Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
220 FOUS30 KWBC 080731 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 331 AM EDT Wed May 8 2024 Day 1 Valid 12Z Wed May 08 2024 - 12Z Thu May 09 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO AND TENNESSEE VALLEYS... A Moderate Risk has been introduced with this update across portions of the Ohio and Tennessee Valleys for the potential of multiple rounds of thunderstorms through early Thursday morning that may lead to several/numerous instances of flash flooding, some of which could be significant across southern KY and middle TN. Current radar imagery shows a few lines of thunderstorms moving south of the Ohio River into KY, generally along/ahead of a cold front that is draped across portions of Indiana southwestward through southern Illinois. Aloft, the deep closed upper low remains centered over the Northern High Plains with a broad area of southwesterly flow stretching from the Southern Plains to the lower Great Lakes. Embedded in that flow is a weak shortwave analyzed over portions of AR/MO, moving northeast. The expectation is that the current convection will continue to sag south/southeast through KY, settling across southern KY near the TN border by the start of the period before stalling out with the loss the best nocturnal LLJ. This is expected to lay out a quasi- zonal boundary that will be the focus for additional thunderstorm development through the day as the airmass reloads with the deepening low pressure over the Mid-MS Valley. The environment today will be characterized by high PWs (1.5"+) and a zone of sufficient instability (MUCAPE > 2000 J/kg). Most importantly will be the increasingly favorable setup for training convection especially by later this evening/tonight across middle TN and southern KY with the deep layer mean flow becoming oriented parallel to the storm motions, helping to foster backbuilding and training convection. The probabilistic data continues to support the higher end rainfall and rain rate potential with the 00Z HREF probabilities for 5" increasing to near 50% straddling along the KY/TN border. This is supported by moderate values of 2" hourly rainfall total probabilities later today/this evening. Finally, the HREF EAS probabilities for 3" have increased from the previous forecast to near 40 percent, again mainly along the KY/TN border region. All this together supports the potential for multi-inch totals through the period and potential for more significant instances of flash flooding. Elsewhere, the Slight Risk encompasses areas further north into IN and OH where some hi-res guidance suggests convection may develop that far north and be robust enough to support isolated but intense rain rates, and some of these areas are a bit more saturated due to recent heavier rainfall. Further southwest along the frontal boundary, deeper and more intense convection is likely to develop along the frontal boundary into AR but the more isolated nature and higher FFGs may limit scope of any flooding there. ...Upper Midwest... Compact area of deeper convection is expected to develop along an inverted trough underneath the core of the upper low in the Midwest that will be capable of producing localized heavy rainfall and flash flooding. A narrow axis of higher moisture is expected, with PWs above 1-1.25" from far southeast South Dakota through Iowa and southern Minnesota. Given this greater moisture, broad forcing, and steepening lapse rates, pockets of slow-moving and pivoting showers and thunderstorms are likely and the latest HREF probabilities support isolated/scattered instances of hourly totals in excess of 1" and 24-hr totals up to 2" in places. Given this, there is at least some flash flood risk so the Marginal Risk for the area continues to look good. Taylor Day 2 Valid 12Z Thu May 09 2024 - 12Z Fri May 10 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS TO THE LOWER MISSISSIPPI VALLEY... ...Eastern Texas to Lower Mid-MS Valley... The strong spring system over the Central U.S. will begin to elongate/stretch in a lengthy, positively tilted shortwave trough by the start of the forecast period and sink southward toward the region. Earlier convection from the previous day is likely to push outflow boundaries through the region while another embedded shortwave trough is expected to move through in the southern stream. Altogether, this should put the region in a favorable region of forcing for ascent. Deeper moisture is expected to pool along/ahead of the surface boundaries and will be characterized by PWs greater than 1.5", which is around 2 std above the climatological normal for this time of year. With sufficient instability developing that will overlap with the better forcing and moisture, rounds of thunderstorms are expected to develop along the boundary initially over eastern TX before slipping south/southeast toward the Gulf Coast across portions of LA/MS/AL. By this point in the storm system evolution, the flow is likely to be more parallel to the overall storm motions and this could favor some repeating rounds or training convection particularly over portions of MS/AL. The environmental ingredients will support intense rain rates over 1-2"/hr at times and the latest probabilistic data shows potential for isolated 2-4" totals in a relatively short period of time. While there remains some uncertainty in the placement of the heaviest, no significant changes were made to the Slight Risk area and the message for scattered instances of flash flooding. ...Central Appalachians to the Mid-Atlantic... Low pressure is expected to track from the lower Ohio Valley to the eastern Great Lakes over the forecast period as shortwave trough energy aloft and height falls over the region. A strong southwesterly flow in the low levels will bring higher moisture, characterized by dewpoints in the upper 50s to lower 60s and PWs above 1", especially from the Central Appalachians and southern areas of the Northeast. With the uncapped and warm/moist environment, there should be scattered to numerous showers and thunderstorms that develop that will be capable of producing isolated/localized rain rates in excess of 1-1.5"/hr at times. This could cause isolated flash flooding, particularly for some of the areas that have seen recent heavy rainfall and more saturated ground conditions as well as urban locations. ...Midwest to Western Great Lakes... Deeper moisture pooled along an inverted trough / TROWAL slowly moving through the region will be the primary focus for embedded heavy rain cores that may lead to flash flooding. Tall/skinny CAPE profiles, an axis of higher/deeper moisture, and warm cloud depths will promote efficient rain producing showers and thunderstorms that could total 1-2" in spots and a Marginal Risk was introduced for portions of this region. Day 3 Valid 12Z Fri May 10 2024 - 12Z Sat May 11 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST U.S.... The advancing frontal boundary across the Southeast U.S. will be the primary focus for deep convection Friday into Friday afternoon and will be capable of producing rain rates in excess of 1-1.5"/hr at times. Showers and thunderstorms are expected to be ongoing at the start of the period, lingering from the deep, organized convective line from the previous day. This activity will be in the presence of a sufficiently high moisture pool, characterized by dewpoints well into the 60s and low 70s and a PW axis of 1.5-1.75". A subtle shortwave trough noted in the flow will be the main large scale forcing mechanism to drive the convection that looks to track through southeast Alabama, southern Georgia, northern Florida and then eventually off the Southeast coast. Some localized heavier rain totals and rain rates will be possible that could pose an isolated flash flood risk. Taylor 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