Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 23 2023 16:01:34 FOUS30 KWBC 231601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sun Apr 23 2023 Day 1 Valid 16Z Sun Apr 23 2023 - 12Z Mon Apr 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF SOUTH TEXAS... ....16Z Update... Only a couple adjustments were made to the current outlook areas over south Texas given observation and model trends. The first was to trim the north/west sides of the MRGL and SLGT risk areas as the ongoing MCS has progressed southeastward and cleared these regions. The other was to adjust the SLGT risk a bit further southeast along the coast given updated hi-res model guidance which shows the southern end of the complex progressing south-southeasterly along the Gulf Coast mid-late afternoon. MRMS QPEs this morning have shown areas of 2"+ per hour as well as estimated shorter-term 15-minute rates as high as an 1" with the ongoing storms. The notable high rain rates will be of particular concern for urban areas, with ongoing flash flood warnings in the Laredo and Corpus Christi areas. However, the risk will end rather quickly later this afternoon as the complex is forecast to clear the coast over the next few hours. Moderate to locally heavy rain is also expected to continue over New England through the period, with some localized totals over 2" possible. However, current MRMS estimates as well as hourly rain rates in the updated hi-res guidance remain under 0.5" per hour, signaling that the steady rain is unlikely to result in any notable risk other than some localized street ponding.=20 Putnam ....South Texas... Few changes were made to the ERO for Sunday with this morning's update. Scattered showers and thunderstorms are currently developing north of the Marginal Risk area in central Texas ahead of a southward moving cold front. As the front moves into South Texas this morning, a surface low tracking along the Mexican side of the Rio Grande will pick up quite a bit more Gulf moisture as a subtle shortwave in the right entrance region of a jet over the Mid-South all move over the region. This will support widespread development of showers and thunderstorms all across the region, which will all track southeastward with time. The individual storms will congeal into a Mesoscale Convective System (MCS) over the next few hours, with rainfall rates in the stronger elements potentially exceeding 2 inches per hour at times. Fortunately, the heaviest rain will be with the leading convection, which is forecast by all of the CAMs to race southeastward into the Gulf by midday, if not sooner. Thus, the threat for flooding is largely confined to the morning/early afternoon (if the MCS moves slower than forecast), with only lingering nuisance shower activity expected behind the MCS. The only change of note for this update was a small nudge of the southern end of the Slight Risk area north out of Kenedy County. FFG values in this area are among the highest in the country, over 5 inches in an hour, which is highly unlikely to occur given the expected movement of the strongest convection through the county later this morning and insufficient instability to achieve rainfall rates that high. Otherwise, the greatest flash flooding risk remains in the urbanized areas of coastal south TX, especially Corpus Christi. ....New England... A slow moving cold front will continue inching eastward across New England today. Embedded storms have been producing rain briefly approaching 1 inch per hour, though for most areas rainfall rates have remained at a half inch per hour or less. Nuisance flooding is possible especially in urban areas from New Haven, CT, Providence, RI, and Boston, MA through the day. Since for most areas the rain will remain light enough not to be a flash flooding threat, no Marginal Risk area has been introduced with this forecast update. However, the area will continue to be monitored today. Wegman Day 2 Valid 12Z Mon Apr 24 2023 - 12Z Tue Apr 25 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS... No significant changes were made to the Marginal Risk area encompassing much of KS and OK with this morning's forecast. A weak shortwave will move across the Marginal Risk area, which will support scattered showers developing along a front. The showers will be slow-moving, but will only support light rain. Behind this initial round, isolated to widely scattered stronger storms are expected to develop, particularly across the TX and OK Panhandles region. While these storms will be isolated, any stronger ones have the potential to cause isolated flash flooding if they move over sensitive areas. This is most likely overnight Monday night, as it will take until Monday night for any instability (which will still remain under 500 J/kg) to advect northward into this region. This remains a low-end Marginal Risk, as much of the area has well below normal soil moisture supporting high FFGs, and organized convection is not expected. A downgrade of the Marginal Risk area is possible with future updates. Wegman Day 3 Valid 12Z Tue Apr 25 2023 - 12Z Wed Apr 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE ARKLATEX REGION THROUGH EASTERN OKLAHOMA AND FOR PORTIONS OF THE COLORADO FRONT RANGE... ....Arklatex Region... A shortwave moving through the flow across the Slight Risk area will help force convection as instability increases to around 1,000 J/kg Tuesday. A preexisting LLJ over central TX & OK will shift eastward through the day as a potent mid-level low ejects out of the Rockies across TX. Ahead of the low the flow will turn south to southeasterly, drawing the LLJ northward. The LLJ will also advect more unstable and moisture-laden Gulf air north into the Slight Risk area. While there is good agreement in the models that there will be swaths of precipitation exceeding 3 inches over the Slight Risk area, there's very poor agreement on where those swaths of higher rainfall totals will develop. Thus, the Slight Risk is largely covering the area of highest forecast rainfall totals, but it's expected that any flash flooding will occur with those higher and more localized swaths of higher rainfall, and not the entire Slight Risk area. Of course, as agreement (hopefully) improves with subsequent guidance, the Slight Risk area is likely to be refined to cover a smaller area. The inherited surrounding Marginal Risk area was trimmed towards the west quite a bit out of AL and eastern MS, as agreement that the focus for rain will be further west is also good. ....Colorado Front Range... The plume of moisture associated with the LLJ over the Arklatex Region will turn eastward into eastern CO on Tuesday. This easterly flow will also advect instability as high as 1,000 J/kg across eastern CO as well. The mid-level low will increase 850 mb surface flow to 15 kts along the Front Range. This orthogonal flow to the Front Range, the approach of a strong positively-tilted shortwave trough in the upper levels, and that instability will optimize precipitation rates in the region. There is much better agreement in the guidance that this will result in multiple inches of rain along the Front Range and adjacent foothills, as the upslope and instability work in tandem to increase total forecasted rainfall. The consensus rainfall totals have more than doubled from previous runs over a wide sampling of the guidance, which increases forecast confidence. Thus, a Slight Risk area was introduced with this forecast update. The elevation of the rain/snow line will need to be monitored, as it may approach some of the higher foothills, especially Tuesday night. Finally, this region has low FFGs, with hourly values between 1 and 1.5 inches over a large area of the northern Colorado Front Range. Localized areas around Denver are even lower. These rainfall rates are quite possible along the mountains as the upslope flow is supported by the atmospheric instability. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!97h_JSxeX9T7zC0pBkjtPtfeLrt9x2fhae-_-dVHOnDo= L7ihSG5ITfTmUn8x6uG7mH8Mp0fD-CylrXTeR8MlcR8nPAk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!97h_JSxeX9T7zC0pBkjtPtfeLrt9x2fhae-_-dVHOnDo= L7ihSG5ITfTmUn8x6uG7mH8Mp0fD-CylrXTeR8MlXBjaip0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!97h_JSxeX9T7zC0pBkjtPtfeLrt9x2fhae-_-dVHOnDo= L7ihSG5ITfTmUn8x6uG7mH8Mp0fD-CylrXTeR8MlSLrO4Og$=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) .