Quantitative Precipitation Forecast
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366 FOUS30 KWBC 020823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Thu May 2 2024 Day 1 Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHEAST TEXAS... ...Southeast Texas... A complex scenario currently unfolding across southeast TX within the confines of areas that recently received very heavy rainfall over the weekend. There are two distinct areas of interest with a very heavy thunderstorm cluster focused east of Bryan/College Station near Lake Livingston and surrounds. A secondary area is currently moving through central TX Hill Country along the I-35 corridor between Austin/San Antonio. Hi-res deterministic has struggled to gain footing on the exact evolution of these two areas of focus with some missing the bill altogether, parlaying the complexity of the setup. Based on current mesoanalysis, there is a west to east SBCAPE gradient oriented from the Hill Country west of I-35 over into southeast TX and the LA border. Rich, Gulf air has been pulled northward within the north-south oriented 30-35kt LLJ positioned over the eastern 1/3rd of the state. PWATs are analyzed between 1.75-2" across the area of interest which has aided in significant rainfall rates between 3-6"/hr at times within the eastern most cell organization prompting totals to reach over 5" within the areas impacted. This will only continue through the morning as an expected cold pool is projected to push out of the eastern thunderstorm complex and move to the southeast over the course of the morning. Rates of over 2"/hr are almost a certainty with the current environment with the 00z HREF signaling neighborhood probabilities over 50% through much of the area between Bryan/College Station down to just north of Beaumont. The complex will eventually lose steam and rates will dwindle back to manageable levels, but not before a swath of 3+" of precip is laid in wake of the complex. This is not the entire story, however as there are other questions that are having a hard time being resolved. There are some instances of a boundary being left behind as remnants and helping to produce backbuilding across the same areas that will have been hit recently. This comes in conjunction that the energy left from the secondary complex over central TX continuing its progression eastward as it maintains headway with the mean steering flow. If that were to occur, additional rainfall exceeding 1"/hr would be possible to add to the copious amounts being dropped currently and through the morning. To make matters more intriguing, some guidance has another complex developing out over west TX that eventually grows upscale this evening and moves over the same areas hit this morning. This is well documented on the 00z HREF 3hr mean QPF fields which would exacerbate flooding potential further. This is a significant impact scenario unfolding due to the compounding nature of the precip over a water-logged area after 6-12" of rain on Sunday, as well as any additional rains within the latter part of the period. With collaboration from impacted WFOs across southeast TX to the LA border, a Moderate Risk was added to the new D1 with the western extent over Brazos county, extending eastward to Tyler/Hardin/Jasper near the LA state line. The southern periphery was the hardest to nail down, but utilizing the HREF probability fields and mean QPF footprint, have extended the MDT down to Montgomery and Liberty counties with the extreme northeastern tip of Harris county also included. A higher end SLGT is still forecast around the periphery, including the Houston metro, but that comes with uncertainty on the behavior of the outflow and eventual progression of the complex, as well as magnitude as it moves towards the Gulf coast. ...Mississippi Valley and Southern Plains... Current radar/sat composite shows two distinct convective clusters propagating eastward across the central plains with the northern periphery of the first "wave" extending all the way towards the IA/MN border with roots back to northern KS. The second "wave" is smaller, but following in the footsteps of the first organized cluster with majority of its footprint based over northern KS into southern NE. Both of these organized convective regimes will move to the east and northeast through the beginning of the period with locally heavy rainfall capable of 0.75-1.5" over the area it traverses. This will set the stage for places across IA and northern MO to see some overlap from convective development in the afternoon hrs along the leading edge of a cold front that will motion to the east and southeast the latter portion of the period. Pretty consistent signal amongst all CAMs and globals for a line of heavy QPF extending from far eastern KS up through northern MO into IA during the afternoon hrs today with a general 1.5-3" of rain falling within the span of 3-6 hrs when it occurs. Rates are currently progged to be within that 1-2"/hr range with the 00z HREF neighborhood probability for at least 1"/hr between 25-50% with the primary area located over northern MO into southern IA. 2"/hr rates are lower, but certainly non-zero within the 10-20% range over the same area, so that gives us a lower and upper quartile to work with in terms of expectation. FFGs are currently running between that 1-2"/hr marker for exceedance, so it`s within the zone of opportunity to cause some flash flooding potential within the rates alone. A SLGT risk was maintained from previous forecast with minor modifications made on the flanks of the risk area. Across the southern plains, that same cold front will interact with area convection forming ahead of the boundary, acting as a focal point as it moves south through OK into the Arklatex by the end of the period with some guidance keen on a MCS developing within the vicinity of I-40 to the Red River. There`s significant uncertainty on the expected evolution of any convection that does develop, but the higher probability is thunderstorms will develop and produce locally heavy rainfall within a core of higher theta-E`s as any weak surface capping breaks with diurnal destabilization. Totals of 1-3" will be plausible across much of OK into the Lower Mississippi Valley near the Arklatex over into the AR/TN border near Memphis. Best prospects reside further west into OK which has been the beneficiary of heavy precip in recent days leading to lower FFGs and flood concerns. This continues a very unsettled period the past several days. Kleebauer Day 2 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... ...Southern Plains and Southeast... A trio of areas extending from the southern plains through the Southeast US will draw attention for the period. The primary area of focus will reside in the Lower Mississippi Valley where some guidance is now suggesting the development of convection upstream within the axis of an approach cold front, developing upscale and maneuvering through the ArklaTex into LA by the beginning of the forecast period. Locally heavy rains would be a certainty if this evolution occurred and will need to be monitored closely with regards to timing and magnitude as a stronger complex or later arrival time may put the area into a higher risk (Slight) given the antecedent conditions expected upon arrival. HREF blended mean QPF resides around 1.5-2" for the area encompassing the ArklaTex through northern LA which could be sufficient for an upgrade. However, the deterrent is an inordinate amount of spread in the hi- res for where any complex would descend upon with some much further west and others barely showing much at all. Based on the grand ensemble blend, including global deterministic, the prospects are higher for something to affect the area, so the MRGL risk was maintained with wording for possible upgrades. ...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. Current QPF maxima is within the vicinity of southern NE into northern KS, but the expectation is for multiple pockets of higher QPF embedded within the complex as it moves to the southeast. As of now, the system looks to remain progressive, but will monitor closely as we approach the event in question to see if we can hone in on any specific area for potential upgrades, or maintain continuity. ...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 D3. As a result, kept continuity with the MRGL risk from previous forecast package. Kleebauer Day 3 Valid 12Z Sat May 04 2024 - 12Z Sun May 05 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR WEST AND NORTH TEXAS INTO SOUTHWEST OKLAHOMA... ...Southern Plains... A cold front will move south through the course of Saturday with sights on north TX by the afternoon hours as it progresses out of the plains. A dryline will be focused over west TX by the morning and will begin mixing eastward through peak diurnal instability. Aloft, a shortwave will eject northeast out of MX which will provide areal ascent within the confines of where the dryline and cold front will intersect. Guidance is in agreement on a rapid development of convection in-of the western rolling plains, northeast Permian Basin up to the Red River with heavy rainfall likely along and east of the dryline as dew points in the upper-60s and low-70s will exist on the moist side of the boundary. The convergence of the front and dryline will induce a large cluster of heavy thunderstorms capable of significant rainfall and rates upwards of 1.5-2"/hr based on local climo and expected PWAT anomalies of 2-2.5 deviations above normal. Current rainfall totals are already pushing 2.5-3" on the ensemble bias corrected QPF with likely higher totals once we factor in CAMs later into the equation. A SLGT risk was maintained and expanded to encompass the 1"+ contours within the ensemble bias corrected forecast and WPC Guidance which is factoring in higher localized totals within area convection. Would not be surprised to see this be labeled a higher end SLGT, or an even higher risk given the early signals and the forecasted synoptic evolution. ...Central and Southern Appalachians A cold front will approach from the west with slow progression through the Ohio and Tennessee Valleys before aligning parallel to the Appalachian front by mid-day Saturday. A broad ridge to the east will provide a convergent signature within a band of elevated PWATs along and east of the terrain in the Mid-Atlantic. Modest instability and upper forcing along with the surface convergence pattern will lead to scattered convection over portions of the western Carolinas up through western Pennsylvania through most of the period, especially the afternoon and evening Saturday. Locally heavy rainfall will be plausible in the setup with weak steering flow aloft allowing for meandering and/or training cells within the confines of the complex terrain in the interior. FFG indices are relatively low, but not significant so, so the threat currently is fairly minor, but within the threshold for a MRGL risk that was inherited from prior forecast packages. Another instance of continuity with a better analysis in future forecast cycles with the help of CAMs and associated ensemble(s). ...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, but does have merit given the signal at leads. Maintained continuity from previous forecast and will assess whether it needs adjusting, or removal pending future trends in guidance. 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