Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 11 2024 15:49:09 FOUS30 KWBC 111548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1148 AM EDT Thu Apr 11 2024 Day 1 Valid 16Z Thu Apr 11 2024 - 12Z Fri Apr 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PART OF THE OHIO VALLEY EASTWARD INTO PORTIONS OF THE CENTRAL APPALACHIANS AND MID-ATLANTIC... ....16Z Update... One of the primary changes to the D1 ERO includes the removal of=20 the SLGT risk across portions of FL and southeast GA as the threat=20 of flash flood potential is dwindling as rates have subsided within the past hour. A line of convection is still ongoing and will=20 press to the east over the next 2-4 hours with locally heavy=20 rainfall possible within the organized line. The best chance=20 remains over the urban corridors of northeast FL like Jacksonville=20 where runoff is more common. A MRGL was maintained for the area,=20 but will likely be removed by the next update (01z).=20 Further north, have scaled back the coverage of the MRGL risk on the southwest portion of the previous forecast as our upper low makes headway to the northeast. Rates are capped below 1"/hr across all of TN/MS and western KY where scattered showers and storms will present totals bordering low-end FFG exceedance criteria for the 3 and 6 hour intervals. Progressive nature of the upper low will create a less favorable dynamic by later this afternoon with rainfall shifting northeastward in time. Expect this to be fine tuned, as well with the next update. Greatest wholesale change was the expansion of the SLGT risk over the Ohio Valley to the southwest to align with the latest instability fields within the latest hi-res suite, as well as the favored probabilistic signatures for both FFG exceedance and expected rates within this afternoons convective development. Large scale forcing will be the primary driver for the area encompassing the Ohio Valley as the region works within the eastern flank of the progressive shortwave trough with a strong diffluent signature aimed over eastern KY, OH, and WV. The strongest signal for convection and higher rates of at least 1"/hr is located over eastern KY up through western WV into eastern OH with latest HREF neighborhood probabilities between 25-40% for the 1"/hr marker. The 2"/hr threshold is fairly non-existent which caps the threat into the SLGT risk territory with any upgrade very unlikely. A lot of that has to do with the thermodynamics limited with tons of cloud cover currently impacting the region ahead of the troughs approach.=20 The lower FFGs in place and enough large scale forcing will be just enough to provide the opportunity, thus have maintained the overall structure of the previous SLGT risk with an expansion on the western flank and removal of the SLGT over the Blue Ridge in VA/MD given recent trends. Heaviest rains will be focused along the crest of the Blue Ridge with valley areas remaining less favorable for enough precip to cause flash flood concerns, but enough to maintain the MRGL.=20=20 Kleebauer ....Previous Discussion... A complex and broad mid-latitude cyclone will produce a broad shield of rainfall across the eastern United States...with several regions where rainfall rates/amounts will threaten (if not exceed) flash flood guidance. One area is over portions of the eastern Ohio Valley into parts of the central Appalachians and adjacent Mid- Atlantic region where the 11/00Z HREF shows in excess of 30 percent neighborhood probability of 1 inch per hour rainfall rate and even a 5 percent neighborhood probability for 2 inch per hour rates by late afternoon. Synoptically, there is strong moisture transport into the region and precipitable water values approaching 2 standardized anomalies greater than climatology for this time of year. Flash flood guidance was lowered in parts of the area recently...making the possibility of flash flood exceedance a possibility. Maintained the previously issued Marginal Risk area without too many changes given the sensitivity of urban areas. Main question was how much convection will be lingering in the Southeast U.S at the beginning of the Day 1 period. Models appear to be underdone with amounts and too fast here...so will maintain the risk area primarily for the first few hours of the morning. Bann Day 2 Valid 12Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN NEW ENGLAND... Made only minor modifications to the previously-issued Slight and Marginal Risk area. There was a bit of an eastward expansion based on the latest WPC deterministic QPF and the NBM. But the northern and western periphery remained in place without any real changes where the risk of flooding was due to a combination of rainfall and snow melt. There has been some run to run variation but the overall idea of strong moisture transport of deep moisture and higher dewpoints at low levels from the Atlantic that gets directed normally to the terrain has remained fairly consistent. Thus saw little or no reason to make any meaningful removal from the Slight/Marginal Risk areas due to the low-end excessive rainfall threat across northern New England. Bann Day 3 Valid 12Z Sat Apr 13 2024 - 12Z Sun Apr 14 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG A PORTION OF THE CALIFORNIA COAST... Generally light to locally moderate rainfall should gradually be working its way southward from southwest Oregon into northern and central California by the time the Day 3 period begins on Saturday morning as low pressure over the eastern North Pacific eases southward. On-shore flow ahead of the low and associated upper level trough will support some one quarter- to isolated on half- inch per 6 hours along the coastal ranges. Thinking is that the on-shore flow will be shunted southward with time, but there could be an up-tick in rainfall rates as cold mid-level temperatures near the center of the upper low results due to steepening low level lapse rates at time of maximum daytime heating. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_grSa_LM2ebPWJgEWDt5wdV7TlQ-tI9pKOFxS3ExjC4H= fLaq_JsPIvQE-WsEF5qQx72o7mOTsmeitWGjszXCKtdINZA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_grSa_LM2ebPWJgEWDt5wdV7TlQ-tI9pKOFxS3ExjC4H= fLaq_JsPIvQE-WsEF5qQx72o7mOTsmeitWGjszXCR4XFNHs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_grSa_LM2ebPWJgEWDt5wdV7TlQ-tI9pKOFxS3ExjC4H= fLaq_JsPIvQE-WsEF5qQx72o7mOTsmeitWGjszXCHjY_99s$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .