Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 28 2023 22:24:32 FOUS30 KWBC 282224 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 523 PM EST Tue Feb 28 2023 Day 1 Valid 16Z 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 WA/OR to the northern CA/NV border, westerly low-level inflow (at 850 hPa) picks up out of the Pacific at ~45 kts just prior to 12Z Wednesday and precipitable water values rise towards 0.75". CAPE is expected to rise into the 100-200 J/kg range along the coast while dropping off quickly with inland extent. The ingredients above indicate the potential for hourly rain totals up to ~0.5", which is hinted at by some members of the 12Z HREF for non-frozen precipitation. However, low snow levels expected with this system, no higher than 4500 feet, should keep the majority of higher precipitation in the form of snow. The chance of excessive rainfall, which would be primarily for burn scars, is non-zero but probably still below 5% on a neighborhood basis. While increases in moisture, instability, or snow/freezing levels could lead to an upgrade to a Marginal Risk for southernmost California at some point today, current thinking is to not include a risk area. Otto/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 The large scale pattern will feature a strengthening 500 mb low advancing eastward across the southwestern U.S. through Thursday morning, with the 12Z GFS indicating standardized height anomalies between -3 and -4 over Arizona and New Mexico. Meanwhile, anomalous ridging will be in place (as it has been much of 2023) from the Gulf of Mexico into the Caribbean, allowing low level moisture return from the western Gulf into the Lower Mississippi Valley while mid-upper level moisture streams downstream from the subtropical eastern Pacific into the southern U.S. While the timing of the 12Z GFS appears a bit fast with the 500 mb closed low by the end of the period (12Z Thursday), it is reasonable with the overall evolution of fronts, moisture and upper forcing. According to the 12Z GFS, precipitable water values are expected to rise into the 1.25-1.75 inch range (2 to 3 standardized anomaly) from northeastern Texas into the Ohio and Tennessee Valleys. The increase in moisture is likely to support MLCAPE of roughly 500 to 1000+ J/kg with MUCAPE up to about 1500 J/kg from Arkansas into the Tennessee Valley. Warm advection showers early in the period ahead of a returning low level boundary lifting north across Tennessee will be followed by a more robust surge in showers/thunderstorms between 21-00Z ahead of/along a slow moving cold front. Southwesterly 850 mb winds are forecast to back while increasing into the 35-50 kt range over the Lower Mississippi Valley in conjunction to a lead shortwave originating from western Texas. Steering flow roughly parallel to the initiating boundary will support a favorable pattern for training. Sufficient speed sheer will be present for organized storms and repeating rounds of heavy rain appear a good possibility. Rainfall rates of 1-2 in/hr are likely but locally higher totals will be possible and a decent consensus of models indicate the potential for 2-4 inches from eastern Arkansas into Tennessee and northern Mississippi/Alabama. Portions of this region have seen above average rainfall over the past few weeks which has lowered flash flood guidance (FFG) lower than average. However, the lowest FFGs are along the Ohio/Kentucky border into the central Appalachians with 1 inch in 3 hours, accounting for a northward expansion of the Marginal Risk compared to continuity. Other changes included trimming the Marginal from northeastern Texas where QPF and probabilistic guidance didn't higher totals compared to what would be needed based on FFG and antecedent rainfall. Otto 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... The strong closed low from Thuesday morning will eject across the Southern Plains, reaching in the vicinity of the Ozarks Friday morning. Increasing magnitudes of divergence and diffluence will be likely ahead of the advancing closed low across portions of the Southern Plains into the Mid-Mississippi Valley. At the surface, a low will strengthen as it moves from the Red River Valley into Missouri or Arkansas. Anomalous precipitable water values of 1.25 to 1.75 inches will remain ahead of the system's cold front and along the eastward extending warm front. Strong forcing for ascent combined with a favorable pattern for training/repeating storms with steering flow parallel to low level convergence axis will promote a longer duration heavy rainfall event with rainfall rates of at least 1-2 in/hr. Much of the 12Z guidance supports 24 hour totals of 3-5 inches and potential for 5+ inches is certainly plausible from the eastern Red River Valley into the Lower Ohio Valley. The latest guidance showed timing differences with the closed low with the 12Z GFS fastest, though the 12Z NAM has trended faster. Meanwhile, the ECMWF/UKMET/CMC have trended slower or remained steady with the timing of the closed low, translating into fairly large placement differences with where the heaviest rainfall will occur. The ECMWF/UKMET/CMC deterministic and/or probabilisitc QPF has better probabilities for heavier rainfall to the south and/or west of the GEFS, SREF and NAM. While the southern and eastern extent of the risk areas remained unchanged from continuity, some expansion to the north and west were made to account for some of the placement differences in QPF. While Moderate Risk impacts continue to appear likely, shifts in location may occur until confidence increases with the timing/placement of the upper low. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9THFI2ADf47LmJViP8eRUCemD-tvackBZeNruxh6kSkc= 4VzITmXcm51k9efNnmIlCJt5cpEAc-segMRzvb3Ag6PsTz0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9THFI2ADf47LmJViP8eRUCemD-tvackBZeNruxh6kSkc= 4VzITmXcm51k9efNnmIlCJt5cpEAc-segMRzvb3AITb7hWQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9THFI2ADf47LmJViP8eRUCemD-tvackBZeNruxh6kSkc= 4VzITmXcm51k9efNnmIlCJt5cpEAc-segMRzvb3Af5FINO8$=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) .