Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 09 2023 15:58:11 FOUS30 KWBC 091558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Wed Aug 09 2023 Day 1 Valid 12Z 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 THE SOUTHERN APPALACHIANS... ....Southern Appalachians/Gulf Coast States... Convection should be ongoing at the start of the period over the southern Appalachians, located ahead of an eastward advancing mid-level shortwave. WSW low level flow of 30-40 kt is forecast by the 00Z model consensus to be pointed into the southern Appalachians along with precipitable water values of approximately 1.7 to 2.0 inches just upstream across portions of TN, AL and GA. Due to the high moisture forecast, sizable CAPE of at least 1000-2000 J/kg is expected to be in place, supporting rainfall rates of 1-2 inches per hour or less than an hour. A broader footprint for 1-2 inches appears probable along with localized potential for small 3 to 4 inch totals embedded within. The shortwave is expected to reach the Mid-Atlantic coast by 00Z Friday, likely dissipating the flash flood threat for much of the southern Appalachians but seasonably high precipitable water values will remain near a remnant frontal boundary extending from the Carolinas to the Lower Mississippi Valley. WNW mid-level flow will be in place (quasi-parallel to the front) with the potential for convective development within the unstable and moist airmass. Recent QPF guidance does not paint a solid picture for excessive rainfall across the remaining Gulf Coast states, but localized QPF maxima are present in the 00Z model guidance and in the higher resolution GEM_regional, FV3 and NAM_nest solutions. Latest thinking is for spotty 2-4 inch rainfall maxima to occur within the broader Marginal Risk area. ....Mid-Atlantic into the Northeast... A pair of shortwaves will advance eastward on Thursday, one over the southern/central Appalachians and the other across southern Ontario/Quebec. Moisture return ahead of these features and with a warm front lifting northward is expected to support precipitable water values climbing into the 1.5 to 2.0 inch range for the NYC metro and southern/eastern New England. While the degree of instability is questionable, there appears to be a 2-4 hour window for training cells over any given area of the northern Mid-Atlantic into New England from roughly 18-06Z, perhaps lingering a bit longer for coastal ME. Recent heavy rain may play a role in flash flooding given increased sensitivity/soil saturation. ....Upper Midwest... The Marginal Risk area was expanded slightly to cover the latest spread across the eastern Dakotas into MM/IA. Increasing low level ascent across a slow moving warm front in southern MN is expected to contribute to the development of thunderstorms ahead of an eastward advancing shortwave from the north-central U.S. Precipitable water values are forecast to reach into the 1.2 to 1.5 inch range in the pre-convective environment and fairly strong jet divergence/diffluence will accompany the increasing low level jet for the evening/early overnight time frame. Instability appears to be one possible limiting factor with limited CAPE forecast for northern portions of the Marginal Risk area but this is where the better forcing for ascent will be located. Better instability across southern locations into Iowa may support higher rainfall rates but forcing will be more limited with southward extent. The end result may be a fairly localized flash flood threat for the region, focused over urban areas and other locations with poor drainage. ....Four Corners Region... The focus for convection Thursday afternoon will shift a bit further to the north as weakly anomalous moisture spreads into NV, UT and CO. There will also be the added factor of increased lift within the right entrance region of a zonally oriented jet streak aloft to track through UT/CO during peak heating. Once again, the signal for heavy rain is limited, but models tend to struggle with QPF in this region of the U.S. given the small scale nature to convection and local forcing near the terrain. Otto Day 3 Valid 12Z Fri Aug 11 2023 - 12Z Sat Aug 12 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR ARIZONA/NEW MEXICO...THE UPPER MIDWEST AND FROM THE OZARKS TO THE SOUTHEASTERN U.S. COAST... ....Ozarks, Lower Mississippi Valley into the Southeast... WNW flow in the mid-levels will be present from the central/southern Plains to the Southeast coast, located north of an elongated ridge centered over the west-central Gulf Coast. A remnant frontal boundary is expected to reside from the Southern Plains into the Carolinas with high precipitable water values nearby, locally in excess of 2 inches per the 00Z model consensus. Given portions of this region have been wet or will be seeing rainfall leading up to the start of the period on Friday, at least a localized flash flood threat will exist across this region, aided by any small scale vorticity maxima approaching from upstream, on the north side of the mid-level ridge. ....Upper Midwest... A longwave trough over the north-central U.S. will edge eastward from Friday into Saturday with an embedded shortwave tracking from the Dakotas into the Great Lakes region. Weakly anomalous moisture (+1 to +2 standardized precipitable water anomalies) is expected to advect northward into the Upper Midwest ahead of an advancing cold front over the central U.S., and north of what should be a fairly well-defined warm front extending ESE from the Upper Mississippi Valley into the Upper Ohio Valley. Daytime heating, increasing upper level ascent ahead of the shortwave trough and associated jets combined with increasing 850 mb winds in the 20-35 kt range should yield thunderstorms with areas of heavy rain. West to east repeating of storms will be possible, although similar to the previous day across the Upper Mississippi Valley, instability remains uncertain...especially with northward extent to support excessive rainfall totals. For now, a somewhat broad Marginal Risk was drawn to account for localized flash flood potential from localized 2-3 inch totals, perhaps locally above 3 inches through Saturday morning. ....Arizona/New Mexico... The 500 mb pattern from the 00Z models shows good agreement and continuity from previous runs for a closed low along the central CA coast and oval-shaped ridge centered over the west-central Gulf Coast. Southerly flow with embedded shortwave activity is forecast to move northward from Mexico into eastern AZ/western NM and upper level jet right entrance region lift may be in play for the region with jet energy directed from central AZ into CO. Precipitable water anomalies of +1 to +2 are also forecast to be present along the AZ/NM border which should help to support rainfall rates of 1-2 in/hr if sufficient instability is able to develop with daytime heating. A Marginal Risk was introduced to account for what appears to be the greatest probability of locally excessive rainfall within the highest QPF maxima depicted by the 00Z guidance. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_NsCjgXXA_-ljwl210xITSnZgXbupfmbmDLGAC30P4Q8= 1n2tTtQI91ycLiRqf0WcXmYR-Leumwz87TIllvMrnb8rbqc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_NsCjgXXA_-ljwl210xITSnZgXbupfmbmDLGAC30P4Q8= 1n2tTtQI91ycLiRqf0WcXmYR-Leumwz87TIllvMrL7YghBo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_NsCjgXXA_-ljwl210xITSnZgXbupfmbmDLGAC30P4Q8= 1n2tTtQI91ycLiRqf0WcXmYR-Leumwz87TIllvMrocGRTTE$=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) .