Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 03 2025 19:30:36 FOUS30 KWBC 031930 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 230 PM EST Mon Mar 3 2025 Day 1 Valid 16Z Mon Mar 03 2025 - 12Z Tue Mar 04 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS... ....16z update... 12z Hi-Res CAMs continue to depict run to run consistency with small adjustments. Timing of initialization still appears to be peaking in the 03-06z time window and from north (KS) to south northern TX. CIRA LPW sfc-850mb moisture shows the dry slot from yesterday's low level cyclone has started to erode with return moisture coming through the Concho Valley with the dry line orienting more WSW to ENE lifting north with Tds increasing from 10-20F into 50-low 60s. This orientation is a bit flatter to the deeper steering flow once cells do develop/mature after 06z and may allow for a tad higher potential for repeating/increased duration of those intense rainfall rates. Still forward speeds will limit overall totals and flash flooding is more likely to be limited to urban/typical poor drainage areas that could be very quickly inundated. Still overall expected totals are not likely to induce large areas of flooding conditions and a Marginal Risk continues to be appropriate risk level. Gallina ---Prior Discussion--- Robust convective development is expected tonight across portions of north central TX into central OK along a cold front. As impressive forcing (strong mid-level shortwave and upper-level divergence) overruns this front convection should quickly develop by 06z. Activity will likely organize into a line pretty quickly and then track eastward across north TX and OK. The expected quick eastward motion of the cold front will likely limit the extent of the flash flood risk. However with a 50-60kt south southwesterly low-level jet developing and deep layer flow largely parallel to the front, there should be some opportunity for brief convective training. While the duration of training will not be very long, it may locally be enough to exceed 2" in an hour of rainfall. Setups like this tend to overachieve rainfall wise, and that will likely be the case this time as well. Thus we still tend to think the new 00z HREF guidance is too dry (with the PMM suggesting localized totals of ~1.5"), and there may still be an uptick in model QPF in future runs (with totals in vicinity of one of the wetter models, the 00z GEM-Reg, still quite possible). Overall, not anticipating a significant flash flood threat, but localized, mainly urban, flash flooding is a possibility. Churchill/Chenard Day 2 Valid 12Z Tue Mar 04 2025 - 12Z Wed Mar 05 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MIDWEST AND PORTIONS OF THE MID-SOUTH INTO THE=20 SOUTHEAST... A pattern change will bring widespread precipitation to much of the Central and Southeast CONUS, as a cut-off low/upper-level trough and associated surface cyclone progress across the middle portion of the country. QPF maxima continue to be expected across two general regions, in and around the Mid-South (primarily in associated with precipitation from the broad open warm sector and subsequent cold frontal passage) and much of IA into northern MO (mainly from the pivoting low pressure center as it occludes). While flash flood impact coverage should remain somewhat limited in both areas (low-end Slight risk probabilities) with relatively dry antecedent conditions and a rather progressive storm system motion, the very dynamic nature of the system is still likely to bring periods of strong convection with localized totals nearing 2". While a portion of the QPF over IA is likely to be in the form of snow on the back-end of the low, the expected 2" total will also be relatively long duration, as rates will be limited to 0.5"/hr with a relative lack of instability. Over the Mid-South and Southeast, any localized 2" totals could occur over a much shorter period of time (as little as an hour) as instability which be much more plentiful. Barring any major changes in the forecast strength and progression of the storm system, impacts are likely to be capped at the Slight threshold with continued refinement to the outlook with the addition of more CAM data over the next day or so. Churchill 2100 UTC update =46rom eastern portions of the Southern Plains toward the Lower MS Valley...expect a well defined convective line to be present across eastern OK at the beginning of the day 2 period, 1200 UTC March 4. The previous slight risk area was expanded back to the west by approx 40 miles to match where the well defined squall line may be at 1200 UTC. This line should become increasingly progressive with time day 2, with the greatest excessive rainfall threat being early day 2 and for the farther west areas of the risk region. We did take out the slight risk area to the east of the AR/MS border region where the increased progression of the squall line should reduce the excessive rainfall threat. This does show up in the HREF neighborhood probabilities of 1"+ amounts in a 6 hr period, lessening with time after 0000 UTC March 5. In the vicinity of the KS/NE/IA/MO border area...we extending the slight risk area westward to cover far southeast NE and far northeast KS and decreased the slight risk coverage across south central to southeast IA. For the western extension, this was to cover where hi res guidance shows the potential for the pivot of the heavy rains in the comma head/deformation precip band in the 1800 UTC March 4 to 0600 UTC March 5 period. There is still some spread as to exactly where this pivots, resulting in a relatively large area. To the east of the pivot region, a dry slot expanding northeastward should decrease the excessive threat and is the reason for the trimming of the slight risk area over south central to southeast IA. Oravec Day 3 Valid 12Z Wed Mar 05 2025 - 12Z Thu Mar 06 2025 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF=20 SOUTHERN CALIFORNIA AND THE NORTHEAST... ....Southern California... A cyclone moving towards the region at the base of an amplifying and progressive West Coast upper level trough brings precipitable water values near the coastal ranges above 1", with 0.75"+ extending into the deserts. While the strength of the low-level inflow is modest -- 25+ kts -- the system has some instability to work with which could allow for hourly rain totals to reach 0.5", which would be most problematic in burn scars. Overall totals appear to be capped near 3". The inherited Marginal Risk appears reasonable, so changes to the area depicted were minimal. ....Northern Mid-Atlantic States/Southern New England... A progressive deep layer cyclone moves through the Great Lakes into Southeast Canada, bringing of shot of 60 kt inflow at 850 hPa into the region with precipitable water values rising to 1-1.25". Portions of PA, NY, and the southern coast of New England appear to have some instability to work with, as MU CAPE of 250-500 J/kg is forecast to be available. Along with low- to mid-level frontogenesis, hourly rain totals to 1.25" should be possible. While most of this area has had below average precipitation this past week, a good portion of New England north of CT/RI/southeast MA have 1-2" of snow water equivalent on the ground, so melting snow should be occurring as well considering the strength of the warm air advection which could raise both temperatures and dew points well into the 40s. The inherited Marginal Risk both fits the most recent QPF guidance and the ideas above, so changes to the area depicted were minimal. Roth 2100 UTC update. No changes made to the previous marginal risk areas over coastal Southern California into the upslope of the southern Sierra or across the northern Mid-Atlantic, southeast NY State into southern New England. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5FsY0c0hSoBQyByKfm6CCykKyk2JBPahTfiFW_tRLyS8= EeCA0uPa559rwvx6vOF5c2s-gMeVv1wAh4br9g05ZfVfwf4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5FsY0c0hSoBQyByKfm6CCykKyk2JBPahTfiFW_tRLyS8= EeCA0uPa559rwvx6vOF5c2s-gMeVv1wAh4br9g05hGQGfdY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5FsY0c0hSoBQyByKfm6CCykKyk2JBPahTfiFW_tRLyS8= EeCA0uPa559rwvx6vOF5c2s-gMeVv1wAh4br9g05PKCG2-g$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .