Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 08 2023 08:57:43 FOUS30 KWBC 080857 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 457 AM EDT Tue Aug 08 2023 Day 1 Valid 12Z Tue Aug 08 2023 - 12Z Wed Aug 09 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN NEW ENGLAND... ....Northeast U.S.... On-going heavy to excessive rainfall across portions of New England will continue into the afternoon as deepening low pressure north of New York draws anomalously deep layer moisture and associated instability into New England with the synoptic scale flow pattern still able to support back building storms. Timing of the low and associated precipitation has changed a bit and required adjusting the Moderate Risk area. With storms impacting the terrain, particularly the White Mountains of New Hampshire into western Maine, the combination of rainfall and orographics should result in enhanced rainfall totals and the Moderate risk was largely unchanged. With models beginning to latch of to the speed of the back edge of the precipitation area...trimmed the Moderate out of some areas where QPF had become quite low. One other change was to extend the Slight Risk farther south where CAPE values were closer to 1000 J per kg and the potential for warm rain processes was through a particularly great depth. The western portion of both the Slight and Marginal Risk areas were adjusted. ....Central Plains to the South/Southeast U.S.... The Slight Risk area was maintained over the Plains from Colorado into Kansas and parts of adjoining Kansas. Thunderstorm development in Oklahoma during the overnight hours prompted an introduction of a Slight risk area extending from Oklahoma into Arkansas. It also allowed for the Marginal Risk area in the Plains to be joined with the Southern U.S. Marginal Risk area.=20 Southwest U.S... West to to southwest flow aloft will draw additional moisture from the Gulf of California into parts of Arizona...leading to increased coverage of late afternoon and evening showers and thunderstorms over/near some of the higher terrain in mainly in Arizona. There may be a few downpours from storms if the instability is sufficiently high to generate some storms. Bann Day 2 Valid 12Z Wed Aug 09 2023 - 12Z Thu Aug 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO VALLEY INTO THE SOUTHEAST U.S.... ....Illinois into the Tennessee Valley and portions of the Southeast... Numerical guidance still shows that organized convection should develop during the afternoon or evening as shortwave energy initially over Kansas/Nebraska heads eastward...leading to height falls aloft overspreading the Mid-Mississippi Valley by 10/00Z.=20 Gulf moisture gets pulled northward during the day with both the GFS and NAM showing precipitable water values exceeding 2 inches from central/south central Missouri into Mississippi and Alabama by 09/18Z while the flow aloft ahead of the strengthening shortwave becomes increasingly difluent. This is similar to what was shown by the models on Monday but displaced a bit west. The flow aloft is still expected to become difluent on the east-side of an open wave at 300 mb. With moisture flux some 4+ standard anomalies above climatology...2 to 4 inch amounts still seem like a reasonable range of amounts. As such...a Slight risk area still seems appropriate.=20 Southwest U.S... Similar to Tuesday...west to to southwest flow aloft will draw additional moisture from the Gulf of California into parts of Arizona...leading to increased coverage of late afternoon and evening showers and thunderstorms over/near some of the higher terrain in mainly in Arizona. There may be a few downpours from storms if the instability is sufficiently high to generate some storms. Bann Day 3 Valid 12Z Thu Aug 10 2023 - 12Z Fri Aug 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE EASTERN UNITED STATES... ....Eastern U.S.... Surface low pressure...initially associated with the convection over the central U.S. on Wednesday...is expected to move towards the eastern seaboard on Thursday ahead of a cold front dropping out of Canada into the Great Lakes. This will set the stage for increasing coverage of showers and thunderstorms over the eastern third of the country with one area of showers and thunderstorms associated with the surface low pressure center and a second farther south. Precipitation amounts should be fairly light...however rainfall associated with the low could overlap with areas that were soaked recently has the potential for isolated flash flooding or run off problems. Farther south...the area from the Tennessee Valley towards the Carolinas should have a seasonally moist airmass and enough dynamics to support rainfall rates sufficient to produce isolate flooding without such wet antecedent conditions. At this point with timing being so broad...opted for one broad Marginal area given the proximity of the two regions and uncertainty of timing. ....Upper Midwest... Placed a Marginal Risk area across portions of the Upper Midwest in a region where instability is expected to develop ahead of a developing surface low. With moisture pooling over the eastern Dakotas during the period...and precipitable water values increasing to more than 1.6 inches...convection that develops should have sufficiently strong instability to support locally heavy downpours by evening. Given uncertainty in placement and even the coverage of storms precludes more than a Marginal risk area at this point. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_Ma3IAhiegUpmbpWnrKqyDQjgS4u-sh-TZ9D3mHQKlLk= 7MccnvpBUZGPEP8GFmZZO7TnXzotgbiLQwmWGKv1fXF5g9U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_Ma3IAhiegUpmbpWnrKqyDQjgS4u-sh-TZ9D3mHQKlLk= 7MccnvpBUZGPEP8GFmZZO7TnXzotgbiLQwmWGKv1YNq3l40$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_Ma3IAhiegUpmbpWnrKqyDQjgS4u-sh-TZ9D3mHQKlLk= 7MccnvpBUZGPEP8GFmZZO7TnXzotgbiLQwmWGKv1FI5k7Lc$=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) .