BBS: TELESC.NET.BR Assunto: HVYRAIN: Excessive Rainfall Discussion De: COD Weather Processor Data: Mon, 27 Apr 2026 07:50:53 +0000 ----------------------------------------------------------- FOUS30 KWBC 270750 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 350 AM EDT Mon Apr 27 2026 Day 1 Valid 12Z Mon Apr 27 2026 - 12Z Tue Apr 28 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST... Multiple rounds of convection are expected today ahead of a cold front extending from an anomalously deep spring low. Significant northward moisture transport will result in a plume of PWs above 1 inch as far north as Lake Superior, and above 1.5 inches as far north as Chicago. Along the entire length of the cold front, these values will generally approach the 99th percentile for late April and, combined with moderate to strong instability, should support hourly rain rates in the 1 to 2 inch range. A limiting factor for flash flooding will be the dominant convective mode -- very likely to be relatively narrow squall lines, forced by a steadily advancing cold front, that will limit the overall duration of the heavy rain rates.=20 However, an area from northern and central Missouri, into much of=20 Illinois and Indiana, will have a greater potential for training=20 convection. This will be due to a significant upstream complex of=20 thunderstorms over northeast Kansas as of 07Z, which should shift=20 east over the remainder of the morning. The orientation of the=20 convective line is closer to parallel with the deep layer shear=20 vector, which will favor training over northern Missouri at the start of the period. Eventually, this activity may produce an outflow boundary oriented in a west-east fashion that would focus subsequent convective development in a more favorable configuration for additional training. The broad Slight Risk was maintained and adjusted to better fit the latest guidance where probabilities of 1 inch per hour rain rates are highest. Further south, a Marginal Risk was extended from Arkansas and Tennessee southwest into central Texas to account for a conditional threat of localized heavy rainfall and flash flooding. The environment in these areas will be characterized by strong instability (CAPE in excess of 3000 j/kg) and abundant deep moisture (PW between 1.5 and 1.8 inches). The QPF signal in guidance is inconsistent, but enough to indicate a threat for isolated to scattered thunderstorms, primarily in the afternoon and evening. The combination of the aforementioned instability with strong deep layer shear could lead to a few slow-moving=20 supercells, which has been a theme in recent days. It is a classic conditional threat -- while much of this region further south along the front could remain dry, any storms that become organized and rooted in the very unstable boundary layer could produce localized very heavy rainfall.=20 Lamers Day 2 Valid 12Z Tue Apr 28 2026 - 12Z Wed Apr 29 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX INTO THE MID SOUTH... A favorable environment will exist for numerous organized=20 thunderstorms over the southern U.S. on Tuesday, with a broad warm sector containing very strong instability (CAPE over 3000 j/kg), deep moisture (PW climbing into the 1.6 to 2.0 inch range), and strong deep layer shear (0-6km bulk shear 40-60 knots). In that sort of environment, rain rates should reach 2 inches per hour. The open question is what sort of mesoscale boundaries may exist to focus convective development, and whether or not some of these will be oriented in a west-east fashion (parallel to the bulk shear vectors). Any prominent boundaries that are favorably oriented=20 could easily focus a corridor of significant rainfall and flash=20 flooding in this sort of environment. However, an equally plausible scenario would be more scattered clusters and lines of=20 thunderstorms that can produce localized heavy rainfall and flash=20 flooding, but fail to focus the convection in a way that leads to=20 more significant totals. For now, it seems reasonable to maintain=20 the broad Slight Risk and wait for greater confidence in the=20 mesoscale details.=20 Lamers Day 3 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 threat of excessive rainfall and flash flooding seems to be increasing in the most recent set of model guidance, due to a=20 continuation of strong instability and the arrival of a plume of=20 unusual amounts of mid-level moisture. Backward trajectory=20 analysis indicates that moisture around 3km above the surface=20 originated in the tropical central Pacific, advected into the southern U.S. by an anomalously strong subtropical jet (300mb winds around 60+ knots) extending about 6000 mi. from the Marshall=20 Islands, to just south of Hawaii, all the way to Texas. This should push PW values over the whole region above 1.7 inches, and above 2 inches in portions of Texas, which is quite unusual. For instance, a 2+ inch PW has only been observed on an upper air sounding at=20 Corpus Christi 3 times prior to May 1st (36 year period of record;=20 since 1990). Although model QPF is not exceptionally high at this=20 point, that may largely be a function of a lack of high-res=20 guidance from convection-allowing models. The environment (when=20 also considering strong bulk shear to around 50 knots) would favor=20 organized thunderstorms with very high rain rates, potentially in=20 excess of 2 inches per hour. Given that the previous outlook had=20 probabilities below 5 percent, the main change with this update is=20 to introduce a Marginal Risk, but a further upgrade may be=20 required, especially across parts of Texas. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7DrlUHD12MMEbEWLtjwtiYM5kh24nVNs6GJBBgQQqRrS= 3GtsR60uHd35mimOt1yXXOvu2BY38ft6qttdt90b3rlvSsg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7DrlUHD12MMEbEWLtjwtiYM5kh24nVNs6GJBBgQQqRrS= 3GtsR60uHd35mimOt1yXXOvu2BY38ft6qttdt90b1k9HfmI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7DrlUHD12MMEbEWLtjwtiYM5kh24nVNs6GJBBgQQqRrS= 3GtsR60uHd35mimOt1yXXOvu2BY38ft6qttdt90bkXW03Vc$=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. 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