Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 04 2025 00:45:21 FOUS30 KWBC 040045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 745 PM EST Mon Mar 3 2025 Day 1 Valid 01Z Tue Mar 04 2025 - 12Z Tue Mar 04 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS... Forecast generally remains on track with convection expected to initiate along the cold front over central OK/KS around 03z-04z.=20 This activity should quickly grow upscale as impressive forcing=20 (strong mid-level shortwave and upper-level divergence) overruns=20 this front. The expected quick eastward motion of the cold front=20 will likely limit the extent of the flash flood risk. However, the=20 strong low-level jet and deep layer flow largely parallel to the=20 front may allow for some brief convective training. While the=20 duration of training will not be very long, it may locally be=20 enough to approach 2" in an hour of rainfall. An increase in low=20 level moisture transport after ~10z should support a further=20 expansion of convection from northeast TX into eastern OK early=20 Tuesday morning, with an increased cell merger threat and an uptick in hourly rainfall possible. Still forward speeds will limit=20 overall totals and flash flooding is more likely to be limited to=20 urban/typical poor drainage areas that could be very quickly=20 inundated. 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 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 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!7iseTFDP5QIMoEbMhLA-_VbBI0k7U_xoY61FLAg1ud4A= 8QN-hJyCVN-cDC5wZ8b1QtzgnYHd0Jiz8YGwGWC-UTB6pEQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7iseTFDP5QIMoEbMhLA-_VbBI0k7U_xoY61FLAg1ud4A= 8QN-hJyCVN-cDC5wZ8b1QtzgnYHd0Jiz8YGwGWC-EPKig2I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7iseTFDP5QIMoEbMhLA-_VbBI0k7U_xoY61FLAg1ud4A= 8QN-hJyCVN-cDC5wZ8b1QtzgnYHd0Jiz8YGwGWC-3sJBQfg$=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) .