Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 20 2025 00:45:39 FOUS30 KWBC 200045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 845 PM EDT Mon May 19 2025 Day 1 Valid 01Z Tue May 20 2025 - 12Z Tue May 20 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN OKLAHOMA, NORTHWEST ARKANSAS, AS WELL AS SOUTHERN AND=20 NORTHWEST MISSOURI... ....Oklahoma through the Ozarks... Large scale forcing along with a very unstable environment has caused a proliferation of heavy convection with roots embedded in a formidable theta_E ridge alignment situated from Eastern TX all the way up into Northern MO. Current WV satellite indicates a strong shortwave ejecting out of the Southern Rockies making headway through the TX Panhandle allowing a strong upper ascent pattern to take place downstream. A sharp dryline oriented south to north across Central NE down through the eastern side of the TX Big Bend is a delineation point for the western edge of not only the most unstable airmass downstream of the mean trough, but a western foci for convective development. Smaller mid-level perturbations out ahead of the shortwave have helped initiate a swath of convection from the Red River up through Eastern NE with the core of heaviest precipitation focused south over OK/Eastern KS/Western MO where regional PWATs are hovering between the 90th and 98th percentiles as indicated via PWAT climatology from multiple sites (KFWD/KLZK/KSGF). This environment has generated a line of heavy thunderstorms oriented southwest to northeast following the mean flow with training convection occurring over much of Southeast OK up towards the Northwest AR border.=20 A broad expanse of 2-4" with locally higher has occurred within=20 the confines of that area with storms continuing to move through=20 the same locations. Totals of 3-6" with perhaps a few pockets of=20 6+" are forecast for the event total with heavy convection shifting focus further northeast through the evening. Strong LLJ positioned over the Lower Mississippi Valley will aid in the convective=20 posture that is forecast within Eastern OK up into the Ozarks of=20 AR/MO, an area that has been highlighted since the initial D1=20 forecast last evening. Hourly CAMs, both HRRR and RRFS are now in=20 lock-step on the expected convective evolution with heavy=20 thunderstorms likely overnight up into the above locations,=20 continuing northeast towards the Mississippi River basin overnight=20 with sights on areas near and south of the St. Louis metro.=20 Instability magnitudes will be lower within those zones, so=20 anticipating a drop off in hourly rates, but still expected 1-2"/hr maximum, enough to provide scattered instances of flash flooding=20 in that zone. The primary areas of interest will reside from Southeast OK up through the AR/MO Ozarks with a northern extension of the heaviest precip into the Springfield, MO area and surrounds. Considering the anticipated convective evolution and totals between 2-4" (or higher locally) across the above zones, the previous MDT risk was adjusted further west to encompass areas already getting affected by heavy precip and the expected areas off to the northeast. For more information on this setup, please see MPD #0290. ....Northwest Missouri... A second Moderate Risk was initiated in coordination with the local Kansas City WFO as more heavy thunderstorms are anticipated to maneuver eastward out of Northeast KS and affect areas that have seen significant rainfall and ongoing flooding within the north sideof the metro up close to the IA/MO line. Latest HRRR is=20 handling the area convection pretty well and is outlining the=20 secondary push of the activity to west of the metro area. There's=20 also potential for this convection to turn into a MCV as indicated=20 by a few of the CAMs outputs. This could exacerbate the flash flood scenario with activity lingering over an area that will be fully=20 saturated by the early morning time frame. There's still some=20 question as to intensity of the approaching storms this evening,=20 but the environment is ripe and with area FFG's at the lowest they=20 could be at this juncture, a targeted MDT risk for additional=20 totals of up to 1-3" on top of the already 2-5" has been added=20 across Northwest MO.=20 Other areas of note for potential flash flooding, but on a more localized basis include; Southern IA, Omaha metro, and Southwest=20 IL. These areas have been highlighted by multiple CAMs outputs for locally elevated rainfall totals stemming from convection moving into the area, most of which will be nocturnal.=20 Kleebauer Day 2 Valid 12Z Tue May 20 2025 - 12Z Wed May 21 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, OHIO, AND TENNESSEE VALLEYS... 20Z Update: 12z guidance remained pretty consistent within its run to run variance on the expected evolution across the Central and eventually Eastern U.S. CAMs output was strikingly similar in many regards with only deviations being the relative maxima across the Ohio Valley. In any case, the setup remains coincident with a SLGT risk prospect as the greatest forcing will occur the period prior with a step down in ascent magnitude moving into Tuesday. The corridor of interest remains over the Ohio River Basin down through KY and TN where instability will be greatest and the frontal positioning overhead would allow for steady motion of convection over the area. The good news is the instability field will be meager compared to what we saw during the D1 so the threat of widespread flash flood prospects are less likely. That said, antecedent wet soils across the region will promote a slightly better flash flood opportunity due to higher runoff capabilities as moisture absorption will be harder to occur given the soil moisture percentiles between 60-80% as indicated via NASA SPoRT. Areal average of 1-2" with locally as high as 4" are forecast across the Central Ohio Valley encompassing Southern IL/IN/OH down through the northern half of KY. This is coincident with a modest neighborhood probability (30-50%) for >3" with a sharp decline towards >5". Expected rates between 1-2"/hr max will curb the higher end scenarios, but the combination of saturated grounds and locally heavy rainfall will still induce scattered bouts of flash flooding. The previous SLGT was generally maintained given the above variables. Kleebauer ...Previous Discussion.. The deep trough across the middle of the country and its 2 separate shortwaves forcing the heavy rainfall from the Day 1/Monday period will phase the shortwaves by the start of the Day 2/Tuesday period, and also quickly weaken as the flow quickly becomes zonal. The result will be a widespread shearing front and surface low which will propagate east-southeastward from southern Iowa to northern West Virginia through the period. The low tracking somewhat parallel to its leading front will result in an elongated area of rain stretching from the northern Plains to the Mid-Atlantic. The greatest forcing will be near the surface low and the phased and weakening shortwave energy in the upper levels rapidly flattening to zonal flow. The further north and west along the line you go, the more "used up", the moisture and instability will be, favoring more stratiform rain, especially across Minnesota and Wisconsin. Due to the lack of instability which would be critical for heavy rain rates in this area, the Slight Risk was trimmed out of those states with this update. Elsewhere the Slight remains largely unchanged, especially for portions of Kentucky and West Virginia, which have been hard hit with recent storms, thus have sensitive soils to additional rainfall. The long duration of the rainfall may play as big a role as individual storms' rainfall rates in many areas. The surrounding Marginal Risk area was expanded in the Mid- Atlantic, particularly across Virginia, where recent heavy rains may have made soils in the most sensitive areas prone to isolated instances of flash flooding, even as far east as the coast. Elsewhere, only minor changes were made. Wegman Day 3 Valid 12Z Wed May 21 2025 - 12Z Thu May 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC AND DEEP SOUTH TEXAS... 20Z Update: There was little change in the inherited D3 MRGL across the Mid Atlantic with the best threat likely over Central and Western PA due to complex terrain and heaviest QPF footprint, and along the DC to NYC corridor due to saturated soils leftover from prior rainfall and the urbanization factors that exist to promote higher runoff potential. A general 1-2" with locally up to 3" is forecast across the region, but much of the rainfall will be of stratiform variety leading to flash flood concerns more in the isolated category within the areas mentioned above. For South TX, jury is still out on exactly how the convection within Coahuila will evolve, but considering the proxy of the threat and the fact that what does transpire will be fairly significant, the threat was still deemed close enough for potential impacts to validate a continuation of the MRGL risk inherited. Will monitor closely in the following forecast cycles. Kleebauer ...Previous Discussion.. The same storm system that brought heavy rains from the Plains to the Midwest today and Tuesday will shift east into the Mid-Atlantic and portions of the Northeast on Wednesday. However, lack of instability will greatly cut down on heavy rainfall rates in most areas by Wednesday. This should mean most of the rainfall is stratiform north of the Mason-Dixon line. Across Virginia however, marginal amounts of instability may help fuel a few stronger thunderstorms capable of isolated instances of flash flooding. Once again though, previous days' rain will likely be a major, if not dominant factor in the amount of flash flooding, as rates are not expected to get very high. Urban and small stream flooding along the I-95 corridor from DC to NYC will be a concern on Wednesday. For Deep South Texas, as with previous days, expect the strongest storms and the heaviest rainfall to remain over the mountains of northern Mexico on Wednesday. By Wednesday afternoon however, some of those storms may try to drift off the mountains and across the Rio Grande, leading to isolated instances of flash flooding as ample instability will allow the storms to become quite strong and capable of heavy rain. Considerable uncertainty remains as to how far east into Deep South Texas the storms will get, so the inherited Marginal was left as is with no changes for this forecast update. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7vMw2NUYt5ddYGoBOBqdMbc3Yfrz7onN0lgWORJTBqnY= SMHJjgFtm4IR-GdaTnIV0HruycCNMFITK2yQDg9QoNCsgSU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7vMw2NUYt5ddYGoBOBqdMbc3Yfrz7onN0lgWORJTBqnY= SMHJjgFtm4IR-GdaTnIV0HruycCNMFITK2yQDg9Qi_pDwyY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7vMw2NUYt5ddYGoBOBqdMbc3Yfrz7onN0lgWORJTBqnY= SMHJjgFtm4IR-GdaTnIV0HruycCNMFITK2yQDg9QOq3xhYU$=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. 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