Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 16 2023 22:17:27 FOUS30 KWBC 162217 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 616 PM EDT Sun Jul 16 2023 Day 1 Valid 2153Z Sun Jul 16 2023 - 12Z Mon Jul 17 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN NEW ENGLAND... ....Northeast... We were able to trim back the MDT risk area with this update. Recent observational trends indicate the most consolidated area of heavy rainfall will be from eastern Long Island into RI and eastern MA through the evening hours. Thus we opted to focus the MDT risk here, where 1-2" an hour rainfall will continue in spots for the next few hours. This area of more concentrated heavy rainfall may clip portions of DownEast ME tonight...and will take another look for the 01z update whether the MDT needs to be expanded into this area as well...but for now think the higher end Slight risk should cover it. Over the interior Northeast no longer thought the MDT risk was necessary, but we will maintain a Slight risk. Isolated to scattered convection will continue near and ahead of a frontal boundary, although cloud cover has generally hindered destabilization and generally kept activity localized and rather shallow in nature. Still think we may see a few cells become more intense as we go through the evening hours, and where these more intense cells do develop a quick 1-3" of rain is possible. Given the antecedent conditions over this region, this will likely be enough to drive a localized flash flood risk. Given the sensitive antecedent conditions, if a cell or two is able to grow in intensity (as recent HRRR runs suggest is possible), then still can not rule out an isolated significant flash flood, but the coverage of any higher end impacts should be pretty low. For that reason think a Slight risk should cover this conditional threat ....Gulf Coast and Southeast... A Slight risk was maintained for a good portion of LA into southwest MS. An impressive overlap of instability and PWs is noted across the region...MLCAPE of 3000-5000 j/kg and PWs around 2.25". This type of environment would favor vigorous updrafts, while also supporting efficient and effective rainfall producers. Thus it will only take a brief merging of cells to result in a quick 2-3"+ of rainfall, and recent radar trends suggest cell mergers are on the uptick. Expect scattered flash flooding to remain a concern over the next several hours as convection continues across this region. ....Central Plains through southern IL... Widely scattered showers and thunderstorms will continue across this corridor into tonight. Cells should be quick moving and not expecting anything more than a widely isolated flash flood risk at this time. Will evaluate this region closer for the 01z update to see if any more trimming of the risk area is warranted. Chenard Day 2 Valid 12Z Mon Jul 17 2023 - 12Z Tue Jul 18 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE MID-MISSOURI RIVER VALLEY, OHIO VALLEY TO THE EASTERN GREAT LAKES, AS WELL AS MUCH OF THE FLORIDA PENINSULA, SOUTHERN LOUSIANA, AND SOUTHEAST ARIZONA/SOUTHWEST NEW MEXICO... 2030Z Update... Made only minor updates based on the 12Z guidance, with no significant changes from the previous forecast. Pereira Previous Discussion... No major changes were made to the various Marginal Risk areas across the country on Monday. A secondary front pushing south and east across the Midwest will provide the focus for widely scattered showers and storms over the Ohio Valley today. Since the main plume of moisture responsible for the heavy rain Sunday in the Northeast will be well off the coast by Monday, this secondary front will have much less access to atmospheric moisture. This will dramatically cut down on the storm coverage and intensity, which in turn will limit the flash flooding threat as well. Nonetheless, isolated training/backbuilding storms can't be ruled out, with the strongest signal from southeast OH into western PA. The Marginal Risk was maintained across this area. The southern end of the same front will be between forcings, and therefore the signal for heavy rain has diminished, thus the biggest change from inherited was to drop much of MO and IL from the Marginal Risk area. Meanwhile further northwest across the Mid-Missouri River Valley, a developing LLJ will begin pushing the front east as a warm front Monday night. This interface will be the focal point for scattered storms, primarily over SD and northeastern NE. Isolated flash flooding can't be ruled out. This same LLJ will be much more influential and impactful further south and east on Day 3/Tuesday. The FL Peninsula will remain on the southern end of the impressive moisture plume now entirely off the rest of the East Coast. Thus, scattered but largely disorganized convection may still produce flash flooding since any storms will have ample moisture to draw from. The greatest threat will be along the Treasure Coast. A weaker signal persists over southeastern LA, but with possible flash flooding from Day 1/Sunday, opted to leave the Marginal as a low-end threat for Monday. This is the lowest confidence of the various Marginal Risk areas. Finally, weak monsoonal flow may produce a renewed round of afternoon showers and thunderstorms over southeastern AZ and southwestern New Mexico. This area was hit by convection the past couple days, so an isolated flash flooding threat can't be ruled out. Wegman Day 3 Valid 12Z Tue Jul 18 2023 - 12Z Wed Jul 19 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE LOWER MISSOURI VALLEY TO THE LOWER OHIO VALLEY AS WELL AS FOR PORTIONS OF THE NORTHEAST... 2030Z Update... No significant changes from the previous forecast. Models continue to show a strong signal for heavy rains developing across portions of the lower Missouri, mid Mississippi, and lower Ohio valleys this period. However, there remained enough location uncertainty to hold off on an upgrade to a Moderate Risk for now. Pereira Previous Discussion... ....Lower Missouri Valley to the Lower Ohio Valley... An MCS at the nose of strengthening LLJ is expected to develop over eastern MO and IL Tuesday evening, which will intensify as it slowly moves east across IL and IN Tuesday night. The LLJ will strengthen to 50 kts in the mid-levels Tuesday night as it advects plentiful Gulf moisture northeastward into a warm front draped across the area. This front will be the primary source of lift for strong and slow-moving thunderstorms capable of over 2 inch per hour rainfall rates in the strongest storms. While the area has been about average for rainfall over the past 2 weeks based on AHPS data, the potential for very heavy rain would likely overcome these antecedent conditions should the heavy rain persist for a long enough period of time. Since the front will be slow moving, this will support training thunderstorms which will move southeastward with time. The most persistent areas of rain may see rainfall totals exceed 5 inches for storm total rainfall from this event. There is considerable uncertainty as to where the MCS is likely to develop. The areas mentioned above are most likely to see the greatest rainfall, but the spread covers from central MO east to southwestern OH. Assuming the signal remains high for localized heavy rain, and with better consistency among the guidance with future forecast cycles, a Moderate Risk upgrade is possible. For now the high-end Slight Risk encompasses from St. Louis east across southern IL and southwestern IN. ....Northeast... The front pushing east across the Mid-Atlantic will have considerable upper level support from a deepening negatively tilted longwave trough. However, moisture will be limited as the primary moisture plume will be well off the coast. Thus, while storms will have the potential to grow strong with the strong forcing, the relative lack of moisture will limit strength to some extent, but moreso coverage. Thus, the Slight Risk is primarily driven by highly favorable antecedent conditions that will be ongoing after today's rainfall event. Tuesday's storms will be less strong and with less coverage, but since FFGs are likely to be low, widely scattered flooding is still possible. The Slight Risk was expanded southward to include all of the Hudson Valley and into northern NJ with this update, following the axis of heaviest rainfall. ....South Florida... Typical sea breeze convection will be ongoing with the stationary tropical air mass persisting. The Marginal Risk was nudged northward to include more of central FL with this update. ....Southeast AZ/Southwest NM... Monsoonal flow is likely to result in another round of afternoon showers and thunderstorms over the same area as previous days. Isolated flash flooding in sensitive areas can't be ruled out. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4s7b3so76Sezov_VeEvVUOpykeXufjv6W8TvGhRkUOwF= IFt1g3DO65beX9Iz6qEwg7V-2DSL3Knlg5bjwxDDlwaqpm8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4s7b3so76Sezov_VeEvVUOpykeXufjv6W8TvGhRkUOwF= IFt1g3DO65beX9Iz6qEwg7V-2DSL3Knlg5bjwxDDEl-3nUs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4s7b3so76Sezov_VeEvVUOpykeXufjv6W8TvGhRkUOwF= IFt1g3DO65beX9Iz6qEwg7V-2DSL3Knlg5bjwxDDFIzwLC8$=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) .