Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 10 2023 00:59:49 FOUS30 KWBC 100059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 858 PM EDT Wed Aug 09 2023 Day 1 Valid 01Z Thu Aug 10 2023 - 12Z Thu Aug 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE MID-MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS.... ....Mid-Mississippi, Tennessee, and Ohio River Valley Regions... For the 01z ERO update, the broad Slight Risk was split into two separate regions (the MS/TN Valley and the OH Valley), with a break across central portions of KY. This expected bi-modal distribution of precipitation has become increasingly clear throughout the afternoon, and there is enough confidence now to split the risk areas (as separate convective clusters are already emerging). Focusing on the MS/TN Valley first, the stage is being set for impressive upscale growth of convection into a progressive MCS overnight (tracking from northern AR and southern MO into the Mid-South). Storms have already initiated across far northeast OK into southwest MO in association with a potent shortwave just upstream over eastern KS, and some west-to-east training is already occurring with fairly slow east-southeastward propagation of convection initially (with MRMS indicating 1-2"/hr rainfall rates). Downstream of the current convection, a pool of anomalous tropospheric moisture (with PWs of 2.0-2.2", over the 90th percentile) and rebuilding instability (per ML CAPE of 1000-3000 J/kg) via a strengthening low-level jet (20-40 kts of SW flow at 850 mb) is occurring. This environment will continue to support rainfall rates of 1-2"/hr (and locally higher where convection can sufficiently train) as the MCS grows upscale. Given the expectation for a forward propagating MCS, a favored location for training (and thus flash flooding) would typically be on the trailing southward flank of the developing cold pool, though this potential looks fairly low as the low-level inflow is largely orthogonal to the expected outflow boundary (oriented from NW to SE). Even still, the 18z HREF indicates a fairly broad area with potential for localized 3" exceedance through 12z (per 40-km neighborhood probabilities of 10-30%), focused mainly from southern MO and northern AR through the Mid-South (northeast AR, West TN, northern MS, and into northeastern AL). It is also worth noting that farther east across this Slight risk area (into northern AL, northwestern GA, and Middle/East TN), a prior MCS (having traversed the Mid-South during the day) is resulting in areas of convection that are ongoing this evening (with storm activity most vigorous over central AL). This activity should gradually come to an end as it merges and propagates southeastward over the next several hours, and the aforementioned developing MCS will then track into some of these same areas toward dawn (having matured and entering a weakening stage with the veering low-level jet). The broad low-level jet across the central CONUS is also driving anomalous tropospheric moisture farther northeastward into the OH Valley, where scattered instances of flash flooding also appear to be likely overnight. The potential for strong to severe storms are less likely in this region, but tall, skinny CAPE profiles (with ML CAPE of 500-1000 J/kg) and PW values near or just above the 90th percentile for this time of year will continue to favor some efficient rainfall from south-central IN/IL into southwestern OH. A broad area of stratiform rainfall with embedded convection is ongoing across much of IL/IN/OH, and convection will tend to focus along the southern periphery of this area. MRMS (and weather station observations) indicate hourly rain rates of 1-2 inches in areas of convection, and these rates will continue to be possible overnight (with localized west-to-east training). A decrease in available instability will gradually lessen the risk further to the east toward the central Appalachians overnight, although it won't completely eliminate it (with the potential for localized flash flood concerns continuing overnight). ....Arizona/New Mexico... Weakly anomalous moisture is in place across west-central AZ into southwestern NM (PWATs of 1.0-1.3"), driving scattered convective activity with sub-hourly (15-min) rainfall totals of up to 1". This activity is expected to continue for at least a few more hours, as instability remains (ML CAPE of 500-1000 J/kg). A Marginal risk remains in place for this convective activity, as any flash flooding should remain localized and confined to particularly sensitive areas (like dry washes and burn scars). Churchill Day 2 Valid 12Z Thu Aug 10 2023 - 12Z Fri Aug 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST... ....Mid-Atlantic and Northeast... Model guidance has indicated an increasing heavy rainfall threat on Thursday from E PA and N NJ into S NY, CT, RI, and MA. This appears to be associated with a prominent and now better-resolved mesoscale convective vortex (MCV) in the Midwest. As of 18 UTC on Wednesday, this MCV was pushing through eastern Missouri, and models continue the general trajectory through the Upper Ohio Valley tonight and into the Northeast tomorrow. There is still quite a bit of variability in the modeled convective patterns and QPF, and that seems to largely be due to differences in how strong the MCV is, and how much it will interact with a potentially coupled jet structure, which could lead to deepening of a low-mid level cyclone near southern New England. In general, the model preference was for an intermediate approach -- not favoring very intense surface lows (for the middle of summer!) below 990mb as depicted on the 3km NAM, WRF-ARW, and WRF-ARW2 (with heavier QPF displaced further north and inland), but also not favoring scenarios showing no MCV continuity or surface low development (with a much weaker QPF signal overall). Therefore, the 12Z HRRR and 12Z ECMWF seemed to represent a more plausible scenario, with enhanced rainfall potential in the aforementioned area from E PA and N NJ into SNE. An ingredients-based perspective using those models supports potential for hourly rain rates in the 1-2 inch range, and potentially sustained for a couple hours if training can occur. Therefore, those areas were upgraded to a Slight Risk in coordination with local WFOs. Some things to monitor for potential future changes... (1) Whether the Slight Risk may need to be expanded further north and/or northeast into portions of New Hampshire and Maine, if the interaction of the MCV and jet structure leads to a deeper low and focused heavy rainfall further north; and (2) If the Slight Risk may need to be extended further back to the west into the central Appalachians, if focused convection appears more likely earlier in the day. ....Interior Southeast U.S... The convective pattern and heavy rainfall threats over the Southeast on Thursday and Thursday evening will be strongly influenced by how convection evolves tonight, and any resulting outflow boundaries. Therefore, there is still some uncertainty. A notable trend in the ensemble model guidance today was to shift the stronger QPF signal further south, from near the TN-NC border region more into N MS, N AL, and N GA. This was also reflected in our machine learning guidance, based on the 12Z GEFS. Strong instability and PWs around or in excess of 2 inches would support potentially intense rainfall rates, and veered low-level inflow from a westerly direction could favor backbuilding given a likely upstream CAPE max somewhere near N MS. Given the uncertainty, it's possible that the risk areas may continue to shift, but in general, the expectation is for a corridor of enhanced heavy rainfall, and potentially flash flooding, somewhere in the region. ....Southern Nevada, Southern Utah, Northern Arizona... Scattered diurnal convection is expected in the Southwest. The 12Z HREF maintains small probabilities of 1hr rainfall exceeding flash flood guidance with the most intense rain, and this seems reasonable. The introduction of higher probabilities is precluded by a lack of very strong instability or anomalous moisture, so the broad Marginal Risk was maintained. ....Upper Midwest... The Marginal Risk was removed for the Upper Midwest region. Although convection is expected, hi-res models uniformly show it to be rather progressive and unlikely to last very long at any one location. Flash flood guidance is also higher across this region than the remainder of the North-Central U.S., and much of the region is also in drought. Instantaneous rain rates may be high, but the probability of exceeding flash flood guidance, while non-zero, should generally be less than 5 percent in this region given a lack of persistence of the high rain rates. Lamers Day 3 Valid 12Z Fri Aug 11 2023 - 12Z Sat Aug 12 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL IN THE SOUTHEAST, GREAT LAKES, DESERT SOUTHWEST, AND CENTRAL SIERRA NEVADA... ....Greater Southeast Region... A broad Marginal Risk was maintained over the Southeast region, generally in the vicinity of where a slow-moving or stalled front is expected to be located. The front may be positioned relatively far south and closer to the Gulf Coast in the vicinity of AL and GA, where consecutive days of convective outflows may reinforce cooler air in the interior Southeast. However, given that the frontal position will be crucial to the location of more intense convection, there is considerable uncertainty more than 48 hours in the future. The Marginal Risk placement relied on ensemble probabilities of higher QPF. South of the front, strong instability is expected, with a continued presence of PWs near or above 2 inches. This will continue to support localized higher rain rates potentially reaching 2 inches per hour. As the mesoscale environment becomes more certain over the next day or so, it wouldn't be surprising to see a more focused area of higher probabilities, and thus an upgrade to a Slight Risk, somewhere in this larger region. ....Upper Midwest... The size of the Marginal Risk was reduced in this region, and focused more around urbanized areas in the central Great Lakes region, where flash flood guidance also happens to be a little lower and the ongoing drought is not so severe. However, as in the Day 2 period, convection developing along and ahead of an advancing cold front is expected to be fairly progressive and therefore any heavier rain rates seem unlikely to be sustained at any one location for very long. It's possible that this Marginal Risk could be further shifted, reduced in size, or eliminated if future model runs continue the trends of showing relatively narrow convective bands advancing quickly to the east. For the time being, there appears to be enough of a threat of 1-2 inch per hour rain rates coinciding with some larger urban areas or regions of lower FFG to continue the Marginal Risk. ....Eastern Arizona and Western New Mexico... A northward surge of moisture, increasing precipitable water values to slightly above average, and the entrance region of an anticyclonically curved, strengthening, upper level jet streak may combine to lead to an increase in convective coverage near the AZ-NM border region on Friday. One limiting factor may be available instability, as most models are indicating less than 1000 j/kg of MUCAPE, which seems to be due to a relatively dry and deep sub-cloud layer, with marginal surface dewpoints and LCLs over 700mb. Instability tends to be a factor more highly correlated to flash flood activity in the Southwest region, so overall the threat looks to be fairly isolated at this time. Therefore, the Marginal Risk was maintained. ....Central Sierra Nevada... A Marginal Risk area was added for the Central Sierra Nevada region, where there is expected to be a surge of deeper moisture and a lingering mid-level vorticity center from the now-decayed tropical cyclone (Eugene) in the East Pacific. The GEFS and ECMWF ensembles were in fairly good agreement with placing an area of higher QPF in this region, and the PWs are forecast to be above the 95th percentile for mid-August in both ensemble systems. Although instability is not projected to be very strong, models do show potential for pockets of over 500 j/kg of MUCAPE, which is not insignificant at higher elevation. The main question will be whether denser cloud cover associated with the deeper moisture limits heating and instability more than models are currently anticipating. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6M7dqMTzu60VrC1ex2gLXe3Wx6uzGsKbdBlC1YZHE7Zw= u-rnlqT2ZKS6ADh2ehbaEhndqJm-kFcHKiCEMVj6Dq6yOjE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6M7dqMTzu60VrC1ex2gLXe3Wx6uzGsKbdBlC1YZHE7Zw= u-rnlqT2ZKS6ADh2ehbaEhndqJm-kFcHKiCEMVj6XH22_QM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6M7dqMTzu60VrC1ex2gLXe3Wx6uzGsKbdBlC1YZHE7Zw= u-rnlqT2ZKS6ADh2ehbaEhndqJm-kFcHKiCEMVj6tYHa1O0$=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) .