Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 29 2023 01:09:41 FOUS30 KWBC 290109 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 909 PM EDT Fri Apr 28 2023 Day 1 Valid 01Z Sat Apr 29 2023 - 12Z Sat Apr 29 2023 ....THERE IS A MARGINAL RISK OVER PORTIONS OF THE EASTERN OHIO VALLEY/MID ATLANTIC AND MUCH OF EASTERN TEXAS/ADJACENT PLAINS... ....Eastern Ohio Valley to portions and portions of the Central Appalachians... Maintained the Marginal Risk area which encompassed portions of eastern Ohio and far western Pennsylvania into far eastern West Virginia as convection makes its way eastward out of the Ohio Valley towards a region of lowered Flash Flood Guidance where 1-hour Flash Flood Guidance is ranges from marginally above an inch in PA and in the 0.8 to 1.0 inch range in the upslope region of West Virginia and in the 1 to 1.5 inch range per 3-hour period. Highest 1-hour rainfall rates from the CAM guidance is below 1 inch per hour...so the Marginal highlights the hydrologic sensitivity more than rapid onset from an uptick in rainfall intensity before the risk diminishes by late evening. ....EASTERN TEXAS INTO PARTS OF EASTERN OK AND NORTHWEST ARKANSAS... The latest guidance continues to suggest an increase in the potential for heavy rainfall across eastern Oklahoma and further north/east across western Arkansas as a secondary boundary and embedded shortwave energy taps moisture streaming northward from a line of active convection/squall line farther south in Texas.=20 Followed the 18Z HREF idea of increased probabilities and increased areal coverage of 1+ inch per hour over portions of the region expanding the Marginal Risk area a bit northward and eastward across eastern Oklahoma into Arkansas and northwestern Arkansas. Higher rates are expected in Texas while movement of the storms farther north should be a bit slower. In either case...a Marginal Risk area should still be adequate due to the progressive nature of the convection...and problems being confined more in regions of poor drainage. Bann Day 2 Valid 12Z Sat Apr 29 2023 - 12Z Sun Apr 30 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA... ....Southeastern US... 21Z update... The latest guidance continues to show 2 axes of QPF across the Southeast, the heavier and more dominant one along the Gulf Coast and into Georgia (within the Slight Risk area) and another one spanning from Mississippi to far southwest North Carolina. Consensus with the northern band has areal averages of 1 to 3+ inches and the local FFG are as low as 1/1.5 in and 0.25 in along the North/South Carolina border in the mountains. Given the potential to reach or exceed FFGs and the soil sensitivity, the Marginal Risk area was extended northward to cover this part of the country. Additionally, the Slight Risk had a minor tweak eastward across eastern Georgia to reflect the latest trends. Campbell A developing low over the Mississippi Valley and a pair of upper level shortwaves moving northeast across the Southeast will provide the forcing as highly anomalous Gulf moisture to 3 sigma above normal with PWATs approaching 2 inches, and a 50-60 kt LLJ by late Saturday night over the FL Panhandle will all combine over the Slight Risk area to result in multiple rounds of thunderstorms each capable of rainfall rates exceeding 2 inches/hour, which will move northeast out of the Gulf and into southeastern GA and SC. The first round of storms will be with a weaker front moving across the area Saturday afternoon, then the second will follow it with the stronger surface front late Saturday night. Instability will increase through this time as greater instability is advected northward with the Gulf moisture on the strengthening LLJ. No significant changes were made with this morning's forecast update. Multiple rounds of storms are expected, most of which will be overnight Saturday night across the Slight Risk area. While each storm is likely to be fairly quickly moving, repeated storms over this area which just saw thunderstorms move through yesterday are likely to result in isolated to widely scattered flash flooding, especially in urbanized and poor drainage areas, such as Tallahassee, Jacksonville, and Valdosta. ....Southern New England, New York City Metro Area, Long Island, and the Hudson Valley... 21Z update... With QPF trends showing decreased amounts across parts of Southeast New York the Marginal Risk area was trimmed over this part of the region. Still expecting the heavier rainfall to be over coastal areas. Campbell The same front that is likely to cause flooding over the Mid-Atlantic today will continue drifting northward into New York and Southern New England Saturday. The front will be significantly weaker on Saturday than today, though 30-40 kt southerly winds will bring 1 inch PWAT atmospheric moisture into the area. The forcing in the upper levels are likely to remain well west of the area, and the separation of the moisture and forcing should help to mitigate any heavy rainfall. This remains a lower end Marginal risk, and decreasing rainfall totals the past few forecast cycles may have this Marginal canceled with future updates. Wegman Day 3 Valid 12Z Sun Apr 30 2023 - 12Z Mon May 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTHERN MAINE, EASTERN NEW HAMPSHIRE, SOUTHERN NEW YORK AND CENTRAL/EASTERN PENNSYLVANIA... ....New England and the Mid-Atlantic... 21z update... With the increasing trend for training storms with heavy rain and potential flooding impacts, an upgrade to a Slight Risk was coordinated with the local forecast offices. The new Slight Risk covers much of central/eastern Pennsylvania and south-central New York areas where the 3hr FFG is as low as 1/1.5 inches and guidance depicts 1 to 3+ inches falling during this period. The Slight Risk area further east still highlights the appropriate level at this time, only minor adjustments were made across Maine with the eastern boundary of the Slight and the northern bound of the Marginal Risk. Campbell In coordination with GYX/Gray, ME forecast office, a Slight Risk upgrade was introduced with this morning forecast update. An area of low pressure over the Southeast that helps force the convection there Saturday night will race up the East Coast and then curve westward over New York State towards a second low over the Great Lakes Sunday night. Once the low turns westward, the warm front ahead of the low, in tandem with vigorous upper level forcing from a negatively tilted longwave trough tracking northeastward through the Mid-Atlantic will invigorate the southeasterly flow off the Atlantic to a large degree. The flow will increase to as strong as 60 kt at 850 mb, with the LLJ advecting PWATs to 1.25 inches into the Slight Risk area. Add in significant topography in the White Mountains of NH and southern ME, and it's likely there will be 3 inches or more of rain, much of which falling after midnight Sunday. This large amount of rain falling in an area with steep topography will result in rapid increases in local small creeks and streams. Thus, the possibility of flash flooding in this area is more than just isolated. Somewhat lesser amounts of rain are expected both north of this region in central Maine, as the front that stalls out and causes the locally heavier rain over southern Maine doesn't get that far north, and further west there will be lower availability of moisture, despite similar upper level forcing closer to the center of the longwave trough. Further, across the northern Mid-Atlantic, especially where a bit heavier rain is expected over northeastern PA and much of NY, it's been relatively dry, so the 1-2 inches of rain, locally heavier, will fall over a longer period of time over an area that can better handle those amounts of rain. The Marginal Risk area was expanded southward across the Delmarva and northern VA with this update, largely to encompass the same areas expected to pick up multiple inches of rain today, and are also likely to see an additional inch or so of rain again on Sunday. Given much of the heaviest rain in the Slight Risk area is expected around the Day 3/Day 4 transition around 12Z Monday, any increased speed of the front may nudge the heaviest rainfall rates and chances of flooding into the Day 4/Monday ERO period. ....Florida... The trailing end of the front and the low over the Southeast will be over the middle of Florida from the western Panhandle through the northern Peninsula on Sunday. With the parent low tracking northeast early in the day, showers and thunderstorms associated with the trailing cold front will develop in the morning and persist through peak heating, likely diminishing in intensity, coverage, and duration by the time the storms reach southern Florida after sunset Sunday evening. With storm motion parallel to the front (northeastward) and Corfidi vectors at 10 kt or less over the Marginal Risk area in FL, along with instability to 1500 J/kg Sunday morning ahead of the storms, the likelihood for training storms is significant. Since the storms will have plenty of moisture to work with, expect heavy rain rates to 2 inches/hour associated with the storms. Depending on how the storms ongoing Saturday night evolve, an upgrade over the northern portion of the area is possible if the storms from Saturday night are slow to clear the Day 2/Saturday Slight Risk area. Thus, unlike in New England, the timing for the main event in this area will be first thing Sunday morning, with a likely early cancellation of the Marginal Risk area by Sunday afternoon. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_vR3IingpH3Lve585UAk9I17qRq19npCKqhq_n4aEE7N= WP1cDvGVdISTVkFGmwQpKo87Qt9l43lUvvPe_pU9kvqhzKs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_vR3IingpH3Lve585UAk9I17qRq19npCKqhq_n4aEE7N= WP1cDvGVdISTVkFGmwQpKo87Qt9l43lUvvPe_pU9kbIZo4c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_vR3IingpH3Lve585UAk9I17qRq19npCKqhq_n4aEE7N= WP1cDvGVdISTVkFGmwQpKo87Qt9l43lUvvPe_pU9vONheRE$=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) .