Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 14 2022 00:44:38 FOUS30 KWBC 140044 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 743 PM EST Tue Dec 13 2022 Day 1 Valid 01Z Wed Dec 14 2022 - 12Z Wed Dec 14 2022 .....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY... At 00Z, a Pacific cold front/dryline was analyzed southward from eastern Oklahoma into east-central Texas while a NNE-SSW oriented pre-frontal line of thunderstorms stretched from eastern Arkansas into northern Louisiana and eastern Texas. The leading outflow boundary associated with the axis of thunderstorms has been acting as an effective front with a 10-15 degree drop in surface dewpoint in its wake. Thunderstorms during Tuesday afternoon/early evening have been largely progressive toward the northeast with a general eastward translation of the thunderstorm axis, but with embedded areas of short term training, rainfall rates of 1-2 in/hr have occurred in the broad vicinity of the ArkLaTex. Within areas of training, localized peak hourly rainfall totals in excess of 2 inches have been observed as well. As a shortwave, currently seen on water vapor imagery over eastern Arizona, tracks eastward into western Texas overnight, the effective front is expected to stall from southeastern Texas into central Louisiana as a surface wave forms near the upper Texas coast between 09-12Z. The boundary is forecast to stall in an orientation that matches the mean steering flow (SW to NE), supporting an increased risk for training areas of heavy rain from central/southern Louisiana into western Mississippi. Precipitable water values currently near 1.5 inches (00Z soundings from LCH and LIX) will likely increase through late tonight, nearing 1.7 or 1.8 inches by 12Z along the Gulf Coast as 30-40 kt of flow at 850 mb allows a continued source region of moisture/instability from the relatively warm Gulf of Mexico. Aloft, flow should be diffluent to the southeast of a 130-140 kt jet max located on the southeast side of a large closed low in the central/northern Plains, helping to support vertical lift. The greatest concern for training thunderstorms appears to begin in the 06-12Z time frame in the vicinity of the central LA/MS border with a southwest to northeast stripe of heavy rain that is expected to peak in the 3-5 inch range, driven by rainfall rates of at least 1 to 2 in/hr, but rates of 2-3 in/hr cannot be ruled out locally. Farther north from the Arkansas/Mississippi border into the mid-Mississippi Valley, instability is forecast by recent runs of the RAP to remain below ~500 J/kg, which should limit rainfall intensity. However, the threat for training will exist across these northern locations as well where northern portions of the axis of heavy rain push out ahead of those to the south, setting up repeating/training of heavy rain with peak rainfall rates of 1-2 in/hr. Flash flood guidance lowers with northward extent as well, as low as 2 inches in 3 hours in the vicinity of the Missouri Bootheel. Adjustments for this update were mainly to trim back the western edge of the Marginal and Slight Risk areas over northeastern Texas, southeastern Oklahoma and western Arkansas where heavy rainfall has ended. Otto Day 2 Valid 12Z Wed Dec 14 2022 - 12Z Thu Dec 15 2022 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN LOUISIANA TO CENTRAL ALABAMA... 21Z update... The latest guidance is suggesting that there will be redevelopment across southern Louisiana once the initial line passes. This will reinvigorate convection across the Lower Mississippi Valley/Central Gulf Coast region allowing hourly rates to flourish from 1 to 2.5 inches/hour while slowing their eastward progression. A broad area of 1 to 3 inches will extend from the Central Gulf Coast to as far north as the southern Ohio. There is a growing signal for an axis of 2 to 4 inches to align from south-central Louisiana to central Alabama with very isolated maximums of 6 to 8 inches possible. A Moderate Risk was hoisted over portions of southern/eastern Louisiana, southern Mississippi and western Alabama given the favorable environment for sustained, slow moving convection and enhanced moisture supply. The Marginal and Slight Risk areas were expanded further north to reflect the latest WPC QPF and latest trends. Campbell A broad slight risk area was maintained with some minor modifications over the upcoming day 2 period from the Central Gulf coast, northeastward into the southern Appalachians-southern OH Valley. The day 1 heavy precip event stretching from the eastern portions of the Southern Plains into the Lower to Mid Mississippi Valley will continue its eastward march during day 2, stretching from the Central Gulf coast into the Southern Appalachians. The overall favorable large scale pattern for organized frontal and pre-frontal convection will continue during day 2, with strong frontal convergence and well defined upper difluence continuing in the axis of above average PW values, along with continued anomalous 850-700 mb moisture flux coming off the central to eastern Gulf of Mexico ahead of the associated eastward moving cold front. We contemplated upgrading to a moderate risk area for portions of far northeast Mississippi, northern Alabama into northern Georgia where recent heavy rains have increased stream flows and lowered flash flood guidance values. Our concerns are that models may still be too far to the north with their respective max precip areas. The greatest instability for the day 2 time period is expected farther to the south and closer to the immediate Gulf coast. This may support the heaviest precip falling farther to the south than models suggest, across areas that have not had as much precip recently and subsequently have lower stream flow and higher flash flood guidance values. For these reasons, no changes were made with the risk level, keeping it at slight for the time being. Hurley/Oravec Day 3 Valid 12Z Thu Dec 15 2022 - 12Z Fri Dec 16 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6gvmZvv3FedJ-j2zVuocDvZdaosz9UCH7m7WKxDj2opR= 99IcsQ4og2tgUB0XPzbsIaa-CcMPGBz95EpnfA31MXkWuZQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6gvmZvv3FedJ-j2zVuocDvZdaosz9UCH7m7WKxDj2opR= 99IcsQ4og2tgUB0XPzbsIaa-CcMPGBz95EpnfA31UkfoKcU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6gvmZvv3FedJ-j2zVuocDvZdaosz9UCH7m7WKxDj2opR= 99IcsQ4og2tgUB0XPzbsIaa-CcMPGBz95EpnfA314CDbBhk$=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) .