Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 09 2023 20:19:16 FOUS30 KWBC 092019 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 417 PM EDT Wed Aug 09 2023 Day 1 Valid 16Z Wed Aug 09 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, Ohio, and Tennessee River Valley Regions... For the 16 UTC ERO update, the broad Slight Risk was maintained over much of the region, with a few changes to account for ongoing convective trends. A prominent mesoscale convective vortex (MCV) is evident on radar composite imagery just southeast of the Kansas City metro area, and was advancing steadily to the east. Forward progression seemed to be a little faster than model guidance was indicating, and the MCV could reach the Mississippi River north of St. Louis by about 19Z, which would roughly coincide with the timing of most favorable conditions for deep convection downstream. An increase in convection will be aided by a coupled jet signature already favoring significant upper level divergence this afternoon and evening ahead of the advancing MCV. There are several lingering uncertainties that prevented a targeted upgrade to a Moderate Risk, although we will continue to monitor for an unscheduled update this afternoon. We do think that the probability of flash flooding is in the upper end of the Slight Risk range from central Illinois into central and southern Indiana and southwest Ohio. The first concern is related to downstream cloud debris that may limit destabilization, although visible satellite imagery does show some filtered sunshine, and RAP analysis already shows moderate instability in place. Second, hi-res models are very much struggling to adequately capture the significant ongoing coverage of convection in northern Missouri ahead of the MCV, so there are obviously questions about how reliable their output (including the HREF ensemble) will be downstream into the Ohio Valley. And third, models do show the highest PWs arriving more with the MCV than ahead of it. Therefore, deeper moisture and a deepening warm cloud layer may actually not ideally align with the better forcing. Nevertheless, tall, skinny CAPE profiles and PW values near or just above the 90th percentile for this time of year will favor some efficient rainfall, and focused ascent ahead of the MCV could favor areas of training convection for a couple hours. Hourly rain rates of 1-2 inches will be possible. A decrease in available instability after 00-03 UTC 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. Another area of heightened concern continues in the southern portion of the broad Slight Risk area. Our assessment continues to be that the probability of flash flooding is in the upper end of the Slight Risk range from the Ozarks of southern Missouri and northern Arkansas southeast into western Tennessee and northern Mississippi. Confidence is not sufficiently high for a Moderate Risk upgrade in these areas either, but this is another region that will be monitored closely for an unscheduled update. Flash flood guidance is a little higher in these areas, but ongoing convection has been producing corridors of 1-2 inch per hour rain rates already, and this may prime ground conditions for an additional round of convection. The next round of convection will likely initiate later today ahead of an advancing shortwave, currently evident on water vapor satellite channels in northwest Kansas. PWs over 2 inches (over the 90th percentile) and ample instability should support rain rates over 2 inches per hour if and when convection can sufficiently train. One of the main lingering uncertainties is that the setup for later today and tonight is only somewhat favorable for training convection. Hi-res models indicate the potential 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. However, this is more likely when the instability maximum is situated more clearly upstream, and the low-level inflow is oriented more parallel to the outflow boundary, rather than orthogonal as is forecast in this case. Additionally, model forecasts do show some potential for a decrease in precipitation efficiency with some mid-level dry air after 21-00 UTC. Nevertheless, more veered low-level flow or changes in the convective evolution in the next few hours with the early round (16-20 UTC) could support a targeted upgrade to a Moderate Risk later today. ....Arizona/New Mexico... At 16 UTC, there were no major changes to the outlook in the Southwest. What follows is the previous discussion. Weakly anomalous moisture is expected to be in place across AZ (standardized precipitable water values of 1 to 1.5) this afternoon, while higher anomalies (+2 to +3) reside over the southern CA coast. While cloud cover during daytime heating remains a bit uncertain, it seems clouds will be more likely over CA during the early afternoon, associated with remnant moisture from former Hurricane Eugene in the eastern Pacific. Thunderstorms are expected to initiate along the Mogollon Rim with daytime heating (likely 18Z+) followed by additional convection over the higher terrain of southeastern AZ. Forecasts from a majority of the 00Z guidance suggest MLCAPE of 500-1500 J/kg during the afternoon with forecast soundings showing fairly moist low to mid level profiles. While storm motions don't appear overly slow overall, there is at least a low-end threat for isolated flash flooding with the convective peak later in the day. Lamers/Otto 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!4hOeIb8cPcQXgKvLqFRLBJz2ZQhHCbB0-I22QJLDXAOP= sdc-oqjfTtBBRJg7LxT-q8mae9_GCXs9yhx0SFVYoJTC9o0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4hOeIb8cPcQXgKvLqFRLBJz2ZQhHCbB0-I22QJLDXAOP= sdc-oqjfTtBBRJg7LxT-q8mae9_GCXs9yhx0SFVYlus2osc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4hOeIb8cPcQXgKvLqFRLBJz2ZQhHCbB0-I22QJLDXAOP= sdc-oqjfTtBBRJg7LxT-q8mae9_GCXs9yhx0SFVYUqbawfA$=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) .