Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 02 2023 20:26:54 FOUS30 KWBC 022026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 PM EST Thu Mar 02 2023 Day 1 Valid 16Z Thu Mar 02 2023 - 12Z Fri Mar 03 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM NORTH TEXAS AND EASTERN OKLAHOMA INTO ARKANSAS AND THE MID-MISSISSIPPI VALLEY... ...16Z update... Scattered showers and thunderstorms were present at 1530Z across portions of north-central TX within the warm/moist sector of a frontal system located across TX with a quasi-stationary front extending northeastward from a weak low between ABI and MWL, with a dryline extending southwestward from the low into portions of western TX. Shower activity was also noted farther east into portions of MS/AL/GA, to the north of a remnant frontal boundary, reinforced by outflow. The 12Z FWD sounding sampled fairly steep lapse rates between 700-500 mb near 8 C/km. As a strong closed low over AZ/NM reaches western TX later this afternoon, convective initiation is expected over central to northern TX, ahead of a strengthening surface low and near/south of the eastward extending warm front in the 18-21Z time frame per the 12Z hires consensus. Some of these early cells could generate a flash flood risk given the available shear as moisture and upper level dynamic forcing increase. Minor adjustments to the southwest were made to the existing risk areas across northern TX along with a small narrowing of the Moderate Risk in AR to account for the latest 12Z hires model guidance. Forecast reasoning in the previous discussion still remains on track. Otto ....previous discussion follows... 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 Fri Mar 03 2023 - 12Z Sat Mar 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL THROUGHOUT MUCH OF THE OHIO VALLEY... A potent, negatively tilted, mid to upper level shortwave is likely to translate northeastward from the MO/AR border into the OH Valley through Friday evening, followed by weakening into the Northeast through Saturday morning. At the surface, a deep surface low will track into IN/OH Friday afternoon with an occluded cyclone sweeping eastward to the south of the surface low. A triple point low is forecast to develop over the Mid-Atlantic region with the main warm conveyor shifting to the East Coast for 00Z with rapid low level moisture transport from the northern Mid-Atlantic into Long Island and nearby locations. Heavy rain is expected to be ongoing at the start of the period (12Z Friday) across the Lower Ohio Valley where training and repeating of rain is expected, with 1-2 in/hr rainfall rates just ahead of the surface low track along an eastward extending baroclinic zone, at the nose of a 50-70 kt 850 mb jet. While there is some uncertainty with how quickly heavy rain from the Day 1 period will shift east for the beginning of Day 2, strong upper level diffluence and coupled divergence will be present over the OH Valley within the left exit region of a 130-150 kt jet near 300 mb and the right entrance region of an anticyclonically curved jet stretching west to east across the Upper Midwest into the Northeast. Instability is an unknown, with only ~500 J/kg MUCAPE forecast over southern IL/IN/OH during the day, not considering the more aggressive 12Z NAM which had a less agreeable surface to mid-level low low track. Beyond ~00Z, instability is expected to only be a couple hundred J/kg as precipitation moves toward the eastern U.S. Portions of the risk areas have received well above average precipitation over the past 2 weeks which may play a role in runoff given elevated soil moisture content. The Slight Risk was drawn for the highest probabilities of 2 to 4 inches over the 24 hour period (12Z HREF and global guidance), centered just north of the OH River. Northern portions of the outlook area were trimmed southward a bit to account for likely frozen p-types, but there is enough uncertainty with the track of snow/ice to keep a Marginal Risk from portions of central IL into northern IN and northwestern OH. Farther east and beyond 00Z, the threat for excessive rainfall appears low but enough to support possible exceedance of the low 3 and 6-hr FFG in place across parts of the northern Mid-Atlantic region where rainfall rates locally over 1 in/hr and 1 to 2 inches in 3 hours will be possible. Across southern areas (TN into northern MS/AL/GA), the Marginal Risk was left in place given heavy rainfall which fell Wednesday night into this afternoon. While a progressive line of showers/thunderstorms along/ahead of the cold front is expected, very localized flooding could occur if a stronger cell or two is able to materialize in advance of the main line and briefly train over a sensitive location, posing a risk for very isolated flooding. Otto Day 3 Valid 12Z Sat Mar 04 2023 - 12Z Sun Mar 05 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 late Friday into Saturday. A shortwave on the south side of the mid-level closed low is likely to impact the coast of OR into northern CA at the start of the Day 3 period (Saturday morning) with a round of locally heavy rain. A second round of heavy rain is anticipated in the 00-12Z Sunday window, tied to another mid-level shortwave rounding the base of the closed low. Each shortwave is forecast to be accompanied by a surge in 850 mb winds of 40 to 50 kt, with wind direction ranging from S to W over the period, impacting the lower elevations of the Coastal Ranges. While precipitable water values do not look to exceed 0.5" along the coast and snow levels will be near 2000 ft ASL, lowering into Sunday morning, a localized threat for flooding will exist given the potential for locally heavy rain from the two rounds of precipitation. Steep lapse rates will be present from 850 mb to 500 mb (or above 500 mb given a low tropopause height) which should promote a showery regime with embedded heavier cores. 24 hour totals of 1-2 inches are expected along the coast but with localized totals near or just above 3 inches possible. Recent rain/snow has helped soil moisture to be above average for the region and WFO EKA noted minor flooding on some area roadways, so the additional rainfall from Saturday into Sunday could renew localized flooding concerns. As the event draws nearer, future forecast guidance may support adjustments or complete removal of the Marginal Risk area. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4R2sJD9uX5aejhiRcXkJ6IitCGc-YQgws3EPyWmVmQCa= FpEbZG3o3nndLPSD4g4HM_uhkXcfeHQgnkoyytHhHCN6_T4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4R2sJD9uX5aejhiRcXkJ6IitCGc-YQgws3EPyWmVmQCa= FpEbZG3o3nndLPSD4g4HM_uhkXcfeHQgnkoyytHhrKOqEVM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4R2sJD9uX5aejhiRcXkJ6IitCGc-YQgws3EPyWmVmQCa= FpEbZG3o3nndLPSD4g4HM_uhkXcfeHQgnkoyytHh7pCgEEE$=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) .