Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 12 2023 20:28:40 FOUS30 KWBC 122028 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 PM EDT Sat Aug 12 2023 Day 1 Valid 1916Z Sat Aug 12 2023 - 12Z Sun Aug 13 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST AS WELL AS CENTRAL PLAINS... ....South-Central Colorado east through the Oklahoma Panhandle and Southwest Kansas... An MCS is expected to develop across the Slight Risk area this afternoon through this evening. This is along a stalled out front draped west to east from Colorado all the way through the Northeast. Convection is likely to develop all along the front at various times throughout the day. Additional rainfall will develop with the MCS across the Slight Risk area. Easterly mid-level flow will advect Gulf moisture up into the southeast Colorado High Plains, meanwhile, MUCAPE values will spike above 3,000 J/kg over much of KS. A vort max will track northeastward out of the 4 Corners region and into the Panhandles by late this afternoon and this evening. Increasing 850 mb winds between 03-06Z above 30 kt will likely coincide with expanding convection, out ahead of an expected MCS advancing eastward from the High Plains, supporting potential for repeating and training. The combination of all of these ingredients has led to shower and thunderstorm development across CO, which prompted the issuance of Mesoscale Precipitation Discussion #911. The convection will quickly merge into an MCS by 23Z as the storms approach the CO/KS/OK junction area. As the storms move into the moisture and easterly low-level flow, the developing MCS may allow for cell mergers and backbuilding to locally enhance rainfall amounts over the area. While this area has been relatively dry in recent days per NASA Sport imagery, expect the storms to have ample moisture to produce rainfall rates as high as 2 inches per hour at times. Each individual cell should be moving along at a fast enough speed to keep the time any one area is seeing heavy rain low, but the aforementioned cell mergers may lengthen that time locally. The previous Slight Risk over southeastern Colorado/southwestern Kansas was extended eastward into southwestern Missouri to account for expected heavy rain overnight, likely continuing through 12Z Sunday for Missouri/Arkansas. 12Z HREF probabilities for spotty 2-4 inch totals across southeastern Kansas into southwestern Missouri were 40-60 percent, overlapping with some lower FFG across the region. ....Northeast... A fast-moving shortwave trough will cross northern NY through New England this afternoon into tonight. North of the Slight Risk area, the shortwave should be moving plenty fast enough to keep associated shower and thunderstorm activity also moving at a rapid pace. Thus, despite very favorable antecedent conditions in the form of wet soils across northern NY and northern New England, the Marginal was kept in place. In the Slight risk area, leftover energy on the southern periphery of the shortwave will linger back across the region. With increasing moisture advecting in from the south with PWATs approaching 1.75 inches by tonight. The influx of moisture and stalled out upper level energy will support backbuilding and training convection developing to the lee of the Catskills and tracking east across the Hudson Valley and into CT. The storms will be capable of 1 to 2 inch per hour rainfall rates, and somewhat favorable antecedent conditions could result in more widely scattered flash flooding across this area. With general agreement on this synoptic setup across the area, the Slight Risk was hoisted with this update. Given heavy rain over northeastern Ohio into western Pennsylvania over the past 24 hours (1 to 3+ inches through 12Z), FFG values have lowered to roughly 1 inch (or less) per hour and 1-2 inches in 3 hours for the region. Another round of thunderstorms is expected this afternoon ahead of a progressive cold front to move through Ohio and Pennsylvania from the northwest, with increasing moisture and instability with daytime heating, rainfall rates of 1-2 in/hr appear likely with areas of short term training...despite an overall tendency for storms to track toward the south and east with the mean flow. ....Mogollon Rim/Great Basin... Renewed convection along the Mogollon Rim associated with the monsoon is expected to develop across northern AZ into far west-central NM. Storms in recent days have been successful at generating flash flooding in limited areas, and a repeat day as yesterday is likely once again as far as overall storm coverage is concerned. ....Central Sierra Nevada... No changes were made to the inherited Marginal Risk area. QPF values are underwhelming for 24-hour totals in this area, but the second straight day of convection in this area could support additional isolated flash flooding this afternoon and evening. Roth/Otto/Wegman Day 2 Valid 12Z Sun Aug 13 2023 - 12Z Mon Aug 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EASTERN KANSAS, SOUTHERN AND CENTRAL MISSOURI, AND THE LOWER OHIO RIVER VALLEY... ....Eastern Kansas through the Ohio Valley... Soil moisture values have increased across the KS/MO region relative to a week ago given heavy rain on Wednesday and Saturday morning. An additional round of heavy rain is possible tonight (Saturday night) into Sunday morning, followed by yet another round Sunday night. There could be the need for an upgrade to Moderate Risk across southeastern KS into southwestern MO if there is enough overlap of Saturday night's rain with the Sunday night event. Given some shifts in the model guidance over the past 24 hours, and unknowns of QPF heading through Sunday morning, will hold off yet again on a potential upgrade to a Moderate Risk for portions of MO, possibly southeastern KS. This area can likely be revisited for the Saturday night forecast cycle with higher confidence given the first of the two expected rounds of heavy rain over the next 36 hours will be ongoing. Areas of heavy rain are expected to be ongoing across KS/MO at the start of the period, but moving eastward into the Lower Ohio Valley by noon. As a result of the morning rains, an outflow boundary may set up in the vicinity of the Ozarks and serve as a focus for additional thunderstorms with daytime heating. However, the approach of a potent shortwave trough from the Northern Plains will send a cold front southeastward into KS/MO Sunday evening with an increasing low level jet (20 to 30+ kt) out ahead of the front from OK/KS into AR/MO and a much better chance for thunderstorms. Upper level diffluence and right-entrance-region jet divergence are expected to enhance lift across the region as the upper trough approaches late Sunday. While the cold front itself will be progressive, concerns about west-east training exist out ahead of the front within an axis of forecast precipitable water values in excess of 2 inches from eastern KS into MO and the Lower Ohio Valley. Outside of the FV3, there is good support from the 12Z components of the 12Z HREF for 3-5 inches from southeastern KS into southwestern MO for the 24 hour period ending 12Z Monday. Subsequently, Therefore, the Slight Risk in place on the previous ERO issuance was shifted south and west. Farther east into the Ohio Valley, a Slight Risk remains were multiple rounds of convection will be possible, atop portions of fairly wet antecedent conditions. While heavy rainfall doesn't appear to be as widespread compared to locations farther west, there will remain the potential for some 2-4 inch totals through the 24 hour period. ....Upper Midwest... Trends in the 12Z deterministic models have shifted a mid-level shortwave trough southward over the Northern Plains/Midwest Saturday night. As a result, northern portions of North Dakota into the Upper Mississippi Valley are expected to have little or zero instability in place to support high rainfall rates with the latest model suite shifting the QPF axis southward, which prompted southward adjustments to the existing Marginal Risk. A strengthening surface low will develop out ahead of the north-central U.S. shortwave and track across the upper Midwest, with an east-west oriented front extending from the northern Mid-Atlantc into IA/IL and low level moisture transport across the frontal boundary. The heaviest rainfall is expected just north and east of the low center from SD across south-central MN and into WI. While localized rainfall totals exceeding 2 inches are likely, that rainfall should be both spread out over a somewhat longer duration with 1-2 inches in 3 hours time. Portions of MN into WI have been exceptionally dry recently, so much of the rainfall is likely to be beneficial to the area but wetter antecedent conditions exist over eastern SD. Impacts from rainfall are not expected to be widespread enough to warrant a Slight Risk, but the area will continue to be monitored. ....Maine... A Marginal Risk was introduced for this update in part due to 7-day rainfall anomalies of 300 to 600 percent over Maine. South of a closed mid-level low moving through Quebec, a 6-12 hour window for heavy rain will exist ahead of a frontal boundary with instability of 1000+ J/kg forecast for portions of southern/coastal Maine. Rainfall rates of 1-2 in/hr will be possible which could locally exceed area FFG. Beyond 00Z, the front and threat for heavy rain should shift offshore, ending any remaining threat for flash flooding. ....Central Sierra Nevada... No changes were made to the Marginal Risk area in the Sierras with this update. Little change in moisture anomalies are forecast for the region on Sunday compared to Saturday although storm motions may be a bit higher on Sunday. A localized risk of flash flooding will likely remain for the Sierra Nevada on Sunday but trends will have to be monitored for possible expansion into adjacent areas of the Southwest. Otto/Wegman Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9-lt7ltkqtNycvrXGezOemlFzJwo8RwyJ2yBg1UjjfJ8= 7wwreXWze4mA4Cm74BhO1RT41_b2ZhSEbMPXXUmnYI_KXpU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9-lt7ltkqtNycvrXGezOemlFzJwo8RwyJ2yBg1UjjfJ8= 7wwreXWze4mA4Cm74BhO1RT41_b2ZhSEbMPXXUmnaGhcXB4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9-lt7ltkqtNycvrXGezOemlFzJwo8RwyJ2yBg1UjjfJ8= 7wwreXWze4mA4Cm74BhO1RT41_b2ZhSEbMPXXUmnFBI6MqI$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .