Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jun 05 2023 20:11:28 FOUS30 KWBC 052011 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 410 PM 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 FROM THE FRONT RANGE OF THE ROCKIES THROUGH THE SOUTHERN HIGH PLAINS... ....Southern High Plains through the Northern Rockies... Once again, the High Plains will be sandwiched between a ridge of high pressure to the northeast, and a strengthening area of low pressure across the Pacific Coast. This will continue to draw moisture and instability northward beneath a diffuse mid-level trough, with the favorable thermodynamics for heavy rain continuing across a large area. PWs are forecast to reach +1 to +2 standard deviations above the climo mean from west TX through MT, overlapping MUCAPE of 500-1500 J/kg, locally higher. With weak impulses lifting northward within the flow, any place could experience diurnal convection once again Tuesday, with storm motions generally around 10-15 kts in the overall weakly forced environment. Since much of this region has experienced 7-day rainfall that is 200-400% of normal, FFG is low (less than 1.5"/3hrs in many areas) and streamflows are already high according to USGS. This suggest that any slow moving cell with rain rates of 1+"/hr could produce runoff and flash flooding. While the signal for organized activity is modest on D2, there does appear to be a need for an expansion of the SLGT risk inherited for northeast NM and the TX Panhandle. From CO southward through west TX, there is a good model consensus for higher 0-6km bulk shear reaching 20-30 kts to help organize storms, resulting in potentially higher rain rates of longer duration. This is supported by an axis of modest 24-hr probabilities for more than 3" of rain from southern CO through the El Paso area of TX. Additionally, some of this same area will likely experience heavy rain on Monday, making the soils even more susceptible to flash flood concerns. After coordination with the area WFOs, the SLGT risk was expanded northward along the Front Range of the Rockies and southward to the Sacramento Mountains to account for this greater potential, enhanced even moreso the sensitive burn scars across the area. Note as well the CSU first-guess field indicates the potential need for a small SLGT risk across southern MT where the HREF probabilities also suggest a narrow corridor of higher 3"/24hr and 3-hr FFG exceedance probabilities. The simulated reflectivity for that area does develop a more organized cluster of storms, and heavy rain rates are likely. However, this is a pretty small geographic area that would likely need to fall directly on top of the 72-hr MRMS footprint from heavy rain to more than an isolated threat. For that reason have opted to maintain just a MRGL D2, but note this could require a future increase to SLGT if the signal persists or becomes more impressive. ....Upper Midwest and Northern Plains... The cold front from Monday is expected to stall and become nearly stationary draped north to south across ND/MN as it encounters an expansive ridge to the west to block its progression. This front will remain in a region of favorable thermodynamics for heavy rainfall, with MUCAPE forecast to reach 1000-2000 J/kg coincident with PWs exceeding 1.5" in a narrow corridor, 2-3 standard deviations above the climo mean according to NAEFS. This will likely manifest as scattered storms as reflected by high-res simulated reflectivity, tracking slowly along the front to support short term training. With rain rates possibly exceeding 1"/hr at times, any training could produce locally 2-3" of rainfall as reflected by the HREF neighborhood and EAS probabilities. This rain falling atop some more saturated soils from additional rain on D1 could result in isolated flash flooding. ....Central California through the Northern Great Basin... The anomalously strong closed low moving onshore CA D1 will move little during 2, basically spinning in place through the period. This will allow for persistent synoptic ascent downstream, with upper diffluence, mid-level divergence, and some upslope flow driving ascent. At the same time, instability and moisture will remain favorable with PWs at +2 standard deviations, and MUCAPE above the 90th percentile according to the SPC sounding climatology. The overlap of this forcing and thermodynamics is likely to drive another day of scattered to widespread showers and even isolated thunderstorms, with the HREF indicating a better chance of 0.75-1"/hr rates than what occurs on Monday. Additionally, with rainfall expected on D1, this will further increase the pre-conditioning of the soils so even where recent rainfall has been modest, the chance for runoff will begin to increase. With a signal from the ensembles for 1-3" of rainfall expanding into OR/ID where recent rainfall has been more substantial, the MRGL risk was expanded just slightly north and east from inherited where any heavy rain rates could quickly result in runoff, with isolated flash flooding also possible, especially over recent sensitive burn scars. ....Gulf Coast... Continued extreme thermodynamics with MUCAPE above 2000 J/kg and PWs above 1.75 inches in an otherwise weakly forced environment will support another day of slow moving, heavy rain producing, convection. Synoptic ascent will be generally modest with a weak jet streak positioned to the south, but subtle mid-level impulses, possibly residual vorticity from D1 convection, could re-ignite thunderstorms during the aftn. This will likely be aided by sea breeze convergence and storm interactions as storm motions fall to just around 5 kts in the pulse environment. Although the signal for heavy rainfall is a little less robust here than on D1, rainfall rates of 2"/hr in these slow moving storms could still locally produce more than 3" of rain as shown by HREF neighborhood probabilities. Where these slow moving storms are most intense, especially if they fall atop an urban area or overlap with the heaviest rain footprint from Monday, isolated flash flooding could occur. Weiss Day 3 Valid 12Z Wed Jun 07 2023 - 12Z Thu Jun 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE FRONT RANGE OF THE ROCKIES THROUGH THE SOUTHERN HIGH PLAINS AND FOR PARTS OF MONTANA... ....Southern High Plains through the Northern Rockies... The large scale pattern will remain generally unchanged on Wednesday, but subtle changes occurring late will begin to enhance the excessive rain threat once again. The large ridge across the Plains will slowly weaken D3 in response to anomalously deep troughs spreading across the Northeast and California. The latter of these will begin to drive height falls into the Intermountain West late in the period, while shortwaves embedded within the persistent weak trough axis from earlier in the week continue to lift northwestward. As the region gets squeezed between these features, moisture will become more impressive, noted by PW anomalies on the NAEFS reaching +3 sigma, while instability climbs to around 1000 J/kg, and as high as 2000 J/kg in the Southern High Plains, with another pocket of higher MUCAPE progged across MT. It is these two areas of higher instability that could support more organized or more intense convection on Wednesday. From the Front Range of the Rockies southward through the Sacramento Mountains, and extending eastward into the High Plains, the GEFS/ECENS/SREF probabilities all indicate an area of better heavy rainfall potential, likely due to convergence along a surface trough beneath concentrated mid-level energy. This will occur atop soils that are fully saturated from recent rainfall noted by high percentiles in the USGS streamflow gauges, and FFG generally 1.5"/3hrs or less. The inherited SLGT risk was expanded to cover this area to account for the better ensemble signal across these saturated soils, including recent burn scars, which could support more scattered instances of flash flooding. A second SLGT risk was added for south-central MT after coordination with WFO BYZ. Although the signal for heavy rainfall here increases just beyond D3, there is good model consensus for an axis of heavy rain as a shortwave drifts northward into anomalous thermodynamics. While the spatial coverage of heavy rain may be somewhat limited D3, the ensemble probabilities overlap the recent heavy-rain footprint noted by 72-hr MRMS rainfall, which could result in a higher flash flood risk. ....Northern Sierra/Northern California into portions of the Great Basin... The strong closed low across CA on D2 will begin to slowly fill on Wednesday, but again move very little through the D3 forecast period. This will produce a continued favorable environment for showers and isolated thunderstorms as mid-level divergence and upper diffluence overlap within an environment characterized by 0.75-1" PWs and anomalous MUCAPE above 1000 J/kg. Rainfall rates within this convection will likely again eclipse 0.5-1"/hr, and although the best environment may shift slightly northward on Wednesday, the overall setup and position of the excessive rain outlooks is mostly consistent with previous days. This suggests that any place the heavier rain rates can impact will be susceptible to runoff as antecedent soil conditions continue to moisten. ....Coastal Southeast Florida... The inherited MRGL risk area was maintained for primarily the Gold Coast of Florida. More favorable exit region jet-level diffluence will overlap a weak shortwave moving northeast across the peninsula through increasingly confluent 500mb flow. This enhanced deep layer ascent should result in increasing coverage of diurnal convection, with rain rates above 1"/hr possibly focusing along the Atlantic coast where the mean SW flow interacts with the westward advancing sea breeze to slow storm motions. This area has been saturated as of late, but the greatest risk will be across the urban corridor where any slow moving storm could result in considerable runoff and isolated flash flooding. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5DIsDS18ldwQ5nvAxmAT_u__jGJ43ZVCVW4dWw2X2Mi9= jfV8S0yZ8QJSgmFsjKsPaEHDR93fByTFdp6UX0L15i-tAfM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5DIsDS18ldwQ5nvAxmAT_u__jGJ43ZVCVW4dWw2X2Mi9= jfV8S0yZ8QJSgmFsjKsPaEHDR93fByTFdp6UX0L1Rsj5qfs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5DIsDS18ldwQ5nvAxmAT_u__jGJ43ZVCVW4dWw2X2Mi9= jfV8S0yZ8QJSgmFsjKsPaEHDR93fByTFdp6UX0L1-exh2JI$=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) .