Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 13 2023 20:27:31 FOUS30 KWBC 132027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Thu Apr 13 2023 Day 1 Valid 16Z Thu Apr 13 2023 - 12Z Fri Apr 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR ATLANTIC COASTAL SOUTHEAST FLORIDA... ....16Z Update... ....South Florida... In coordination with the MFL/Miami, FL forecast office, a Slight Risk area was introduced for the urban corridor around Ft. Lauderdale and Miami. After the incredible amounts of rain for a small portion of the coast between Hollywood and Ft. Lauderdale yesterday evening, FFGs have bottomed out at 1/4 inch in 6 hours in the hardest hit areas. Standing water remained around the Ft. Lauderdale airport this morning, and as such any amount of rain is likely to quickly worsen the flooding situation there. While areas further south towards Miami have gotten less rain, and therefore have higher FFGs, the wet pattern where areas are approaching 10 inches of rain in the last 72 hours makes this area susceptible as well. Given those extremely favorable antecedent conditions, the CAMs guidance is in very good agreement that convection will break out across the middle of the FL Peninsula from Lake Okeechobee south. Those showers and thunderstorms are likely to drift northeastward as the predominant steering flow has shifted out of the southwest. Once the storms reach the coast, there's some indication (though far less agreement than the formation of the storms themselves) that the storms may slow or even stall along the coast. The combination of this possibility of stalling out convection, and even the high likelihood that moving storms traverse the area necessitated the Slight Risk upgrade for this evening. Since much of the CAMs guidance suggests that the storms will continue moving off the coast, the more likely outcome is for 1 to as much as 2 inches of rain falls along the I-95 corridor in south Florida, but certainly the threat of stalling out convection potentially repeating in the same areas as yesterday will continue to be monitored. ....North Florida and the Southeast... The inherited Marginal Risk area largely remains in place for this afternoon's update. Given the ongoing squall line moving across the eastern Panhandle and the threat for severe weather, the Marginal Risk was expanded to include all of north Florida with this update. Meanwhile the main area of rain is north and west of the vertically stacked low moving into southern AL, thus, the Marginal Risk remains in place over MS. The leading front draped from southeast GA to far northern AL offers an axis along which showers and a few thunderstorms are training this afternoon. Thus far there are no big heavy rain-makers along this feature except offshore of north FL, but again the potential storms containing rainfall rates over 2 inches per hour with training could lead to an isolated flash flooding risk. Wegman ....Previous Discussion... The closed, deep layered low centered over southern Louisiana early this morning will begin to lift north/northeastward today into tonight, taking on a negative tilt later in the period as it becomes absorbed ahead of the approaching longwave trough back over the Plains. A broad area of ascent will be found across the Southeast along and downstream of the advancing low while at the surface, a warm front begins to lift northward through Florida into Georgia. Moist easterly flow ahead of that boundary and then southwesterly flow in its wake overrunning the boundary will bring a surge of higher moisture through the region, characterized by PWs of 1.25-1.5". With roughly 1000 J/kg of CAPE present overlapping with the higher moisture, the ingredients are in place for pockets of locally heavy rainfall and intense rain rates (locally exceeding 1-1.5"/hr) from the central Gulf Coast through portions of the Southeast. While most of the convection is likely to be somewhat progressive with a fast mean flow, the unidirectional mean flow and higher 850 mb inflow suggests some potential for backbuilding. Localized totals of 2-4" appear likely within the Marginal Risk. Generally this is over areas with higher flash flood guidance, thus only expected isolated/localized instances of flash flooding. For this issuance, the Marginal Risk was expanded to cover the urbanized corridor of southeast Florida, where extreme rainfall occurred yesterday/last evening. While the 00Z hi-res guidance and HREF suggests greater risk of intense rainfall will be further north along the FL Peninsula, there is a signal that a trailing convergence area could focus additional afternoon/evening thunderstorms again over S. Florida. Taylor Day 2 Valid 12Z Fri Apr 14 2023 - 12Z Sat Apr 15 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CAROLINAS... ....2030Z Update... No changes were made to the Marginal Risk area over the Carolinas on Friday. Instability has increased a bit over the area, with some guidance up as high as 1,500 J/kg of MUCAPE as the storms are moving through. PWATS also remain as high as 1.5 inches, which is in the 75th percentile of climatological norms. CAMs guidance of the evolution of the event shows an initial round of rain that may have some embedded convective elements moving through early Friday morning, perhaps during the Day 1 period, but that rainfall is highly unlikely to be impactful by itself. It will mostly work to prime the soils for the potentially more significant storms that will move through during peak heating Friday afternoon. As the upper level low approaches the Carolinas, expect multiple rounds of storms to move through, though with somewhat limited organization. The key is that those storms will be moving northeastward, in the same direction as the upper low itself, which will increase the likelihood that multiple cells of heavy rain moving over portions of the Marginal Risk area. FFGs particularly in the western portions of the Marginal Risk area are somewhat lower from previous rains, and it's this portion of central NC and SC, roughly parallel to and east of the I-85 corridor that stands the highest risk for isolated flash flooding, if multiple rounds of storms move over this area in quick succession, which is a distinct possibility. As mentioned yesterday, there remains a somewhat higher likelihood for greater areal coverage of 1 inch or more of rain further east closer to the coast of southeastern NC from Cape Hatteras south to the SC border, but FFGs in this area are significantly higher than further inland, which should cut down on the flash flooding risk. Wegman ....Previous Discussion... The deep mid/upper level low will begin to open up and weaken as it progresses further to the northeast Friday into Friday night, though with the anomalous moisture in place (upwards of 1.5" PWs) and available instability (1000 J/kg CAPE), locally intense rainfall will be possible across initially the SC Piedmont initially the reaching the NC Piedmont late morning. A secondary round of convection is possible during the afternoon/early evening associated with the main upper level shortwave moving overhead. The combination of steeper lapse rates and peak heating could bring another round of heavier showers and thunderstorms. Rain rates of 1-2"/hr at times both in the morning and afternoon rounds will be possible and the latest deterministic and ensemble guidance shows potential for isolated 2-3" totals. Recent rainfall has primed the soils somewhat and makes the area a little more susceptible to flooding with both soil moisture and streamflows running 75-90 percent above average for mid-April. The combination of the wetter soils over the more urbanized areas of the SC/NC Piedmont may lead to a few instances of flash flooding during the period. Taylor Day 3 Valid 12Z Sat Apr 15 2023 - 12Z Sun Apr 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE LOWER AND MIDDLE MISSISSIPPI RIVER VALLEY... ....2030Z Update... ....Lower to Mid-Mississippi River Valley... The Marginal Risk area was expanded northward by over 1.5 states width with this update, largely following the Mississippi River to now include eastern AR & MO and also western IL, KY, and TN. This corridor is along and ahead of a strong, but rather slow-moving cold front. The most impressive aspect of this event will be the instability, as MUCAPE values may approach as high as 3,000 J/kg across this area Saturday afternoon. Corfidi vectors are very weak, at 5-10 kts out of the southwest Saturday afternoon. This means storms will be very slow-moving and with an at least semi-parallel orientation to the front, may support some training. The extreme instability, in addition to supporting large hail, will by default also promote very heavy rainfall rates from those same storms. Thus, the combination of extreme instability, slow storm motion semi-parallel to the front, PWATs to 1.25 inches, which is roughly in the 75th percentile of climatology, and a local minimum of FFGs around the MS River in this region all support the Marginal Risk, with certainly some potential for a Slight Risk upgrade with future updates. It's certainly worth noting that the Marginal Risk area roughly overlaps with the Storm Prediction Center's Slight Risk area for the mid-Mississippi River Valley. No changes were made to the inherited Marginal Risk area across LA and southern MS, and the points made in the previous discussion below all remain valid. In this region, it's highly likely that any one location will pick up more rainfall than points further north, but this will be offset by much higher FFGs in this region, so the Marginal Risk remains unchanged in this region. ....Western U.P. of Michigan, northern Wisconsin and the Arrowhead of Minnesota... In this region, rainfall totals further north along the same front causing severe weather further south will be much lower. However there are many portions of this area where FFGs are extremely low (1/4 inch in 6 hours) due to snowmelt. While the rain-snow line may play a role in northeastern Minnesota, rain will be the dominant precipitation type further east. 24-hour forecast rainfall totals are expected to generally range between 1/2 and 3/4 inch through early Sunday morning, but the potential with moisture advection of exceeding the 1/4 inch in 6 hour threshold for meeting FFGs is significant. There is unanimous model agreement that temperatures will remain above freezing in this area between now and the rainfall on Saturday, so there's no reason to expect snowmelt will not continue around the shores of Lake Superior. While the rain is never expected to be heavy, the low FFGs and the forecast rainfall alone are worthy of consideration of a Marginal Risk introduction for this area with future forecast updates. Wegman ....Previous Discussion... Locally heavy rainfall capable of producing flash flooding will be possible Saturday into Saturday night across portions of the Lower Mississippi River Valley. The synoptic setup is expected to feature subtropical jet positioned through the Gulf with an embedded shortwave trough expected to pass through during the first part of the period helping to provide the large scale forcing for ascent. Meanwhile, low level southerly flow ahead of that wave should draw up anomalously high moisture, characterized by PWs exceeding 1.5", possibly as high as 1.75" across the southern portions of Louisiana, Mississippi, and Alabama. With upwards of 30-35 kts of inflow at 850 mb and plenty of instability, locally intense rainfall will be possible. Later in the period, Saturday night, a stronger shortwave trough originating out of the Central Rockies will approach the region. Convection is expected to develop along an advancing cold front between 00Z-12Z Sunday, likely over portions of the ArkLaTex eastward toward central Mississippi. The environment characterized by strong low level moisture transport and sufficient instability will support locally heavy rainfall of 1" to locally 3" over an area that is more sensitive/susceptible to heavy rainfall with recent rainfall departures running 200-300 percent of normal and streamflows above normal as well. Given the expected heavy rainfall over areas with wetter antecedent conditions, the Marginal Risk was expanded northward to include portions of northern Louisiana into west-central Mississippi for the possibility of isolated/scattered instances of flash flooding. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_iR2uQoOA6LicjMG3-u5LgGm1KbyA3PRSc_h1zmqq4Fy= N1xUiQUsn_zUAKuduEqc3_9y-c06T_tSsRCX6DcpnXixyT0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_iR2uQoOA6LicjMG3-u5LgGm1KbyA3PRSc_h1zmqq4Fy= N1xUiQUsn_zUAKuduEqc3_9y-c06T_tSsRCX6DcpHm47TMo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_iR2uQoOA6LicjMG3-u5LgGm1KbyA3PRSc_h1zmqq4Fy= N1xUiQUsn_zUAKuduEqc3_9y-c06T_tSsRCX6Dcp4PM64To$=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) .