Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 26 2023 08:47:36 FOUS30 KWBC 260847 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 446 AM EDT Sun Mar 26 2023 Day 1 Valid 12Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL ALABAMA AND GEORGIA... There has been quite a notable flip-flop in all of the guidance as pertains to the weather expected to unfold across portions of the Southeast today. A stalled out front remains in place across the Slight and Marginal Risk areas from Louisiana to southern North Carolina. This front is already serving as a focus for convection from central MS through far western GA. The convective activity is only expected to increase for the rest of the predawn hours and into the morning. 30-40 kts of southwesterly 850 mb flow is drawing plentiful Gulf moisture northward into the front. Meanwhile, a pair of subtle disturbances are expected to track eastward along the front, causing local increases in both convective coverage and severity. This is probably the largest change from 24 hours ago, when we had eliminated even a Marginal risk of flash flooding! These disturbances acting to organize the convection along the front now will be quite effective at taking advantage of as much as 2,500 J/kg of MUCAPE and somewhat unusual levels of atmospheric moisture at 2 sigma above normal and PWATs exceeding 1.5 inches. Uncertainty remains above average given the rainfall event is already beginning, as to where the heaviest rain is expected to fall. While most of the CAMs have highlighted the Slight Risk area, the primary area of uncertainty is how a subtle upper level disturbance interacts with the ongoing convection and associated cold pools this evening. The wave will track well north and west of the area, but nonetheless will have enough impact on the ongoing thunderstorms to result in a local increase in storm coverage and intensity. Some of the CAMs suggest this will result in the heaviest convection displaced well to the north of the surface front, highlighting Birmingham and Atlanta, whereas others think the primary forcing will stay with the front further south, resulting simply in an area of strong thunderstorms as the latest salvo in the wave train. The impacts from this difference would be how severe the likely ongoing flooding can get if the stronger storms track over the same areas, or if they're sufficiently displaced to the north to result in fewer impacts (despite the lower FFGs due to yesterday's rain across northern AL). If the former scenario pans out where the storms stay with the main surface front, then there's some potential for extreme rainfall totals exceeding 6 inches (as depicted in the 00Z HRRR primarily), and the potential need for an additional upgrade to a Moderate Risk. As regards training storms, the potential is very high, and in fact is already happening in the Slight risk area of eastern AL/western GA along the front. These storms will continue training at least through the morning hours, at which point the overall convective activity will wane a bit with the typical weakening of the LLJ. Convection then increases again with the approach of the second wave in the late afternoon and evening hours. For all convection today, the motion of the storms will be east-northeastward, which is parallel to the surface front that is not expected to move much during the day. Thus, the flow orienting parallel to the front supports continued training through the day. Already, currently ongoing severe thunderstorm activity is producing local rainfall rates up to 1.5 inches per hour, and that intensity of convection is expected to continue over the next several hours, and again late this afternoon and evening. For the surrounding Marginal Risk, expansions were made in all directions with this morning's update. Some convection has already developed in northwestern Alabama and central and southern Mississippi as of this writing, and continued scattered convective development is likely through the day in both areas. The increased coverage and intensity required an eastward expansion of the Marginal Risk to the Carolina coast, as all of the convection will track along this corridor (albeit likely in a somewhat weaker state than areas further west), and therefore training convection once again will be the dominant factor favoring development of flash flooding, particularly if those stronger storms move over urbanized areas. ....Potential for Future Risk Changes... Depending on storm coverage through the morning, and if guidance remains consistent on continued training storms in much of the Slight Risk area late this afternoon and this evening, a Moderate Risk may be needed. Wegman Day 2 Valid 12Z Mon Mar 27 2023 - 12Z Tue Mar 28 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. By Monday morning, most of the convective activity associated with the front in the Southeast will have drifted south of the main areas impacted by the strong storms today, i.e. closer to the Gulf Coast. With little in the way of overlap from Day 1 and good agreement overall that convective coverage and intensity will be greatly reduced as compared with Sunday's storms, a somewhat low confidence forecast of no flash flooding remains for this forecast update. However, should there be any northward trend with the forecast convection on Monday, then a Marginal Risk will be needed. Wegman Day 3 Valid 12Z Tue Mar 28 2023 - 12Z Wed Mar 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE CENTRAL CALIFORNIA COAST AND ADJACENT MOUNTAINS... ....Central California Coast... In coordination with the MTR/Monterey, CA and LOX/Los Angeles, CA forecast offices, the inherited Slight Risk area was expanded both north and south with this morning's forecast update. An upper level polar cutoff low begins the day Tuesday morning centered off the Oregon coast, and tracks southeastward to be centered over the northern California coast by Wednesday morning. A 100 kt westerly jet streak wrapping around the low will direct a plume of Pacific moisture and precipitation into California as it tracks south. PWATs will approach 3/4 inch, which is about 2 sigma above the climatological normal. This will bring with it a brief increase in snow levels as high as 7,000 ft during the day Tuesday. The attendant surface low will follow a similar track as the upper level cutoff low through this period, making it a vertically stacked low, that will result in largely unidirectional flow throughout the column south of the low. Since the vertically stacked low has polar origins, it will have plenty of cold air and a lack of plentiful Pacific moisture with it. Thus, this rainfall event is not expected to be anywhere close to as intense as the atmospheric rivers impacting the state in recent weeks. Nevertheless, the residual impacts from the atmospheric rivers remain across coastal central California, and the injection of up to 3 inches of rain into the coastal mountains are still expected to cause or worsen flooding impacts as that rainwater flows down the mountains and swells already high rivers. Thus, the Slight Risk was expanded due to increasing forecasted rainfall through the Bay area and especially the mountains north of there, and expanded south to include the westernmost Transverse Ranges of Santa Barbara County. Further inland, slightly lower snow levels, much higher mountains, and more recovery time from the last round of atmospheric rivers were factors that led to the continuance of the Marginal Risk, as much of the heaviest precipitation is likely to fall as snow. The highly favorable antecedent conditions for flash flooding remain a significant factor in the Slight Risk area. ....Potential for Future Risk Changes... Despite the expansion of the Slight Risk area, at this point it appears further upgrades to the Slight Risk area are unlikely, as precipitation totals have remained fairly steady, and the lack of tropical moisture will preclude rainfall rates more typical of true atmospheric rivers. ....Gulf Coast through Coastal Georgia... The Marginal Risk area along the Gulf Coast is largely unchanged from the Day 4 update yesterday. There has been a small southward shift in most of the guidance, so the immediate Gulf Coast and points up to 100 miles inland remain in the Marginal Risk area for this update. The front will be in roughly in the same place Tuesday as it was on Day 2/Monday. Thus, expect Monday's rainfall to have lowered FFGs a bit in the Marginal Risk area by the time we get to Day 3/Tuesday. Forecasted rainfall is a little higher in this area than on Day 2/Monday, particularly around southern Mississippi. Thus, think continued convection in the same areas on Tuesday as on Monday warrants the continued Marginal Risk for the same stalled out front. ....Potential for Future Risk Changes... As with the current Day 1 update, if future model trends move higher for forecasted rainfall, it's very possible a Slight Risk may be needed for portions of the Marginal Risk area on Tuesday. Since a large component to any upgrades will be the rainfall distribution in this area on Day 2/Monday, confidence and current forecasted rainfall are low as to where any upgrades would be. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8U8zJXPnzGoFNjdne-gQnHHIt-ck4v8B8DUZIX4mWk8S= Zg7qFmCaUrC5XZuBLcxGdBzsOkkorVZrcTAMIE4u55ehnFI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8U8zJXPnzGoFNjdne-gQnHHIt-ck4v8B8DUZIX4mWk8S= Zg7qFmCaUrC5XZuBLcxGdBzsOkkorVZrcTAMIE4uNWRbmgY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8U8zJXPnzGoFNjdne-gQnHHIt-ck4v8B8DUZIX4mWk8S= Zg7qFmCaUrC5XZuBLcxGdBzsOkkorVZrcTAMIE4ulOR7aWQ$=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) .