Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 13 2023 07:26:50 FOUS30 KWBC 130726 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 325 AM EDT Thu Jul 13 2023 Day 1 Valid 12Z Thu Jul 13 2023 - 12Z Fri Jul 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF CENTRAL MISSISSIPPI AND SOUTHERN VERMONT...=20 =20 ....Northeast...=20 The 00z guidance has changed from earlier, which led to some morphing of the risk areas across the region. For the Moderate Risk area in and near southern southern VT, there is=20 potential for rounds of heavy rain over very saturated terrain. Broad mid-level northwest flow will continue across the Great Plains and Mississippi Valley, lifting into more southwest flow across the Appalachians and Northeast as the broad and expansive mid-level trough centered near the Hudson Bay persists. This will provide an environment allowing waves of convection moving through the flow. Most of this development on Thursday will occur along a slow-moving cold front and leading pre-frontal trough. PWs will rise to 1.5-1.75" on W/SW 850mb inflow of 20 kts, with ML CAPE surging during peak heating south of the front to as high as 3000 J/kg. The Moderate Risk area has the potential for overlapping of sufficient moisture, instability, and mean southwest flow parallel to the associated frontal boundary promoting possible cell training. Much of Vermont and New England remains vulnerable due to recent extreme rainfall, where areas of 300-600% of the seven day average rainfall has fallen. The above ingredients support 2"/hr totals, which could total up to 5" locally where training manages to persist for 2-3 hours. Within the Moderate risk area, expect several instances of flash flooding and impacts due to rapid runoff.=20 =20 ....Mid-South to Central Gulf Coast...=20 A Moderate Risk remains over central MS to account for potentially significant rainfall occurring early in the period, expected to begin by 12z Thursday. A noticeable upper shortwave on water vapor imagery is forecast to be drifting, and potentially reinvigorating, as it crosses MS/LA Thursday morning. The low level inflow is expected to be roughly double the mean 850-400 hPa wind, which is very efficient from a heavy rain perspective and generally a feature of warm core lows. The 00z mesoscale guidance suggests similar potential to what occurred over southern AR/northern LA Tuesday night into Wednesday morning, but there are early signs that is evolving differently than 24 hours ago as it is roughly 4-5 hours slower in forming, there is a larger angle between the 850 hPa inflow and mean 850-400 hPa wind, less (but still sufficient) instability available upwind, and initial convection is not training. The exact location is still not evident as convective coverage remains sparse and the MUCAPE gradient is slowly shifting northeast, but the Moderate Risk area is somewhat south of the mesoscale guidance due to the guidance's usual north to northeast bias with organized convection. Portions of this region have received over 300% of their average seven day rainfall, so soils should have some degree of saturation. As the shortwave continues progressing from the Mid-Mississippi Valley and LA, the threat for excessive rainfall during the afternoon and evening hours on Thursday is expected to shift south and east towards the north-central and northeast Gulf Coasts. Here, the 00z HREF probabilities of at least 3"+ and 5"+ in 24 hours continue to support a Slight Risk, which was somewhat reconfigured based on the latest guidance.=20 =20 ....Mid-Mississippi Valley through the Central Appalachians...=20 Convective trends among the global models and CAMs has become a bit more organized and intense from the Ohio Valley into the central Appalachians. Although much of the activity Thursday, which should be widespread, will be generally progressive, effective bulk shear of 30-40 kts will allow for storms to organize into multicells and then train to the northeast, with additional enhanced ascent occurring where flow can favorably upslope into terrain. The favorable thermodynamics across this region will support hourly rain totals to 2", which through brief training could produce 2-4" of rain or locally higher amounts in some areas, which would be particularly problematic in the coal fields of WV and KY. Confidence in the exact evolution and development remains modest. Spotty areas have seen above normal rainfall over the past week in this region. The Slight Risk area was drawn to best reflect the highest QPF probabilities overlapping some of the most sensitive soils.=20 ....Central to Southern High Plains...=20 Broad northwest flow on the upwind side of an expansive mid-level trough will drive multiple shortwaves down across the region on Thursday. These impulses will combine with modest upper diffluence within the tail of a departing jet streak to the east and convergence along a weakening front/surface trough moving across the area. A departing MCS in the morning could also produce some priming rainfall, but the greater risk for excessive rain will occur in the evening/at night. During that time, east-southeast low-level inflow will surge to 20-30 kts as a wave of low pressure develops across the High Plains along a stationary front over Colorado. This will draw PWs above 1.5 inches northward, with values as high as 1-1.25" in the High Plains of CO, combined with resupply of MUCAPE approaching 2000 J/kg. This moist ascent will drive a theta-e ridge northward, supporting expanding convection along a surface trough as another impulse moves overhead. Timing of the impulse and its affect on the impinging surface boundary is in question, and if they do not overlap the coverage of storms could be less. The Marginal Risk area captures the heavy rain signals seen in the 00z guidance, which remain scattered with not great agreement in placement. Hourly rain totals to 2.5" and local amounts to 4" are possible wherever cells can train/merge or mesocyclones can form. Some areas in the Marginal Risk have seen greater than 600% of their average seven day rainfall, so soils should be rather saturated. Slight Risk impacts cannot be ruled out.=20 ....Near the southern AZ/NM border... Precipitable water values southern AZ remain 1.5"+ this period, with rather weak flow, implying slow cell motions for any thunderstorms that might develop. ML CAPE should rise to 1000+ J/kg. Temperatures at 700 hPa are near 15C around the time of convective initiation, implying modest mid-level capping. This should allow for isolated to widely scattered thunderstorms with locally heavy rainfall, like has been seen over the past few days. Raised a Marginal Risk area for this region. =20 Roth Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-1f2_CyZrHi2EAI1vk_UDYQorbRE1eF83dd1KDYhRpzj= LYJNvd8F0FGBKWCQCGhgFpJt7wZ2qbCraAZHiEP1ZdFb-6k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-1f2_CyZrHi2EAI1vk_UDYQorbRE1eF83dd1KDYhRpzj= LYJNvd8F0FGBKWCQCGhgFpJt7wZ2qbCraAZHiEP19JMqeIM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-1f2_CyZrHi2EAI1vk_UDYQorbRE1eF83dd1KDYhRpzj= LYJNvd8F0FGBKWCQCGhgFpJt7wZ2qbCraAZHiEP1_mxp_cs$=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) .