Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 19 2023 00:42:15 FOUS30 KWBC 190042 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 841 PM EDT Mon Sep 18 2023 Day 1 Valid 01Z Tue Sep 19 2023 - 12Z Tue Sep 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF NEW ENGLAND... ....New England... Maintained the Slight Risk of excessive rainfall across portions of New England as low pressure continues to show low pressure making a slow northeastward progression. Saw little reason to modify the boundaries of either the Marginal or Slight risk areas given where early evening radar showed the better reflectvities and at least some overlap with the area that recently got rainfall from Lee. The Slight risk area encompasses the area of 1.5+ inch rainfall in the 12 hour period by both the 18/22Z HRRR and the 18/12Z ARW2 although the HRRR has been trending drier and a bit farther west with each run since 18/20Z. So far...rainfall rates have been generally half an inch or less over the southeastern portion of New England in part due to the limited instability. Agree with the previous outlook that there will be at least some embedded convection despite the modest instability given the mesoscale forcing. That combined with the sensitive soils in place was enough to maintain the Slight Risk with only minor adjustments based on early evening radar trends. ....Southern Florida Atlantic Coast... Maintained the Marginal Risk in south Florida but with some realignment for the few remaining hours that convection should be active. The airmass south of a cold front was characterized by precipitable water values around 2 inches and 2000 J/kg of MLCAPE along with a warm cloud layer aloft of 12k ft to 14k ft. Activity around the major metropolitan areas have diminished but new echoes were growing upscale to the north of their. The 12Z HREF did seem to capture the possibility of their formation and gradual eastward translation. So kept a Marginal risk area in south Florida but shifted the area based on radar and cooling cloud tops atop the newest convection on satellite imagery. Bann Day 2 Valid 12Z Tue Sep 19 2023 - 12Z Wed Sep 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS OKLAHOMA... 2030Z Update... ....Oklahoma... The synoptic and mesoscale breakdown is broken down well from overnight shift. Primary adjustment today was to extend the Slight Risk farther north into southeast Kansas. The nose of the LLJ (NAEFS shows 850mb winds over north TX/central OK above the 90th climatological percentile) will be placed over southeast Kansas with strong 850mb theta-e advection into the lower Missouri River Valley. The new 12Z HREF does show probabilities up to 20-30% for 24-hr QPF > 5" for Tuesday, the bulk of which comes Thursday evening and into the overnight hours. These factors went into why the Slight Risk was expanded farther norther into southeast Kansas. ....South Florida... Another day with a stalled front nearby, another day for thunderstorms capable of producing excessive rainfall rates of 2-3"/hr. Area averaged guidance soundings over South Florida showed MLCAPE of 1,500 J/kg available, as well as PWs near 2" and averaged low-mid level RH values near 80%. Compared to the Day 1 Outlook, parts of southwest FL are also at risk for torrential downpours. This is also following what will be a busy Monday when it comes to storm coverage, so some soils could be a little more saturated and more prone to flash flooding. In collaboration with MFL, did include much of the CWA under the Marginal Risk. Slightly farther up the coast into MLB's southern zones, the 12Z HREF showed up to 40-60% probabilities for 24-hr QPF > 5", which is a good signal for storms likely containing excessive rainfall rates over a densely populated area. For these reasons, introduced a Marginal Risk for most of South Florida. Mullinax ---Previous Discussion--- ....Oklahoma... Surface pattern will shift abruptly with a deep moisture flux over the southern plains stemming from strong southerly flow brought on by a pressure gradient between low pressure development over West TX and surface ridging east of the Mississippi. A textbook return flow pattern will advect Gulf moisture through the plains with global standardized PWAT anomalies reaching 1-1.5 standard deviations above normal by later Tuesday afternoon. Theta-E ridge positioned from east TX up through central and eastern Oklahoma will be the primary focus for where convective concerns will arise later Tuesday afternoon and evening as a solid instability axis over the above area will work in tandem with a shortwave approaching from the west along the northern periphery of an upper ridge centered over Chihuahua. As the shortwave approaches, large scale ascent will increase and convection will fire along the western edge of the theta-E gradient in place over northwest OK and southern KS. Bulk shear around 35-40 kts present will be enough to maintain decent structures for any storms that do develop leading to multi-cell and discrete supercell modes within the period of initiation. Nocturnal LLJ will ramp up after 00z with storms over OK taking on more upscale growth and propagating through central and eastern OK by nightfall. Forecast bufr soundings over central OK paint an ominous picture of the overall environment, especially the wind field and propagation points. Hodographs from point soundings near and north of OKC show a potential for back-building convective field based on the proposed veering winds within the mean flow, and as SPC stated on their latest risk update, the ability for storms to back-build from outflow boundaries left over from earlier convection could lead to training elements that would exacerbate flooding potential. Ensemble bias-corrected QPF and all global deterministic pin a large area of 1.5-2.5" of rainfall over central OK up towards the KS border. CIPS analogs were also pinpointing this area for heavy rainfall potential with 3 out of the top 5 analogs for Tue-Wed producing widespread 2-3+" totals in their respective evolution(s). With such a robust synoptic and mesoscale signature backed by prior analog based data, felt it was worthy to upgrade central and eastern OK to a Slight Risk with agreement from the Norman WFO.=20=20=20=20 Kleebauer Day 3 Valid 12Z Wed Sep 20 2023 - 12Z Thu Sep 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS, MISSISSIPPI VALLEY, AND NORTHERN MIDWEST... 2030Z Update... ....Eastern ID, Western WY, northern UT/NV... An amplifying upper trough over the Northwest will direct positive vorticity advection (PVA) over the Snake River Valley and Northern Rockies. NAEFS is showing signs of an increasingly anomalous event beginning to take shape Wednesday night. NAEFS shows by 00Z Thursday, 500-700mb heights dip below the 10th climatological percentile, IVT values within the Marginal Risk area rise to 200 kg/m/s (above the 90th climatological percentile) and PWs by 06Z Thursday are reaching the 90th climatological percentile. Latest global guidance also hinted at ~250-500 J/kg of MUCAPE Wednesday evening. Lastly, NASA SPoRT-LIS showed as high as 80-90% soil saturation in parts of the region, suggesting there are some soils that remain sensitive to excessive rainfall rates. PWs are the primary limiting factor as they likely will top out between 0.5-0.75". However, the strong synoptic scale lift combined with brisk 500-700mb winds aiding in topographic ascent could lead to hourly rainfall rates approaching 0.75-1.0". Given the lingering sensitive soils, went ahead and issued a Marginal Risk for parts of the interior Northwest. ....Upper Midwest... Strong 850mb theta-e advection will intersect the frontal boundary positioned over eastern ND and western MN to trigger numerous showers and storms throughout the region. A diffluent 250-500mb pattern aloft supports strong upper level divergence overhead and should support upscale growth of storms throughout the region. MUCAPE will rise to 500-1,000 J/jg with PWs rising between 0.9-1.1" for parts of the region, which is about 1.5-2 SDs above normal according to NAEFS. Effective bulk vertical wind shear will range between 20-30 knots to keep storms more organized. The Marginal Risk has been shifted farther west given recent QPF trends pointing to heavier amounts out into the Dakotas. The Dakotas also have lower FFGs compared to those in north-central MN, giving a little more confidence that rainfall in the Dakotas may have better odds of resulting in flash flooding. Still, the areal extent for possible flash flooding will be quite limited given the at-risk region has seen 5-10% of normal rainfall over the past 7-days. The previous discussion for the Southern Plains/Mississippi Valley still looks to be on track, so maintained the previous forecast reasoning for the Marginal Risk there below. Mullinax ---Previous Discussion--- ....Southern Plains/Mississippi Valley... Guidance is in fairly good agreement a remnant MCV originating from the previous evening convection will lead to more convective enhancement across the across the mid-Mississippi Valley Wednesday afternoon and evening. The shortwave that will lead to the flooding concerns over OK the prior period is hinting at closing off or sharpening while entering the Ozarks area of MO/AR as seen on the forecast 500mb vorticity reflections. This would produce large scale ascent near and under the parent MCV which will occur within a zone of prime instability on the western flank of a surface ridge over the eastern CONUS. The question becomes where the MCV goes after crossing into the Mississippi Valley which at this lead is tough to discern given the propensity for convective complexes to shift courses at the last minute based on changes in the downstream environment away from the convective impact. As of now, ensemble mean 500mb forecast shows the best chance for flash flooding concerns from convective initiation will be over eastern OK into southwest MO where guidance has the closed mid-level reflection, leading to the center of the MRGL risk located within those bounds and extended on the northern and southern periphery due to other guidance showing convective risks away from the primary area of interest. A stalled front over the Red River into the Lower Mississippi Valley will also be a trigger for diurnal convective risks within a very moist, unstable environment. FFG's are lower across much of the outlined MRGL risk area, but hints of heavier rate potential could pose from flash flooding concerns, especially in small towns and urban corridors. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!89fzQndzvrd9mr_UwKodHBiOc_IizpEwbtyXjLGNq29i= zzCnH0WynOhbodbM0fbrsziyFdJtUJbhEIkQBC3CRBGdRNo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!89fzQndzvrd9mr_UwKodHBiOc_IizpEwbtyXjLGNq29i= zzCnH0WynOhbodbM0fbrsziyFdJtUJbhEIkQBC3CWUUA4Aw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!89fzQndzvrd9mr_UwKodHBiOc_IizpEwbtyXjLGNq29i= zzCnH0WynOhbodbM0fbrsziyFdJtUJbhEIkQBC3CTBmMH9I$=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) .