Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jun 03 2023 19:28:42 FOUS30 KWBC 031928 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 327 PM EDT Sat Jun 03 2023 Day 1 Valid 16Z Sat Jun 03 2023 - 12Z Sun Jun 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS AND PORTIONS OF THE NORTHERN ROCKIES... ....Southern/Central Plains... Another active day of convection is expected from the Central Plains to the Mexico/Texas border, but with somewhat more diffuse organization than previous days. Mid-level troughing elongated across the Plains combined with modest left-exit jet level diffluence and a residual surface trough will drive deep layer ascent through today. This expansive lift will work across an environment with favorable thermodynamics for heavy rainfall noted by NAEFS PW anomalies of around +1 standard deviation coincident with MUCAPE of 1000-2000 J/kg, higher across southern Texas. As bulk shear increases to 30-35 kts concurrent with peak instability, this could result in some storm interactions/more organized clusters moving generally 10-15 kts as progged by the layer Corfidi vectors. Although the signal for training overall is modest today, with rainfall rates possibly reaching 2"/hr at times, any short term training or repeated rounds could quickly result in runoff issues, especially where soils are most saturated from 14-day rainfall that has been more than 300% of normal across much of west/central Texas into western OK and southern KS. Although the model signal is vague for any more organized areas of convection leading to more focused heavy rainfall, modest HREF probabilities for more than 3" invof the TX Panhandle continue to warrant the inherited SLGT risk. This is driven strongly by the antecedent conditions from recent rainfall leading to compromised FFG as low as 0.5-1.5"/3hrs which has a 20-40% chance of exceedance. Any storm that moves across this region could quickly lead to additional runoff concerns. While the rest of the area remains in a broad MRGL risk, there is potential that thunderstorms blossoming over the Serranias del Burro will congeal into an MCS with rain rates exceeding 2"/hr and push E/SE into the RGV or western TX Hill Country tonight. Some of these areas received as much as 1-3" of rain yesterday according to MRMS, but at this time the consensus signal for rainfall is a bit south of that footprint, which seems reasonable as low-level inflow is progged to back more to the south, likely resulting in any MCS diving to the southeast and missing the lower FFG to the north. There is still uncertainty into this evolution, and if it appears this MCS will track across similar areas to yesterday a targeted SLGT risk may be needed from the Big Bend to the Hill Country, but at this time the uncertainty into placement and speed precludes any upgrade. ....Northern/Central Rockies... The amplified mid-level pattern becomes a bit more strung out today as an expansive high-pressure ridge to the east expands and retrogrades slightly to the west. This will force the best meridional moisture transport and confluent flow westward from the past few days, with the best overlap of ascent and forcing shifting NW from Colorado into central/western MT and ID. There is good agreement that within this confluent flow, periodic shortwaves and associated vorticity maxima will lift northward, enhancing deep layer ascent already present through upper diffluence. With the best forcing becoming slightly displaced today from the best moisture transport, using the 850mb winds as proxy, this should result in more modest coverage of convection, with slightly less intensity. Still, PW anomalies of +1 standard deviation (+2 in MT) according to NAEFS will supply sufficient moisture for heavy rain in any of these thunderstorms today. Storms that do develop today will generally move W/NW within mean 0-6km winds of 10-15 kts, but terrain features and any storm interactions will likely lower cell translations in some areas. Rain rates progged by the HREF will generally be 0.5-1"/hr today, but could exceed 1"/hr where instability is greatest across parts of MT/ID or where any upslope can locally enhance ascent. This suggests the inherited MRGL risk is appropriate for much of the area with little change. However, the SLGT risk was trimmed from the south, to cover just areas where HREF probabilities for 1"/hr rainfall are greatest thanks to better overlap with instability, and to account for the footprint of the 24-hr MRMS QPE which has lowered FFG to just 0.25-0.75"/3hrs. ....Florida... No significant changes to the inherited MRGL risk as Tropical Storm Arlene continues to weaken and drop southward over the eastern Gulf of Mexico. NHC has this system decaying into a remnant low late tonight, and the guidance indicates the residual vorticity max associated with it will re-curve into the broad Southeast trough and move near the FL Straits before D1 is over. This subtly enhanced ascent combined with the continued modest diffluence aloft and a surface trough/convergence axis will once again provide sufficient ascent for scattered aftn/eve convection. The focus will once again be across the southern peninsula where weak bulk shear and slow storm motions will provide a setup which supports merging/interacting cells with rain rates of more than 2"/hr. This area has been very active the past few days noted by MRMS 72-hr QPE of 4-7" in a stripe from near Tampa to Miami resulting in above normal streamflow according to USGS gauges. With another similar day expected today, any slow moving convection, which will likely have rain rates above 2"/hr at times, could again lead to isolated runoff or flash flood instances. Weiss Day 2 Valid 12Z Sun Jun 04 2023 - 12Z Mon Jun 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS AND NORTHERN ROCKIES... ....Southern Plains through the Northern Rockies... Weakening expansive trough positioned NW to SE from the Northern Rockies through the Southern Plains will continue to provide an environment favorable for scattered to widespread aftn convection. This trough will become even more elongated with time on Sunday, while aloft an axis of deformation shears from the Central Rockies through Texas, providing additional deep layer ascent. Once again, the simulated reflectivity from the guidance suggests primarily scattered showers and thunderstorms, but weak vorticity maxes embedded within the flow could locally organize more substantial convection somewhere across the area. This region has been quite wet the past two weeks noted by AHPS 14-day rainfall that is generally 150-300% of normal, with some areas >600% near the TX Panhandle, or less than 50% for eastern KS/OK/TX. Where rainfall has been above normal, FFG is compromised to widespread 1-1.5"/3hrs, but in some places is as low as 0.25"/3hrs. This suggests that any scattered storm which =3D=3Dfalls over a sensitive area could produce runoff/flash flooding, which is reflected by a large but scattered signal for 3-hr FFG exceedance in the HREF, driving the expansive MRGL risk area. Embedded within the MRGL risk there is an indication for two smaller regions with a targeted higher excessive rain threat. The first is across the Panhandle of TX and into western OK where 72-hr rainfall on MRMS has been as high as 5-8", resulting in antecedent streamflow that is above the 90th percentile according to USGS. In this area, even a disorganized heavy rain producing cell could result in flash flooding, and the HREF exceedance probabilities peak around 30-40%. The other region is invof WY/MT border where recent rainfall has also been excessive resulting in more favorable conditions for rapid runoff. This area also has a narrow overlap of aligned mean winds/Corfidi vectors with 25-35 kts of bulk shear and 1000 J/kg MUCAPE, suggesting the potential for some storm organization and training, and where the PQPF first guess field indicates the need for a narrow SLGT risk. ....Florida... Another day of scattered to widespread afternoon thunderstorms is likely across the southern half of the Florida Peninsula on Sunday. The residual vorticity associated from the remnants of Tropical Storm Arlene will be exiting to the east early in the forecast period, but a secondary shortwave embedded within the larger scale trough may move across the region during the evening. This will combine with a persistent but weakening upper jet to provide deep layer ascent. Although PWs are progged to slowly dry out during D2, there is still likely to be an extended period of MUCAPE > 1000 J/kg with PWs > 1.5 inches to support aftn/eve thunderstorms with rain rates of 1-2"/hr drifting slowly northeast on 5-10 kts of 0-6km mean winds. Although the simulated reflectivity for Sunday features lesser coverage of storms, the bulk shear magnitudes are progged to climb to 25-30 kts, at least marginally supportive for some organization/clusters to support these efficient rain rates. If any of these heavy rates fall atop soils that are primed from several recent days of heavy rain, it could result in isolated runoff issues leading to flash flooding. Weiss Day 3 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... Broad but weakening elongated trough will persist across the High Plains and into the Northern Rockies Monday, continuing a pattern that has changed little during the past week. This will continue to support scattered afternoon convection from Texas through Montana, with ascent provided via subtle impulses moving through the flow and weak jet level diffluence. Thermodynamically, the environment will remain favorable for heavy rain rates as PWs of +1 to +2 standard deviations above the climo mean coincide with at least modest MUCAPE of around 500 J/kg. While the ensemble signals for heavy rain rates on D3 are generally weaker and less widespread, it is likely some areas will still receive more than 1 inch of rain, which could fall atop saturated soils from the recent heavy rainfall. However, the signals are modest enough to preclude much more than a MRGL risk for most areas. The exception is across the High Plains of NM into the TX Panhandle where the SREF indicates at least a 5-10% chance of 3" of rain, which will fall atop soils saturated from 3-day rainfall of 5-8" resulting in FFG that is 0.75-1.5"/3hrs. Confidence in scattered flash flooding is not extremely high, but the inherited SLGT risk was maintained primarily for the antecedent soil conditions, as even the CSU first guess field has just a MRGL over part of the area, and this was coordinated with WFOs AMA/LUB/ABQ. Should the model signals continue to temper, a removal or trimming of this SLGT risk area may be required with later updates. ....Sierra Nevada... A closed mid-level low approaching from the Pacific will move onshore central CA late Monday evening. Downstream of this feature, height falls and mid-level divergence will intensify across the Sierra, with ascent being aided by modest upper diffluence. Through this evolution, moisture will increase across CA and Great Basin noted by PWs rising to +1 to +2 standard deviations above the climo mean according to NAEFS, coincident with a corridor of MUCAPE exceeding 1000 J/kg. This should support afternoon convection, with weak mean winds supporting terrain tied development with slow storm motions. Although recent rainfall here has been very low, USGS streamflow is still above the 90th percentile in many locations implying still saturated soils. This suggests that as storms move slowly across the area with 0.5"/hr or greater rates, rapid runoff could occur. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6444dDlrG-yZm6JCj311f06-khN-t4quAKvetOlA4oQg= DVdseEmtAM0A1A0RSl1mkAKXXi8wJJkI57ecVQdzcJ3EAmA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6444dDlrG-yZm6JCj311f06-khN-t4quAKvetOlA4oQg= DVdseEmtAM0A1A0RSl1mkAKXXi8wJJkI57ecVQdze4lAeGI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6444dDlrG-yZm6JCj311f06-khN-t4quAKvetOlA4oQg= DVdseEmtAM0A1A0RSl1mkAKXXi8wJJkI57ecVQdzEPgrXAY$=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) .