FOUS30 KWBC 200100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 800 PM EST Wed Nov 19 2025 Day 1 Valid 01Z Thu Nov 20 2025 - 12Z Thu Nov 20 2025 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTH-CENTRAL TEXAS, INCLUDING PORTIONS OF THE HILL COUNTRY... ...Texas... A targeted Moderate risk was maintained across portions of south central TX, including portions of the Hill Country. The 18z HREF and REFS show increasing probabilities of 1"/hr rainfall by 04z-05z, increasing further after 06z. The pattern is favorable for localized areas of heavy rainfall by late tonight into the day Thursday. PWs will be above the 90th percentile, with abundant low level moisture advecting in from the Gulf, and mid and upper level moisture out of the Pacific ahead of the approaching upper low. Instability will be sufficient deep convection and high rates...thus the extent of the flash flood risk will come down to the longevity of these higher rates. Cells should initially be rather quick moving of to the Northeast, however as low level flow increases we should tend to see an increasing backbuilding/training component to the convection. This should be enough to result in an uptick in the flash flood risk. Both the 18z HREF and REFS have high neighborhood probabilities of exceeding 3" by 12z Thursday over the Moderate risk area. The most recent 23z run of the HRRR as rainfall of 3-5" over this same period. The rainfall continues into Thursday, with another uptick in rainfall intensity and potential backbuilding convection by mid morning into the early afternoon as large scale low level convergence increases. Thus it still appears like the highest chances for significant flash flooding is after 12z Thursday, when this 2nd round convection potentially trains over areas that have already received heavy rainfall and are thus saturated. In fact, the REFS probabilities of exceeding the 24hr 100yr ARI are as high as 25%, with the HREF around 10%. This is indicative of the higher end impact potential Thursday should both rounds of convection materialize over the same portion of the Hill Country. Even though the higher probability threat is likely tomorrow, the flash flood risk tonight should not be understated. While the model consensus of upwards of 3-5" through 12z would result in some flash flooding, there is certainly a possibility that enhanced backbuilding results in more localized totals over 5" in just a couple hours...which would likely be enough to produce significant flash flooding over these more flood prone areas. Thus a Moderate risk remains for both tonight and tomorrow across this corridor. The Moderate risk areas are quite small, but in reality the axis of most extreme rainfall and higher end impacts will likely be even narrower than these risk areas. However predicting exactly where this occurs is not feasible, warranting the slightly larger risk areas to account for location uncertainty. ...Southwest... A Marginal risk was maintained for portions of AZ and NM. The approaching upper low will result in continued showers and embedded thunderstorms into the overnight period. The highest rainfall rate potential is likely this evening over central AZ, but this should be isolated and generally on a downward trend into the overnight. The highest areal averaged rainfall (~1") will be across portions of southwest NM, but lower instability should keep these rates mostly at 0.5"/hr or less. Thus overall the flash flood threat should stay isolated in nature. Chenard Day 2 Valid 12Z Thu Nov 20 2025 - 12Z Fri Nov 21 2025 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTH-CENTRAL TEXAS, INCLUDING PORTIONS OF THE HILL COUNTRY... ...20z Update... A couple of significant changes to the outlook areas for this update. This first is to introduce a targeted Moderate Risk upgrade for the same areas as detailed for Day 1. This is due to increasing likelihood for localized 5"+ storm totals beyond the 12z Thursday Day 1/2 cut- off, which may result in numerous instances of flash flooding in a relatively small area (with higher confidence overall in locally significant flash flooding as opposed to the total areal coverage). While 6-hr FFG exceedance probs from the HREF are below 20% through the end of Day 1, they increase meaningfully to near 60% going into Day 2 (through 18z Thu). The second significant change was to split up the larger Slight Risk area into two distinct areas, 1) South-Central TX and 2) eastern OK into northern AR and southern MO. This reflects the consensus of the latest CAM guidance which suggests a large gap in convection across North TX during the period (with questions still lingering regarding the eastward extent of both the South-Central TX and Ozarks clusters of convection into the later half of Day 2). Churchill ..Previous Discussions.. ...Southern Plains... The environment continues to becomes more supportive of storms capable of producing heavy to potentially excessive rainfall as the flow aloft backs over portions of Texas and Oklahoma and becomes increasingly diffluent as an upper trough approaches the region. At lower levels...an airmass with precipitable water values gets drawn northward an converges along a surface front over parts of Texas and Oklahoma. Excessive rainfall is possible by means of intense rainfall rates by individual storms in regions with the highest amount of instability (presumably ahead of the approaching cold front) and by training of cells/multiple rounds of convection (more of a factor where synoptic scale forcing and mesoscale forcing act in tandem farther north). The axis of highest precipitable water values... 1.5-1.75"... sets up over the eastern half of Texas into southeastern-most Oklahoma and into Arkansas. Those values are well above average for mid-November. Localized rainfall amounts up to 2.5" in an hour with additional local totals of 5" possible within this set up which would result in scattered instances of flash flooding. ...California... Maintained a Marginal Risk area from continuity for portions of coastal Southern California as the next area of low pressure over the eastern North Pacific drops southward off the California coast and brings another round of rainfall Thursday into early Friday. Rainfall amounts at this point look to be pretty modest but the rain will be falling on areas where antecedent conditions have been wet for the better part of 5 or 6 days. That may result in additional localized flooding on Thursday or Thursday night. Bann Day 3 Valid 12Z Fri Nov 21 2025 - 12Z Sat Nov 22 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO VALLEY AND PORTIONS OF THE SOUTHWEST US... 20Z Update: Limited changes necessary from the previous forecast due to general run-to-run continuity. Heaviest rainfall still trending closer to the southern Peninsular Range with an extension of convection back over the southeast CA deserts and southwest AZ for the MRGL over the west. Moderate to locally heavy rain possible for the Ohio Valley. Steady forward propagation of broad QPF shield should limit the necessity for more than the current MRGL risk. Will assess the southern portion of the risk area closely in the coming days as this region, extending into the Southeast CONUS will be most coincident with a purely convective regime as instability is prevalent within a broad, but modest theta_E depiction. This area would promote higher rate potential, but could be a case where forcing is not enough to activate the better thermodynamic environment. For now, maintain relatively continuity of the MRGL with some minor adjustments overall. Kleebauer ..Previous Discussions.. ...Ohio Valley... Rain will be spreading northward and eastward from the Plains on Day 2 into the Ohio Valley on Friday and Friday night in association with the surface low coming out of the mid-Mississippi Valley. Between above normal levels of moisture being transported into the region from the south and flow aloft becoming confluent... there is still the possibility for isolated downpours that lead to spotty problems from run-off despite the set up becoming less favorable with time. ...Southwest US... A mid- and upper-level trough with an embedded closed low will continue to drop southward on Friday...with an embedded closed low expected near the US border with Mexico by 00Z on Saturday. Showers and embedded thunderstorms should be developing in the highly diffluent flow to the east of the trough where precipitable water values approaching an inch rotate into southern California and western Arizona. Opted to remain at a Marginal threshold given the timing of the best upper support but a Slight may still be needed. 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