Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
341 FOUS30 KWBC 032028 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 PM EDT Fri May 3 2024 Day 1 Valid 16Z Fri May 03 2024 - 12Z Sat May 04 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHEAST TEXAS... ...16Z Update... ...Southeast Texas... In coordination with HGX/Houston, TX; LCH/Lake Charles, LA; and FWD/Fort Worth, TX forecast offices, a Moderate Risk upgrade was included with this update for portions of southeast Texas. Persistent backbuilding storms producing 1-2 inches per hour rainfall rates has been occurring over areas that have been pushing a foot of new rain over the past 48 hours. For more details please see the corresponding MPD 216 at www.wpc.ncep.noaa.gov/metwatch/metwatch_mpd_multi.php?md=0216&yr=2024. ...Arklatex... Another bigger change made with this update was to trim down the inherited Slight out of the Arklatex region, confining it to the extremely hard hit southeast Texas, and into southwest Louisiana. The ongoing MCS at the time of this writing over eastern Arkansas and northern Louisiana is weakening and breaking into component convection. While those stronger storms will likely hang on a bit longer, the increasingly isolated nature of the convection suggests that any potential resultant flash flooding will be isolated, and more indicative of a Marginal. Further, once the MCS weakens, dissipates and pushes east into Mississippi, guidance is in good agreement that that will be the end of the day`s (and tonight`s) rain for that area. Of course, there`s always potential in this very moisture-rich environment that popup convection could redevelop, but without much CAMs indication of that, that too suggests the flash flooding threat is waning. ...Central Plains... The Marginal Risk area in Kansas has been expanded a bit to the south in deference to the newest 12Z HRRR suggesting a line of convection will develop this afternoon and tonight over the heart of the state. This in turn would limit development further north across Nebraska. Once again the CAMs have been performing poorly so the Marginal area is really a hedge to cover the wide range of possibilities of convective evolution. For Kansas, the southwestern part of the state remains very dry as far as soil moisture conditions are concerned, so the Marginal was to cover potential training convection. Further south into Oklahoma, guidance has been trending downward for most of the state, so the Marginal was removed with this update. Dry line convection likely across west Texas may sneak across the Red River into Oklahoma, so that potential will need to continue to be monitored. ...Central Appalachians... Shower activity from Tennessee to Ohio may develop into more robust, albeit scattered convection across the Marginal Risk area this afternoon and evening. Soils are still a bit sensitive here, but the rainfall is not expected to be notable, so the Marginal covers the isolated flash flooding risk here. Wegman Day 2 Valid 12Z Sat May 04 2024 - 12Z Sun May 05 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF CENTRAL AND NORTH TEXAS... ...2030Z Update... ...Southern Plains... Model guidance has shifted south quite a bit with the most recent updates. The general consensus now is for storms to form along the dry line in West Texas, then develop upscale and towards the east in central Texas. As the storms develop, there will be cell mergers and other interactions, as well as possible training that could lead to local rainfall totals to 5 inches. Further, soils in this area have been saturated due to rainfall from thunderstorms as recently as yesterday. Thus, the Slight Risk area was expanded south to include much of central Texas. The surrounding Marginal was expanded further south to account for likely guidance changes prior to the event. Most of the flash flood producing rainfall is expected overnight Saturday night, continuing into Sunday morning. On the northern side of the Slight, what happens further south will be critical as to how much rain falls along the Red River. Since this area too has nearly saturated soils, the northern side of the Slight was trimmed less on the north side than it was expanded on the south. Nonetheless, the northern side of the Slight has more uncertainty. ...Sacramento Valley of California... The inherited Marginal was shifted a few miles west to move the area off of the higher elevations of the Sierras. Unseasonably cold air associated with an upper level low will keep most of the precipitation at the higher elevations and passes through the Sierras as snow. Any rainfall at the lower elevations will be confined to the valley. The event is a strong one for this time of year, and so abundance of moisture still keeps the area as a low end flash flooding threat. Wegman ...Previous Discussion... Height falls aloft ejecting eastward from the Southern Rockies and the presence of a dryline over West Texas will foster an environment for thunderstorms capable of producing heavy rainfall by later in the day on Saturday that persists into Saturday night/early Sunday morning. Precipitable water values will have increased to nearly 1,5 inches...some 2 to 2.5 standardized anomalies greater than climatology...over portions of central and west central Texas by 00Z Sunday in response to a period of southeasterly flow off of the Gulf of Mexico. Consensus of model guidance QPF in the range of 1 to 3 inches seems reasonable...although the UKMET was a notable outlier. ...Northern Sierra Nevada Foothills... A fairly robust IVT pulse will slide down the west coast with southwesterly flow aimed orthogonally to the northern and central Sierra`s by Saturday. Higher elevations will be greeted with more snow, but the adjacent foothills will remain warm enough to benefit from all rainfall. Rates will be borderline overall, but the prospects for over 1" of precip has grown in the past succession of runs with the ensemble bias corrected guidance now upwards of 2" now forecast within the elevations below 8000` MSL, falling mainly in a span of 8-12 hours. The threat is on the lower end of MRGL. Maintained continuity from previous forecast and will assess whether it needs adjusting, or removal pending future trends in guidance. Bann Day 3 Valid 12Z Sun May 05 2024 - 12Z Mon May 06 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHERN MISSOURI TO NEAR THE UPPER TEXAS COAST... ...2030Z Update... As the guidance has shifted southward on Saturday, the same has occurred on Sunday. Storms will likely be ongoing in portions of east Texas at the start of the period Sunday morning, and will continue pushing east into the morning. It appears likely that there will be an east-west-oriented boundary along which storms will be training on the south end of a more progressive line of storms. There is inherent and typical uncertainty with where that line will set up, especially with recent large southward shifts in the guidance. Thus, to cover the large changes in the guidance, as well as very recent and ongoing heavy rain across southeast Texas, the Slight was expanded well south of inherited, and likely will be upped at least to a higher-end Slight in Texas if the current shifts in the guidance hold, with a distinct possibility for a Moderate upgrade with hopefully agreeable CAMs guidance. For the Ozarks region, significant uncertainty also exists, as the area will be on the northern side of the strong shortwave trough driving all of the convective activity. Much like much of the lower Mississippi and points west, the area also has average to above average soil moisture, which in turn will support the development of flash flooding given enough rainfall. The northern side of the storm complexes often get cut off from the Gulf moisture supporting them in favor of storms further south and closer to the Gulf. This increases the uncertainty in the forecast for this area. For now, guidance spread has led to an expansion of the Slight into south-central Missouri and north-central Arkansas...as well as the surrounding Marginal as far east as Alabama. These are likely to change due to the highly variable forecasts of convection for this area. Wegman ...Previous Discussion... The potential for excessive rainfall makes its way eastward on Sunday in association with a closed low aloft and a cold front at the surface. Model QPF was not as heavy as forecast on Day 2 as shifting low level flow results in weaker low level moisture flux convergence along the front. Despite model QPF generally being 2 inches or less...opted for a Slight Risk given the wet antecedent conditions over portions of eastern Texas and eastern Oklahoma. Bann 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