Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 13 2024 15:58:44 FOUS30 KWBC 131558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sat Jul 13 2024 Day 1 Valid 16Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE EASTERN SEABOARD, UPPER MIDWEST, PINEY WOODS, AND FOUR CORNERS REGIONS... ....16Z Update Summary... Biggest change for the morning update was to shrink the western extent of the MRGL across the east coast as the frontal boundary will continue to slide eastward over the course of the afternoon with the convective pattern accompanying also ejecting off the Atlantic coast by later today. The MRGL was maintained for areas where thunderstorms are ongoing or can be expected for another 2-4 hrs before the setup dwindles. Elsewhere, only minor adjustments made to respective risk areas to account for the latest probability fields and radar trends the past few hrs. More information within each sub-heading below. ....Missouri... Radar and satellite composite indicate our convective threat across central MO is waning, albeit slowly as a few cells are still dropping locally heavy rainfall in-of the outlined area. The threat will continue to weaken over the course of the morning and afternoon, however a few convective pulses are still plausible considering the environmental priming due to daytime destabilization coupled with the attendant shortwave energy overhead. Rates generally 1-1.5"/hr max, so the upper potential is relatively capped with the MRGL risk area being sufficient to=20 cover the threat. ....Eastern Seaboard... Frontal boundary is slowly pressing eastward this morning and continue to meander very close to the Atlantic coast for the next 2-4 hrs allowing for isolated pulse convective concerns from the SC/GA border all the way up into coastal Southern New England. The primary areas of interest the next few hours are across RI and Southeast MA with a more consolidated mid-level vorticity maxima currently maintaining a moderate to heavy multi-cell cluster moving through the region. Rates are generally <1"/hr which will limit the flash flood threat in the grand scheme, but wanted to keep some continuity while convection was ongoing. The risk area will likely be nil by the next forecast update given the current progression.=20 The second area of focus is the Southern Delmarva down into the Hampton Roads portions of Southeast VA where a combination of frontal positioning and favorable antecedent conditions for flash flooding pose a short term threat before conditions improve with the boundary sliding off the eastern sea board. This area has seen some pretty decent totals the past 24-36 hrs with the lower FFG signature well-established within the latest 12z update of 1/3/6 hr indices. Like the threat over New England, this area will likely be subject to a nil ERO by the next round of updates as the front finally clears the coast, so this maintenance was more for the immediate term while convection is ongoing.=20 ....Upper Midwest... A more developed complex of storms is currently moving through northern WI this morning with an expectation of that system decaying as we move through the next 2-4 hrs. Upstream is a more formidable shortwave trough with energy ejecting quickly east-southeastward out of southern Saskatchewan that will be the main player for later this afternoon and evening as it makes headway into the Upper Mississippi Valley. 12z CAMs were in agreement on a complex developing over northern MN after the 23/00z time frame, however the eventual motion of the complex is subject to variability from the various CAMs outputs, documented well within the HREF QPF mean indicating a fairly widespread, smooth distribution of 1-1.25" totals within the recent output. Theta-E gradient will lie from central MN down through the western half of WI which is likely where any upstream complex would try to anchor once it begins pushing downstream within the mean flow.=20 The highest probabilities for at least 1" are across the western=20 half of the Arrowhead down through northwestern WI with=20 neighborhood probs between 70-90+% within the latest HREF. At least 2" neighborhood probs are also fairly solid between 50-70%, but=20 the >3" signals drop off precipitously with a thinner band of=20 25-40% probs nestled between MSP and DLH in Northeastern MN to the=20 WI border. This is likely due to the past rainfall coupled with=20 what is expected later this evening with the secondary complex.=20 Progressive nature of the complex this evening will also limit=20 significant flood potential, however there will still be scattered=20 instances up across the outlined risk area thanks to rates between=20 1-2"/hr still plausible within any storm development. If the 12z=20 KABR sounding is any indication what will be advecting into the=20 region, there will be plentiful instability to work within along=20 the northern edge of the theta-E gradient, as well as increasingly=20 better shear with the approach of the shortwave.=20 The previous MRGL risk was relatively maintained, but will make mention that a targeted upgrade is possible for portions of Northeastern MN and Northwest WI pending the convective evolution of the expected MCS over the area. Further downstream will have a shot at a future upgrade as well, but the variability of what occurs further northwest leads less confidence on a suggested target area of opportunity. There was still enough of a threat to warrant the MRGL risk, so maintained continuity. ....Piney Woods of Southeast Texas... Widely scattered but slow moving storms may cause isolated flash flooding in portions of southeast Texas again later today due in part to some sensitivity of the soils post-Beryl. Rates generally between 1-2"/hr in the heavier cells, but the threat will remain very diurnal with chances dwindling after 23/00z.=20 ....Four Corners... Monsoonal moisture may cause slow moving storms to form along the terrain which may cause isolated flash flooding from the Mogollon Rim over to the Sangre de Cristos of central NM. There has been little change in the overall coverage and intensity of the storms in the guidance in the Marginal Risk area, however a targeted area of mid-level moisture advection across portions of NV will lead to a slightly better threat of isolated flash flooding concerns within parts of the region. In coordination with the Las Vegas WFO, have expanded the MRGL risk a bit more to the northwest to account for the threat later this afternoon and early evening.=20 Kleebauer Day 2 Valid 12Z Sun Jul 14 2024 - 12Z Mon Jul 15 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS AND THE GREAT LAKES AS WELL AS FOR PORTIONS OF THE SOUTHWEST... ....Northern Plains and Great Lakes... On Sunday the axis of rainfall across the region will shift a bit towards the east across the Great Lakes on the periphery of the building ridge across the middle of the country. Organization of the storms remains poor in the guidance, so any areas where a little training may result in higher rainfall totals remains obscured. Thus, while a small area of Slight risk impacts may be embedded in the Marginal risk region, uncertainty is too great to separate out the area of that risk at this time. Further, since there's good agreement that there will be more rain in portions of the area (Wisconsin) today, should higher rainfall totals occur then those areas may be able to be highlighted with a Slight with future updates. ....Southwest... Atmospheric moisture will increase across the region as compared to today by Sunday. This will result in both an increase in coverage and intensity of the resultant storms across this area today. That said, the trends in the guidance have been flat. There's somewhat higher certainty that storms will concentrate along the Mogollon Rim of Arizona, so that region is in a higher-end Marginal with potential for an upgrade to a Slight with future updates. Meanwhile the signals for heavy rain have decreased in far southern Arizona. Wegman Day 3 Valid 12Z Mon Jul 15 2024 - 12Z Tue Jul 16 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER GREAT LAKES AND FOUR CORNERS REGION... ....Upper Great Lakes... "Ridge running" storms will continue across the upper Great Lakes again on Monday. The storms should be fast-moving which will greatly limit the flash flooding potential, but antecedent conditions after potentially multiple afternoons of storms for some areas may support a future Slight risk for this area. Nonetheless there remains too much uncertainty for such an upgrade at this time. ....Four Corners... Monsoonal flow may shift a little bit east away from northwestern Arizona and southern Utah and focus closer to the AZ/NM border while extending north across much of central Colorado. Storms that form will once again likely be tied to the terrain, which with burn scars may result in locally significant flash flooding if the storms are strong enough and remain stationary, but with still minimal organization there is not enough certainty to upgrade to a Slight at this time, but the potential is there for future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8KXOSFn6h65xXwMWCYhj2ggy1DwnlGWlGcJdAOZtAmep= ZCYpbdC_Gd-fuhB71-NgFrBoF71CKoQ0SENo1G94Ozfdw50$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8KXOSFn6h65xXwMWCYhj2ggy1DwnlGWlGcJdAOZtAmep= ZCYpbdC_Gd-fuhB71-NgFrBoF71CKoQ0SENo1G945mP4bew$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8KXOSFn6h65xXwMWCYhj2ggy1DwnlGWlGcJdAOZtAmep= ZCYpbdC_Gd-fuhB71-NgFrBoF71CKoQ0SENo1G94XC9Jpu4$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .