Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
031 FOUS30 KWBC 031558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sun Aug 3 2025 Day 1 Valid 16Z Sun Aug 03 2025 - 12Z Mon Aug 04 2025 ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST/GULF COAST AND SOUTHERN PLAINS... ...Southeast/Gulf Coast... 16Z Update... Two areas of rain this morning will modify the overall environment, but another active day of convection with scattered flash flooding is likely for portions of the Southeast. PWs on the morning 12Z soundings from KLIX to KCHS are a bit lower than 24 hours prior, but still widespread around/above 2 inches, and supportive of heavy rainfall rates for which the HREF indicates has a 40-60% chance of exceeding 2"/hr. Instability this morning is modest to the north due to the wedge/overrunning rain, but should increase within clearing, especially from southern AL through the Panhandle of FL and into southern GA. Here, the signals for heavy rain are greatest as reflected by both HREF and REFS probabilities for 5"/24hrs reaching above 50%, and this is also overlapping vulnerable soils from heavy rainfall and a few instances of flash flooding from Saturday. For this reason there still appears to be an elevated excessive rain risk for portions of the Southeast, and only cosmetic adjustments were made to the inherited, primarily to focus more significantly on the FL Panhandle. Relevant Portion of previous discussion: The cold front will continue to waver along the Gulf Coast as a stationary front in response to subtle additional height falls as the trough extending from the Northeast continues to subtly amplify. The trough axis may reach the Gulf Coast by evening, but flatten at the same time in response to an elongated ridge draped across the Gulf. A constant pool of anomalous PW values (above 2.25 inches with deep column saturation) noted via moist- adiabatic lapse rates through the depth of the column indicating tall-skinny CAPE which is progged to reach above 1000 J/kg during peak heating. Convergence along the front into these thermodynamics will support another day of slow moving convection noted by 0-6km mean winds of just 5-10 kts pointed to the east, with rainfall rates likely exceeding 2 inches/hour at times (HREF 50-60% chance) supported by warm cloud depths nearing 15,000 ft to produce efficient warm- rain collision processes. Shortwave energy is expected to rotate beneath the trough, tracking from the Florida Panhandle into eastern Georgia. This features may support an increase in areal coverage of convection and possibly organizing into clusters/multi-cells. With weak storm motions in place, any collisions/organization during this time could briefly pulse up rain rates even more significantly, while also leading to longer temporal duration or repeating rounds of these rain rates. Otherwise, scattered thunderstorms with intense rain rates could produce instances of flash flooding. Models are depicting a secondary area of maximum rainfall potential along a westward advancing surface trough through northern Alabama into central Tennessee where 3 inches/24hr probabilities peak at 40-50%, but this correlated with 3-hr FFG exceedance that is just 10-15%. ...Plains... 16Z Update... Biggest change with this update was to remove the SLGT risk from North Texas where weakening MCV this morning will track with more modest rain rates as it decays into the afternoon. Additional MCSs are probable this evening as vorticity impulses swinging through the mid-level W/NW flow interacts with both a stationary front and an increasing LLJ late. While any of these MCSs will contain heavy rain rates, they should also remain progressive as they dive SE into the moisture, so unless they overlap saturated soils from antecedent rainfall, the flash flood risk appears more isolated than scattered. This is reflected by HREF probabilities for 3-hr FFG exceedance that reach only 5-10%, suggesting just small adjustments to the large MRGL risk area are needed. Relevant portion of previous discussion: The stationary front stretching from Montana to Texas is expected to weaken, leaving behind a weak convergence boundary as it decays. Mid-level flow upstream of this feature will gradually become more W/NW as it gets squeezed between an amplifying ridge over the Southwest and a trough across Canada. Within this pinched flow, multiple shortwaves/vorticity impulses will shift eastward, interacting with the front to cause another day of convective development across the High Plains and into the Plains, with several MCSs possible beneath any of the stronger impulses. The exact location and track for any of the MCSs that develop remain uncertain however there is a decent signal for storms to have rainfall rates of 1-2 inches/hour. While the CAMs are generally suggestive of forward propagating features that will limit the duration of heavy rainfall, locally any of these could produce some short-term training, especially where any MCVs and lingering boundaries can track. Weiss/Campbell Day 2 Valid 12Z Mon Aug 04 2025 - 12Z Tue Aug 05 2025 ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE SOUTHEAST... ...Southeast... A surge in moisture ahead of the longwave trough axis will maintain unsettled conditions and widespread showers/thunderstorms from southern Alabama through Georgia and coastal South Carolina. While there is a degree of uncertainty, a signal arises for an axis of heaviest QPF to streak across the Gulf Coast into Georgia. PW values likely continuing at 2-2.25 inches overlapping MUCAPE of more than 1000 J/kg, rainfall rates of 1-2+ inches/hour are likely within any convection that develops and then tracks northeast. This setup may be conducive for some repeating rounds. A Slight Risk was hoisted for this period for the Florida Panhandle and west-central Georgia. ...Northern Plains... The closed mid-level low from the Day 1 period will continue on is northeastward track as it weakens, leaving a lingering boundary in its wake. Local rainfall rates in excess of 1 inch/hour are possible, with a secondary surge in convection possible late in response to 850mb winds reaching 20-30 kts converging into the region. Rain accumulations during the Day 1 periods will likely lower FFG across the region, thus there will be isolated excessive rainfall impacts. ...Northern Rockies... During this period a shortwave advancing through the Intermountain West will aid in the amplification of a trough shifting over the Northern High Plains. In response, this will drive enhanced ascent into the already broad synoptically forced lift, helping to spawn a weak wave of low pressure to track into the High Plains by late afternoon/evening. Fairly progressive showers and thunderstorms will move from the Northern Rockies to the Northern High Plains. PW anomalies of +1 to +1.5 sigma above the climo mean is forecast, which may lead to rainfall rates reaching 1 inch/hour sporadically thus increasing the threat for local flash flooding concerns. Campbell/Weiss Day 3 Valid 12Z Tue Aug 05 2025 - 12Z Wed Aug 06 2025 ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN THE SOUTHEAST... ...Southeast... Gulf moisture will continue to surge north/northeast over the stalled west-east orientated frontal boundary. Showers and thunderstorms will continue across much of the Southeast with the highest QPF expected to focus over eastern Alabama and central Georgia where accumulations of 2 to 3 inches will be possible. This multi-day event will result in lowered FFG and increased threat for flash flooding. A Slight Risk area was raised for this period covering eastern Alabama and much of central/northern Georgia. ...Northern Plains and Minnesota... The upper-level trough and surface frontal system mentioned during Day 2 will continue to track across the northern tier, shifting showers and thunderstorms to the east. Guidance is showing areal averages of 1 to 2 inches across the region with isolated maximums possibly up to 3 inches, with the higher totals closer in proximity to the International border. A broad Marginal Risk was maintained for this period. First guess fields hint a small area may need a Slight Risk, but confidence on where is low at this time so opted to stick with just a Marginal for this issuance. Campbell 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