Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
627 FOUS30 KWBC 081548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1148 AM EDT Wed May 8 2024 Day 1 Valid 16Z 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... ...16Z Update... Current radar/sat composite shows two distinct areas of impact this morning; a west-east elongated boundary with training bouts of convection across southern KY into northern TN, and a prolific supercell propagating to the east-southeast across MO. The supercell over MO is important in the overall evolution of flooding prospects across the Ohio and Tennessee Valleys as the mesocyclone from this cell will pivot southeast, becoming the main focal point for congealing convection with expected upscale growth into a significant MCS that is well-stated with the latest SPC D1 Moderate Risk for severe. Rainfall across southern KY into northern TN has already produced localized totals of 1-2" with another 1-3" anticipated over the course of the morning and early afternoon before the main precip shield makes headway into the region this evening through the overnight time frame (For more information on the current rainfall, see MPD #0239). The compounding factor creates a significant threat for flash flooding potential as areal priming from the first wave of rainfall will degrade the regional FFG`s leading to a proned zone for further impact. This generally lies within the confines of the remnant boundary where the current precip is ongoing, and where the MCS will become rooted later in the period. Theta-E gradient is well-documented given the latest RAP surface analysis with some modest advection anticipated across the Mid- Mississippi Valley within the lee of a surface low analyzed near the KS/MO border. This will create a general zone of recovery over western TN into far southwest KY down through the Lower Mississippi Valley with MLCAPE indices likely between 2500-4000 J/kg across the aforementioned area, and 2000-3000 J/kg for points further east across TN and northern MS/AL. The western locations will see an increase in convective coverage with locally heavy rainfall initiating between 18-21z allowing for a modest flash flood threat based on discrete convection alone. As we move into the early evening hours, general convective pattern will shift from discrete convection to more multi-cell and organized cell structures with aid from the primary mesocyclone diving southeast out of MO along the theta-E gradient. That`s when the convective mode will favor upscale growth and a significant swath of very heavy rainfall is anticipated through the path of the expected MCS. Probability fields reflect the anticipated impacts this evening within the bounds of the latest HREF EAS signals for at least 3" of total precip the next 24 hrs, as well as a significant output from the 10 and 100-year ARI exceedance probabilities. The former is now running between 60-80% across the border of KY/TN to the northeast of I-65 with a large swath of at least 40% probability expanding west-northwest to east-southeast between Paducah to about 75 miles east of Nashville. The 10-year ARI exceedance probabilities are fairly similar to the 3" EAS markers with 100-year ARI settling between 20-35% in that zone to the northeast of I-65. This correlates to the area that will see dual impact from the morning wave of convection and the expected MCS overnight leading to HREF blended mean totals approaching 6-8" locally. An expanse of 3-4.5" mean totals encompass the entire area from southwest KY through northern and central TN leading to a robust areal coverage of heavy rainfall and the necessity to expand the MDT risk a bit further south to reflect the trends. The southern periphery of the SLGT was also expanded to account for recent trends in guidance bringing the southern and southwest extent of the primary MCS into northern MS/AL, as well as northern GA as the bowing segment is forecast to propagate to the southeast within that sharp theta-E gradient, tapping into the favorable environment on the southern extent of the complex. A secondary MCS is possible given the latest HRRR as a shortwave ejects out of TX behind the primary shortwave responsible for the setup. This will be a little less severe and doesn`t favor the prolific rainfall totals from it`s predecessor, however additional rainfall over areas that have been impacted would add, "Insult to injury" and could exacerbate the flood threat that is forecast to occur across TN. The previous MDT was maintained with minor expansion on the western and southern edges of the risk area. There is a non-zero chance for a targeted High Risk somewhere within the confines of TN, but this will be more of a nowcast as we monitor the evolution of the prospectus MCS and the QPF footprint left in wake of the disturbance. Regardless, a significant flash flood threat is likely within the bounds of the MDT risk and could be locally significant even within areas bounded by the SLGT risk. Elsewhere, the threat across the Midwest did not feature any significant chances that would allow from much deviation from the previous forecast. Continuity was maintained for the locally heavy rainfall threat under the guide of the inverted trough and upper forcing situated across IA/MN/SD. Kleebauer ...Previous Discussion... 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