Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 28 2023 08:35:34 FOUS30 KWBC 280835 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 434 AM EDT Mon Aug 28 2023 Day 1 Valid 12Z Mon Aug 28 2023 - 12Z Tue Aug 29 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE SOUTHEAST AND SOUTHERN MID-ATLANTIC... ....Southeast U.S into Southern Mid-Atlantic... Stalled frontal boundary across the southeast U.S will again be the focal point for a corridor of heavy rainfall during peak diurnal convection on Monday afternoon and evening. 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. 28/00Z HREF CAM consensus is for two general areas of precip maxima; the southern Appalachians to Upland portions of South Carolina down through northern Georgia, and southeast Virginia and northeast North Carolina. Saw little need to make changes except to trim the far southern extent out of the area. It does not preclude isolate run-off problems in urban areas or in areas with poor drainage...but the overall concern for excessive rainfall is near the front farther north. ....Florida Peninsula... Kept the Marginal Risk area over parts of the west and southwest Florida peninsula coast for continued consensus from the latest guidance about leading/outer bands of Idalia late Monday night...which is generally limited to this more sensitive urban corridor from Tampa Bay through Naples. Diurnally forced activity across the rest of the southern peninsula is expected as tropical moisture streams into the area ahead of Idalia. As mentioned previously...any excessive rain concern elsewhere across the Florida peninsula should be isolated given the recent generally dry weather and high flash flood guidance. ....Colorado through West Texas.... Late day 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 rdge axis approaches. This will help focus activity closer to the eastern slopes of the southern Rockies than recently. In general...a localized excessive rainfall event is anticipated...aA localized Slight Risk may be warranted should a focus area on Monday repeat over areas of heavy rain through tonight. In general...a localized excessive rain risk is anticipated. 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. Should the moisture/instability surge make it farther north, areas farther north would need a Marginal Risk. ....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!4URM2AW2ffhzIjeI0-fF0F7oH-4gAV8juHdR2mOkXvht= TcXo33UNzbSs34cyykeKkN5diG3MNkx0_ohocBcwh-ytl20$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4URM2AW2ffhzIjeI0-fF0F7oH-4gAV8juHdR2mOkXvht= TcXo33UNzbSs34cyykeKkN5diG3MNkx0_ohocBcwH54kNog$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4URM2AW2ffhzIjeI0-fF0F7oH-4gAV8juHdR2mOkXvht= TcXo33UNzbSs34cyykeKkN5diG3MNkx0_ohocBcwQbpJPSM$=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) .