Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 10 2023 15:57:39 FOUS30 KWBC 101557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1056 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 FLASH, RAPID-ONSET, RIVER, SMALL STREAM, AND URBANIZED FLOODING EXPECTED IN THE HIGH RISK AREAS... ....16Z Update... Upgrades to the risk areas continue this morning south of the main event ongoing across Central California. Guidance continues to trend upward in forecasted rainfall into Southern California, especially the 13Z NBM into Transverse and Peninsular Ranges. With more scattered rainfall totals today into tonight over 3 inches into those ranges, the threat for flooding in both the mountains and the urbanized valleys downstream of those mountains has increased. Thus, the Moderate Risk area has been expanded to include all of the Transverse Ranges and the northern end of the Peninsular Ranges, with the Slight Risk expanded to include almost all of the Los Angeles and San Diego Basins. In the urbanized valleys, expect the combination of around an inch of rain with aforementioned higher amounts in the neighboring mountains, to combine with rapid snowmelt to result in scattered flash and urbanized flooding in these areas, especially tonight. With reports coming in of flash flooding into the valleys east of the Sierras, in coordination with the VEF/Las Vegas, NV forecast office, the Slight Risk area was expanded to include more of the area seeing leeside impacts. The High Risk areas remain largely unchanged with this update, with only some small expansions of the coastal area to include more of the ranges of Santa Barbara County. As of the time of this writing, a Flash Flood Emergency with high water rescues ongoing has been issued around Springville. Storm total rainfall in the high risk areas is already widespread over 5 inches, with isolated reports as high as 8.5 inches in the Sierras. Finally, for a little bit of good news, the axis of heaviest rainfall associated with the atmospheric river has shifted south of the Bay area, so San Francisco, its immediate environs, and all points north along the coast have been removed from all the risk areas for today, though certainly upstream flooding through the Central Valley will continue even if the main area of rain has ended. All risk areas along the coast have been trimmed southward a bit around Monterey Bay and the surrounding mountains. Guidance suggests that while the heaviest rain will continue moving south light to moderate rain will continue around Monterey Bay and points south into tonight. Wegman ....Previous Discussion... 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 about 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. 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 lesser 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 Valid 12Z Sun Mar 12 2023 - 12Z Mon Mar 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA AND THE SOUTHEAST U.S.... ....California... One...or more...shortwaves embedded within the broader scale trough will help the mid-level flow become increasingly zonal from the eastern North Pacific Ocean into the northern portion of California on Sunday. This should result in an increasing areal coverage of generally light to possibly moderate rainfall and renew flooding concerns in an already water-logged area. Spaghetti plots of SREF QPF area show potential more than 2 inches in the complex terrain in the along the coast and into the northern Sierra...while the GEFS is not as wet. With antecedent conditions being so wet and some uncertainty in when the next upstream system approaches the area (tending to think Monday)...will have an area similar to the Day 2 outlook area but increase it to a Slight Risk, ....Southeast... The spread in model solutions on Day 2 persists into Day 3 as the system makes its way eastward...with the ensembles tending to show a broad north to south spread with NMM members of the SREF being farther north extending from the Tennessee Valley eastward into the Carolinas while the ARW members were largely confined to the southern end of the Appalachians into the Southeast U.S.. Only part of the area has experienced above normal rainfall lately...and a Marginal was placed to cover the overlap between the lower flash flood guidance and where the NMM/ARW members and the GEFS/ECMWF showed some common ground. Future adjustments are likely assuming the timing and placement get in better agreement. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7KcMxJPn3AhK31-qVAWNqc7RI9TWVam8rsIP1R5OOXww= it6jTB7P-VTIZP41mtSddTdMpZlb36E-BXYq8N5wm3lchmc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7KcMxJPn3AhK31-qVAWNqc7RI9TWVam8rsIP1R5OOXww= it6jTB7P-VTIZP41mtSddTdMpZlb36E-BXYq8N5wSyGMVOU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7KcMxJPn3AhK31-qVAWNqc7RI9TWVam8rsIP1R5OOXww= it6jTB7P-VTIZP41mtSddTdMpZlb36E-BXYq8N5wgp5Lp6M$=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) .