Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 11 2025 19:25:32 FOUS30 KWBC 111925 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 225 PM EST Tue Feb 11 2025 Day 1 Valid 16Z Tue Feb 11 2025 - 12Z Wed Feb 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE MID-SOUTH INTO EAST TEXAS... Some adjustments were made to the Slight and Marginal Risk areas based on the latest guidance and observational trends. First, the Slight and Marginal risks were expanded towards the southwest based on the latest observational and model (12z CAM) trends. This included an expansion towards the Houston metro region, given that the probabilities for localized 5" exceedance are maximized (near 10%, per 40-km neighborhood HREF probs) across portions of East TX (and trends are edging towards the Upper TX Coast itself, particularly with the 12z HRRR being a southern outlier). While instability will be maximized across the region, rainfall totals are still rather spread out between two distinct rounds (one currently ongoing and largely beneficial, but will prime the soils for possible isolated to scattered flash flooding with the second round expected late this evening). Farther northeast into northern LA, central MS, and far southern AR, instability will be more limited, but probabilities for 3" exceedance remain high (40-60%, per 40-km neighborhood HREF probs) and the Slight was maintained. Even farther northeast into northern AL, the CAMs have trended lower with expected rainfall (with little to no instability) and longer term, spread out rainfall totals of 1-2" should result in little to no impact (though localized flood impacts cannot be ruled out). The Slight was removed from this region to confine it to the areas where 3" exceedance are possible. Churchill ....Previous Discussion... Few changes needed to the old Day 2 ERO as it propagates into the Day1 period. Main focus for heavy rainfall and the potential for excessive rainfall will be along and ahead of a cold front associated with a developing synoptic scale low pressure system within a broad/fairly flat flow pattern. Inclusion of more CAM guidance supports the idea of higher intensity rainfall extending as far west as eastern Texas...a tendency we often see convection get organized farther southwest than expected in events like this, and there is growing QPF signal in the 12z ECMWF, UKMET and Gem Reg over this area. The Slight risk area extended northeastward where there is a bit of instability that develops later in the period but looks to be lacking initially. At expect the flash flood risk to pick up more over this area as we head into day 2. Bann Day 2 Valid 12Z Wed Feb 12 2025 - 12Z Thu Feb 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE TENNESSEE VALLEY INTO THE MID-SOUTH... ....Tennessee Valley into the Mid-South... Another wave of low pressure will move through the area Wednesday into Wednesday night, bringing additional widespread precipitation to the region. Increasing low level moisture combined with the favorable upper level support should result in robust rainfall with embedded convection. An axis of 2 to 3 inches is expected during the 24-hour period from portions of Louisiana through southwest North Carolina. Embedded within that will be the possibility of higher amounts, most likely over portions of Alabama and Georgia where the overlap of available instability and potential for training/repeating rounds of rainfall appear most likely. Combined with the preceding day rainfall, 48-hour totals exceeding 3-5" will be possible and this will likely lead to excessive runoff and increasing soil saturation. As a result, a higher end Slight Risk in the ERO is maintained, with the greatest risk for flash flooding over portions AL/GA. ....California Coast... The beginning of the active/wet period for the West Coast begins late in the period as the initial surge of atmospheric moisture arrives after 00Z Thursday across coastal central California. This will bring the initial threat of flooding, though the bulk/peak of it will be in the Day 3 /Thursday/ period.=20=20 Taylor Day 3 Valid 12Z Thu Feb 13 2025 - 12Z Fri Feb 14 2025 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL PARTS OF THE SOUTHERN CALIFORNIA COAST... ....California... The peak of the atmospheric river is expected during this period with the initial focus on coastal central California at the start=20 of the period, then the best IVT shifts southward through the=20 period, reaching southern California by the end of the period=20 Thursday night. IVT values from the GFS peak at around 600-700=20 kg/m/s initially along the central coast then settle around=20 400-500 kg/m/s as the AR reaches SoCal. The greatest QPF amounts=20 will be confined to the coastal terrain areas as well as for the=20 Sierra. Total amounts for the 24-hour period look to be between=20 1-3" with the favored terrain areas and peaks topping out between=20 4-5".=20 The primary concern is the potential for damaging and potentially life-threatening burn scar flash flooding and debris flows, particularly for southern California where the Moderate Risk (Level 3/4) is in place. Even outside the burn scar regions, the intense rain rates and rainfall totals over the highly urbanized area could bring scattered to numerous instances of flash flooding. Elsewhere, a broad Slight Risk (Level 2/4) exists for much of the coastal=20 ranges as well as the Central Valley and lower elevations of the=20 Sierra Nevada.=20 There is a Moderate Risk of excessive rainfall over portions of=20 the Southern California coast embedded within a broader Slight risk of excessive rainfall covering much of the central and southern=20 California coast. An atmospheric river is expected to skirt the=20 coast as it makes its way south and east. The primary concern is=20 for damaging flooding and debris flow from the numerous fresh burn=20 scars in the region given the 24-hour QPF in the 1.5 to 3.5 inch=20 range...with potentially localized higher amounts. In=20 addition...the area is highly urbanized and an environment in which much of the rainfall will turn immediately to runoff. Farther=20 north...the Slight risk was extended into portions of the=20 Sacramento Valley and San Joaquin Valley...where QPF is not=20 expected to be as great but the conditions have been wetter and=20 soils are more saturated than soils farther south. ....Florida Panhandle and Southern Georgia... Lingering heavy rainfall will continue Thursday across portions of the Florida Panhandle and southern Georgia, ahead of the approaching cold front. Rainfall amounts look to be generally in the 1-3" range, with some potential for localized rain rates sufficiently high enough to cause some urban/localized flooding concerns.=20 Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5R9c1xHXaDnp1gTTjyGRXmFp1yugiXJwDMeJcNVCiZ9Q= leFn0RchbKxjyWnnklSMKWdvz7O92zHBLlntopQilcx488I$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5R9c1xHXaDnp1gTTjyGRXmFp1yugiXJwDMeJcNVCiZ9Q= leFn0RchbKxjyWnnklSMKWdvz7O92zHBLlntopQiTIEbGs0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5R9c1xHXaDnp1gTTjyGRXmFp1yugiXJwDMeJcNVCiZ9Q= leFn0RchbKxjyWnnklSMKWdvz7O92zHBLlntopQiQkl4MxM$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .