Quantitative Precipitation Forecast
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Issued by NWS
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101 FOUS30 KWBC 020059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Wed May 1 2024 Day 1 Valid 01Z Thu May 02 2024 - 12Z Thu May 02 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL AND EASTERN TEXAS... ...01Z Update... A few changes to note with this update: CAMs guidance on the whole has been doing quite poorly resolving the various areas/cells of convection across the middle of the country. As such, certainty on the coverage and nature of any resultant flash flooding is reduced. Thus far, the nearly stationary storms causing flash flooding, all in Texas, have been largely isolated...but due to abundant moisture and snail`s-pace movement, isolated significant flooding with those cells appears likely. The Moderate Risk area in eastern Texas has been shrunk a bit and lifted north, based largely on where the current thunderstorm activity has developed. The latest guidance that seems to have even a reasonable depiction of the current convective activity suggests that the storms in west Texas will continue moving east and eventually interact with the nearly stationary cells in central/east Texas. This interaction is most likely to occur in the Moderate Risk area, which while low-confidence, remains in effect. The Slight Risk area towards the Texas Panhandle is largely to cover the ongoing convection that has been slow moving up to this point, but recent radar trends have suggested they are beginning to move east. The area ahead of them around the southwest corner of Oklahoma was hit hard by storms a few days ago, so the ongoing storms, while expected to weaken, will be moving into a more favorable environment for flooding, so the Slight was expanded north and west. Meanwhile, it was shrunk out of eastern Oklahoma as once the storms organize, they`ll be faster-moving and less likely to produce flash flooding. Up north into the central Plains, a lack of instability has resulted in a larger area of mostly lighter rain, though a few stronger storms are in western KS and NE. These may also organize over the next few hours, but will be very fast moving. Given about average soil moisture conditions into eastern Nebraska and surrounding areas, in coordination with the impacted offices, the Slight was downgraded to a Marginal. Wegman Day 2 Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE MISSISSIPPI VALLEY EXTENDING FROM THE NORTHERN MIDWEST DOWN TOWARDS THE GULF COAST... ...2030Z Update... ...Midwest... Trimmed down the far northern and western flanks of the Marginal and Slight in the Midwest where probabilistic guidance in heavier QPF totals >1" were decreasing and instability will be lower to the north of the surface low track through the Upper Mississippi Valley Thursday evening. The area most likely to see the heaviest rainfall will be closer to the triple point low, which in this case would favor an area from southern IA on north and east through eastern IA and into southwest WI. NAEFS shows this designated area sporting >90th climatological percentile PWs (1.3-1.5") between 18Z Thurs and 00Z Fri, as well as IVTs above the 97.5 climatological percentile (500-700 kg/m/s) in that same period. 12Z HREF shows MUCAPE between 500-1,000 J/kg will also be present. Hourly rainfall rates up to 2"/hr are possible in the strong est storms, which would support the potential for slash flooding in these areas. Farther south, a negatively tilted 500-700mb axis emerging out of the Ozarks will still be capable of producing strong-to-severe thunderstorms that could track across much of western MO and eastern KS. Localized rainfall rates surpassing 2"/hr cannot be ruled out given PWs topping 1.5", MUCAPE approaching 1,500 J/kg, and mean 1000-500mb RH values as high as 90%. ...Southern Plains and Lower Mississippi Valley... The expansive Slight Risk in the South Central U.S. was expanded a little farther west more into central TX and central OK where latest CAMs guidance has slowed down the cold front approaching from the northwest. Farther south, there remains some uncertainty on the position and duration of the ongoing cluster of storms in the early-mid morning hours Thursday. Should storms stick around longer into the morning hours along and north of I-10, there could be the need for either an additional expansion of the Slight Risk or an upgrade to Moderate Risk should flash flooding be more extensive and linger beyond 12Z Thursday. Recent rainfall in western Louisiana (2-5" above normal over the past 7 days) has left soils in the area more sensitive to 2-3"/hr rainfall rates, which are supported with PWs just short of 2.0" and skinny MLCAPE up to 1,000 J/kg present. Overall, the Slight Risk remains on track but the setup in eastern TX and western LA will be closely monitored overnight. Mullinax ---Previous Discussion--- ...Midwest... Previous forecast still has merit with little deviation from past issuance. Thus, maintained continuity from the last discussion with some minor changes to details within the discussion below.... Overnight convection from this evening will continue to march northeastward as the congealed cold pools from the initial cell mergers will advance through the central Midwest with sights on northern IA into WI and MN on Thursday morning. The complex will eventually peter out as it moves northward into a less favorable environment which will put an end to the flooding threat as we move into the afternoon. Later in afternoon and evening, a cold front will propagate to the east out of the central plains with a redevelopment of convection over eastern KS, IA, and northwest MO. This will allow for the secondary peak of flash flooding potential as thunderstorms migrate through the mid and upper Mississippi Valley, impacting some areas that were just affected in the last 24 hrs. With the lower FFGs expected, the threshold for flooding will be lower than climatological norms for some areas within the expected secondary convective regime. Totals are currently within the 1.5-3" range on guidance which is significant enough to warrant at least the current SLGT risk. ...Southern Plains and Lower Mississippi Valley... As stated previously in the D1 outlook, our complex of thunderstorms in TX will migrate eastward into the Lower Mississippi Valley with heavy rainfall likely within the western half of LA into southern AR. The setup has now trended towards a "front loaded" forecast where the primary rainfall in the period that would induce flash flood concerns will occur during Thursday morning and early afternoon before dissipating due to the loss of sufficient upper forcing. The back end of the latest CAMs and associated HREF signal the opportunity for upwards of 2-3" of rainfall during that short time frame Thursday AM which will be enough to cause problems given the overlap from the previous weekend event that dropped copious amounts of rainfall over southeast TX into western LA. 00z HREF blended mean has an areal extent of 1.5-2" with probability matched mean upwards of 3" along the TX/LA border. HREF neighborhood probability for rates exceeding 2"/hr are also upwards of 30-40% within the same areas, correlating with the forecasted rainfall in that 2-3" zone. The setup really comes down to the timing of the complex in TX as to whether a reintroduction of a MDT risk will be needed after it was downgraded and shifted one period earlier. This will be assessed in future forecast updates as we closely monitor radar and hi-res guidance trends over the course of today and overnight into Thursday. For now, the SLGT risk will remain with wording of it being on the higher end of the risk threshold. Further north across KS and OK, the cold front to the north will move south with a push from high pressure nosing in through the plains in wake of the low pressure moving to the north. An area of convection will likely form over the central plains and move south with the boundary leading to more locally heavy rainfall in association with the area of convection. The threat is less aggressive compared to what is expected further south, but the chances are still prevalent. The SLGT risk was maintained from the previous D3 connecting the two primary areas of interest to the north and south. Kleebauer Day 3 Valid 12Z Fri May 03 2024 - 12Z Sat May 04 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF TEXAS, THE LOWER MISSISSIPPI VALLEY, TENNESSEE VALLEY, CENTRAL PLAINS, AND THE PACIFIC NORTHWEST... ...2030Z Update... Little in the way of change for the inherited Marginal Risks aside from minor adjustments based on the latest QPF. There was some consideration to adding a Marginal Risk in portions of the Great Lakes given the >90th climatological percentile PWs present according to the ECMWF SAT, but thunderstorm activity looks fairly progressive and confidence in sufficient instability being present was not high as of this forecast cycle. Mullinax ---Previous Discussion--- ...Southern Plains and Southeast... A multitude of shortwaves will ripple through the flow out of the subtropical jet with convective development across portions of TX through the Lower Mississippi Valley up into the Tennessee Valley. The convection over TX will have the best potential for flash flood concerns due to the regional instability and alignment of a stalled frontal boundary forecast across the central and eastern portion of the state with the dryline positioned over west TX. QPF distribution is scattered in nature with the maxima shifting all over the place pending deterministic. Considering the spread, the MRGL risk inherited was maintained, but there is potential for an upgrade within the two area boundaries due to the features being a focal point for training convection and surface convergence. Further east, widely scattered thunderstorms will produce heavy rainfall at times with the best opportunity occurring earlier in the period when the strongest shortwave progresses through the region(s). Ensemble bias corrected QPF is consistent with a maximum across LA which will have gone through some significant precip prior, so that will be another area to monitor in time for potential upgrades. Forecast QPF is still marginal for any upgrade potential, but CAMs will shed more light as we move closer to the time period of interest. ...Central Plains... A fairly robust mid-level perturbation will eject east-southeast out of WY/CO with convection initiating over the Front Range, eventually congealing into a MCS as it migrates eastward through southern NE into KS. Models are in agreement of at least some modest QPF with 1-2" scattered across the aforementioned areas and QPF maxima running upwards of 3" at this time. These MCS setups are notorious for localized flooding concerns, so wanted to make sure to add a Marginal Risk area to cover for the potential. As we move closer, the MRGL risk will likely be shifted around to account for the trends in guidance, especially within D1 when the CAMs and HREF provide more favor. ...Pacific Northwest... A relatively weak IVT pulse will enter coastal OR and northwest CA by the second half of the period with locally heavy rainfall expected through Saturday morning. IVT index is running at an "AR 1" which signals a weaker atmospheric river event, but one that can still produce modest rainfall totals to the coastal plain. Ensemble trends have come up a bit from previous forecasts with QPF running between 1.5-2.5 over the span of 12 hrs (00-12z Saturday), with a bit more as we head into D4. As a result, kept continuity with the MRGL risk from previous forecast package. 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