Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 10 2023 08:29:05 FOUS30 KWBC 100828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 327 AM EST Fri Mar 10 2023 Day 1 Valid 12Z Fri Mar 10 2023 - 12Z Sat Mar 11 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE FOOTHILLS OF THE CENTRAL CALIFORNIA SIERRAS AND COASTAL RANGES... Widespread heavy to excessive rainfall will continue through the day on Friday over an area already soaked by a prolonged wet period and by recent rainfall from a passage of several waves.=20 With a fetch of moisture originating south of 30 degrees North latitude...precipitable water values of 1.5 inches were abt 50 nmi off the California coast between the Bay area and the Transverse Ranges and this airmass will continue to be drawn inland. The latest numerical guidance continues to show the focus of heavies rainfall working its way southward along the Coastal Ranges as the channel of best on-shore flow gets redirected with time, This keeps one QPF bullseye focused near the coast and another in the upslope regions of the Sierra Nevada foothills with WPC deterministic pretty close in terms of placement and amounts in the 4 to 9 inch range which leaves room for isolated higher amounts approaching a foot after the start of the Day 1 period.=20 Highest amounts are where the flow aligns with the terrain to focus a prolonged period of QPF as well as areas that have enhanced rainfall amounts due to instability that the HRRR redevelops during the afternoon with MUCAPE values at or slightly higher than 250 J per kg north of the Transverse Ranges. By early evening, however, precipitation rates begin to wave as the moisture transport of deep moisture becomes weaker and directed farther south. Antecedent conditions remain incredibly favorable with a well-above-normal snowpack in the Sierras and soils remain near saturated area-wide via NASA Sport imagery so the combination of rain and snowmelt (especially below 5500 ft) pose the potential for widespread flooding is considerable, especially within the High Risk areas although the risk of flooding/rapid inundation remains a concern in areas of lesseer QPF given the antecedent conditions which would result in much of the water turning immediately to runoff in the Slight and Moderate Risk areas. The strong atmospheric river should push IVT values greater than 400 kg/m*s past the Sierra Nevada and into the central Great Basin. While rainfall rates should remain meager and under 0.5" per hour, the combination of rainfall amounts up to an inch and snowmelt could lead to ponding of water and rises to creeks, streams, and rivers. The latest HREF 24-hour QPF probabilities for exceeding 100 year ARI are also elevated for this area. Bann Day 2 Valid 12Z Sat Mar 11 2023 - 12Z Sun Mar 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL/NORTHERN CALIFORNIA AND THE MID-SOUTH... ....California... Lingering shower activity will continue in the Sierra mountains of central and Northern California especially, but to nowhere near the degree as from Friday into the early morning hours on Saturday. The challenge will be what lingering impacts there will be as the area gets mostly light rain on Saturday in an area that will have very little tolerance for any additional rainfall. Given the very real potential for even the 1-2.5 inches of new rainfall Saturday...there is concern that any on-going flooding concerns could be made worse in addition to possibly resulting in additional flooding concerns. While there remains some potential for a Slight Risk upgrade in the central and northern Sierras as snow levels rise a little bit through the 6-7k feet range during this period...that was not done at this point. Note that the primary driver of flooding will be antecedent conditions in this area, and not so much additional rainfall. ....Mid-South... There remains a lot of spread in the guidance as to where the heaviest rainfall will be seen in this area on Saturday. For the most part, the significant rainfall will hold off until late Saturday night. The GEFS keeps its greatest moisture profile back across Arkansas and northern Mississippi into the overnight hours Saturday/early morning hours Sunday...while the areal coverage from the ECENS extended from southern Arkansas into the Tennessee Valley (with the 10/00Z GFS lending more support to the ECENS placement than it doe to its on ensembles). Regardless...recent heavy rainfall has suppressed flash flood guidance in the area.=20 As a result, a broad Marginal still is valid to depict the uncertainty in placement. As mentioned previously...the Marginal Risk area may need to be shifted from its present location but the chances of needing an upgrade are low. Bann Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6UysKQMeeHMoKXZbqxP5H87yGhN-KLuWCDYBxd6h9kty= 1C2zA1LHA81ripauzyzCYVrUd7_GrqbrhE5-c-sUIafc3a0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6UysKQMeeHMoKXZbqxP5H87yGhN-KLuWCDYBxd6h9kty= 1C2zA1LHA81ripauzyzCYVrUd7_GrqbrhE5-c-sUduRRFmA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6UysKQMeeHMoKXZbqxP5H87yGhN-KLuWCDYBxd6h9kty= 1C2zA1LHA81ripauzyzCYVrUd7_GrqbrhE5-c-sUWGqFeks$=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) .