Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 23 2023 20:15:22 FOUS30 KWBC 232015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Wed Aug 23 2023 Day 1 Valid 16Z Wed Aug 23 2023 - 12Z Thu Aug 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST AND THE GREAT LAKES/UPPER OHIO VALLEY... 16Z Update: Across the Texas Big Bend and the Southwest, few changes were needed to the ERO risk areas as rainfall associated with Harold continues to move westward. Some trimming of the back edge of the Marginal and Slight Risk areas were done to match the latest radar trends and based on the 12Z model guidance. The moisture and associated mid/upper level energy will begin to move into the Southwest later today through tonight. With ample moisture and expected development of instability, localized 0.75-1" hourly totals will be possible across portions of southern New Mexico through portions of central/northern Arizona and perhaps into southern Utah. The Slight Risk covers the greater threat for total amounts exceeding 1-1.5" and is tied to the latest HREF probabilities. For the Great Lakes and upper Ohio Valley, a more challenging and uncertain forecast with both ongoing convection this morning, mainly elevated across eastern Michigan now moving into northeast Ohio and far northwest Pennsylvania and another round later tonight. The current activity should continue to slide southeast through the afternoon with some gradual weakening over the next several hours. Then later tonight into early Thursday morning, another round of elevated training convection is likely with the surging low level jet expected. 12Z model guidance still has spatial differences (mainly west/east oriented) but consensus on eastern/northeastern Ohio and northwest Pennsylvania seems to be favored and this is backed by the 12Z HREF probabilities for hourly totals 2"+ beginning to ramp up after 04Z this evening. Given the favored setup for training/backbuilding convection, localized 3-5"+ totals will be possible and may lead to scattered instances of flash flooding. Taylor ---previous discussion--- ....Texas Big Bend Region into the Southwest... The mid to upper level shortwave associated with the remnants of tropical cyclone Harold will continue to move around the southwestern periphery of the central U.S. high, lifting north from northwestern Mexico into the southwestern U.S. today.=20 Showers and storms fed by an axis of deep southeasterly flow and moisture will likely be ongoing across portions of the Big Bend and West Texas this morning, producing locally heavy amounts and isolated to scattered flash flooding concerns. As the system continues farther north, its associated moisture and the more typical monsoonal moisture are expected to increase shower and thunderstorm coverage and a greater threat for isolated flash flooding across portions of the Southwest from southern New Mexico to southern Utah. A Slight Risk was maintained from West Texas into southern New Mexico. PWs at or above 1.75 inches (1 to 2 standard deviations above normal) in addition to enhanced forcing east of the center are expected to support heavy rain and flash flooding translating northwest across this area today.=20 ....Great Lakes to the upper Ohio Valley... Ongoing elevated convection over the northern Great Lakes is expected to continue and may produce some locally heavy amounts as it drops southeast into the eastern Great Lakes and upper Ohio Valley early in the period. However, the greater threat for heavy rain and flash flooding is forecast to develop late today into the overnight hours as the low level jet intensifies and storms ignite once again east of a slow-moving warm front. Storms that develop will have ample moisture at their disposal, with models showing PWs of 1.75-2.25 inches pooling on the nose of 40+ kt westerly inflow. While there are still plenty of differences in the details, at least some of the guidance, including the 00Z HRRR and ARWs, indicated a period of northwest to southeast training storms along the warm front from southeast Michigan to northeast Ohio and northwest Pennsylvania before a more organized system develops and moves across the region. While uncertainty remains, felt there was enough of a growing consensus to upgrade a portion of the area to a Slight Risk. The Slight Risk generally highlights the area where the HREF shows higher probabilities for rainfall amounts exceeding 2 inches. The lower resolution NAM, GFS, and ECMWF also indicate heavy amounts developing across this region as well. Pereira Day 2 Valid 12Z Thu Aug 24 2023 - 12Z Fri Aug 25 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE SOUTHWEST AND NORTHEAST.. ....Southwestern U.S.... The mid-level remnants of Harold and monsoonal moisture spreading north along of the west side of the central U.S. ridge will support increasing shower and thunderstorm coverage across the Four Corners states and southern Nevada on Thursday, with PW anomalies of 1 to 3 standard deviations above normal forecast within much of the highlighted Marginal Risk area. Given the complex terrain and the center of some of the greater PW anomalies (2 to 3 standard deviations above normal), a Slight Risk remains in place for portions of southern Utah and now extended across portions of western Colorado. Across western Colorado, guidance suggests a ribbon of locally higher instability (MLCAPE > 1000 J/kg) may develop on the gradient of the higher PW axis, supporting more intense and perhaps more widespread precipitation capable of producing rain rates in excess of 1"/hr. ....Lower Great Lakes and upper Ohio Valley to the Northeast... A very unsettled and active pattern will continue across the Great Lakes, upper Ohio Valley, and portions of the Northeast again on Thursday and Thursday night. Convection is likely to be ongoing at the start of the period Thursday morning, supported by the nocturnal low-level jet, with the most recent guidance suggesting northeast Ohio, northwest PA, and perhaps southwest NY most likely area of seeing more intense rainfall rates and totals. Then later in the period, mainly afternoon/evening through early Friday morning, continued unsettled weather and development of additional thunderstorms is expected. PW anomalies ramp up considerably and synoptic setup favors some training/backbuilding thunderstorms. If enough instability can develop, more intense rain rates will be possible. Based on the 12Z deterministic guidance and the 12Z HREF neighborhood probabilities and EAS probabilities, a Slight Risk was introduced across portions of Pennsylvania and extreme southern tier of NY where 2-3" amounts (locally higher) are possible. Some overlap with convection tonight (Day 1 period) could bring 2-day totals above 5-6" locally, particularly over northeastern Ohio, northwestern to north-central PA and extreme southwest to southern tier of NY. =20 Taylor Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!415buQx8rvmJFymJEmJhu6VEdFOb6XSRN8cFL1D2zB3Y= ZdWNmHKskssetcq64f8iWwJlI1U8yk8qGFzhSD0Bix3fme8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!415buQx8rvmJFymJEmJhu6VEdFOb6XSRN8cFL1D2zB3Y= ZdWNmHKskssetcq64f8iWwJlI1U8yk8qGFzhSD0B2WPdjt8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!415buQx8rvmJFymJEmJhu6VEdFOb6XSRN8cFL1D2zB3Y= ZdWNmHKskssetcq64f8iWwJlI1U8yk8qGFzhSD0BqG7fo3o$=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) .