Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
784 FOUS30 KWBC 100058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 858 PM EDT Thu May 9 2024 Day 1 Valid 01Z Fri May 10 2024 - 12Z Fri May 10 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS THROUGH THE SOUTHEAST U.S... ...Southern U.S.... Isolated to scattered flash flooding is expected across a wide swath of the Southern U.S. tonight...stretching from eastern TX into southern GA. MLCAPE values ranging from 3000-5000k j/kg and PWs around 1.75" supports deep convection capable of intense rainfall rates. As we go through the evening hours convective coverage should increase as mid/upper forcing ramps up and low level convergence increases as a cold front drifts southward. Multiple areas of convection are likely across this swath through the overnight hours. Eventually we may see a more dominant squall line, but before that we are likely to see numerous convective clusters. Individual cell motions will generally be fairly quick, however some areas are likely to see multiple rounds of convection. Also, as activity grows upscale overnight a west to east training axis is a possibility somewhere from LA into AL. Not seeing a strong or consistent enough signal for a MDT risk upgrade, especially since a lot of this region has relatively high FFG that will need to be overcome. However this is likely a higher end Slight risk event and scattered flash flooding is expected overnight across this entire corridor. The robust instability and moisture will support hourly rainfall as high as ~3". So even storms that are moving along at a decent clip will have the potential to result in urban flash flood concerns. East central TX into west central LA is the corridor that has the most saturated conditions and highest streamflows. There is growing concern for a locally significant flash flood threat in this vicinity through the evening hours. Observational trends suggest a period of colliding and training convection is possible over a narrow corridor during the next several hours...with the 00z experimental WoFS indicating a 40-70% chance of exceeding 5" of rain, and a 90th percentile QPF of ~8". Any rainfall swath of this magnitude will be pretty narrow, but the potential is there for a localized significant flash flood given the antecedent conditions over this area. ...Mid Atlantic... MPD #253 describes the localized flash flood risk over portions of WV/PA and far western MD this evening. Brief convective training segments could locally drop 1-2" of rain within an hour, resulting in an isolated flash flood risk through the evening hours. Chenard Day 2 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.... ...20Z Update... Very little change in the previous MRGL risk across southeast AL, southern GA, and northern FL as the expected synoptic and mesoscale evolution have deviated little since the last forecast. The primary target will be across southwest GA through northern FL between Tallahassee and Jacksonville. Propagating MCS across the Southeast U.S will continue its progression into GA by the morning with locally heavy rainfall between 1-2"/hr possible before the primary outflow exits off the eastern seaboard. Secondary outflow(s) from associated convection could very well contribute to deviated motion away from the mean flow with some guidance introducing secondary and tertiary maxes within the FL Panhandle. The latest HREF probabilistic output for 1" and 2"/hr rates indicates that opportunity with small areas of 20-40% probs for the 1"/hr rates and 10-20% for the 2"/hr within the Panhandle east of Panama City over towards Jacksonville. This allowed for a minor southern nudge of the risk area to account for the chance that the core of the precip ends up further south than the mean QPF forecast. Otherwise, areas within GA are expected to see another wave of precip with a footprint of 25-35% neighborhood probabilities for at least 3" within the zone of southwest GA, an area that has seen significant rainfall and slightly lower FFGs compared to the surrounding locales. Overall, the threat is low to medium end of the MRGL risk considering the convective nature of the precip and expected overlap from the D1 rainfall associated with another MCS dropping southeast through GA currently. Thus, continuity was maintained in the grand scheme with just those minor adjustments based on the probability fields and blended mean QPF. Kleebauer ...Previous Discussion... Only subtle changes were made to the Marginal Risk outlook area from yesterday`s Day 3 ERO. 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. Hurley/Taylor Day 3 Valid 12Z Sat May 11 2024 - 12Z Sun May 12 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF WEST TEXAS... Despite a marginal airmass in wake of a cold front to begin the weekend, a low-end MRGL risk for heavy rainfall will exist within the confines of the Concho Valley and adjacent areas of the Permian Basin across west TX. A stout upper low across the Great Basin will pivot eastward to the Four Corners with a prevailing diffluent signature focused across west TX and NM allowing for multiple mid- level perturbations to eject northeast out of the mean flow. Theta-E ridge located over southwest TX in conjunction with the passing vorticity maxima will help initiate area convection with focal points generally tied to the topography near the Davis and Glass Mountains to the west of the Pecos River. Efficient mid-level shear will aid in preservation of convection as it motions to the northeast with enough large scale ascent to continue it`s progression well to the northeast of the initiation point(s). Environment will be conducive for locally heavy rainfall within a general spread of 1.1-1.5" PWATs, a solid 1-2 standard deviations above normal based on the latest NAEFS standardized anomalies. The second mid-level vort max is being depicted as the most prolific with best prospects for more widespread convective coverage, specially for areas east of the Pecos River. Ensemble bias corrected QPF output is generally between 0.75-1" with some embedded maxes around 1.25-1.5" which is fairly aggressive at these leads without any input from relevant CAMs suite. ML output from both the GFS/ECMWF Graphcast indicates a relative max across the area outlined with even some potential for the risk area to be expanded west, pending the convective spread for areas within the zone of better theta-E`s. Have introduced a Marginal Risk for the aforementioned locations with future forecast iterations potentially expanding in any given direction pending favor dynamically or thermodynamically. Kleebauer 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