Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 02 2023 08:13:39 FOUS30 KWBC 020813 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 313 AM EST Thu Mar 02 2023 Day 1 Valid 01Z Thu Mar 02 2023 - 12Z Thu Mar 02 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM NORTH TEXAS AND EASTERN OKLAHOMA INTO ARKANSAS AND THE MID-MISSISSIPPI VALLEY... An exceptionally deep upper low emerging into the southern Plains will spawn a surface low over northeast TX that will deepen to levels that would fall below the CFSR for early March according to NAEFS. Synoptically, the upper low emerging out of NM will provide excellent diffluent flow over the South-Central U.S. with the nose of a 120 kt 500mb jet aimed at northern TX, eastern OK, and into the Ozarks by Thursday evening. At lower levels, a deep 850mb moisture fetch is set to intersect a slowly lifting warm front while the dryline to the west also acts to initiate thunderstorms in the afternoon. Anomalous PWs (NAEFS shows >90th climatological percentile) will be present across the region with 1.2-1.6" PWs over the eastern half of TX and into the Lower MS Valley. The 850-300mb layer looks to be quite saturated beyond 18Z Thursday with >90% RH values from northeast TX and eastern OK into the Middle MS River Valley. In addition, the 00Z HREF showed mean MUCAPE values >1,000 J/kg from the DFW metro area on north and east through southeast OK, southern AR, and northwest MS Thursday afternoon and evening. What further bolsters the flash flood threat is the mean layer 850-300mb flow paralleling the intersection of the developing warm front, the dryline, and the eventual cold front tracking across central TX Thursday evening. As convection fires along the warm from across northeast TX and southeast OK, the combination of 50-75 knots of effective bulk shear and elongated hodographs in much of the Moderate Risk area can foster and maintain supercells. Thunderstorms such as supercells that contain organized, strong mesocyclones are efficient rainfall producers, especially within the environment they are likely to encounter Thursday afternoon and evening when warm cloud layers could range between 10,000-13,000 feet in depth. Hourly rainfall rates will likely surpass 1.5"/hr with perhaps some of the most intense rates approaching 2"/hr in some cells. Given these rainfall rates and potential training, rainfall amounts of 3-5" are forecast within the stripe of most intense training convection with localized amounts up to 8" possible. By Thursday night, the line will congeal along the lifting warm front resulting in training convection over portions of the mid-MS Valley Thursday night. While there likely will not be as much instability available, the combination of ~500-750 J/kg of skinny MUCAPE, anomalous moisture, and highly curved hodographs should maintain efficient rainfall producing thunderstorms. The 00Z HREF did depict a 40-50% probability area for 6-hr QPF > 6-hr FFGs between 06-12Z Fri from Jonesboro, AR and north of the Memphis metro area to the Mississippi-Ohio Confluence. Soil saturation in eastern OK and central AR are closer to neutral according the NASA SPoRT-LIS 0-40cm soil moisture percentiles, but some areas (northeast OK, central AR) have 7-day precipitation anomalies of 200-300% according to AHPS. This forecast update did trim off some of the northern flanks of the Marginal and Slight Risks areas which go from eastern KS to the Ohio Valley where instability will be far more lacking than the southern tier of the risks areas. Mullinax Day 2 Valid 12Z Thu Mar 02 2023 - 12Z Fri Mar 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL THROUGHOUT MUCH OF THE LOWER OHIO VALLEY... As the strong wave of low pressure makes its way into the mid-MS River Valley early Friday morning, a robust 60 knot 850mb jet will deliver copious amounts of Gulf of Mexico moisture to the TN and OH Valley early Friday morning, then direct it into the Lower Great Lakes and northern Mid-Atlantic Friday afternoon. Soundings throughout the OH Valley Friday morning are well saturated with mean 850-300mb moisture percentages >90%. PWs Friday morning will approach 1.0" (>90th climatological percentile according the NAEFS 12-18Z Fri) with similar values in the northern Mid-Atlantic Friday afternoon. The biggest limiting factor wil be instability, which will be highest (roughly 500 J/kg MUCAPE) from the OH River Valley on south into the Mid-South where MUCAPE has a better chance to approach 1,000 J/kg. The mean 850-300mb flow will run parallel to the warm front over the OH Valley this morning where the best potential for training convection exists. Latest 00Z HREF show 50-70% probabilities for 6-hr QPF > 6-hr FFGs over southern IN and along the border into north-central KY. Instability will drop of as the best forcing and convection approaches the upper OH Valley, but 6-hr FFGs are as low as 1.0-2.0" in these areas. Solis are also a little more saturated in north-central OH and western PA where AHPS depicts 200-400% of normal rainfall over the last 7-days. The Mid-Atlantic will struggle to see the excessive rainfall rates their neighbors in the OH Valley will, but the anomalous moisture overhead and strong vertical ascent from strong isentropic ascent should yield modest 0.25-0.50"/hr rates in some areas. This could lead to some nuisance flooding and/or street ponding in poor drainage areas and more urbanized areas. Otherwise, did start to trim off some of the northern extent of the Marginal and Slight Risks areas over the Lower Great Lakes where recent trends have not only helped to further reduce available instability, but also become cold enough aloft for snow to be the primary p-type. Did expand the Marginal risk farther south to include more of MS and AL where multiple days worth of rainfall leading up to Friday has saturated the soils further. The cold front will be progressive, but given the strong thunderstorms and more saturated soils, opted to bring the Marginal Risk south due to the potential for very localized flooding. Mullinax Day 3 Valid 12Z Fri Mar 03 2023 - 12Z Sat Mar 04 2023 ....THERE IS A MARGINAL RISK FOR COASTAL AREAS OF SOUTHERN OREGON AND NORTHERN CALIFORNIA... The busy pattern in the Northwest looks to continue as another deep upper low develops off the Pacific Northwest coast. As the first wave of low pressure occludes off the coast, a second forming wave of low pressure will approach the southern coast of OR and the northern coast of CA Saturday afternoon. It will deliver a surge of 850-700mb moisture flux embedded within WSW flow, leading to some upslope enhancement into the Coastal Range. Latest deterministic guidance suggests 1.5-3" worth of rainfall is forecast, which happens to occur in areas where NASA SPoRT-LIS shows soil moisture percentiles in the 0-10cm layer ranges between the 70-90th percentile. Some of these areas have also seen snow in recent weeks at unusually lower elevations. NOHRSC does show up to 1-4" worth of snow water equivalent in some of the lower elevations <2,000 feet, which be more prone to melting this time around. Did maintain the Marginal Risk this forecast period, but it is worth noting there is the potential for snow at elevations >2,000'. Should snow levels drop further and snow become the primary precipitation type closer to the coast, the Marginal Risk may need further adjusting or could be dropped all together in subsequent forecasts. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!470PPycf10imifOfW0msHQCWD_v6qL353ajvf-38mRFp= KoHOgzEHrhX8WpPbd-aAI8tOuRlgoaZcANic6tULlkI1xOI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!470PPycf10imifOfW0msHQCWD_v6qL353ajvf-38mRFp= KoHOgzEHrhX8WpPbd-aAI8tOuRlgoaZcANic6tULVLSn6CI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!470PPycf10imifOfW0msHQCWD_v6qL353ajvf-38mRFp= KoHOgzEHrhX8WpPbd-aAI8tOuRlgoaZcANic6tULfi0yZZI$=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. 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