Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 03 2023 20:02:41 FOUS30 KWBC 032002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 401 PM EDT Mon Apr 03 2023 Day 1 Valid 16Z Mon Apr 03 2023 - 12Z Tue Apr 04 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL IN AND AROUND SOUTHERN AL, SOUTHWEST GA AND THE FL PANHANDLE, AND ACROSS SOUTH CENTRAL MI... 16z Update: The Marginal risk looks in good shape over southern AL/GA. Localized 2" in an hour rain will continue along the west to east training line, with totals locally exceeding 3". There is enough of a southward propagation that amounts over 5" still seem unlikely (outside of an isolated occurrence). Thus, overall think the flash flood risk remains localized in nature. Will need to watch the convection developing in the westerly flow behind the main line, as some of this activity could intensity and try to organize. But at the moment think this will remain pretty small scale in nature and probably not persist long enough at any one location for anything more than a localized flood threat. We did opt to introduce a Marginal risk across portions of south central MI for tonight, where a band of elevated convection is likely along an axis of frontogenesis. Not much instability, but just enough to get some locally higher rates embedded within the broader rain shield. Generally looking like hourly rain will peak around 0.4", with event totals generally 1-2" along a narrow corridor. In general, exceedance of 1,3,6 hr FFG currently seems unlikely...however this part of MI is experiencing elevated streamflows and above average soil saturation. Thus runoff should be pretty efficient from this rain, and some flooding impacts are possible...especially over any more sensitive basins and/or urban locations. Chenard ....Previous Discussion... No changes were made to the Marginal Risk area across portions of the Southeast Monday. The mesoscale convective complex (MCC) currently tracking across MS and AL will continue tracking southeast, as the heaviest convection follows the instability towards the Gulf Coast. The 25-35 kt LLJ has turned more southwesterly, and will continue weakening through the day into tonight. There remains some 850 mb moisture convergence along and ahead of the MCC, which is likely to continue supporting heavy convection into the morning. There is good agreement among the CAMs that the convection tracking southeast across southern AL and into southwest GA and the FL Panhandle will cover a smaller area with time, as the lighter rain to the north of the convection largely dissipates by mid-morning. HREF probabilities remain similar to the previous discussion, where neighborhood probabilities of 3" or more peak at over 60%, and 5" or more are around 30% in the southeast corner of AL. HREF EAS probabilities for 3" are closer to 15%. The combination of the expected shrinking coverage of heavy rain with time and these similar HREF probabilities suggest that the Marginal Risk remains in place. It appears likely that by sunset, much of the convection will have dissipated. Thus, with the focus for any potential flash flooding rains focused on the first half of the day, it appears unlikely that a Slight Risk will be needed at this point, but trends will continue to be monitored. Wegman Day 2 Valid 12Z Tue Apr 04 2023 - 12Z Wed Apr 05 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE MISSISSIPPI VALLEY AND GREAT LAKES REGION... ....19z Update: Generally just minor adjustments to the two Marginal risk areas. With the southern risk area from TX into MO the cold front should remain fairly progressive through 12z Wednesday. Thus any more substantial flood risk would probably require a more robust development of convection along the pre-frontal trough. This earlier activity could certainly exhibit some southwest to northeast training, with eventual merging into the cold frontal convection later at night. For now think the risk of flooding remains Marginal, however will continue to monitor trends. Over WI/MI there are some signs of a QPF uptick and PWs are impressive. While some heavier rains may get into the deeper snow pack areas, the higher low level temps/dewpoints do not look to get quite that far north through 12z Wed. Thus will maintain the Marginal, although there may be a narrow corridor from southeast WI into north central MI that could pick up some 2"+ totals. Chenard ....Previous Discussion... ....WI and MI... A rapidly strengthening low will track from the CO/KS border Tuesday morning to near the WI/MN border by Wednesday morning. While the low's primary impacts will continue to be on the cold/blizzard side of the low, the threat for flash flooding in this area persists. With the low track now turning more northerly with time by Tuesday night, it appears more likely now that the warm front will continue to make progress northward, into areas that have maintained a healthy snow pack to this point. Thus, while more snowmelt is added to the equation, offsetting that will be the warm front being more likely to continue moving northward across the U.P. and into Canada. Thus, overall rainfall in the Marginal Risk area has come down a little bit. Since this area has been hit with precipitation of all kinds lately, the warm up associated with the warm sector of this low is likely to contribute to flooding concerns related to the rainwater flowing over frozen ground, resulting in less absorption into the nearly saturated soils, which is in addition to any snowmelt. Thus, FFGs in this area are quite low, and even a period of steady moderate rainfall rates may cause localized flash flooding issues. According to NOHRSC, there is a tight gradient between no snow from roughly Milwaukee south in WI and south of Clare in the middle of the L.P., to there being a foot of snow or more by Green Bay and up towards Traverse City. Much of the Marginal Risk area does not have any snow on the ground, and the rainfall totals of around an inch on average for northern portions of the Marginal risk area are likely to be absorbed where they fall on the deep snow pack, so the main concern for snowmelt is the small gradient area with less than a foot of snow on the ground. Fortunately this being a relatively small area due to the tight gradient of snow pack will work in favor of keeping flooding concerns due to snowmelt down a bit. For now, the Marginal risk was maintained due to the offsetting of somewhat less rainfall over more area potentially impacted by snowmelt concerns, but any increase in rainfall amounts, especially over the somewhat lesser snow pack on the northern tip of the L.P. and the low FFGs could result in the need for an upgrade. The Marginal Risk area was trimmed a bit mostly on the southern side out of nearly all of IA and IL as the best forcing and best chances for heavy rain are now expected to be further north and east. ....East TX to Southern IL... The greatest threat for rains capable of producing flash flooding in this area will be tied to the movement and strength of convection that develops along the cold front as it moves eastward across the Marginal Risk area late Tuesday night. By Tuesday night, the entire atmospheric column will be unidirectional southwesterly. As such, this will favor not only a slowing of the eastward progress of the cold front, but also the potential for training convection ahead of it. The LLJ will increase in intensity to 60 kt across AR and into IL, as PWATs increase to over 1.5 inches. This healthy amount of energy all tracking along a relatively narrow corridor will also favor more intense training thunderstorms. The two factors working against each other in this case will be forcing and cold frontal movement. The further north you go, the better the forcing will be as the upper level low pivots away into Canada, but that same forcing will allow more of an easterly push behind the cold front which will keep it moving, making it less likely convection will be able to train over the same areas. On the flip side, further south into Texas, there will be less forcing allowing thunderstorms to both form and grow more intense, but the front becoming nearly stationary will support those storms training over the same areas. Thus, there is some potential here too to need a Slight Risk, but where such an area would be drawn is the biggest question. Much of the Marginal Risk area got hit recently with heavy rains, from the Arklatex region earlier this evening, to central AR up to southern IL just 2 nights ago. Thus, soils are largely saturated, and certainly could be prone to flash flooding should training thunderstorms occur over these same areas. The Marginal Risk was expanded south further into Texas to account for a higher likelihood of training convection in these areas Tuesday night. No significant changes were made to the Marginal risk area in the north. Wegman Day 3 Valid 12Z Wed Apr 05 2023 - 12Z Thu Apr 06 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE UPPER TEXAS COAST INTO KENTUCKY... 19z Update: Main change to the Slight risk was a modest expansion northward into KY based on latest guidance trends. Forward progression of the cold front really slows by Wednesday into Wednesday night across the Slight risk area, which is supportive of training or multiple convective rounds. The stronger dynamics lift to the north (supporting the stalling front), however the right entrance of the upper jet should provide ascent over the front through the period. With low level flow weakening and the instability corridor becoming quite narrow, there is some question regarding CAPE persistence over the risk area. Some threat that instability gets eroded resulting in less intense convection as the period progresses. Given that uncertainty think maintaining the Slight risk is the way to go for now. However this event certainly has some higher end Slight potential given the slowing front and anomalous moisture, and can not rule out a smaller MDT being needed at some point. As mentioned below, as we get into the timeframe of more of the CAMs we'll be able to make a better assessment on whether any upgrade might be needed. To the north, the Marginal risk was expanded across the OH Valley and into portions of the Great lakes and interior Northeast. Convection will be pretty progressive here, but briefly intense rainfall rates could cause localized issues. In northern lower MI the true warm sector temps/dewpoints finally should overrun the higher snowpack which could result in some runoff issues. Same goes for portions of Upstate NY, where heavier rain rates and warmer air over snow may result in some runoff issues as well. Chenard ....Previous Discussion... The same cold front as discussed on Day 2/Tuesday continues impacting the area into Day 3/Wednesday. While northern portions of the cold front across the Midwest and Northeast continue moving eastward through the period, the southern portion of the front continues to remain largely stuck in place. The influx of Gulf moisture into the front is expected to increase through this period, as PWATs exceed 1.75 inches along portions of the TX coast into LA during the day Wednesday. In fact, the flow southeast of the front turns towards the southeast (i.e. orthogonal) to the axis of the front, which in turn may begin moving the front in Texas slowly towards the northwest with time by Wednesday night. Note however that the flow is only about 10-15 kt, so any movement of the front will be S...L...O...W. Meanwhile, broad southwesterly flow perpendicular to the front will continue through Wednesday night along northern portions of the Slight Risk area from MS northeast into TN. Further, the forcing in the upper levels behind the front is also expected to strengthen on Wednesday as compared with Tuesday. As even colder air wraps around the low across the northern Plains, the upper jet will be invigorated into hyperdrive, increasing in intensity to over 150 kt out of the southwest from MO through MI. This puts the Slight Risk area in the favorable right entrance region of the jet, which will only further enhance overall lift, which will result in stronger thunderstorms. Finally, many portions of the Slight Risk area from northern LA and MS tonight further north into TN just 2 nights ago were slammed with severe weather and training thunderstorms. Thus, soils in these areas are already at or near saturation, and the prospect of a stalled out cold front that could result in 2-4 inches (locally higher) of rain over these same areas only further exacerbates the flash flooding risk. Should rainfall amounts continue to increase, which is a distinct possibility especially once the CAMs (which handle convection better than the global models, hence their name) get a chance to analyze the atmospheric conditions and expected resulting thunderstorm activity, a Moderate Risk upgrade is quite possible with future shifts. This will once again be largely dependent on if the front remains largely stuck, and the ample Gulf moisture continues to be forecast to be rapidly advected along the front by a strong LLJ. Stay tuned. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9dz3JLmWQ31aCS1SC9WP7wE-bRBwEq_j6ccdJqyIAZ_z= 5m461efC_y1_5EhGfQ_6-ZIvqAWZyJvTqFPJ9dxFdo7yMjk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9dz3JLmWQ31aCS1SC9WP7wE-bRBwEq_j6ccdJqyIAZ_z= 5m461efC_y1_5EhGfQ_6-ZIvqAWZyJvTqFPJ9dxFh9LASzQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9dz3JLmWQ31aCS1SC9WP7wE-bRBwEq_j6ccdJqyIAZ_z= 5m461efC_y1_5EhGfQ_6-ZIvqAWZyJvTqFPJ9dxFNzAdvDE$=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) .