Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Oct 03 2023 20:01:26 FOUS30 KWBC 032001 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Tue Oct 03 2023 Day 1 Valid 16Z Tue Oct 03 2023 - 12Z Wed Oct 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE TEXAS COAST... ....16Z Update... Only some minor adjustments for the SLGT risk across the Texas Gulf coast where primary convergence signal will be located in a narrow corridor from eastern Houston down into Matagorda. Surface analysis pins a weak trough over the coastal plain of TX with a persistent onshore flow within the southeast periphery of the surface ridge extending across the Gulf. PWAT's at 12z for the KCRP sounding were a touch above 2" with guidance indicating slightly higher PWAT indices to the NE around Houston (2.1-2.3") which puts the area within the 1.5-2.0 deviations above normal territory. This is the kind of environment that is conducive for locally heavy rainfall with rates pushing 2-3"/hr, which is already the case within the first wave of precip in the area. 12z HREF mean QPF is an area of 1.75-2.25" with blended mean QPF footprint showing a swath of 2.5-3.5" located just to the southeast of Houston proper. Deterministic QPF is gaudy with local 5-7" totals being depicted anywhere within the SLGT risk zone forecast. The main period of interest continues to be between 18-00z when local convective risk peaks as the core of the moisture flux from the Gulf moves ashore as noted across all hi-res guidance and 12z HREF probabilistic measures for 2"/3-hr and 3"/3-hr with 60-70% and 40-50% respectively for each indicator. Despite the area being dry over the past several days/weeks, the urban corridor within Houston and surrounding suburbs, as well as anticipated very heavy rainfall rates maintains a prominent signal for enhanced flash flooding concerns over the course of the afternoon and early evening hours. Thus, have maintained the previous Slight Risk issuance with adjustments made to the eastern and western extension based on 12z HREF blended mean QPF footprint, as well as HREF probability outline for 12-hr precip between 18z-06z of at least 40% or greater. Locally heavy rainfall from convection over the central plains will offer local flash flood potential, mainly within urban areas and places where rainfall rates can reach up to 2-3"/hr over an hour or two period. Storm motions will be fairly progressive across the plains, so the prospects for training precipitation is lower, but certainly non-zero given boundary behavior from outflow generation. High FFG's are the main detriment to raising the MRGL to a higher risk category, but pending the evolution of convection, there could be a targeted SLGT somewhere within the confines of central KS up through the eastern half of NE. Dryline activity over west TX will create a localized QPF maximum likely within the confines of the Stockton Plateau up into the southern Permian Basin. A solid signal for heavy rainfall has been forecast over the past few successions of hi-res deterministic and associated ensemble guidance. This likely stems from elevated moisture field ahead of the dryline and supercell storm modes capable of enhanced rainfall. This threat will be weighted towards the front end of the MRGL placement into TX with initiation likely after 18z and ending as early as 00z when convection kicks east and cold-pool environment helps decay convective longevity. Kleebauer ....Texas Coast... Southeasterly flow from the Gulf of Mexico will interact with a coastal trough to produce showers and thunderstorms this afternoon and early evening. Heavy rainfall is possible within the Slight Risk area, between Houston and Corpus Christi this afternoon, when instability will be at its peak (1000-1500J/Kg). There will be plenty of moisture present, despite the lack of dynamic support, with PWATs between 2-2.5" along the Texas coast. HREF neighborhood exceedance probabilities of 2-5" are reasonably high at this time so those were used as a proxy for this day 1 Slight Risk area. ....Great Plains and Upper Midwest... A cold front moving across the Great Plains will act as a focus for showers and thunderstorms today. Some locally heavy rain could cause flash flooding, particularly over parts of eastern South Dakota/Nebraska, but considering how dry the region is, that threat is low for the most part. However, there's an increasing signal for heavy rainfall to develop over portions of west Texas south of Midland late this afternoon and evening. HREF exceedance probabilities of over 2" are high for that area and there will be substantial instability in place (1500-2000J/Kg) so an afternoon upgrade may be necessary if upward trends continue. Kebede Day 2 Valid 12Z Wed Oct 04 2023 - 12Z Thu Oct 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS AND THE LOWER/MIDDLE MISSISSIPPI VALLEY... ....2030Z Update... The overall synoptic pattern is still on track for Wednesday into Thursday AM with an increase in heavy rainfall potential beginning late-Wednesday afternoon, carrying through the end of the D2 period. Models are in agreement on placement of a robust QPF axis extending from southern MO into central TX with a bullseye of the heaviest rainfall over eastern OK and northwest AR in due part to an expected MCS migrating across OK and the adjacent Red River Valley. 12z HREF mean QPF and ensmeble bias-corrected QPF across the above areas are hefty with a widespread 2-3" footprint, and embedded totals likely over 4" within the core of heaviest rainfall. The root cause for the potential stems from an anomalous airmass advecting northward from a forecasted strong LLJ located over the southern plains. GEFS/NAEFS PWAT anomalies are approaching 2-2.5 devaitions above normal for the setup which shows up very well when looking at forecast bufr soundings out of Tulsa and OUN where PWATs are surging above 1.5", something typically found in these highly amplified patterns. The other notion for the potential is the 12z HREF progs for 1-hr and 3-hr rates/totals where probabilities are approaching 50-60% for a large area encompassing north TX up through eastern OK for 2-3"/hr rates and 3"/3-hr totals. Total forecast QPF is between 3.5-4.25" across southeast OK where most guidance has the 24 maximum, including the ensemble bias-corrected QPF which is very aggressive in its presentation. After collaboration with all the local WFO's within the SLGT, have decided to maintain the Slight Risk given the area FFG guidance is still fairly high for the area, which is climatologically favored to have less major flooding concerns due to soil type and higher flood thresholds. Locally higher impact flooding will still be possible, especially over terrain focused in the Ozarks and towards the OK border. Urban corridors within the SLGT are also at risk, especially within persistent heavy rain bands that are embedded within the core of the MCS. Overall, this is a a "higher end" SLGT risk for the region with full agreement from the local FO's. Kleebauer ....Southern Plains and Lower/Middle Mississippi Valley... There continues to be reasonably high confidence of an excessive rainfall event unfolding across eastern Oklahoma, western Arkansas and northeastern Texas on Wednesday. Embedded shortwave energy within an amplified upper trough will provide ample forcing for waves of convection to propagate across Oklahoma and north Texas beginning tonight and continuing on Wednesday. Plenty divergence aloft and sufficient low level jet support will promote forcing for heavy rainfall. Instability between 500-1000J/Kg and PWATs between 1.75-2.25" should support rain rates of over 1.5" with locally higher amounts. We're expecting between 3-5" of 24hr QPF for much of the slight risk area with locally higher amounts possible. An upgrade could be possible if things like the ensemble means (GEFS and ECE) come into better agreement with respect to the axis of heaviest precip. The GEFS favors a more northerly solution while the ECE is farther to the south. 24hr exceedance probabilities of over 2" remain high though for both ensemble means. Some heavy rainfall may also develop over parts of Texas' central Gulf Coast where they're expected to receive some antecedent heavy rain today, which will make their soils vulnerable to runoff. Kebede Day 3 Valid 12Z Thu Oct 05 2023 - 12Z Fri Oct 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF TEXAS, THE ARKLATEX, AND THE MIDWEST.. ....2000Z Update... The previous SLGT risk was dropped for the D3 time frame as the QPF forecast has continued to slowly trend down with the heaviest precip now being aligned over the southwest Rio Grande Valley (RGV) between Laredo and Zapata and more localized heavy rainfall to the northeast over east-central TX. There's still an opportunity for an upgrade to a targeted SLGT over the RGV if guidance trends better for convergence over south TX and heavier precip signals re-appear within the population center along the river valley. Until then, the current MRGL should suffice, and was in agreement with some of the local WFO's within the risk area. An additional Marginal Risk was added to the D3 time frame across part of the Midwest. More on this potential below.... ....Midwest... A digging upper trough over the northern plains will create a favorable jet streak pattern across the Midwest with the central Midwest area over IL/IN/southern MI in the best ascent focused under the influence of the right-entrance region of a departing speed max. Shortwave energy ejecting to the northeast out of the Mid-Mississippi Valley will also traverse overhead creating a better dynamical structure for the affected area with widespread showers and storms propagating northeast within the main steering pattern ahead of the mean trough. Local QPF maximum between 1.5-2" is now forecast with even the ensemble bias-corrected QPF indicating the potential for 2+" over northern IN where the best ascent and low-level instability axis is located, meaning more convective potential. Rich Gulf moisture advecting ahead of the mean trough and attendant cold front will lead to PWATs approaching 1.5-2 deviations above normal, more than sufficient for heavy rainfall potential for a 3-6 hr period when precip moves through the area. Locally heavy rainfall will affect urban corridors the most, especially places like northwest IN over into the Chicago south side where FFG's are low due to urban sprawl on tip of Lake Michigan. A Marginal Risk was introduced given the threat. Kleebauer ....Southern Texas... There's a decreasing signal for heavy rain over parts of southern Texas along the southern periphery of a cold front on Thursday. Upper-level energy associated with the deep parent low in Canada will swing through Texas and provide forcing for showers and thunderstorms to develop across southern portions of the state. The guidance has trended farther north with the favorable jet dynamics, however this does not mean that the excessive threat does not exist as there are still some areas which have received heavy rain recently and are susceptible to flash flooding within the current risk area. The latest guidance shifting the axis of heaviest rainfall into northern Mexico just south of the Rio Grande among other factors has lead to a decrease in risk from a SLGT to a Marginal Risk across the state, extending north into the Arklatex, mainly from residual impact from the morning MCS expected on Thursday. Kebede/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7iAeASq8exQ0gL3LxpcKLBoOFK6NIKZHbzd1HqOT254v= Gt5NgQR0aSnRTjMRJrfy1cXfsEnqL6_CdAID-9eHROhpoQU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7iAeASq8exQ0gL3LxpcKLBoOFK6NIKZHbzd1HqOT254v= Gt5NgQR0aSnRTjMRJrfy1cXfsEnqL6_CdAID-9eH079HrVs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7iAeASq8exQ0gL3LxpcKLBoOFK6NIKZHbzd1HqOT254v= Gt5NgQR0aSnRTjMRJrfy1cXfsEnqL6_CdAID-9eHPAl5dxA$=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) .