Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 19 2023 20:05:33 FOUS30 KWBC 192005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 404 PM EDT Tue Sep 19 2023 Day 1 Valid 16Z Tue Sep 19 2023 - 12Z Wed Sep 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF THE SOUTHERN PLAINS AND MISSOURI VALLEY... ....16Z Update... ....Eastern Oklahoma Area... Upon review of the latest 12Z guidance, the inherited Slight Risk area was contracted quite a bit with this update to highlight all of eastern OK, and very small segments of the neighboring states. Visible satellite imagery shows much of the new Slight Risk area remains under dense overcast, with radar showing some individual cells of heavy rains, but those are very widely scattered. NASA Sport imagery shows much of eastern OK remains under abnormally dry conditions, with soil moisture generally between 5 and 15 percent, or 2 to 10 percent of normal for this time of year. Thus, anywhere but the heaviest rainfall will likely be beneficial to the areas impacted. So far today, with any heavier cells being small and transient, flash flooding has not been a concern except when one of the cells happens to move over an urbanized/flood-sensitive area. For the rest of today, clearing skies in western OK should allow for more robust instability to develop from west to east, this will allow severe thunderstorms to develop. They're likely to congeal into an MCS tonight over eastern OK, which...depending on how much instability can be advected into the area, may result in widely scattered instances of flash flooding from OKC east to the MO/AR border. The forcing very quickly cuts off just east of OK into MO/AR, so any storms that form will struggle mightily making much eastward progress once into MO/AR. For eastern KS and far western MO, instability will be the primary factor limiting heavy rainfall development, as any storms further south into OK will likely have all of the instability to use. Thus, the northern side of the resulting MCS is likely to result in a longer-duration light to moderate rainfall, unlikely to result in anything but isolated flash flooding. Thus, the Slight was trimmed on the north side, in good agreement with the CAMs, as expected rainfall totals are likely to remain at an inch or less. For western OK, storm motions are expected to be fast enough to keep any flash flooding limited until the MCS forms further east. Up across IA/IL, the rainfall ongoing so far has been widespread light rain, falling over areas that have been even drier than points further south. While there is some backbuilding of some locally heavier cells over IA, the overall complex is weakening. Later convective activity this evening is likely to remain isolated, and given the very dry soils any storms will be moving over, flash flooding appears so unlikely as to not need a Marginal, so that area's Marginal Risk was also dropped. ....Florida... No changes were made to the Marginal Risk area in FL as the forecast remains steady state in that area. Refer to the below discussion for more details. Wegman ....Previous Discussion... ....Southern Plains/Missouri Valley... WV satellite imagery this evening shows our next disturbance traversing the southern Rockies with convection already in-of the TX Panhandle. This shortwave will become the primary focus for later this afternoon and evening as large scale ascent within a favorable thermodynamic environment will trigger a round of strong to severe thunderstorms across the Southern Plains, migrating eastward during an expected upscale growth convective mode for the back end of the D1 time frame. A sharp increase in regional moisture is already occurring thanks to a prominent 35-40 kt LLJ bisecting the plains as the area lies on the western flank of a sprawling surface ridge analyzed over the Tennessee Valley, and the eastern side of developing low pressure along the lee of the Rockies. PWATs running between 0.75-1" over the plains east of a line from SJT to DDC will see a rapid increase towards 1 standard deviation above climatological norm by the time we reach this afternoon. This will greatly increase regional instability over much of east TX up through OK and KS which will play a role in the convective pattern later today. 2000-2500 J/kg of MLCAPE is forecast to the east of the dryline centered over the western rolling plains up through southwest KS. As the mid-level perturbation exits the Rockies to the plains, this will garner the triggering mechanism to initiate the first round of thunderstorms across the southern plains along and to the east of the dryline (western edge of the theta-E gradient). Multi-cell and discrete supercells will be the initial modes of convection when the storms begin, but increasing LLJ pattern over central and eastern OK up into eastern KS will promote a shift to more organized cell clusters promoting heavy rainfall coverage over a larger area. This evening and overnight into Wednesday AM will be the primary time frame of interest with regards to flash flooding potential as the expected complex will provide the coverage and magnitude necessary to bring more widespread impacts to the areas above. Hi-res ensemble (HREF) and associated deterministic are all fairly aggressive with the totals across the aforementioned area of interest, especially over east OK up through southeast KS where HREF neighborhood probabilities of >2"/24 hrs have shot up near 40% with a signal for 1-2"/3 hrs between 00-06z Wednesday up to a large area of 20-40%. Deterministic totals are obviously very high with a multi-model consensus of a 4-6" maximum somewhere in the vicinity of the risk area. Bias-corrected ensemble means are also fairly robust with over 1.75" bulls-eyed over NE OK up to the KS line. This actually is in agreement with CIPS analogs placement of max QPF based on the top 5 analogs within the 00z run, which would place the QPF centroid right in the middle of the forecast Slight Risk. The northern and southern expansions are for multi-model consensus on secondary peaks within the QPF field based on convective development and propagation within the spread. There is some deviation on where exactly the complex, or remnant MCV goes by the end of the forecast period, so there could be some shifts in the overall SLGT and adjacent MRGL in subsequent updates. However, will maintain the current positioning based on HREF blended mean QPF and outline of the 20% probability of 2" or greater within the ensemble forecast.=20=20 ....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 2,000 J/kg available, as well as PWs near 2" and averaged low-mid level RH values near 80%. 00z HREF probabilities are once again fairly robust, even for FL standards with signals for 3+" and 5+" totals within the D1 time frame (50-60% and 20-30% respectively). The enhancement of the front and sea breeze convergence creates a favorable scenario for localized flooding, especially in urbanized corridors along the coast(s). There is more of a signal today for the southwest coast being impacted by heavy rain today in due part to a progressive sea breeze propagation from the east and heavy storms forming in-of the Everglades creating subsequent outflows. This is the biggest difference from the last few days, but the impact potential still stands leading to a maintenance of the MRGL risk from previous forecast.=20=20 Kleebauer 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_xd2GO9KGBVCc0oZd5NTf1EQ9ig0f3p947-pv1MsyiM6= kZCHFQvBuCb9dccC1-Ire0g78B5Y2sXiINEjdJBk84x4ZWw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_xd2GO9KGBVCc0oZd5NTf1EQ9ig0f3p947-pv1MsyiM6= kZCHFQvBuCb9dccC1-Ire0g78B5Y2sXiINEjdJBkSxKTOG0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_xd2GO9KGBVCc0oZd5NTf1EQ9ig0f3p947-pv1MsyiM6= kZCHFQvBuCb9dccC1-Ire0g78B5Y2sXiINEjdJBkeG_sDo8$=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) .