Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 09 2023 19:59:56 FOUS30 KWBC 091959 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 258 PM EST Thu Feb 09 2023 Day 1 Valid 16Z Thu Feb 09 2023 - 12Z Fri Feb 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE WESTERN FLORIDA PANHANDLE, SOUTHERN ALABAMA, AND SOUTHWESTERN GEORGIA... A Marginal Risk of excessive rainfall has been maintained for the western FL Panhandle, southern AL, and portions of southwestern GA for the updated Day 1 ERO, due to a stalling cold front moving into the region this morning. The only meaningful change from the prior issuance was to shrink the northern and western extent of the Marginal as precipitation from this morning slides eastward. Precipitable water values ahead of the front values currently range from 1.4-1.6 inches (well above the 90th percentile per TLH sounding climatology), with sufficient instability (MUCAPE of 250-750 J/kg, per the HREF mean) suggesting that rainfall rates may locally approach 2"/hr at times. There is also the possibility of repeating rounds of heavy rainfall, given the stalling of the trailing front (with HREF 3" neighborhood exceedance probabilities as high as 20-30% over the western FL Panhandle). However, antecedent soil conditions in the region are quite dry, with NASA SPoRT 0-40 cm soil moisture values indicated to be as low as the 10-20th percentile. This should significantly limit the areal extent of any flash flooding, along with the fact that any 3" exceedance will likely occur over a period of longer than 6 hours. Nevertheless, poor drainage/urban areas will be the most at risk of realizing any isolated flooding impacts. It's also worth noting that the amount of rainfall that ultimately occurs today will have important implications for Friday and Saturday, as the FL Panhandle and surrounding portions of AL/GA may see many additional rounds of locally heavy rainfall going into the weekend. Churchill/Snell Day 2 Valid 12Z Fri Feb 10 2023 - 12Z Sat Feb 11 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE WESTERN FLORIDA PANHANDLE, SOUTHERN ALABAMA, CENTRAL/SOUTHERN GEORGIA, AND LOWER SOUTH CAROLINA... A stalling frontal zone is expected to result in a lingering axis of significant tropospheric moisture (precipitable water values of 1.3-1.6 inches, above the 90th percentile climatologically per the GEFS) from the FL Panhandle and southern AL northeastward into south-central GA, SC, and southeastern NC on Friday and into 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 (generally less than 250 J/kg of MU CAPE, but near 500 J/kg in the vicinity of the FL/GA border per the HREF mean). Thus 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. The 12z HREF does have non-zero probabilities for greater than 3" in 6 hours along this corridor. The northeastern extent of the Marginal was trimmed with this update to exclude most of North Carolina given the slowing of the approaching upper-level low and weak moisture transport this far north. 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. Churchill/Snell Day 3 Valid 12Z Sat Feb 11 2023 - 12Z Sun Feb 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FLORIDA BIG BEND, SOUTH-CENTRAL TO EASTERN GEORGIA, AND THE SOUTH CAROLINA LOWCOUNTRY... The afternoon update for the Day 3 ERO includes an upgrade to a Slight Risk for parts of the Southeast from the Florida Big Bend to the South Carolina Lowcountry. 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. The upper-level cut-off low is expected to slip eastward across the central Gulf Coast states on Saturday before beginning a turn northeastward by Saturday night, dropping an additional 1-3 inches of rainfall from the Florida Panhandle to southeastern North Carolina. Guidance seems to have a better handle on the speed and strength of the upper-level low with today's 12z cycle, thus the increased confidence. 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. However, the very strong mid-level moisture flux overrunning this quasi-stationary boundary (850 mb winds over 50 kts per the 12z GFS) will likely lead to a large area of moderate rainfall just to the north. Previous rainfall is likely to lower FFG 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 on Day 3 would be handled well by the largely sandy soils. This was also a reason for not extending the Slight Risk into more of eastern South Carolina and North Carolina. Total rainfall amounts of 3-5" (per WPC) 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. This was done in order to incorporate the latest QPF forecast, while also considering this will mainly be a stratiform rain event (and snow across the high elevations of the southern Appalachians) for the precipitation shield well removed from the surface fronts. Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6n5D4-d5PPzzYYrJFYLHrn_wm1Vc0z44m64kqX5gkpUd= AuVkS8IXrnTj8ZQRBs7oU6BR3yRK2ERgdAYvt4dqcL8i82Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6n5D4-d5PPzzYYrJFYLHrn_wm1Vc0z44m64kqX5gkpUd= AuVkS8IXrnTj8ZQRBs7oU6BR3yRK2ERgdAYvt4dqBeWYIT4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6n5D4-d5PPzzYYrJFYLHrn_wm1Vc0z44m64kqX5gkpUd= AuVkS8IXrnTj8ZQRBs7oU6BR3yRK2ERgdAYvt4dqlazHSO8$=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) .