Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jun 05 2023 00:50:11 FOUS30 KWBC 050050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 848 PM EDT Sun Jun 04 2023 Day 1 Valid 01Z Mon Jun 05 2023 - 12Z Mon Jun 05 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN/CENTRAL ROCKIES, THE CENTRAL/SOUTHERN PLAINS, AND CENTRAL/SOUTHERN FLORIDA... ....01z Update... The main change this evening was to remove the Slight Risk area across portions of NM/TX, due to the bulk of the convection waning with any additional heavy downpours expected to be localized (with the atmosphere largely worked over and the latest HRRR runs depicting little additional activity overnight). With a lack of organized convection, a large Marginal Risk area remains across portions of the Northern/Central Rockies into the Central/Southern Plains, in addition to portions of the central/southern Florida (generally encompassing areas with the potential for localized 1-2+" amounts). The HRRR suggests that the bulk of the Marginal Risk areas will see the threat for heavy rainfall end in the next several hours (before 06z), but isolated convection has the best chance of persisting beyond 06z across the Southern High Plains and into much of TX. Please see the latest MPDs for more detailed information. Churchill ....Previous Discussion... ....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... The broad mid-level trough which has been a semi-permanent feature from the Intermountain West into the Southern Plains will continue to become more diffuse Monday as high pressure to the north and south expand, while an impressive closed low moves onshore CA. Beneath this trough, the environment will support another day of scattered thunderstorms as anomalous PWs of more than +1 standard deviation according to NAEFS lifts northwestward from the Gulf of Mexico. Forcing for ascent will again be somewhat modest and drive primarily by weak impulses in the flow overlapping elevated instability, although MUCAPE is progged to be generally just around 500 J/kg across the area Monday. With weak overall forcing in place, convection that does develop will likely be slow moving, and the HREF indicates any of these cells could produce short term 1"+/hr rates, supporting the broad MRGL risk atop soils that remain saturated from recent rainfall. Within this larger scale broad region with potential for excessive rain, there is a focused signal with some better potential across the High Plains of NM. Here, the HREF and SREF both indicate a better chance for more than 3" of rain, and this is atop already saturated soils and some more sensitive burn scars. This lowered FFG is resulting in HREF probabilities exceeding 20% for exceedance as slow moving storms potentially repeat across the area. The SLGT was pulled out of the TX panhandle where signals for heavy rain are much weaker despite the wet conditions experienced during the past week. ....Sierra Nevada... An anomalously strong closed low with 500mb heights falling to around -2 standard deviations according to the NAEFS will lift onshore the central CA coast late Monday night, spreading height falls and downstream divergence into the Great Basin. On the equatorial side of this low, a subtropical jet streak will intensify and shift into the Desert Southwest, placing favorable LFQ diffluence aloft to enhance deep layer ascent. This will help spread higher moisture onshore as well, with PWs progged to reach 0.75 to 1.00 inches, more than 2 standard deviations above the climo mean. This will overlap with MUCAPE that is progged to reach 1000 J/kg to create an environment that will support scattered to widespread showers and even isolated thunderstorms with rain rates progged by the HREF that may at time briefly touch 1"/hr. Although this area has been quite dry recently, USGS streamflow gauges indicate soils that are still extremely saturated, and thus any of these heavy rain producing cells could result in rapid runoff on Monday where locally 1-2" of rainfall is possible. ....Upper Midwest... A cold front dropping southward out of Canada will move through the Upper Midwest during Monday before stalling late in the day. This front will drive ascent through low-level convergence into a favorable thermodynamic environment characterized by PWs nearing 1.5 inches, +2 standard deviations, and a ribbon of 1000-2000 J/kg MUCAPE. Thunderstorms are likely to develop along this front, and then may train along the boundary during the evening due to parallel mean winds. This region has generally been dry during the past week with rainfall generally 50-75% of normal as seen via AHPS. However, USGS streamflow is still quite high near the ND/MN border with FFG just around 1"/3hrs. With rainfall rates of 1"/hr or more likely within stronger cores, any training along the front could produce 2-3" or more of rainfall in a short period of time as reflected by both HREF neighborhood and EAS probabilities. This rain could be sufficient to result in isolated instances of flash flooding. Weiss 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... ....Southern High Plains through the Northern Rockies... Little change across nearly the entirety of the High Plains as strong ridge over the Northern Plains and broad trough across the intermountain west persists. This will result in a continued meridional fetch of moisture from the Gulf of Mexico and Pacific funneling northward, producing an environment that remains favorable for scattered afternoon showers and thunderstorms. The trough responsible for this extended period of wet weather will become a bit more diffuse Tuesday as a strong low moves onshore the CA coast, but subtle negative height anomalies and the WAA will still drive sufficient instability overlapped with the moisture to produce storms capable of 1"/hr rates. This supports the broad MRGL risk from West Texas through the Northern Rockies. However, embedded within this area a small SLGT risk was adjusted cosmetically across northeast NM where recent heavy rain has lowered FFG and raised streamflow, suggesting an area that is more susceptible to runoff and flash flooding. This also includes some sensitive burn scars in the southern Sangre de Cristos. Considered expanding this SLGT risk down into the Sacramento range where model QPF is a bit higher, but the 24-hr probabilities for more than 1" of rain were more modest than points north. Coordinated this with WFO EPZ, and while a SLGT risk may be needed farther south later, the MRGL was preferred at this time. ....Northern Sierra/Northern California into portions of the Great Basin... The anomalous closed upper low which will move onshore CA D2 will generally rotate in place during D3, continuing to provide enhanced ascent throw downstream divergence and upper diffluence to the Sierra and portions of the northern Great Basin. With PWs likely continuing to be above normal, +1 to +2 standard deviations according to NAEFS, and MUCAPE around 1000 J/kg being resupplied from the east, this will support another day of scattered showers and thunderstorms. The coverage of convection Tuesday is forecast to be slightly greater than what occurs Monday, resulting in the expansion of the MRGL risk to cover more of northern CA, NV, and into southern ID/OR. Although the Sierra has been dry, 7-day rainfall across OR/NV/ID has been 100-300% of normal, so any slow moving showers or thunderstorms with heavy rainfall could produce runoff which may result in isolated instances of flash flooding. ....Gulf Coast... Weak flow off the Gulf of Mexico will surge PWs to over 1.5 inches, coincident with MUCAPE during peak heating progged to approach 3000 J/kg. This will provide a favorable environment for aftn thunderstorms beneath a broad mid-level trough within which weak impulses may drop southeastward. This will support storms firing during peak heating along residual boundaries and the sea breeze. These storms will likely move very slowly on just around 5 kts of mean flow, which in the presence of limited bulk shear will support chaotic or at times nearly stationary motions due to boundary interactions and storm mergers. The ECENS/GEFS probabilities indicate a low chance of 3" of rainfall, but this could be underdone by the larger scale ensembles. The environment will support 2+"/hr rain rates, which through slow storm motions atop soils that will be somewhat pre-conditioned from prior rain, could lead to isolated instances of flash flooding, especially in any more urban areas. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8o77sh_qbVPi7xwEH6Fsv_dK2rWw81RCOsI-A6UQSqE_= aK_Qh9sLLFiezLodDnaEybbzGHFWFjXM-FRMh9eNDHhYC2M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8o77sh_qbVPi7xwEH6Fsv_dK2rWw81RCOsI-A6UQSqE_= aK_Qh9sLLFiezLodDnaEybbzGHFWFjXM-FRMh9eNb3OUrto$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8o77sh_qbVPi7xwEH6Fsv_dK2rWw81RCOsI-A6UQSqE_= aK_Qh9sLLFiezLodDnaEybbzGHFWFjXM-FRMh9eNzrZHSI0$=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) .