Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 24 2023 00:25:57 FOUS30 KWBC 240025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 825 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... 01Z Update... Shifted Slight Risk west through south-central Ohio. Expanded Marginal Risk in AZ/UT/NM. ....Eastern Michigan through the upper Ohio Valley... Heavy and repeating activity setting up over Lake Erie/northern Ohio in NWly upper flow and Wly low level flow which is aiding southward propagation of elevated activity that is currently developing over southern Lake Huron and further strengthening over Lake Erie. 21Z/22Z HRRRs (unlike all other HREF CAMs today) have a decent handle on this scenario with activity tracking south along the strong instability gradient down central Ohio overnight. Wly flow continues to increasing moisture with PWs of 2"+ spreading south in advance of the activity through central OH per recent RAP runs. Given the favored setup for training/backbuilding convection, localized 3-5"+ totals are possible which would lead to scattered instances of flash flooding. This warrants shifting the Slight Risk west to be mainly Ohio areas south of Lake Erie to near the Ohio River. Activity may stick to strengthening over the Lakes, but far southeast MI has low enough FFG to warrant including those western shores in the Slight as well. For further info please see MPD 967 and any further followup MPDs for further info overnight. ....Far West Texas Through Utah... The upper level wave associated with the remnants of tropical cyclone Harold is over southwest NM this evening and will continue shifting northwest overnight. Inflow over West Texas and central NM warrants maintenance of the Slight Risk area there with ample moisture, instability, and upper level dynamics warranting expansion of the Marginal Risk over much of AZ, UT, and northwest NM. Please see MPD 966 and any further followup MPDs for further into overnight. Jackson 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 Valid 12Z Fri Aug 25 2023 - 12Z Sat Aug 26 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS THE CENTRAL HIGH PLAINS/ROCKIES.. ....Southwest to the central High Plains.... In addition to the more typical monsoonal showers and thunderstorms, the mid-level remnants of tropical cyclone Harold will continue to spark activity as it moves northeastward along the western periphery of the central U.S. ridge. Organized heavy rainfall is expected to become increasingly likely as the system moves east across southern Wyoming and northern Colorado late Friday into early Saturday. Moisture associated with Harold, along with strengthening low level easterly flow will support deepening moisture over northeastern Colorado, southeastern Wyoming, and the Nebraska Panhandle. The general model consensus shows PW anomalies of 2 to 4 standard deviations above normal covering the region on Friday. Overall the 12Z guidance followed the previous cycle fairly well, still showing good agreement for 1-2" with locally higher amounts centered over this region. The heavier totals are still expected along the upslope region of northeastern Colorado into southeastern Wyoming, along and near the Foothills and Front Range. ....Mid Atlantic... Model guidance seems to be coming together on the idea that a cold front slipping southeastward through the Northeast and Mid-Atlantic will combine with the anomalously high moisture in place to spark additional showers and thunderstorms, some locally heavy, across portions of the coastal areas in the Northeast and Mid-Atlantic. Some timing differences in the frontal passage and also available instability does lower confidence from NY through New England. Further south, the frontal passage timing during the peak heating and a greater axis of higher instability should spark deeper convection across portions of the Mid-Atlantic through northeastern North Carolina. However, some capping may limit the areal extent of the convection but if storms were to develop, locally heavy rain rates would be likely. The scenario for Friday still remains more uncertain but potential exists for localized flooding, particularly over urbanized areas. Pereira/Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_hF-mfdhO9dGYiIwtg6WL0jClr46xwpHdW8zHLa3uQkT= FaVihHbhcfqltW41ebZJGKtt1iBcP8yZvZ9mnziExYihods$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_hF-mfdhO9dGYiIwtg6WL0jClr46xwpHdW8zHLa3uQkT= FaVihHbhcfqltW41ebZJGKtt1iBcP8yZvZ9mnziEc4ViuK8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_hF-mfdhO9dGYiIwtg6WL0jClr46xwpHdW8zHLa3uQkT= FaVihHbhcfqltW41ebZJGKtt1iBcP8yZvZ9mnziEt2UB5Ns$=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) .