Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 07 2023 08:26:00 FOUS30 KWBC 070825 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 425 AM EDT Mon Aug 07 2023 Day 1 Valid 12Z Mon Aug 07 2023 - 12Z Tue Aug 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE NORTHEAST INTO THE MID-ATLANTIC REGION AS WELL AS NORTHEASTERN WASHINGTON AND NORTHERN IDAHO... ....Much of the Northeast U.S.... A potent mid/upper level shortwave trough will make its way eastward from Illinois to portions of the central Appalachians during the day while an anomalously strong jet for the season makes its way from the Ohio Valley into parts of the Mid-Atlantic...helping to keep a feed of deep moisture over the eastern U.S.. Exactly how this interacts with a shortwave trough initially over Michigan before heading north of the border...and any associated height falls over the eastern Great Lakes and Northeast U.S. continues to result in poor run to run consistency...model QPF spread...and associated excessive rainfall potential. With the event moving into the Day 1 timeframe...an increasing amount of CAM guidance is available. The HREF paired-matched mean showed one wave of QPF moving quickly into New England early today before an additional round of active convection capable of intense downpours develops in the afternoon focused farther south which eventually translates/spread north and east. 40 km neighborhood probabilities from the 07/00Z HREF show the highest probabilities for 1-hour QPF to exceed one/two inch(es) developing late afternoon/early evening over parts of the Mid-Atlantic region in proximity to the potent mid/upper level shortwave...although the signal for 1 hour QPF to exceed flash flood guidance/3 hour QPF to exceed flash flood guidance is greater in portions of New York state where Flash Flood Guidance is lowest. For this reason...opted not to make too many adjustments to the northern portion of the Slight Risk area but pulled the southern portion of the Slight Risk in response to the potential for downpours with 2+ inch per hour rates. With so much interaction at the synoptic scale still to be sorted out...additional adjustments one way or the other are still possible. ....Portions of the Northwest U.S.... Maintained the Slight Risk over portions of Washington/Idaho/Montana and Oregon with a fairly subtle southward expansion towards the Blue Mountains. Low pressure prior to the start of Day 1 has drawn moisture into the region from the east.=20 This moisture combined with the slow moving surface low will help fuel another round of afternoon and evening convection. Given the anomalous amount of moisture...with precipitable water values some 150 to 200 percent of normal in northern Idaho...locally heavy downpours that result in flooding are certainly possible. The most vulnerable areas being over and near recent burn scars...although instances of flooding can also occur elsewhere.=20 Thinking is that the NAM is too aggressive with its QPF...a bias with this type of flow pattern... ....Western High Plains... Scattered showers and thunderstorms will develop over parts of Colorado and Wyoming before spreading eastward. Storms will be moving into an airmass with deeper moisture which should allow for an uptick in rainfall rates with time. Locally heavy rainfall rates and isolated heavy rainfall amounts may lead to isolated instances of flooding or at least run-off problems in regions of poor drainage. Bann Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!83zWsMwIYq8G84cMiwuTXsgxjvdAq1cICNTCvZZ3-E-A= aYZhqcHNxK4zOC1n-saOeDP2AHzOyhe-6JBruBCYmAa_xoc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!83zWsMwIYq8G84cMiwuTXsgxjvdAq1cICNTCvZZ3-E-A= aYZhqcHNxK4zOC1n-saOeDP2AHzOyhe-6JBruBCYzQZnSZw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!83zWsMwIYq8G84cMiwuTXsgxjvdAq1cICNTCvZZ3-E-A= aYZhqcHNxK4zOC1n-saOeDP2AHzOyhe-6JBruBCYOjBGZQY$=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) .