Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 28 2023 08:23:46 FOUS30 KWBC 280823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 320 AM EST Tue Feb 28 2023 Day 1 Valid 12Z Tue Feb 28 2023 - 12Z Wed Mar 01 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20=20 Southern California... On Tuesday night, as an upper low drops southward from western WA to the northern CA/NV border, low-level inflow (at 850 hPa) picks up out of the Pacific at ~45 kts and precipitable water values rise towards 0.75". CAPE is expected to rise into the 100-200 J/kg range. The ingredients above indicate the potential for hourly rain totals up to ~0.4", which is hinted at within the 00z HREF probabilities of 0.5", which are towards 30% within the Peninsular and eastern Transverse Ranges of southern CA. Another complication from an excessive rainfall standpoint is the low snow levels expected with this system -- no higher than 4500 feet. The chance of excessive rainfall, which would be primarily for burn scars, is non-zero but probably still below 5% on a neighborhood basis. Any further increase in moisture, instability, or snow/freezing levels could lead to an upgrade to a Marginal Risk for southernmost California. Roth Day 2 Valid 12Z Wed Mar 01 2023 - 12Z Thu Mar 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF TENNESSEE...=20 =20 A low-level boundary wavers around TN this period as one cyclone ejects eastward from the Great Lakes and another moves into the Southern Plains from the West. Precipitable water values are expected to rise into the 1.25-1.75" range, which up to 3 sigmas above the mean for early March per 00z GFS guidance. Inflow from the Gulf is expected to range from 25-45 kts out of the west-southwest at 850 hPa into the region -- with effective bulk shear to match -- which should organize convection when combined with the 500-1500 J/kg of MU CAPE moving in from the south. The flow is generally unidirectional with height, indicating that training bands are possible. The amount of effective bulk shear allows for mesocyclones, which can be efficient rain producers, especially if two or more line up. Temperatures at 700 hPa are generally 3-6C, implying little to no mid-level capping inversion exists. There is a signal in the current guidance for local amounts in the 2-4" range, with the ingredients available supporting hourly rain totals to 2". Flash Flood Guidance values are particularly low in the southern Appalachians and modest across the remainder of the area -- roughly 2"/3 hours. The top 10 cm of soils are between the 60-75% saturated across TN, northern MS, and northernmost AL per NASA SPORT imagery. Widely scattered flash flood issues are anticipated.=20 =20 Roth Day 3 Valid 12Z Thu Mar 02 2023 - 12Z Fri Mar 03 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL ARKANSAS & THE MID-SOUTH... ....ArkLaTex to Mid-South and Lower Ohio Valley... A strong closed low moves from the Southwest towards the MO/AR border this period, allowing for divergence/difluence aloft to develop downstream. Ahead of the system, much of AR remains within 1.25-1.75" precipitable water value range for about 18 hours, which is 3 sigmas above the mean for early March. MU CAPE rises above 2000 J/kg in spots. Convergent inflow at 850 hPa as well as the mean 850-400 hPa flow backs from west-southwest to southwest and eventually south-southwest and its strength lies within the 25-55 kt range, with effective bulk shear to match.=20 The available ingredients allow for both training convective segments and mesocyclones, which can be efficient rain producers when two or more line up. The heavy rainfall pattern should be hemmed in from the south by 6-9C 700 hPa temperatures, implying a mid-level capping inversion exists across portions of LA/closer to the Gulf coast. The guidance advertises local amounts in the 3-7" range. There is some limited overlap of the heavy rainfall within the days 2-3 forecast across southwest TN where flash flood guidance values are already modest. There's a broad area of 60-75% saturation in the top 10 cm of soils from the ArkLaTex through AR, TN, northern MS, and southern KY per NASA SPORT imagery. Along with the heavy rainfall forecast for Thursday and Thursday night, the stripe of heavy rain that fell across Little Rock AR about 4 days ago is of concern. Considering all of the above, increased the threat level to Moderate Risk and included portions of AR, the Bootheel of MO, southermost IL, western TN, and westernmost KY. Eastern portions of the risk areas cover where heavy rainfall is expected on Wednesday/Wednesday night and additional rainfall is forecast Thursday/Thursday night. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6HMO57jF_BQYR_WTzgz0AW55ZRzz0k4ZyL5i_GfAnoRM= 1Ua57QQAJN7GGkld5ZYKRkIokv8YOQtpcswh2J_W2efeQdc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6HMO57jF_BQYR_WTzgz0AW55ZRzz0k4ZyL5i_GfAnoRM= 1Ua57QQAJN7GGkld5ZYKRkIokv8YOQtpcswh2J_WJzVP1MY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6HMO57jF_BQYR_WTzgz0AW55ZRzz0k4ZyL5i_GfAnoRM= 1Ua57QQAJN7GGkld5ZYKRkIokv8YOQtpcswh2J_W-fjwf9w$=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) .