Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jun 04 2023 15:48:33 FOUS30 KWBC 041548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1147 AM EDT Sun Jun 04 2023 Day 1 Valid 16Z Sun Jun 04 2023 - 12Z Mon Jun 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS... ....Southern Plains through the Northern Rockies... Diffuse mid-level trough oriented NW to SE from the Northern Rockies through the Southern Plains will become more strung out and weaker today as expansive mid-level high over the Northern Plains settles to the SW. At the same time, a ridge over Mexico will begin to arc northeast, gradually raising heights across the area. In between, this trough will continue to serve as a focus for afternoon thunderstorms thanks to steeper lapse rates, weak impulses moving through the flow, and a collocated axis of deformation aloft. The thermodynamic environment beneath this ascent will remain favorable for convection with heavy rain, with PWs approaching +2 standard deviations according to the NAEFS ensemble tables, and MUCAPE of 500-1000 J/kg, higher across TX. The combination of deep layer ascent and the favorable thermodynamics should once again result in scattered showers and thunderstorms, although coverage today may be somewhat less intense and widespread than previous days. The high-res is struggling to pinpoint any areas of more focused activity as reflected by simulated reflectivity and 1-hr rainfall probabilities, which suggests the inherited MRGL risk remains supported. Despite that, any heavy rain producing storm, which could have rates above 1"/hr, has the potential to produce isolated runoff/flash flood instances as soils are saturated from 7-day rainfall of generally more than 300% of normal leading to generally reduced FFG. The inherited SLGT risk for the TX Panhandle is driven predominantly by antecedent soil conditions which have been inundated with heavy rain recently. This area has a slightly higher risk for flash flooding as even brief heavy rain could overwhelm the soils, so despite the guidance not showing a clear signal for excessive rain here the SLGT risk is maintained. It was adjusted slightly northwest into NM however to account for some higher rainfall probabilities and to cover some more sensitive burn scars. Considered a small SLGT risk for areas of CO near the Front Range which have also been saturated and have high USGS streamflows currently, but a lack of instability should limit the rain rates despite a model signal for some more significant rainfall accumulations there. ....Florida... Another active day of slow moving convection is expected across the southern Florida peninsula. The 12Z/4 morning sounding at KMFL measured a PW of 1.94 inches, right at the 90th percentile according to the SPC sounding climatology, with deep mid-level moist adiabatic lapse rates and a freezing level nearing 15,000 ft. This is indicative of an environment supporting efficient warm rain processes today, partially thanks to the remnants of Arlene moving across the Straits. Even as SBCAPE climbs above 2000 J/kg this aftn, the CAPE profile should remain "tall and skinny." This will allow for continued efficient collision/coalescence to support rain rates which the HREF indicates could exceed 2"/hr as storms move slowly on 0-6km mean winds of just 5-10 kts. Forcing across the area will be driven by low-level convergence along a weakening front, weak impulses moving within the elongated mid-level trough, and modest upper diffluence as the tail of a weak streak pivots across the peninsula. Together this overlap of forcing and moisture will support scattered or even widespread aftn/eve thunderstorms, with some subtle organization possible on 25 kts of bulk shear. Much of the southern half of Florida has received 200-300% of normal rainfall the past week, and some areas may exceed 3" today. This could result in rapid runoff and isolated flash flooding, most likely across any urban areas or should a slow moving storm impact the most saturated antecedent soils. Weiss Day 2 Valid 12Z Mon Jun 05 2023 - 12Z Tue Jun 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS... ....Southern Plains through the Northern Rockies... With little in the way any significant changes to the overall large scale flow across the central portion of the nation day 2, the day 2 Excessive Rainfall Outlook is very similar to the day 1 outlook. PW values are forecast to remain above average from the Southern to Central Plains through the Central to Northern Rockies. Similar to the day 1 period, shortwave energy embedded in this high PW axis will support another day of widespread scattered convection across these areas. A slight risk area was continued from northeast New Mexico into the Texas/Oklahoma Panhandle region. There continues to be somewhat better agreement that this area will see a better chance of organized heavy precip than other areas in the high PW axis. As stated in the day 1 discussion, isolated flash flooding is possible across the entire area given recent rainfall amounts that have been as much as 400-600% of normal.=20 ....Sierra Nevada... One of the biggest changes to the large scale flow across the U.S. during the day 2 period will be the closed mid to upper level low approaching the central to southern California coastal region.=20 Increasing upper difluence Monday afternoon to the north and northeast of this closed low will support scattered convection through the Sierra, northern California and portions of the Northern Great Basin. With PW anomalies rising to 1 to 2+ standard deviations above the mean in this upper difluent region, locally heavy rainfall amounts of up to an inch possible. With stream flows remaining much above average through the Sierra, isolated flash flooding is possible Monday afternoon into Monday evening. Oravec Day 3 Valid 12Z Tue Jun 06 2023 - 12Z Wed Jun 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS NORTHEASTERN NEW MEXICO... The story remains the same for the upcoming day 3 period with little changes to the overall stagnant blocked pattern across the nation. Another day of widespread scattered convection likely from the Southern to Central High Plains, northward through the Central to Northern Rockies into the Northern High Plains. There is model consensus for a westward shift in the axis of heaviest rains over the Southern High Plains day 3. This will remove portions of central Texas from the marginal risk area. A slight risk area was maintained over northeast New Mexico where model consensus is for a max precip area. This is a region where upper difluence strengthens day 3 well to the east of the upstream coastal California mid to upper level low. This will bring three days of potential heavy rains over this region, warranting the continuation of a slight risk. Similar to the day 1 and 2 periods, given recent heavy rains, 400-600% of normal through portions of this area, isolated flash flooding will again be possible from the additional day 3 heavy rain potential across the remainder of the broad marginal risk area extending into the Central to Northern Rockies and Northern High Plains. There is also a model signal for a precip max along the western Gulf coast from the middle to upper Texas coast into southwest Louisiana.=20 With recent rains not as anomalous across this area as other areas in the Southern Plains, the threat was kept at marginal for the time being. ....Northern Sierra/Northern California into portions of the Great Basin... The closed low that approached the central to southern California coast day 2 is not expected to move much day 3. This will keep a favorable upper difluent pattern in place to the north and northeast of this system and support another day of scattered convection. Consensus is for a broader region of upper difluence compared to previous days, resulting in a broader area of potentially locally heavy rains in what will again be an axis of above average PW values stretching across Northern California into the Great Basin. This is reflected in a larger marginal risk area than on day 2 extending into northern California, southern Oregon, northern Nevada and far southwest Idaho. Stream flows are above average across much of the marginal risk region with isolated flash flooding possible. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!612B6YMfCkDRRUa3DDfF1oSCpn_cdjP0qazXrMcKtukg= 3-v_PQEx88IBA_A-06ufcTVvROwGBcRVBs9K2xLBxKhlgSY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!612B6YMfCkDRRUa3DDfF1oSCpn_cdjP0qazXrMcKtukg= 3-v_PQEx88IBA_A-06ufcTVvROwGBcRVBs9K2xLBE2QnWmk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!612B6YMfCkDRRUa3DDfF1oSCpn_cdjP0qazXrMcKtukg= 3-v_PQEx88IBA_A-06ufcTVvROwGBcRVBs9K2xLBxFB7MWw$=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) .