Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 13 2025 08:12:54 FOUS30 KWBC 130812 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 312 AM EST Thu Feb 13 2025 Day 1 Valid 12Z Thu Feb 13 2025 - 12Z Fri Feb 14 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ....California... Overall, not much change to the previous forecast, with the=20 unfolding atmospheric river expected to peak near the start of the=20 period. A long warm front currently extending along the coast is=20 expected to press inland this morning, with IVTs climbing to around=20 700-800 kg/m/s near the upstream cold front as it approaches the=20 coast later this morning. The strongest moisture flux is expected=20 to center near SF Bay at the start of the period, before steadily=20 shifting south along the coast today. Models continue to show IVTs=20 decreasing as the axis moves south, dropping to around 400-500=20 kg/m/s as the AR reaches Southern California later this afternoon=20 into the evening. Guidance has remained fairly consistent,=20 indicating precipitation amounts of 4-6 inches along the central to southern coastal ranges and the Sierra Nevada, falling mostly as=20 rain below 6000 ft.=20 A Moderate Risk (level 3/4) remains in place across portions=20 Southern California, from Santa Barbara County southeastward into=20 the San Gabriel and San Bernadino mountains, where damaging and=20 potentially life-threatening burn scar flash flooding and debris=20 flows remain a primary concern. Even outside of the burn scar areas,=20 intense rain rates over complex terrain and urban areas may produce=20 scattered to numerous instances of flash flooding. Elsewhere, a=20 broad Slight Risk (Level 2/4) exists for much of the remaining=20 coastal ranges as well as parts of the Central Valley and the lower elevations of the Sierra Nevada.=20 ....Florida Panhandle and Southern Georgia...=20 Lingering heavy rainfall will continue this morning across=20 portions of the Florida Panhandle and southern Georgia ahead of an=20 approaching cold front. HREF neighborhood probabilities indicate=20 that additional rainfall amounts of 1-2 inches are likely within=20 the highlighted area, with some potential for localized rain rates=20 high enough to cause some urban/localized flooding concerns. Pereira Day 2=20 Valid 12Z Fri Feb 14 2025 - 12Z Sat Feb 15 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-SOUTH... ....Mid-South...=20 Although most of the convective activity, and thus heavy rainfall=20 threat, in the region should be concentrated in the Day 3 period=20 (after 12Z Saturday), model guidance does indicate that convection=20 is likely to begin blossoming late Friday Night. Conditions will=20 become increasingly favorable after 06Z Saturday with a strong 55-65=20 knot low-level jet expanding east from the Plains, and significant=20 northward moisture transport leading to rapid moistening of the=20 boundary layer. MUCAPE values of around 500-1000 j/kg should support=20 organized convection with the potential for rainfall rates in excess=20 of 1 inch per hour at times. Although there is a lack of a=20 significant front or boundary to help organize a band of convection,=20 the strength of the LLJ may support backbuilding within a broad zone=20 of developing convective clusters. This may be enough for some brief=20 training and a ramp-up in flash flood potential prior to 12Z=20 Saturday. The 00Z runs of the NAM Nest and FV3 show more widespread=20 convection than other models; if this trend persists as additional=20 hi-res guidance becomes available, an expansion of the Marginal Risk=20 and/or the addition of a Slight Risk area may become warranted with=20 time. ....Southern California...=20 The Marginal Risk was removed for southern California given a more limited QPF signal on the latest guidance, and hi-res models in=20 excellent agreement on a lack of organized convection. The bulk of=20 the threat should be in the Day 1 period. Lamers Day 3 Valid 12Z Sat Feb 15 2025 - 12Z Sun Feb 16 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE TENNESSEE AND OHIO VALLEYS AND THE CENTRAL APPALACHIANS... ....Ohio and Tennessee Valleys... Confidence remains high in significant rainfall-related impacts on=20 Saturday and Saturday Night with model guidance in excellent=20 agreement depicting a swath of 3-6 inches of rainfall in the 24-hour=20 period across the region. Although models vary slightly with the=20 placement, they almost unanimously depict a corridor of heavy rain=20 about 750mi long centered somewhere in TN/KY. This same region has=20 generally seen above average precipitation in recent weeks, with the=20 14-day precip in much of KY, N TN, WV, and W VA at least double the=20 normal values. The confluence of these factors should make the=20 region especially vulnerable to flooding from the predicted=20 rainfall. For additional context on the current hydrologic picture=20 across the region, refer to the National Hydrologic Discussion from=20 the National Water Center at water.noaa.gov. Convection may be ongoing at the beginning of the forecast period=20 and should become increasingly focused along a developing warm front=20 to the east of a surface low that will be emerging out of the Plains=20 on Saturday. The front will be positioned on the nose of very strong=20 SW, or even WSW, inflow, which should favor backbuilding and=20 training and a corridor of very heavy rainfall. The environment will=20 be atypical of mid-February with PWATs reaching around 1.5 inches=20 and MUCAPE of 500-1000 j/kg. For example, the Nashville sounding=20 archive exists back to 1948 and they've only ever reached 1000 j/kg=20 MUCAPE once in February, and 1.5 inch PWATs a few times. The=20 environment will support rainfall rates in excess of 1 inch per=20 hour, and given the potential for training and backbuilding, these=20 rates may be sustained for several hours in some locations. These=20 would be the areas of greatest concern for flash flooding -- not=20 only in terms of likelihood but severity -- particularly if the=20 higher rain rates are sustained over urban areas or vulnerable=20 terrain for extended periods of time. The inherited Moderate Risk was mostly maintained as-is with a=20 slight expansion to the southwest in the vicinity of Memphis, TN;=20 and a more significant expansion eastward into S WV and W VA. The WV- VA border region just received some significant snowfall, and model=20 guidance indicates a fairly high probability of warm, moist air (dew=20 points in excess of 45 degrees) accompanying the rainfall into this=20 area, which may lead to a combination of runoff from rain and snow=20 melt. It's worth noting a couple additional things. First, the amount of=20 rain forecast from E KY into S WV and W VA in particular is quite=20 unusual for the heart of the winter season. Calendar day precip=20 records across that area for January and February are generally in=20 the 2-3 inch range, and NBM probabilities indicate it is likely (60+=20 percent chance) we will see these types of 24-hour rainfall totals=20 in those areas. Second, this relatively rare winter rain event would=20 be delivered to an area that has been quite wet recently, and in=20 some areas has snow on the ground. And third, the high-end scenarios=20 in much of the Moderate Risk area would be significant by historical=20 standards. The NBM 90th percentile from N TN into S/E KY and S WV is=20 on par with the top several wettest days (for any season!) in the=20 station records in the area. This corridor is where the greatest=20 concern for severe impacts exists, and it's possible an upgrade to=20 High Risk may be needed for portions of the area in subsequent=20 updates. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_P91ui_wceXaBtbPdzDS2U2ejXAZ4l7JBKrHAppXy2j3= n75PhydOMS61ECXA_hSACD86zA71nmYnj-U9fKdlvg7HAHc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_P91ui_wceXaBtbPdzDS2U2ejXAZ4l7JBKrHAppXy2j3= n75PhydOMS61ECXA_hSACD86zA71nmYnj-U9fKdl9tWZdKc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_P91ui_wceXaBtbPdzDS2U2ejXAZ4l7JBKrHAppXy2j3= n75PhydOMS61ECXA_hSACD86zA71nmYnj-U9fKdlXXvtJd8$=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. 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