Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Feb 10 2023 20:29:50 FOUS30 KWBC 102029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 PM EST Fri Feb 10 2023 Day 1 Valid 16Z Fri Feb 10 2023 - 12Z Sat Feb 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE EASTERN FLORIDA PANHANDLE INTO FAR SOUTH CENTRAL GEORGIA... ....16Z Update... In coordination with the TAE/Tallahassee, FL forecast office, the Marginal Risk was upgraded to a Slight for the ongoing line of convection across a small area of the FL Panhandle into far southern GA south of Valdosta. Radar storm total imagery and surface observations in the area of heaviest rain suggest storm total rainfall over 5 inches in the Slight Risk area, and the line is nearly stationary. See the associated MPD #61 for additional information. General consensus in the high resolution guidance is for the line to begin moving east and weaken in intensity over the next few hours. Assuming radar trends are supportive, will likely issue a special update later this afternoon to account for this. Elsewhere, expanded the MRGL just a bit to the west along the Gulf Coast to include the Mobile, AL area. Soils are much wetter than further east based on NASA sport imagery, and the forecast of several hours of moderate intensity rain in this area this afternoon and evening from the convection currently east of Louisiana were factors included for the expansion. Wegman ....Previous Discussion... A stalling frontal zone is expected to result in a lingering axis of significant tropospheric moisture (precipitable water values of 1.3-1.7 inches, above the 90th percentile climatologically per the GEFS) from the FL Panhandle and southern AL northeastward into south-central GA, SC, and extreme southeastern NC on Friday and Friday night. Despite the presence of these anomalously high PWATs, large-scale lift is still expected to remain fairly limited (as the aforementioned upper-low slowly digs south into the far western Gulf) with very little instability expected initially. So a fairly narrow Marginal Risk area was mostly maintained, with rainfall totals generally expected to range from 1-2 inches across the area. Localized higher amounts are possible mainly from near and to the west of the Florida Big Bend to south-central Georgia due to the potential for training cells across the stalling frontal zone...although expanded the southern boundary of the Marginal a bit in the northern Florida peninsula in respons to a QPF bullseye shown by the ECMWF and CMC. The 00Z HREF still maintained a non-zero probabilities for greater than 3" in 6 hours along this corridor...while an offsetting concern is that portions of Florida is in D2 drougt. Given the relatively dry antecedent conditions across the Southeast and already limited potential for rainfall to exceed the high FFG, any instances of flash flooding are expected to remain isolated and primarily confined to areas of poor drainage during this time period. However, any rainfall could lead to more sensitive soils ahead of a final surge of precipitation on Saturday. Bann 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8SMNrHDcwLRBVJwTISvReDZyT8T9ysAUqHQno7XwkVol= lVnbwpfBEl3RbCBz3-dt5zLcMTh15LTyGdBuLNj-e0ZHOO0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8SMNrHDcwLRBVJwTISvReDZyT8T9ysAUqHQno7XwkVol= lVnbwpfBEl3RbCBz3-dt5zLcMTh15LTyGdBuLNj-Y9f8uq8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8SMNrHDcwLRBVJwTISvReDZyT8T9ysAUqHQno7XwkVol= lVnbwpfBEl3RbCBz3-dt5zLcMTh15LTyGdBuLNj-76NfUMo$=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) .