Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 28 2023 15:51:08 FOUS30 KWBC 281551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Mon Aug 28 2023 Day 1 Valid 1236Z Mon Aug 28 2023 - 12Z Tue Aug 29 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE SOUTHEAST TO THE CENTRAL MID-ATLANTIC... ....West Virginia... Early morning heavy rain in central WV that developed along a trough along the western flank of the Appalachians has diminished. Additional rains today are forecast to be generally light to moderate, though a remnant MCV could allow a very localized area of heavy rain, including over central WV. A Marginal Risk is maintained for this area of the western flank into southwest PA. ....Southeast U.S into the Central Mid-Atlantic... Southerly low level flow over a stalled front extending from southeast Louisiana into central AL/GA and up the Carolina coast will continue to support areas of heavy rainfall through tonight. The corridor of higher theta-e located along the boundary and weak steering flow will provide an axis in which rainfall rates exceed 2 inches per hour from the most intense convection. Main areas of heavy rainfall featured in the 12Z HREF continue to be southeast VA, where ongoing heavy clusters of thunderstorms are ongoing, the southern Appalachians, and just interior of the central Gulf Coast. Worked with the Marginal and Slight Risk areas to fit this forecast better with the Slight still connecting the southern Apps and the southern Mid-Atlantic across NC where boundary layer moisture convergence is still progged this evening by the RAP. Also, the surge of moisture and some instability into the Chesapeake Bay region continues overnight where the 12Z ARW2/HRRR produce heavy/slow moving activity late tonight. As a result the Slight Risk was expanded north into southern MD and up the DelMarVa a bit. Confidence seems greater that insufficient instability should keep heavy rains from the Philly metro, so less of that area is in the Marginal Risk. ....Florida Peninsula... Diurnally forced, slow moving heavy activity across central and southern portions of the FL Peninsula in the surge of tropical moisture ahead of Idalia warrants an expansion of the Marginal Risk across these areas. PWs in excess of 2.25" are progged over much of the FL peninsula by 00Z, so localized excessive rain seems probable as supported by 12Z HREF CAMs. Outer bands may yet reach the western coast south from Tampa Bay by 12Z, so those areas are still included in the Marginal. ....Colorado through West Texas.... Afternoon and evening convection is expected once again from Colorado into West Texas on Monday downstream of a strong ridge axis. Flow aloft increases as it veers a bit from northwesterly to north-northwesterly as ridge axis approaches. This should help focus activity closer to the eastern slopes of the southern Rockies than yesterday. A localized excessive rainfall event is anticipated. The heaviest rain (1-2") fell yesterday over the Caprock along the TX/NM border, with most guidance generally keeping activity west of there today, so the need for a targeted Slight Risk seems limited at this time. Jackson/Bann Day 2 Valid 12Z Tue Aug 29 2023 - 12Z Wed Aug 30 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE FLORIDA PENINSULA ALONG AND NEAR THE TRACK OF IDALIA... ....Florida into Georgia... There is a Moderate risk of excessive rainfall where Idalia makes landfall and along its path towards the northeast once it is inland late Tuesday night/early Wednesday morning. Considering that the NHC forecast is calling for Idalia to be a well developed hurricane when it makes landfall...intense rainfall rates are expected near the core of the storm and from rain bands ahead of the storm pose a threat of flash flooding. There is still some uncertainty as to exactly when it will make landfall with guidance showing a slowing trend over the past few forecast cycles...meaning a Moderate may not be needed if the arrival comes too late. In addition...there was a subtle eastward shift during the same time which could necessitate a shift in the axis. Check the latest statements and guidance from the National Hurricane Center and local offices for latest information. ....Southeast through Mid-Atlantic... The aforementioned surge of tropical moisture ahead of Idalia is concerning given the persistence of the frontal boundary from the central Gulf Coast through the Carolinas as well as lifting north into the Mid-Atlantic on Tuesday. Convergence of tropical moisture along the front in/near the southern Appalachians may lead to a predecessor type rainfall event over northern Georgia through central/western South Carolina into central North Carolina. This will be in addition to rain that fell on Monday and Monday night...which has made parts of the area more sensitive to heavy rain. A separate Slight Risk area is drawn to cover this risk with the understanding that as we get more into the CAM time range, focusing/upgrading the risk level may become necessary. Farther north, the surge of higher moisture up the Mid-Atlantic in time for the Tuesday diurnal cycle ahead of the cold front expected to move into/through the area Tuesday night warrants an expansion of the Marginal Risk up through the Philadelphia metro area which is where the overlap of greater instability and moisture has its northward extent. At the same time...moisture streaming northward from the North Atlantic should be interacting with a surface cold front across New England at about the same time a shortwave trough approaches from the west. At this point...can not rule out some potential for excessive rainfall.=20 While the driving mechanisms between the two areas were different...it was difficult to justify where one area ended and the next area began in close proximity to each other. The result was a single Marginal risk area that was rather broad and poorly focused. It is presumed that the area(s) can be refined in subsequent outlooks. ....Pacific Northwest.... A strong, negatively tilted upper trough is expected to push into the Pacific Northwest Tuesday with a progressive shortwave maxima rounding the base of the mean trough. Enhanced mid-level forcing generated from strong positive vorticity advection across the Cascades into northern Idaho will generate a rapidly developing area of precipitation within a corridor of modest instability. Despite fast storm motion expected within enhanced mean flow, heavy rainfall will be likely over the aforementioned area within a region of notably steep terrain and low flash flood thresholds. The Marginal Risk is maintained. Bann Day 3 Valid 12Z Wed Aug 30 2023 - 12Z Thu Aug 31 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF FLORIDA NORTHWARD INTO THE CAROLINAS AS IDALIA TRACKS ACROSS THE REGION... ....Florida to the Carolinas... Heavy rainfall will spread northward and eastward in association with Idalia. Given its strength at the beginning of the period...a rather traditional rainfall distribution is expected with intense rainfall rates near the core of the storm and from rain bands feeding into the system. The HAFSA and HAFSB showed rainfall developing out ahead of the center of the storm along/near the Georgia and Carolina coasts later today...and remaining to the northwest of Idalia once it moves offshore. The HAFS track was in good agreement with the official forecast track from NHC both in terms position and amounts...and was useful in crafting the Excessive Rainfall Outlook. Refer to latest statements from the National Hurricane Center for updated information. ....Pacific Northwest.... A strong, negatively tilted upper trough will be about ready to exit the Pacific Northwest early Wednesday as a shortwave maxima begins to lift northward and head north of the border. Initially mid-level forcing will be enhanced from the positive vorticity advection that wanes as the dynamics weaken. Despite fast storm motion...isolated moderate to heavy rainfall may still result in run off problems...especially in areas of steep terrain and low flash flood thresholds. T Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9nOVdMDXSAYBRIJJdNc3uO-OMPFGVxqU5JeOrTodYVug= DPU6l55bfIquRuEYcU_4PLlmi8VT0KCAcbxzpQhxdKlXCxU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9nOVdMDXSAYBRIJJdNc3uO-OMPFGVxqU5JeOrTodYVug= DPU6l55bfIquRuEYcU_4PLlmi8VT0KCAcbxzpQhxdSc5lN4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9nOVdMDXSAYBRIJJdNc3uO-OMPFGVxqU5JeOrTodYVug= DPU6l55bfIquRuEYcU_4PLlmi8VT0KCAcbxzpQhxLjFiUKY$=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) .