BBS: TELESC.NET.BR Assunto: HVYRAIN: Excessive Rainfall Discussion De: COD Weather Processor Data: Tue, 28 Apr 2026 15:58:58 +0000 ----------------------------------------------------------- FOUS30 KWBC 281558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Tue Apr 28 2026 Day 1 Valid 16Z Tue Apr 28 2026 - 12Z Wed Apr 29 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX REGION THROUGH PORTIONS OF THE DEEP SOUTH... ...16Z Update... A plume of deep Gulf moisture will advect northeastward out of the western Gulf across the lower Mississippi Valley today. A stalled out front along the Red River and through central Arkansas is providing the focus for the thunderstorms in southeast Oklahoma. These cells are expected to multiply and expand across much of southern Arkansas, leading to training of cells. Once the cells merge together, they'll become a line that should push southeastward across northern Louisiana and central Mississippi. By then the "training" part should come to an end as the storms move parallel to each other and in a line. A secondary cold front/dry line will approach the stalled front from the north, causing the storms on the northeast side to bow out across the Tennessee Valley. There is some potential for storms to backbuild on the north side of the line, roughly near the IL/MO/KY/TN area of the Mississippi and Ohio Rivers, but significantly less instability there should temper storm intensity a bit, despite soils in some areas having been moistened by recent rains in the area the past few days. =46rom the inherited forecast, the Slight Risk area has been trimmed on the north side to account for the aforementioned bowing segment resulting in significantly less rainfall as compared with any backbuilding storms to the north and especially to the south. The Marginal was trimmed a bit on the north side as well, but largely kept from the Ohio River confluence south. Meanwhile, the Slight was expanded a bit to the west along the Red River dividing Texas and Oklahoma, for expected backbuilding due to an evident vort max in far southwest Oklahoma that is likely to result in renewed thunderstorm development behind the current line of showers and storms moving across southeast Oklahoma and northeast Texas. The HRRR and RRFS 12Z runs were the models most heavily used for the changes made to the outlook. The Marginal was kept very thin on the south side due to aforementioned very dry conditions, high FFGs, and higher likelihood that the training will be done by the time the storms reach central Louisiana and southern Mississippi/Alabama. Wegman ...Previous Discussion... A favorable environment will exist for numerous organized thunderstorms over the southern U.S. today and tonight. Outflow from steadily advancing squall lines over the Tennessee Valley should provide a more distinct focus for renewed convective development over the forecast period. The boundary is likely to be oriented west-east, essentially parallel to 40-50 knot bulk shear vectors. Although the air mass north of the boundary may modify with time, it should still provide low-level convergence and a contrast in instability. South of the boundary, the warm sector will have very strong instability (CAPE over 3000 j/kg) and deep moisture (PW approaching 2 inches; above the 99th percentile for late April). This parameter space is very supportive of high rain rates, potentially in excess of 2 inches per hour in the most intense and organized thunderstorms. If anything, recent hi-res model runs (06Z HRRR, 00Z RRFS in particular) have focused the heavy rainfall signal even more into a corridor approximately 100 miles wide, with 2-4 inch totals and localized maxima even higher. This is consistent with the expected pattern described above, with repeated rounds of thunderstorms along a distinct mesoscale boundary. The main change with this outlook was to narrow the Slight Risk a bit, especially by trimming from the stable (northern) periphery well into the stable side of the mesoscale boundary. It could potentially be focused a bit more, and that will be a consideration later today. A mitigating factor for flash flood impacts could be the severe to extreme drought conditions in place across much of the Slight Risk area. With rain rates potentially in excess of 2 inches per hour, and heavy rain sustained for several hours in some cases, drought conditions may not necessarily prevent impacts but could lead to more isolated instances of flash flooding. This is still consistent with the Slight Risk definition. Flash flood guidance factors in current ground and streamflow conditions, and HREF and REFS probabilities still show about a 15-30 percent chance of exceeding FFG despite the overall dry antecedent setup. Lamers Day 2 Valid 12Z Wed Apr 29 2026 - 12Z Thu Apr 30 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND SOUTHEAST TEXAS TO THE CENTRAL GULF COAST... The outlook for Day 2 has remained mostly unchanged, with a Marginal Risk in a corridor from Texas through Alabama along the western and central Gulf Coast. A threat of excessive rainfall and flash flooding is expected, due to a continuation of strong instability, and the arrival of a plume of unusual amounts of mid- level moisture. Backward trajectory analysis indicates moisture around 3km above the surface can be traced along an anomalously strong subtropical jet extending back into the tropical central Pacific Ocean. This should push PW values over the risk area above 1.7 inches, and above 2 inches in portions of Texas, which is quite unusual for April. Given this, it's possible an upgrade to Slight Risk may be needed in future outlook updates. However, hi-res guidance is just coming into view. While the extended HRRR and RRFS do both show rain rates in this environment reaching (and even exceeding, in some cases) 2 inches per hour, confidence is not high on placement of greater concentrations of convection. Additionally, there doesn't seem to be much signal for backbuilding and training, except perhaps via cell mergers, which could lead to more limited duration of heavy rain. Therefore, the risk level was held at Marginal for now for the entire region. Lamers Day 3 Valid 12Z Thu Apr 30 2026 - 12Z Fri May 01 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN CENTRAL TEXAS... Increasing mid-upper level height falls over New Mexico and Texas, due to a strong shortwave pushing through northern Mexico, should lead to a low-level mass field response with increasing south to southeast inflow across southern and central Texas. That will also lead to a gradual uptick in low-level convergence between the developing low-level jet, and an easterly cool conveyor belt to the north, in a more stable region from Oklahoma into northwest Texas. The result should be more numerous thunderstorms over the course of the Day 3 period, particularly by Thursday Night. Deep moisture will remain firmly in place, although the instability should shift to a narrower CAPE profile with lower equilibrium levels. That trend, in combination with PWs of 1.5 to 1.8 inches, should support increasingly efficient, warm rain processes in convective bands. Furthermore, some training of the lower-topped convective bands may occur. With stronger instability over South Texas, the Rio Grande Valley, and far northeast Mexico, and steadily increasing forcing from the approaching wave upstream, persistent new convective growth upstream of convective bands is plausible. Putting all these factors together, a Slight Risk was introduced over Central Texas. This was placed where drought conditions either do not exist or are much lower on the scale; that also happens to be where some ensemble QPF signal is strongest. It's possible the Slight Risk may trend southeast with time as AI guidance is positioned on the southeast flank of the guidance envelope, and would seem to be a logical corrective to the usual bias of QPF maxima placed too far in the direction of a stable air mass, especially with 2-3 days of lead time. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6uxuRwTWv5yemuXfDTA1r3NrN8FoUP93RyKUQl9_JPWc= Fo51MsEUmgbXWzDd1T_bQzFNxqCKnj_k27n1Vkcply6Lfmk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6uxuRwTWv5yemuXfDTA1r3NrN8FoUP93RyKUQl9_JPWc= Fo51MsEUmgbXWzDd1T_bQzFNxqCKnj_k27n1Vkcp8SauG0k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6uxuRwTWv5yemuXfDTA1r3NrN8FoUP93RyKUQl9_JPWc= Fo51MsEUmgbXWzDd1T_bQzFNxqCKnj_k27n1Vkcp8gTFyOM$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.28-Linux * Origin: Capitol City Online (1:2320/107) ----------------------------------------------------------- [Voltar]