Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 11 2023 00:52:23 FOUS30 KWBC 110052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 750 PM EST Fri Feb 10 2023 Day 1 Valid 01Z Sat Feb 11 2023 - 12Z Sat Feb 11 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE EASTERN FLORIDA PANHANDLE INTO SOUTHERN SOUTH CAROLINA... Limited instability over much of the area should result in lower rainfall rates over most of the region tonight. Based on recent radar, HRRR runs and analyzed instability...it looks like the axis of heaviest rainfall rates will end up over portions of north FL, just southeast of areas hardest hit earlier today. While areas that received 3"+ earlier today will see some additional rainfall overnight, it appears like the better instability axis and thus higher rainfall rate potential, will stay just to the southeast. For that reason we think the risk of additional flash flooding, while non-zero, is below Slight risk levels and can be covered by a Marginal risk. The portions of north FL that could see hourly rainfall of 1-2" have been drier with FFGs quite high, so not thinking the rainfall tonight will pose anything more than a localized flash flood risk. The risk is likely pretty low over southern GA into southern SC, but some locally heavy rainfall is moving into these areas now, so opted to just maintain continuity with the Marginal risk. Generally expecting only some isolated minor flood concerns at most here. By early Saturday some better convection should start to get its act together over the Gulf of Mexico as stronger forcing approaches from the west. Some of this may begin to impact the Gulf Coast by 12z, but for the most part, currently expecting any notable increase in flood impacts from this activity to hold off until after 12z. We do have a Slight risk on Saturday to account for this increased threat by that time. Chenard Day 2 Valid 12Z Sat Feb 11 2023 - 12Z Sun Feb 12 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE EASTERN FLORIDA PANHANDLE THROUGH SOUTHERN SOUTH CAROLINA... ....2030Z Update... This afternoon's update includes a small expansion of the previous Slight Risk area south and west to include areas of the FL Panhandle and far southern GA that are getting soaked with heavy rain today. While much less rain is expected Saturday as compared with today, see no reason even the forecasted lighter rain won't exacerbate already ongoing flooding in the area. Meanwhile much for the rest of the Slight Risk area remains largely unchanged. Antecedent flooding conditions led much of the previous Slight Risk to be maintained in southern South Carolina. The Marginal Risk area across much of NC was removed with this update, in addition to the area having about average soil moisture, most of the rain waits until the parent low is gathering strength off the coast after 00Z, leading to a more likely evolution of a widespread area of light to moderate rain with embedded heavy elements. The quick movement of the low will not allow this area of rain to stay in any one location too long before the precipitation becomes more banded with the bands oriented largely perpendicular to the mean southwesterly flow. This will make the rain become increasingly off-and-on late Saturday night, requiring heavier rainfall rates that are unlikely to materialize. Wegman ....Previous Discussion... Maintained the Slight Risk for portions of the Southeast U.S. that was introduced by yesterday's day shift from the Florida Big Bend to the South Carolina Low-country. This upgrade is supported by converging guidance regarding the upper-level low responsible for much of the atmospheric lift, as well as the location of the forecast heavy rainfall expected to occur over an area forecast to become more saturated by the start of the outlook period due to rain the previous day. There will still be a quasi-stationary boundary draped over portions of the Southeast U.S. with the flow aloft being forced to back in response to a closed mid- and upper-level low that makes its way into the Tennessee Valley on Saturday. This induces a broadening area of precipitation as low pressure forms along the front. Latest guidance shows an additional 1-3 inches of rainfall from the Florida Panhandle to southeastern North Carolina. Despite the expected lack of instability (GFS MU CAPE near 0 J/kg outside of the Florida Peninsula and far southern Georgia), tropospheric moisture levels are expected to remain very elevated, with precipitable water values ranging from 1.25-1.75 inches (near the 90th percentile climatologically) with large-scale lift provided from the traversal of the upper-low (and expected surface cyclogenesis). Strong frontogenesis associated with a draped stationary front from northern Florida to eventually the coastal Carolinas will be the focus for rainfall rates to potentially reach or exceed 2"/hr, otherwise rainfall rates are expected to remain under 2"/hr. It is this additional rainfall on top of what has fallen recently that should result in lower Flash Flood Guidance across much of the Slight Risk area and was highly considered when drawing the area. Otherwise, much of the forecast rainfall and rates generally hovering around or just below 1"/hr would be handled well by the largely sandy soils. WPC 48-hour rainfall amounts of 3-5" between Friday and Saturday is forecast for the region, which may lead to more scattered (as opposed to isolated) flash floods and ponding across roadways/urban areas by the beginning of the weekend. Surrounding the Slight Risk is a larger Marginal Risk that encompasses regions where 1-2" of rainfall are forecast and should lead to mainly localized flooding concerns, which was confined a bit on the northern and western fringes compared to the previous forecast. Bann Day 3 Valid 12Z Sun Feb 12 2023 - 12Z Mon Feb 13 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEASTERN VIRGINIA AND EASTERN NORTH CAROLINA... ....2030Z Update... In coordination with the MHX/Morehead City, NC and AKQ/Wakefield, VA forecast offices, maintained some of the inherited Marginal Risk across SE VA and eastern NC this afternoon. Changes involved trimming much of the southern end of the Marginal Risk area across central NC, as the precipitation will rapidly end there early in the Day 3 period Sunday morning with the advection of the dry slot into the area. Meanwhile, also trimmed the northern end of the inherited Marginal Risk area over the Northern Neck Peninsula of Virginia due to very dry (under 10%) soil moisture based on NASA Sport imagery. Rainfall rates will be quite low during this period, maybe briefly hitting 1/2 inch per hour rates, as the low winds up and moves off the coast of NC. Ahead of the low, a line of stronger storms is likely to impact eastern North Carolina Sunday morning, with rainfall rates rapidly diminishing with the advection of the dry slot after 18Z. This line of stronger storms will be capable of much higher rainfall rates (perhaps briefly exceeding 2 inches per hour), but it will be falling into coastal swamp and sandy soils, which are unlikely to respond with much flash flooding. Any urbanized areas of eastern NC are more likely to see isolated flash flooding. Meanwhile across Virginia, lighter rain will fall over much of the Day 3 period into Sunday night in the wraparound region of the low. While there's high confidence all of Southeast Virginia will see all rain, the longer time period (at least 18 hours) of light to occasionally moderate rain may result in isolated instances of flash flooding, particularly in the urbanized areas of Hampton Roads/Norfolk/Virginia Beach and inland along I-64 through Richmond. Wegman ....Previous Discussion... Rain will continue to move northward from the Southeast U.S. into parts of the southern Mid-Atlantic region in association with a mid- and upper-level low. Present indications are that the higher rainfall rates should be confined close to the coast where the upper level support from a coupled jet structure and some lingering mid-level frontogenesis will offset modest (at best) instability across the region. As such...the main problems should tend to be in areas of poor drainage. Broader coverage of more stratiform rainfall is possible farther west...but neither the rainfall rates nor the rainfall amounts appear to be problematic at this based on input from the local offices...so did not extend the Marginal Risk any farther west or north. The expectation is that the rainfall will be shunted off the coast during the latter part of Day 3 with a corresponding decrease in any excessive rainfall potential. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6jZ8GXQO3-Ygyisu9O95dPRt34XGkkyF50ZNYb_axyrj= jcXG7M1eD20XhM7csPcrLnP-uWK_qVyo0wiu-XZBwrQv3uk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6jZ8GXQO3-Ygyisu9O95dPRt34XGkkyF50ZNYb_axyrj= jcXG7M1eD20XhM7csPcrLnP-uWK_qVyo0wiu-XZBsID-syA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6jZ8GXQO3-Ygyisu9O95dPRt34XGkkyF50ZNYb_axyrj= jcXG7M1eD20XhM7csPcrLnP-uWK_qVyo0wiu-XZB9AVPCAI$=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) .