Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 20 2023 01:08:06 FOUS30 KWBC 200108 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 907 PM EDT Tue Sep 19 2023 Day 1 Valid 01Z Wed Sep 20 2023 - 12Z Wed Sep 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF THE SOUTHERN PLAINS AND WESTERN OZARKS... ....Southern Plains/Missouri Valley... 0100 UTC Update -- Made modifications to both the Slight and Marginal Risk areas based on the latest observational/mesoanalysis trends along with (especially) the latest short range CAM guidance/trends. Not much of a change with the Marginal Risk area -- outside of paring the northern periphery a bit, along with a slight extension south of the Red River in North TX. However we were able to cut a large area of the Slight Risk, outside of the MOKSAROK 4-state area where the guidance (both deterministic and probabilistic) is in a bit more agreement. This is also where the 18Z HREF exceedance probabilities are most elevated, particularly after 03-04Z with the uptick in the LLJ. ....South Florida... As is typical, latest SPC mesoanalysis indicates a general negative dCAPE/dt across S FL following sunset. However given the current radar (trends), along with residual modest deep-layer instability, PWs ~2.25", and sufficient large-scale forcing ahead of the upper trough/south of the surface front (upper divergence with plenty of kinematic support for more organized clusters, with 0-6km bulk shear values of 25-30 kts along with good 925-850mb moisture transport), will maintain a Marginal Risk in the ERO given the potential for localized/isolated flash flooding, mainly over urban areas. Hurley Day 2 Valid 12Z Wed Sep 20 2023 - 12Z Thu Sep 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OZARKS, MISSISSIPPI VALLEY, INTER-MOUNTAIN WEST, AND THE FLORIDA PENINSULA... ....20Z Update... ....Intermountain West/Great Basin... No big changes were made to either the QPF footprint for the large Marginal Risk area across this region. The area of biggest concern remains northeast NV and northwest UT north and west of the Great Salt Lake, for potential upgrades, but since the rainfall footprint is largely unchanged, saw no reason to deviate from inherited. ....Northern Plains... Removed the inherited Marginal Risk for the Dakotas with this update. In addition to a very weak (and decreasing) rainfall forecast in the guidance, much of the eastern Dakotas are very difficult to flood even with more rain. The antecedent dry conditions even further diminish the flash flooding threat. ....Ozarks of MO/AR/OK... A Marginal Risk was maintained with this update, with a bit of trimming on the western side. The ongoing MCS at the start of the period will continue into the morning hours for eastern OK, though with limited instability, the likelihood of heavy rainfall is low. Much of the rain should be over early on across eastern OK, so think most of the impactful rain for that area will be in Day 1/Tonight. Much of the associated rain will shift north into MO for much of the day, though little in the way of heavy rain is expected. A second round of convection may form into a line across eastern OK overnight Wednesday night, which may require further adjustments or potentially an upgrade due to multiple days of heavy rainfall. ....Florida Peninsula... A stalled out frontal boundary with a developing low off the Atlantic coast will provide plenty of forcing for another round of widespread showers and thunderstorms across much of the peninsula again on Wednesday afternoon and evening. PWATs will exceed 2 inches for much of the southern half of the peninsula, which in turn will provide ample moisture for the thunderstorms. The greatest chance of flash flooding will be in urban and flood-sensitive areas. A Marginal Risk area was introduced for much of the Peninsula outside of the Everglades with this update. Wegman ....Previous Discussion... ....Eastern ID, Western WY, northern UT/NV... A deepening upper trough over the western CONUS will create an enhanced area of difluence and attendant vorticity maxima rotating around the general upper circulation positioned over the PAC Northwest. Modest low to mid-level moisture advection within the confines of the Snake River and adjacent terrain will create a pocket of better convective potential Wednesday afternoon and evening as a stronger vorticity maximum pivots through the region providing deep layer ascent during peak diurnal destabilization. Despite the limit on potential due to only a modest moisture profile, enough instability (~500-1000 J/kg SBCAPE) and focused ascent over the topography should provide some heavy rain potential within a zone that is highly prone to flooding due to steep terrain and already elevated soil moisture content as pinpointed by the NASA SPoRT data set indicating areal coverage of 70-90th percentile of climatological norms within the confines of the outlined area. Only minor adjustments were made to the MRGL risk, mainly attributed to the QPF reflection within the NBM and bias-corrected ensemble forecast over the area of best positive vorticity advection (PVA). ....Upper Midwest... MRGL risk from previous forecast was maintained with a slight expansion over the Dakotas and northwest MN to reflect the uncertainty of model guidance where the best convection will develop Wednesday evening. The ingredients for locally heavy rainfall with a zone of lower FFG's is still very much probable as modest theta-E advection into the northern plains will lay the ground work for a zone of potential, mainly along and south of a stalled frontal boundary positioned over the northern plains. Increasing difluence aloft with small vorticity perturbations will provide enough large scale ascent for a round of thunderstorms to develop across central ND over to the MN border. Ensemble means are fairly low with regards to top-end potential for the setup, but some deterministic are fairly robust for localized heavy rainfall within a corridor of low FFG's thanks to a prolonged drought in place. PWATS will also be steadily increasing with GEFS standardized anomalies approaching 1.5-2 standard deviations above normal by the end of the D2 period. Instability in-of the stalled boundary is tame compared to other periods of interest, but increased forcing ahead of a deepening upper trough to the west could provide enough destabilization to generate a period of thunderstorms, mainly Wednesday night into Thursday AM when the best impacts would occur. Thus, will maintain the previous forecast for continuity purposes and assess in subsequent updates. ....Southern Plains/Mississippi Valley... Remnant MCV is anticipated the vicinity of the southern plains up through the mid-Mississippi Valley on Wednesday. There is some discrepancy on the track of the circulation, as well as the convective impacts that are possible during its propagation. There's a mix of deterministic outputs with a maximum focused over eastern OK to western AR, but there's another set of guidance tracking the MCV up into Missouri creating a heavy precip field near Kansas City and the northern half of MO. 00z HREF neighborhood probabilities are actually the most aggressive for totals across eastern OK, primarily in the morning when the complex is still in its infancy compared to where it would be in the afternoon and evening. The 500mb mean height field on ensemble guidance shows a decent signal of at a least a stronger shortwave moving out of OK into MO later Wednesday afternoon with some indications it may close off at points. The guidance on the northern precip maximum have the best 500mb reflection, which makes sense given the more aggressive nature of the evolution. In any case, the MRGL risk was expanded to include both the southern and northern maximum potential as regardless of evolution, the environment remains favorable within a corridor of higher PWATs and large scale forcing associated with the shortwave trough. Kleebauer Day 3 Valid 12Z Thu Sep 21 2023 - 12Z Fri Sep 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEASTERN OKLAHOMA... ....20Z Update... ....Eastern Oklahoma Area... A renewed round of convection is expected Thursday afternoon through the overnight across eastern OK. Expect with the ongoing convection this afternoon and tonight that FFGs will still be depressed by Thursday afternoon (especially with any additional storms on Wednesday). The MCS likely to develop Thursday night will bring more widespread, heavier rains as a 35 kt LLJ develops over OK and advects a moisture-laden airmass with PWATs above 1.75 inches into the area. This will support the convection. In coordination with TSA/Tulsa, OK forecast office, a Slight Risk area was introduced with this update. ....Northern Plains... The inherited Slight Risk area was downgraded to a Marginal Risk with this update. The eastern Dakotas are notoriously difficult to flash flood, and with slightly lesser forecasted rainfall amounts expected, with 24 hour rainfall totals barely exceeding 1 inch, this amount of rain even with any previous days rain is highly unlikely to result in flash flooding. ....Inter-Mountain West... No significant changes were made to the Marginal Risk area. Portions of northeast NV through southeast ID look to be the areas of greatest concern for a potential upgrade, but somewhat lesser forecast rainfall amounts have precluded any upgrades with this update. The area will continue to be monitored. ....Atlantic Coast of Florida... A Marginal Risk was introduced with this update along the coast of Florida. A developing low offshore will advect deep tropical moisture southwestward into the east coast of Florida, where frictional effects will likely result in numerous showers and thunderstorms lined up along the coast. Since the area is heavily urbanized, isolated flash flooding is possible. Wegman ....Previous Discussion... ....Northern Plains... Deep moisture advection pattern continues into D3 across the northern plains where standard anomalies for PWATs over the Dakotas are beginning to breach 2.5-3 deviations above normal which is a great reflection on the ambient environment available across the above area. Large scale ascent on the eastern flank of a deep closed circulation over the west will aid in initiating a large swath of precipitation over the Dakotas, especially focused along a stalled frontal boundary forecast over the region. Bias-corrected ensemble means are already showing a large areal extent of 1-1.5" of precipitation falling on Thursday afternoon and evening with maximum(s) positioned over the area of lowest FFG's. Strong convergence signal on the surface and 850mb frontogen fields across the northern plains from Bismarck, northeast to the Canadian border show the focal point of where the heaviest precipitation would occur. With the highly moist environment in place, heavy rain potential will be at climatological maximum given the expected forcing and overall synoptic scale setup across the plains. Ensemble probabilities of >1" are beginning to spur up towards 20-30% at lead with some non-zero probability for >2" located over northern ND. Considering the large scale synoptics and focused area along the stalled front within a zone of low FFG's with potential priming the day before, have maintained a Slight Risk with some expansion to the Canadian border to adjust for the latest ensemble QPF forecasts and probabilistic data. The southern edge of the SLGT risk over SD was also expanded to the NE border given the increased convective threat late Thursday, but considering the Sand Hill region of NE and it's very high FFG indicators, did not want to extend beyond the border as impacts will likely be muted within that zone. Will make note that another area of interest will be across northern NV as a cold front and increased upper forcing from a strong vorticity maxima providing PVA to the region could pose some increased flash flooding concerns on Thursday morning and afternoon before NVA and dry air advection sweep the precip focus further to the northeast. There's still a bit of uncertainty on the prospects for widespread convective impacts that would necessitate a higher risk for the area, but the enhanced upper forcing within the pattern could very well lead to enough of a signal for an upgrade to a SLGT.=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8W3sRjs5I7vBdquiPlMdZGy-9lnT39jXlB80h-rVGoR4= K9nLPgqzqO6RcGKJ98bazRhqDotc92aAuLHspAaL-GldLBE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8W3sRjs5I7vBdquiPlMdZGy-9lnT39jXlB80h-rVGoR4= K9nLPgqzqO6RcGKJ98bazRhqDotc92aAuLHspAaLXQQmjjc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8W3sRjs5I7vBdquiPlMdZGy-9lnT39jXlB80h-rVGoR4= K9nLPgqzqO6RcGKJ98bazRhqDotc92aAuLHspAaLoJxqrzQ$=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. 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