Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 22 2023 15:58:28 FOUS30 KWBC 221558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Tue Aug 22 2023 Day 1 Valid 12Z Tue Aug 22 2023 - 12Z Wed Aug 23 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE WESTERN U.S. AND SOUTH TEXAS... ....Northwest/Northern Intermountain West/Northern Rockies... Energy associated with a weakening upper low moving into northern California is expected to lift northeast through the northern Intermountain west -- directed by a compact upper low moving into western Washington. This will help to bring additional moisture across the region, fueling another day of showers and storms across the region. The NAM and GFS both show PW anomalies remaining +2 to 3 standard deviations above normal today before the approaching low pushes the deeper moisture farther east by the evening/overnight. In addition to impacting the areas affected by heavier rains from Hilary, this system is expected to bring some more substantial rains farther to the northwest across eastern Washington and northern Idaho. Not much change from the previous outlook, with a Slight Risk extending northward and northeastward from northeastern Oregon and southwestern Idaho, with the highlighted areas in the south more susceptible to flooding given the recent heavy rains. ....Northern Arizona to western Wyoming... An axis of deeper moisture is expected to remain, with increasing mid level flow ahead of the remnant low moving into the Intermountain West helping to draw some deeper moisture north across Utah into western Wyoming. This moisture is likely to support additional rounds of showers and storms, and may create more flash flooding concerns, especially for the complex terrain areas in southern Utah. ....South Texas... Recently upgraded to a tropical storm, Harold is forecast to move from the western Gulf of Mexico into South Texas later today. The 00Z sounding from BRO recorded a PW of 2.25 inches, with recent SPC mesoanalysis indicating PWs above 2.5 inches closer to the storm's center. With its copious moisture, this system is expected to quickly produce 3-5 inches, with locally higher amounts, across South Texas today. Given the dry antecedent conditions, these rains will likely be beneficial for many areas, but excessive for some given their intensity. Therefore, a Slight Risk was maintained across the region. This is forecast to be a progressive storm, with the latest NHC track showing the center moving into northwest Mexico this evening and approaching the Big Bend by tomorrow morning. The Marginal Risk was extended into the Big Bend region. While the heaviest amounts are expected to remain to the south over Mexico through 12Z Wednesday, some locally heavy amounts may begin to spread into this region, which may start to create flooding concerns, especially for areas of more complex terrain. ....Upper Great Lakes... A slow-moving warm front and the reintensification of the low level jet are expected to support another round of elevated storms late Tuesday into early Wednesday. Storms that develop are expected to focus from northern Wisconsin and the U.P. into Lower Michigan. PWs are expected to climb to 1.5-2 inches on the nose of 40+ kt westerly inflow. As with the previous storms, models do not suggest widespread heavy amounts, however training/redeveloping storms may produce an axis or two of locally heavy amounts. Pereira Day 2 Valid 12Z Wed Aug 23 2023 - 12Z Thu Aug 24 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR THE PORTIONS OF SOUTHWEST AND THE GREAT LAKES/UPPER OHIO VALLEY... ....Big Bend Region into the Southwest... Models continue to show a fairly well-defined mid to upper level shortwave associated with the remnants of Tropical Storm Harold moving around the southwestern periphery of the central U.S. high, lifting north from northwestern Mexico into the southwestern U.S. on Wednesday. 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 early in the period, producing locally heavy amounts and isolated 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.=20 ....Great Lakes to the upper Ohio Valley... A warm front and the threat for overnight convection is expected to shift farther east, with areas from southeast lower Michigan to western New York and Pennsylvania most likely to be impacted late Wednesday into early Thursday. PWs are expected climb a little higher with this next round, with the general model consensus showing 1.75 to 2 inches along the leading edge of the low level jet. There is still plenty of uncertainly regarding how the resulting cloud cover from the previous night's convection may impact later convective development, and where and if heavy rains may develop late Wednesday into early Thursday. Therefore, the Marginal Risk was maintained for now. However, an upgrade may be required if the models begin to show increasing agreement for heavier amounts, especially across areas with lower FFGs within the Marginal Risk area. Pereira Day 3 Valid 12Z Thu Aug 24 2023 - 12Z Fri Aug 25 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF SOUTHERN UTAH.. ....Southwestern U.S.... The mid-level remnants of Tropical Storm Harold and monsoonal moisture spreading north 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, a Slight Risk was maintained across portions of southern Utah. ....Lower Great Lakes and upper Ohio Valley to the Northeast... Models show plenty of potential, but also uncertainty, for heavy rainfall across the region. Showers and storms developing Wednesday night may remain ongoing into the early part of the period across New York/Pennsylvania before redeveloping farther to the east ahead of a wave moving through the eastern Great Lakes on Thursday. In its wake, a well-defined cold front will begin to drop south from the Great Lakes, with deep moisture pooling ahead of the front. While models are far from agreement, some do show a notable signal for heavy amounts in the vicinity of the eastern Great Lakes-upper Ohio Valley late Thursday into early Friday.=20 These amounts will be fed by the deep moisture and high instability ahead of the front and the potential for training/redeveloping storms. Similar to Day 2, model spread does not provide confidence for anything beyond a Marginal Risk at this point. However, should the models start to show better agreement, an embedded upgrade may be required in future issuances. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5Mu5vTu4JZvyVy4wLU73anMfF6h-7EvInpO_9q-xGQof= Hmcb5HmS6RXCs2PHxrttQhjOuQ0jXSvWBY7lLPQlggKGdE0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5Mu5vTu4JZvyVy4wLU73anMfF6h-7EvInpO_9q-xGQof= Hmcb5HmS6RXCs2PHxrttQhjOuQ0jXSvWBY7lLPQlErjFcX0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5Mu5vTu4JZvyVy4wLU73anMfF6h-7EvInpO_9q-xGQof= Hmcb5HmS6RXCs2PHxrttQhjOuQ0jXSvWBY7lLPQlVMonoYU$=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. 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