1.5 inches. Previous discussion.. The latest guidance continues to show Gulf moisture being transported northward within an increasing low level flow. The best mid-upper level lift will be present with the best lift occurring 00Z-12Z near northern Mississippi/western and central Tennessee as the associated front advances. Periods of moderate to locally heavy rainfall will be supported within this environment; with a couple of the hi-res guidance showing rain rates of 0.50 inch/hour across parts of Arkansas into Mississippi and western Tennessee during the 12-18Z period. This is also where the swath of higher QPF is expected to occur, with WPC forecasting 1 to 2 inches. During this time isolated locations may experience ponding or rapid runoff but the aerial coverage of flooding is not expected to be widespread. It will however increase soil saturation across the region and lower the FFG from the Southern Plains/Lower Mississippi Valley to the Tennessee Valley. Hurley/Campbell Day 3 Valid 12Z Sat Feb 27 2021 - 12Z Sun Feb 28 2021 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE SOUTHERN PLAINS INTO PORTIONS OF THE LOWER-MID MISSISSIPPI VALLEY, THE NORTHERN TENNESSEE VALLEY, SOUTHERN OHIO VALLEY, AND WESTERN SLOPES OF THE CENTRAL APPALACHIANS... 2030 UTC Update -- Nudged the bottom portion of the Marginal Risk area a bit farther northward based on the latest trends in the guidance, while expanding the outlook area slightly downstream to include more of the lower OH Valley and western WV. Elevated instability north of the warm front at this appears modest at best, averaging ~250-500 j/kg over the outlook area, with pockets closer to 1000 j/kg. This should be more than sufficient to support 1.5+ inches of rainfall within a 3 hour period, especially with some training likely Sat night given the uptick in southwesterly 1000-850 mb flow into a strengthening low-level frontogenetic zone within the right entrance region of an upper level jet streak. Previous discussion below... Scattered to widespread rain will persist during the day 3 period after spreading 1 to 2 inches across the region the day prior. This antecedent moisture will have likely lowered flash flood guidance across the Lower Mississippi and Tennessee Valleys where an additional 1 to 3 inches are forecast. Although the hourly rates across this region are expected to be less than 0.50 inch/hour, precipitation of this magnitude will lead to an increased threat for flash flooding to develop. In coordination with the local forecast offices from far northeast Texas to eastern Kentucky/Tennessee a Marginal Risk area was hoisted. This area will continue to be monitored as the threat of moderate to heavy rainfall continues into the day 4 period. Hurley/Campbell Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt