Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Oct 06 2023 20:28:30 FOUS30 KWBC 062028 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 PM EDT Fri Oct 06 2023 Day 1 Valid 2017Z Fri Oct 06 2023 - 12Z Sat Oct 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN NEW YORK AND NORTHEAST NEW JERSEY... ....2015Z Special Update... Issued a special excessive rainfall outlook in order to introduce a Slight Risk area over southern New York into northern/northeast New Jersey. Earlier this morning, the FV3 and the HRRR started to show the potential for convection over southern New York and adjacent northern/northeastern New Jersey. Additional guidance has arrived since then that bolstered the FV3/HRRR idea of locally heavy rainfall amounts developing in the area later this evening and continuing overnight. The Slight Risk was hoisted to convey the increased chances for heavy rain and that there is at least some potential for excessive rainfall to occur prior to 07/12Z. Bann ....Previous Excessive Rainfall Outlook... ....Northern Pennsylvania into New York... A strong digging high-amplitude upper level trough over the Great Lakes will shift eastward during the day. Increasing southerly winds ahead of a leading cold front will draw warm, moist air from the Southeast northward up the Mid-Atlantic and into New England ahead of the trough. An unimpressive rain shield moving into the Appalachians as of late morning will start to fill in and become enhanced over the Marginal Risk area late this afternoon through tonight as a strong shortwave trough and associated vorticity rounds the base of the trough and begins moving northeast ahead of the trough. The negative tilt to the resultant trough will further enhance divergence aloft, resulting in heavier rain. Initially...moisture availability will be somewhat limited but moisture streaming northward from the system that had been Philippe finally comes into play late in the period. Nonetheless PWATs to 1.25 inches with impressive upper level dynamics should allow for convective rainfall ahead of the main cold front. The 12Z runs of the FV3 and HRRR both showed some potential for problems based on heavy rainfall rates/amounts from far southern New York and northern New Jersey while HREF amounts/neighborhood farther north suggested that problems farther north may need repeating rounds of storms for isolated excessive rainfall threat. CAM guidance suggests most of the heaviest rains will be after sunset. Bann Day 2 Valid 12Z Sat Oct 07 2023 - 12Z Sun Oct 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST... ....2030Z Excessive Rainfall Discussion... Not much change fundamentally with respect to the large scale pattern and overall expectations. There has been an uptick in QPF in places since 0830Z but nothing in a coherent manner requiring significant changes to the on-going Excessive Rainfall Outlook.=20 Perhaps the biggest change was to draw the southern end of the Slight Risk area southward to cover southern New York (including New York City) and into northeast New Jersey given the potential for on-going convection at the start of the period. Beyond that...only minor adjustments were made to the periphery of the outlook areas. Bann ....0830Z Excessive Rainfall Outlook Discussion... The period with the greatest potential for significant flash flooding in the short-term will be on Saturday across portions of the Northeast as a longwave trough over the eastern Great Lakes will continue pivoting northeastward as the embedded shortwave trough elongates southeast to northwestward. This in turn will cause the whole longwave trough to become negatively tilted, which will further enhance divergence across New England. A pair of jet streaks will locally enhance speed divergence even more ahead of the trough. The result will be 2 areas of heavy rains. The western area of the two will be largely associated with the cold front continuing to track eastward from the Mid-Atlantic into New England. Enhanced divergence and the slowing of the eastward progress of the entire pattern...potentially resulting in more numerous instances of flash flooding from multiple rounds of convection. By Saturday night, Vermont may see some wraparound rainfall around the western side of the remnants of Philippe as they get caught up in the main flow ahead of the trough. The second area of heavy rains will be associated directly with the remnants of Philippe as they move into Maine Saturday perhaps as early as Saturday afternoon but then persisting through late Saturday night/early Sunday. The storm will be fast moving thanks to the aforementioned jet streaks helping the storm move along, but the atmosphere will be moisture-laden as all the tropical moisture gathered by the storm rains out over eastern Maine. Thus, the higher rainfall totals of the 2 areas will likely be over eastern Maine. There are some factors that will help mitigate flash flooding risk. First, the fast movement of the storm itself. Second, individual storms embedded within the larger rain area will also be moving quickly in the 45-65 kt LLJ associated with the storms. Third, the soils in the area are around normal for moisture ahead of the storm. As opposed to being already saturated (worst-case scenario), or bone dry (which results in harder soils promoting runoff early on in the event), starting out with near normal moisture should optimize absorption of the rainfall, at least initially. Of course, 3-5 inches of rain may cause widely scattered instances of flash flooding anyway, but they'll be slower to develop and less extreme overall due to optimal soil conditions going in. One factor that may enhance flash flooding potential locally is the peak of fall foliage across New England, which due to the gusty winds will increase leaf-fall rates, which could quickly clog storm drains and culverts. This may result in more widespread instances of flash flooding. Given the other factors working against flash flooding, think the Slight Risk area remains the optimal forecast, though it's a high end Slight for much of eastern Maine. There remains some uncertainty with where the area in between the heavier rainfall will be. There's general consensus it will include RI, eastern MA, eastern NH, and far western ME, but that is not for sure. This sweet spot in between will likely follow or end up just west of the center of Philippe's circulation. A notable dry slot is on satellite even now associated with Philippe, and the added dynamics/jet streaks should promote the dry slot's persistence near the center. Thus, there's expected to be a local minimum of rainfall, which is highlighted by the much lower risk categories in the area mentioned. Again with some uncertainty this area may shift one way or another over the next 24 hours as the track of the center of circulation becomes more clear.=20 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!-TpvtV_zJXsf4Rv_F9v992Jt33ZYa4N4r0T0szvQW9xy= eB6SrX3Cl7zAfZBl3AweIg1RZnwegkbVozuemKnp4ZWGmug$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-TpvtV_zJXsf4Rv_F9v992Jt33ZYa4N4r0T0szvQW9xy= eB6SrX3Cl7zAfZBl3AweIg1RZnwegkbVozuemKnpzKQklM0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-TpvtV_zJXsf4Rv_F9v992Jt33ZYa4N4r0T0szvQW9xy= eB6SrX3Cl7zAfZBl3AweIg1RZnwegkbVozuemKnpWXApemo$=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) .