Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sat Jun 10 2023 15:10:00 FOUS30 KWBC 101827 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 226 PM EDT Sat Jun 10 2023 Day 1 Valid 16Z Sat Jun 10 2023 - 12Z Sun Jun 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PLAINS, ARKLATEX, AND THE MIDWEST... ....16Z Special Update... No big changes were made with this morning's update. The Slight risk area from MT through far northwestern NE was trimmed a row of counties to the southwest with the surrounding Marginal risk area right behind it, as a much drier and certainly much more stable air mass creeps in from the northeast, shutting off all rain chances behind the leading front. The Marginal Risk area in CA/NV was expanded a bit towards the northeast, which includes a strong signal in the HREF for isolated rainfall amounts locally exceeding 1 inch that may exceed FFGs and result in isolated flash flooding across far northern NV and the OR/ID/NV junction area. The Slight Risk area from southeastern NE through northwestern MO was left unchanged with this update, but appears to be a low-end Slight. Instability in this area should only increase between 1,000 and 1,800 J/kg, and widespread low cloud cover over almost the entire Slight Risk area is working to keep those values in check. Much of the CAMs guidance suggests widely scattered to scattered showers and thunderstorms developing in the next few hours, which will track southward, but none of the guidance shows the convection congealing into an MCS, but instead remaining separate as they move southward. The storms could very well produce heavy rainfall rates, but their movement and relatively small coverage should reduce the flash flooding threat in this area. The Marginal risk area over the southern High Plains was trimmed towards the east. For CO into KS, much of the guidance shows one round of storms, that will likely be fast-moving, and thus is unlikely to produce meaningful flooding. Meanwhile over OK, the MCS that was over that area has now shifted off to the eastern part of the state and AR, and only some light shower activity with maybe an isolated thunderstorm are expected through the overnight, so the Marginal Risk area was trimmed back about to where it was with the overnight shift update. The Slight risk area over east Texas looks good, with an enhanced likelihood of severe weather, so only cosmetic expansions to the southwest were included based on the latest guidance. Finally, the eastern end of the Marginal area following east of the Mississippi River was trimmed back to close to the river, as any convection that develops east of the river is likely to be waning from earlier convection. The one exception to this will be over the state of Mississippi, where pulse convective coverage is likely to be more numerous, so the Marginal risk area was trimmed out of the FL Panhandle and all but western AL. Wegman ....Previous Discussion... Northern High Plains... Anomalous moisture continues early on before dropping off and southward as an upper level shortwave potentially splits, with some energy moving north into Canada and some undercutting the ridge and dropping southward. Instability lingers early on, so the existing Slight Risk still makes sense, but it was refined using the latest guidance. Midwest... A shortwave drops south around a forming closed low across southeast Ontario, with the overall system on a strengthening trend. A low- to mid-level low drops southeast and lures PW values up to 1.5-1.75" into portions of KS, MO, NE, and IA. Inflow at 850 hPa and effective bulk shear should be increasing, particularly late in the period when the low-level jet tends to ramp up Saturday night into Sunday morning which portends organized convection. CAPE values upstream are forecast as high as 4000 J/kg on the 21z SREF guidance during daytime heating, falling towards 1000 J/kg overnight. The guidance signal is more scattered than on previous nights in and near the complex border junction of NE/MO/KS/IA, near where mid-level heights/thickness pattern are diffluent (eastern NE, western IA, and western MO), with the guidance (particularly the 00z NAM and 00z Canadian Regional) still showing a signal for local 3-5" amounts, with the 00z HREF guidance showing a non-zero chance for 5"+ amounts. Flash flood guidance values, particularly outside MO, are modest, but the Kansas City urban area would naturally be susceptible to heavy rainfall. The ingredients available could support hourly rain totals to 2.5", which would be a problem in urban areas. The Slight Risk area was re-expanded due to the above, but care was taken not to go too far into IA per their lesser sensitivity mentioned in prior coordination. A broad Marginal Risk area continues to surround the Slight due to some lingering spread as the guidance is no longer coherent if a convective complex is going to form, even if all the ingredients appear to be there for the formation of such a complex. The re-expansion was coordinated with the DMX/Des Moines IA and OAX/Valley NE forecast offices. Portions of CA/NV... An anomalously strong closed low drops into CA Saturday into Sunday. Precipitable water values remain elevated, with moisture increasing in the Central Valley as the cold low develops and moves in. Instability shouldn't be a problem as temperatures aloft decrease. Believe a Marginal Risk of excessive rainfall/flash flood should continue Saturday into Sunday. Hourly rain totals up to 1.5" should be possible where cells backbuild, train, and/or merge. ArkLaTex to the Central Gulf Coast... As a southern stream shortwave phases with a deepening positively tilted upper trough, PWs rise towards 2" along a section of the polar front making some progress eastward. Low-level inflow/effective bulk shear appear to be sufficient for convection with some level of organization, which would support hourly rain totals to 2.5" somewhere between eastern TX, LA, and MS. The NAM/GFS still advertises local 4-6" amounts in eastern TX especially, so the Slight Risk area remains in place in that region. Roth Day 2 Valid 12Z Sun Jun 11 2023 - 12Z Mon Jun 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COLORADO & THE TENNESSEE VALLEY... ....1830Z Update... The only significant change with this afternoon's ERO update was to eliminate the Slight Risk area across the Ohio Valley, while the dynamics of a strong upper level low moving into the area are impressive, especially for this time of year, the lack of instability is a huge detriment to getting rainfall rates needed to cause both significant runoff on the very dry soils in this area, and to get the rates needed for flash flooding. Think instead this area will see a prolonged period of light to moderate rain, which will fall at a slow enough pace that it will have time to soak into the dry soils. Initially the Slight Risk area was trimmed out of northeast Ohio/western Pennsylvania, but with even areas to the southwest which will have some instability (though only to 500 J/kg) not expecting flooding, instead opted to drop the Slight entirely. This was coordinated with the ILN/Wilmington, OH and LMK/Louisville, KY offices. Further south, QPF trends across the TN Valley have come down a bit with the latest guidance trends, but it will be worth noting whether this remains the trend with upcoming guidance. In the meantime, the Slight Risk area was expanded a little towards the southeast down the GA/SC border, given a shift in the local maximum of average rainfall towards the south and east. There should be plenty of instability in this area, but the problem is that any rounds of convection are expected to move through the area quite quickly, so any one area may not see heavy rain for more than a few minutes. This too will limit the flash flooding potential. Given several urban areas including Memphis, Nashville, and Atlanta are in the Slight risk, which all inherently have lower thresholds for flash flooding, have opted to leave the Slight up as a low-end Slight for the time being. The portion of the Slight Risk area over central AL was also trimmed towards the north in coordination with the National Water Center, due to soils being less favorable for flash flooding in that area (minus any urban areas) than points further north into far northern AL. Finally, the Slight risk area over Colorado was nudged a hair towards the west to focus more of the area towards the Front Range. This aligns with the most recent updated average QPF. Any strong or severe thunderstorms over this area will be slow moving with the upslope flow and will struggle to move too far from the mountains. No significant changes were made to the surrounding Marginal risk area over much of the intermountain West, as this area generally has been quite wet, so the lighter forecasted amounts of rain here may still pose an isolated flash flooding threat, particularly on the slopes of any mountains impacted by the shower and thunderstorm activity. Wegman ....Previous Discussion... Great Lakes/Midwest/Ohio Valley/Tennessee Valley... A broad area of locally heavy rainfall is expected in the eastern portion of the country -- much of it welcome because of recent dryness -- as a deepening closed cyclone drops into the Great Lakes and phases with a shortwave moving eastward from the southern Plains. The best signal for heavy rainfall is in the TN Valley, where strengthening low-level inflow, CAPE of 2000+ J/kg, and PWs of 1.5-2" are forecast. There appears to be enough effective bulk shear for convective organization. These ingredients allow for hourly rain totals to 2.5". The 00z UKMET and 00z NAM guidance indicate local 3-4" amounts, while the 00z Canadian Regional advertises local 4-6" amounts. Outside of northernmost GA, the region has seen much below rainfall over the past week, if not two months. However, with the rain rates expected, particularly in urban areas, widely scattered to scattered instances of flash flooding are expected despite the recent dryness, so raised the TN Valley remains in a Slight Risk. To the north, a front moves in while 1.5"+ PWs stay over portions of OH for 12+ hours. Some instability is expected to sneak in to southwest OH from KY with time, but the combination of lowish instability, increasing 850 hPa frontogenesis, and the low-level inflow exceeding the mean 850-400 hPa wind are expected to lead to better than average rainfall efficiency. Within this region that's been dry, some areas for quite a while, there's a concern that soils will be hard/brick-like initially, with rainfall all running off until soils can soften and allow infiltration. With hourly rain totals maxing out in the 1.5" range at the southwest portion of the new Slight Risk, and closer to 0.75-1" in the northeast portion of the new Slight Risk, the three hourly flash flood guidance should be able to be exceeded on a scattered basis, hence the new risk area. Coordination with JKL/Jackson KY, LMK/Louisville KY, and ILN/Wilmington OH forecast offices led to the current Slight Risk configuration. In and near Colorado... Upslope flow behind a frontal zone imports moisture from the southern and central Plains into eastern Colorado, with PWs exceeding 1" in easternmost CO in a col point in the mid-level flow. CAPE is expected to rise towards 1000 J/kg. With the mid-level flow opposing the low-level flow, enough effective bulk shear should exist for organized convection, which could allow hourly rain totals to rise to 2" where cells train and/or merge. Once cold pools becoming dominant, organized convection should plow east to southeast into the low-level flow, increasing storm scale inflow. The guidance has a good signal for local amounts of 2". Flash flood guidance is rather low, so a scattered risk of flash flooding is anticipated as both hourly and three hourly flash flood guidance values are exceeded. The Slight Risk area remains similar to continuity which matches the overnight model guidance. Remainder of the West... An anomalously deep closed low pivoting through southern CA into southern NV will help cause pockets of 1000+ J/kg of CAPE and PWs of 0.75-1.25". This is expected to cause isolated heavy rain concerns particularly at elevation across portions of the West and northern Rockies. Where cells train and/or merge, hourly totals of 1.5" are possible. Considering recent wetness, the existing Marginal Risk area looks reasonable at this time as cell coverage should not be dense enough to cause scattered flash flood issues. Some expansion was made to the Marginal Risk area to account for the 00z HREF guidance. Roth $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .