Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jun 05 2023 15:53:53 FOUS30 KWBC 051553 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Mon Jun 05 2023 Day 1 Valid 16Z Mon Jun 05 2023 - 12Z Tue Jun 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS AND INTO THE FRONT RANGE OF THE ROCKIES... ....Southern Plains through the Northern Rockies... The blocked pattern which has changed very little for the past week will modify only slightly today as a ridge over the Northern Plains and intensifying closed low over CA help to squeeze the diffuse trough across the Intermountain West/Southern Plains. Another day of persistent moist advection within this axis will keep PWs above to well above normal, upon which weak impulses and upper level deformation will drive ascent. The challenge today primarily involves instability which has been forecast to be somewhat modest today, and current cloud cover is quite extensive across NM and into southern CO. Despite that, the guidance is more adamant that instability will surge above 750 J/kg and cloud cover will wane, so it is more likely that another day of scattered to widespread convection will develop, but with again a struggle for any specific focused area. Despite a general vague signal from the ensembles as to where convection may be more prevalent, there are two areas of most concern today. The first is around the inherited SLGT risk from the TX Panhandle into northeast NM. This region has been saturated from 72-hr rainfall according to MRMS which is as much as 5" in some locations, and 7-day rainfall nearing 600% of normal. This has severely compromised FFG to just 0.5-1.5"/3hrs, which the HREF indicates has a 20-30% chance of being exceeded by slow moving storms today. While the total coverage may be tempered by morning cloud cover, the antecedent soils are more than sufficient for the current SLGT risk. However, after coordination with WFO BOU, an extension of this SLGT was made northward into the CO Front Range and foothills, and into extreme southern WY, where recent rain has also been significant and slow moving storms with rain rates exceeding 1"/hr are again possible today. Anywhere these slow moving storms occur could result in runoff issues, but flash flooding may be most likely should a storm drift atop a burn scar or atop the most sensitive soils. The other region of maybe greater risk is along the Gulf Coast of Texas and Louisiana. While this region has been much drier recently, there is an enhanced signal in the ensembles for more than 3 inches of rain today, with potentially more than 5 inches in isolated locations. This will be driven by a very favorable atmosphere with 2000-3000 J/kg of MUCAPE and PWs around 1.75". This will fuel thunderstorms with rain rates exceeding 2"/hr, and storms are likely to be very slow movers on 0-6km mean winds of just around 5 kts with anti-parallel and collapsed Corfidi vectors. Considered a small SLGT risk for this area, but the high FFG and low exceedance probabilities suggest more of an isolated threat today, primarily across urban areas. ....Central California coast range into the Sierra Nevada... Anomalous closed low with 500mb heights falling below -2 standard deviations according to NAEFS will lift onshore the central CA coast today. This feature will spread ascent into CA and the Great Basin as upper diffluence, mid-level divergence, and subtle height falls overlap. At the same time, this will help spread moisture onshore with PW anomalies surging to around 1 inch, and instability being pulled westward noted by MUCAPE exceeding 500 J/kg. This will result in scattered to widespread showers and even isolated thunderstorms with rain rates 0.5"-1" per hour. Despite antecedent dry conditions, streamflows are still quite high, and this could also impact some recent burn scars. This continues to support the expanded MRGL risk from overnight for isolated runoff issues, and only some cosmetic adjustments were made to the inherited area. ....Upper Midwest... A cold front is still progged to drop out of Canada today and move into the Upper Midwest, providing a focus for afternoon thunderstorms. This cold front will drive convergent ascent into an environment with around 1000 J/kg of MUCAPE and PWs of 1.25-1.5", more than 2 standard deviations above the climo mean according to NAEFS, and approaching a daily record according to the SPC sounding climatology. Although the high-res appears to have backed off just slightly in coverage of thunderstorms today, weak storm motions of just 5-10 kts parallel to the front suggest the possibility of some training, with rain rates likely reaching 1"/hr or more as progged by the HREF probabilities. Despite antecedent dry conditions with generally around 50-75% of rainfall the past 2 weeks, USGS streamflow anomalies are still high so FFG is modest. Where any of these slow moving storms train, locally 3" of rainfall could occur, which could result in isolated instances of flash flooding. The inherited MRGL is warranted for this potential, and was adjusted cosmetically for new guidance. Weiss Day 2 Valid 12Z Tue Jun 06 2023 - 12Z Wed Jun 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS... ....Southern High Plains through the Northern Rockies... The stagnant, blocked mid to upper level pattern across large portions of the nation day 1 will persist into the day 2 period.=20 This will keep the Excessive Rainfall Outlook very similar to the day 1 outlook, especially through the Southern to Central High Plains, Central to Northern Rockies into portions of the Northern High Plains. There will once again be an axis of above average PW values with embedded mid to upper level vorts/enhanced upper difluence across these areas. This will support another day of widespread scattered convection with locally heavy rainfall amounts. There continues to be some model signal for organized heavy rains over portions of the Southern High Plains that have seen areas of heavy rains over the past several days. This will continue to support a slight risk area where higher stream flow and soil moisture will increase the risk of additional flash flooding. ....Portions of the Northern Plains... A surface frontal boundary expected to remain stationary day 2 in the vicinity of eastern ND and western MN. An axis of anomalous PW values 2.5"+ standard deviations above the mean and 1000-1500 j/kg MUCAPE in the vicinity of this front will support additional locally heavy rainfall amounts. With stream flows remaining high along the ND/MN border area and potential for some overlap of day 1 heavy rains with day 2 heavy rains, a marginal risk area was added from the previous for this period. This again agrees well with the latest CSU first guess field for the day 2 period which show a small marginal risk along the ND/MN border area. ....Central California coast/Northern Sierra/Northern California into portions of the Great Basin... The strong closed mid to upper low along the Southern CA coast at the end of day 1 will move only slowly inland day 2. This will maintain a very upper difluent pattern to the north and northeast of the closed low from the Central CA coast, northeast through the northern Sierra/northern CA and into the Great Basin. Similar to the day 1 period, the previous marginal risk area was drawn farther to the southwest to the central CA coast region where models are showing some convective potential on the north side of the closed low closer to the central CA coast in a continued axis of much above average PW values, 2+ standard deviations above the mean. Widespread scattered convection also likely farther northeastward across the northern Sierra/northern CA into portions of the Great Basin in the above mentioned region of well defined upper difluence. Similar to the day 1 period, with stream flows remaining above average across the marginal risk area, isolated flash flooding is possible and also a risk for burn scar areas from the past few years. Oravec Day 3 Valid 12Z Wed Jun 07 2023 - 12Z Thu Jun 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS... ....Southern High Plains through the Northern Rockies... With little forecast change to the large scale blocking pattern across the nation, the Excessive Rainfall Outlook will be very similar to the day 2 outlook. Widespread scattered convection is again likely stretching from the Southern to Central High Plains, northward through the Central to Northern Rockies into the Northern High Plains in what will continue to be an axis of above average PW values. There is a model signal for increasing PW anomalies day 3 over the Central to Northern Rockies region where upper difluence also strengthens day 3 ahead of the upstream closed low moving slowly from Southern CA into the Great Basin.=20 There is an increase in model consensus precip across the Central to Northern Rockies which may eventually lead to a greater risk area denoted, but for the time being, the risk is maintained at marginal. Farther to the south, a slight risk was introduced for the same general region of the Southern High Plains as for the day 1 and 2 periods. Although there is again the typical amount of model qpf detail spread for a day 3 forecast, there continues to be a model signal for another round of potentially heavy rains to affect portions of the Southern High Plains from northeast NM into the TX/OK Panhandle/northwest TX region. Given the recent pattern of daily heavy rains, a slight risk is warranted for the potential for additional day 3 rains across the Southern High Plains. ....Northern Sierra/Northern California into portions of the Great Basin... The strong closed low over Southern CA day 2 will continue to move only slowly northeast into the Great Basin day 3. This will again maintain a very upper difluent pattern to its northeast across the northern Sierra, northern CA into the Great Basin. Maintained a marginal risk area across these regions for isolated flash flooding in the persistent axis of much above average PW values, 1 to 2+ standard deviations above the mean. ....Coastal Southeast Florida... Added a marginal risk area for the urban corridor along the Southeast Florida from Miami north to approximately Vero Beach.=20 There is an uptick in model qpf day 3 compared to day 2 as shortwave energy pushes east northeastward from eastern Gulf across South Florida. Enhanced upper difluence forecast to push across South Florida where PW values rise to 1.75-2". With models showing potential for heavy rains, can not rule out isolated urban runoff issues. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_tWi8gfLqZypXbiZF_2szUz6xrouBO0M9w3uiAORPa4y= pIPp_7PAtaBsYsCaYCBkVlm87HrmPX6fIR5hSPX8MJg6Jyw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_tWi8gfLqZypXbiZF_2szUz6xrouBO0M9w3uiAORPa4y= pIPp_7PAtaBsYsCaYCBkVlm87HrmPX6fIR5hSPX8qyyfeT0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_tWi8gfLqZypXbiZF_2szUz6xrouBO0M9w3uiAORPa4y= pIPp_7PAtaBsYsCaYCBkVlm87HrmPX6fIR5hSPX8geXd46o$=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) .