Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 13 2023 00:21:00 FOUS30 KWBC 130020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 720 PM EST Sun Nov 12 2023 Day 1 Valid 01Z Mon Nov 13 2023 - 12Z Mon Nov 13 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN TEXAS... Maintained the Marginal Risk area without significant change over South Texas and the Lower Texas coast. Being as far to the west of the quasi-stationary front over the western Gulf of Mexico has helped keep rainfall rates in check...with hourly rainfall rates generally been under a half-inch (under a quarter of an inch in many locations) due to limited instability and a feed of air with lower equivalent potential temperatures coming in from the northeast. Early evening satellite imagery showed convection extending back into northern Mexico in a region of 400-200 mb divergence associated with the axis of a 70 to 90 kt southerly upper level jet...so continued rain with occasional ebbs and flows of rainfall rates should continue into the late-night. The latest HREF neighborhood probabilities for 1- 3- and 6-hour rainfall exceeding flash flood guidance was confined to the international border region that persisted to at least 06Z. Maintained the Marginal Risk area farther north despite the focus generally being farther south. Given cooling cloud-tops located upstream and given the potential for runoff, especially in poor drainage and urbanized areas, was not inclined to remove the northern portion of Marginal just yet. Bann Day 2 Valid 12Z Mon Nov 13 2023 - 12Z Tue Nov 14 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE MIDDLE TEXAS COAST TO SOUTHERN LOUISIANA... As the previously noted mid-to-upper level trough/low continues to move east, models show a surface low developing along a lingering frontal boundary over the northern Gulf, with the surface wave tracking slowly from the northwestern to the north-central Gulf on Monday into Monday night. Deep southerly flow ahead of the wave will direct increasing moisture into an area of large-scale ascent, supporting organized and potentially heavy rainfall from the middle Texas Coast to southern Louisiana. Limited instability will remain a mitigating factor however, with models indicating that the greater instability and threat for heavy rainfall rates will remain mostly out over the Gulf. However, efficient rates supported by deep warm cloud layers and saturation through the low to mid levels may generate some localized runoff concerns, especially if prolonged periods of rain occur over poor drainage and urbanized areas.=20 For the 12Z update, the existing Marginal Risk area has been maintained with no need for any upgrade to a Slight Risk. The latest CAM guidance strongly supports the core of the heaviest convection should remain offshore, closer to the surface low and where the best instability will reside. The Marginal Risk is generally from the Interstate 10 corridor southward to the Gulf Coast. There was a slight westward extension of the outlook area to include the Corpus Christi area, where the ARW2 and NAM conest have some lingering heavy rainfall close to the Texas Coast in that area.=20 Hamrick/Pereira Day 3 Valid 12Z Tue Nov 14 2023 - 12Z Wed Nov 15 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN LOUISIANA TO THE FLORIDA PANHANDLE... The threat for heavy rainfall is expected to near the central Gulf Coast for Tuesday and Tuesday night as the previously noted low continues to track slowly eastward across the northern Gulf.=20 While moisture and lift will remain sufficient for widespread, organized rainfall, limited instability north of the frontal boundary will confine the greater threat for heavy rainfall rates and amounts mostly to the coast. Not much in the way of change is warranted for the updated Day 3 forecast, with no need for any Slight Risk areas with the highest rainfall rates likely remaining off the coast. Much of this region has been extremely dry over the past two months, so flash flood guidance is high and the ground will be able to absorb much of the rain that falls. Hamrick/Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7U_7PMqTL53P6FSh3GVkHZ0pjzuqqOE_NGSumoW14h8z= 1Kt9G0bLqzHO4aQf2OZEZVpERnXfWuN4Vw3zB28hMvTCabA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7U_7PMqTL53P6FSh3GVkHZ0pjzuqqOE_NGSumoW14h8z= 1Kt9G0bLqzHO4aQf2OZEZVpERnXfWuN4Vw3zB28hCjMYz8Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7U_7PMqTL53P6FSh3GVkHZ0pjzuqqOE_NGSumoW14h8z= 1Kt9G0bLqzHO4aQf2OZEZVpERnXfWuN4Vw3zB28huo31zvQ$=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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