FOUS30 KWBC 040746 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 346 AM EDT Sun Oct 4 2026 Day 1 Valid 12Z Sun Oct 04 2026 - 12Z Mon Oct 05 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER TEXAS COAST... ...Southeast to Gulf Coast... An amplifying mid-level trough will dig and expand across the eastern third of the CONUS today, but height falls will struggle to dig as far as the Gulf Coast due to a ridge centered over the FL Peninsula and backing west into the Gulf. This will result in squeezed mid-level flow from Texas through the Carolinas, and although subtle height falls may eventually expand southeast by Monday morning, the resulting surface front is likely to remain generally in place with just some wavering through the D1 period. This front will serve as an axis for both mass convergence and impressive moisture confluence through the day. The resulting ascent will be aided by the aforementioned weak height falls, but more impressively by intense diffluence within the RRQ of a meridionally arcing and strengthening jet streak pivoting into New England. This deep layer lift will act upon intense thermodynamics thanks to a pool of PWs above 2.25-2.5" (above the 99th percentile according to NAEFS and locally +3 sigma from the climo mean) to support widespread convection, and although instability may be somewhat inhibited to generally 500-1000 J/kg due to cloud cover, the extreme PWs and 15 kft+ warm cloud depths will still support efficient rainfall rates of 2-3"/hr, with short-term rates even higher than that thanks to convective organization on 20-30 kts of bulk shear The guidance is in good agreement this plume of PW will remain mostly unchanged through the D1 period, leading to multiple rounds of heavy precipitation. While the axis of heaviest rainfall does vary amongst the different CAMs, weak impulses drifting from LA to the western FL Panhandle may help develop a wave of low pressure along the front to serve as a focus for locally heavier rainfall, and as Corfidi vectors collapse to around 5 kts and align increasingly parallel to the weak mean flow, this will support upwind development and training of rates. This suggests that some areas will receive 3-5" of rain today, with locally as much as 8" possible from Mobile Bay, AL eastward to the Emerald and Forgotten Coasts along the Florida Panhandle. Considered a small MDT risk for these areas but due to relatively high FFG and modest exceedance probabilities focused right on the coast, and after coordination with WFOs MOB and TAE, opted to keep the enhanced slight risk in place for now and defer a possible upgrade, if needed, to the D1 cycle later today. Farther southwest along the western Gulf Coast and the entire Texas Coast, the MRGL risk was joined to encompass the immediate coast of the entire region due to possible onshore drift of heavy rainfall through the morning/aftn hours. However, the most significant rainfall (both of this area and maybe the entire day) is anticipated along the Lower Texas Coast between Brownsville and Corpus Christi. Here, convection developing over the Gulf beneath mid-level impulses will track SW and into impressive moist inflow, pushing onshore repeatedly thanks to anti-parallel Corfidi vectors. With rainfall rates likely reaching 2-3"/hr, this could result in local maxima rainfall in excess of 10" in a few locations, and both the REFS and HREF indicate a high risk (>50% chance) for at least 8" of rain. This will fall atop locally more sensitive soils too since recent rainfall has been significant in some areas (2-4" last 12 hours, up to 8" last 3 days). While this region generally has elevated FFG due to sandy soils, after coordination with WFO BRO and CRP, raised a targeted MDT risk for the lower Texas Coast where locally significant impacts are possible should any of this heavy rain track across urban areas or more sensitive soils. Finally, there is a tertiary area of locally heavier rainfall possible along the Georgia coast, centered on the vicinity of Savannah, GA. While there is a lot of spatial spread among the various CAMs and associated ensemble clusters (REFS, HREF), there is a distinct signal for locally more significant rainfall across this area as well. This is likely to occur in two rounds - one from pinned sea breeze convection aided by resultant frontogenesis beneath the ageostrophic circulation of the aforementioned jet streak - with a second round possibly occurring later at night as a wave of low pressure moving onshore the FL Panhandle approaches from the SW. Any convection will again contain 2-3"/hr rain rates, and this area has been saturated recently leading to locally compromised FFG of 1-2"/3hrs. This suggests despite the general sandy soils across Georgia coast, scattered flash flood impacts are possible where these locally more vulnerable soils become overwhelmed by repeating rounds of heavy rain. Weiss Day 2 Valid 12Z Mon Oct 05 2026 - 12Z Tue Oct 06 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE LOWER TEXAS COAST... ...Texas Coast... Another day of heavy rainfall is likely across the Lower Texas Coast and into parts of South Texas, likely increasing or exacerbating flash flooding impacts anticipated during D1. A surface front will track southward into the Gulf Sunday and linger along the Lower Texas Coast Monday. While the accompanying ribbon of instability may be quite compressed by Monday, PWs as high as 2.5" and deep warm cloud depths will maintain a favorable thermodynamic environment for heavy rainfall. Additionally, an axis of 700-500mb deformation just offshore will arc back towards the region, resulting in a narrow corridor of enhanced ascent to drive streaks of convection with heavy rain. The HREF (when available) and REFS both indicate a 20-30% chance of rainfall rates exceeding 2"/hr, which through training thanks to continued anti-parallel Corfidi vectors may produce an additional 3-5" with locally higher amounts (REFS 5"/24hr probabilities above 50%). For this reason a SLGT risk has been added for a narrow corridor of the BRO CWA. ...Southeast... A similar environment persists farther east along the Gulf Coast from the Florida Panhandle and extending northeast into coastal South Carolina. A cold front will be drifting slowly southeast across this area as height falls south of an expansive mid-level trough inexorably work their way south towards the Gulf Coast. Weak impulses moving through the flow aloft will help generate waves of low pressure along this front, and where these waves move northeast they will enhance the rainfall through locally accelerated flow and stronger ascent. Regardless, efficient rainfall rates of 2-3"/hr are likely within a plume of exceptional tropical moisture (PWs above 2.25") and deep warm cloud depths. Considered a SLGT risk where the guidance supports areas of heavier rainfall, but there continues to be some uncertainty as to how much instability will persist north of the front (depending on how far south it progresses), and anafrontal rainfall may not be heavy enough for a SLGT risk upgrade. While training is likely along the CAPE gradient which through these rain rates could lead to 2-4" of rain atop saturated soils from D1, opted to maintain the MRGL risk at this time due to this uncertainty. However, a SLGT risk upgrade may be needed if signals continue to support another day of heavy rain on Monday. Weiss Day 3 Valid 12Z Tue Oct 06 2026 - 12Z Wed Oct 07 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF FLORIDA AND SOUTHEAST GEORGIA... The cold front from Monday will stall across the northern Florida Peninsula Tuesday as a stationary front. This front will meander subtly north/south through D3, remaining entrenched within weak mid-level flow and a plume of anomalous PWs progged to be over 2.25", or above the 97th climatological percentile according to NAEFS. Weak mid-level shortwaves traversing slowly atop the boundary will enhance ascent, and despite modest instability, the overlap of sufficient ascent within the deep tropical moisture will result in yet another day of heavy rain as convection trains WSW to ENE on 0-6km mean winds parallel to the boundary. With rain rates expected to reach 1-2"/hr, at least isolated flash flood impacts are expected, especially where rain moves across areas that also experience heavy rain on D2 /Monday./ Effective bulk shear of 20-30 kts may support some modest organization which could enhance the risk to drive a targeted upgrade to a SLGT risk, but at this time maintained the MRGL risk due to uncertainty in the intensity and footprint. Weiss Day 4 and Day 5 Valid 12Z Wed Oct 07 2026 - 12Z Fri Oct 09 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL WEDNESDAY ACROSS PORTIONS OF THE FLORIDA PENINSULA... A stationary surface front will remain draped roughly halfway across the FL Peninsula Tuesday through Thursday and will continue to be a focus for potentially heavy rainfall through at least Wednesday night (current Day 4). The axis of highest PWs (2.25-2.5") and mid-level shortwave energy will be be in the front's vicinity, as well as instability from the nearby Gulf. Deep west/west-southwesterly flow will enhanced the risk for training cells coming off the eastern Gulf across central FL, while anti- parallel flow to the front's north across northern FL bringing in instability from the subtropical Atlantic and could lead to very high precipitation efficiency. Effective bulk shear values over 20kts would allow for some degree of organization on Wednesday. However on Thursday, the guidance shows the front become a little more diffuse as the subtropical ridge and a more DNVA regime begins to build across the FL Peninsula. This would explain why the 00Z operational global QPFs (GFS/ECMWF/UKMet/CMC) are lower on Day 5 compared to Day 4. As a result, at least for now, we did not extend the Marginal Risk area into the Day 5 ERO. Hurley/Roth 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 $$