Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 18 2024 08:36:06 FOUS30 KWBC 180836 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 335 AM EST Sun Feb 18 2024 Day 1 Valid 12Z Sun Feb 18 2024 - 12Z Mon Feb 19 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SANTA LUCIA AND SANTA YNEZ RANGES IN CENTRAL/SOUTHERN CALIFORNIA... ....California... A second stronger system is expected to bring another round of heavy rainfall to portions of CA today. This will be a larger scale trough with a much broader IVT plume directed into CA...thus the latitudinal coverage of rainfall will be quite large with several areas of concern for potential flooding. A Moderate risk remains for the Santa Ynez range in Santa Barbara county, and it was expanded northward into the Santa Lucia range from San Luis Obispo into Monterey counties. This is the corridor where the heaviest rainfall is expected through 12z Monday, with the heavier rates first impacting the Santa Lucia range by 00z, and then shifting southward into the Santa Ynez between 06z-12z Monday. By 12z Monday 00z HREF guidance supports 5"+ amounts over the Santa Lucia and 3"+ amounts over the Santa Ynez (with more to come here after 12z). Initially this rainfall will be driven by increasing IVT oriented perpendicular to the terrain, which will support a steady moderate rainfall, but limited instability should keep rainfall rates from getting too high. However with time some weak instability will try to work into the area from north to south near and just ahead of the cold front. This should result in an uptick in rainfall rates...with 00z HREF data showing over a 70% chance of 0.75"/hr rainfall over the Santa Lucia range. This should be enough to result in some areas of significant flooding.=20 The broad Slight risk over the central/northern CA coast into portions of the Sacramento and San Joaquin valley and into the western slopes of the Sierra remains mainly unchanged. Rainfall will move into coastal areas by 21z, and then spread inland thereafter. The IVT axis is expected to make good inland progress, and may actually slow/stall over portions of the Sacramento and northern San Joaquin valleys. To go along with this IVT and convergence axis, upper level flow is forecast to be very divergent within the left exit region of a strong jet. This combination should result in heavy rainfall over these valley locations, with some weak instability also forecast supporting some embedded convective elements and potential localized 0.5"/hr rainfall. The best potential for these higher rates and flash flooding should be between 00z and 12z when steeper lapse rates overrun the moisture plume. This event could end up resulting in both longer duration flooding from the multi round event, and also more rapid response flash flooding given the aforementioned higher rate potential tonight into Monday morning. With high snow levels, some flooding impacts are also possible even in portions of the central/northern Sierra and spilling over into the eastern foothills/valley locations. ....Florida... A Marginal risk was maintained across portions of the east central and southeast FL coast. Plenty of forcing and moisture for heavy rainfall with an approaching mid level trough, strong upper jet and PW values over the climatological 95th percentile. However instability will again be a limiting factor for flash flooding for the central and northern portions of the state. However a warm or stationary front over southern FL will result in some added low level convergence and some pooling of instability there. High res guidance indicates upwards of 1000 j/kg of CAPE over south FL near and south of this front which will be enough to support some heavier rainfall rates and some flash flood risk. The expectation is that an area of organized convection will move across south FL today, dropping a swath of as much as 3-5" of rain from near Naples to Palm Beach and points south. While hourly rainfall of 1-2"/hr is probable with this activity, even the high QPF from the 06z HRRR is barely getting to 50% of FFG. Thus still thinking any flooding will be localized and confined to any more sensitive urban areas. Chenard Day 2 Valid 12Z Mon Feb 19 2024 - 12Z Tue Feb 20 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SANTA BARBARA AND VENTURA COUNTIES IN SOUTHERN CALIFORNIA... Heavy rainfall will be ongoing across portions of the Transverse Range in Santa Barbara and Ventura counties Monday morning. A MDT risk was thus continued into this day 2 forecast, with areas of significant flooding expected. The forecast rainfall amounts have not changed much, with the 00z high res guidance continuing to support event total rainfall of 4-8" within the favored upslope areas of these counties. Hourly rainfall of 0.5"/hr is all but certain with this event, and 00z HREF data indicates a ~40% chance of 0.75"/hr. While not as extreme as the recent AR event, this should be enough rainfall, especially given the wet antecedent conditions, to produce some significant impacts. The most intense rainfall (when most of these totals will fall) is expected to be between 06z and 21z Monday. Across central and northern CA additional rounds of rainfall are expected. The approach of the large scale trough and low will support continued elevated IVT values into CA through Monday. Along the coastal terrain from just north of San Francisco to Ferndale the duration of favorable upslope flow will be impressive...with steady rainfall continuing for the full 24 hour period. Lower PWs and less forcing will mean lower rainfall rates, but the favorable orographic effects should still result in amounts approaching 1" per 3 hours. Thus over the 24 hour period we could be looking at 3-5" of rain with localized amounts of 5-7". Given that this will be falling on top of increasingly saturated soils and elevated streamflows, additional flooding is likely, and this is considered a higher end Slight risk for these areas. Interestingly most model fields are pretty similar between the GFS and ECMWF, however QPF differs significantly. At this time leaning more towards the heavier ECMWF/ECENS solutions as this appears more reasonable given the prolonged upslope flow and favorable IVT into the terrain. These higher values are also supported by the 00z GEM Reg. It will also be a wet day over the Sacramento Valley locations into the western slopes of the Sierra, where a Slight risk remains. Continued convergence will support showery conditions, and some increased instability given the approaching trough and steeper lapse rates will favor embedded heavier convective segments. Given what should be increasingly saturated and more sensitive conditions, this higher rainfall rate potential supports a continued flash flood risk over these areas. Chenard Day 3 Valid 12Z Tue Feb 20 2024 - 12Z Wed Feb 21 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... The long duration rainfall event over CA begins to wind down by Tuesday, however some flood risk will likely continue. The lingering IVT axis will shift south into far southern CA, and while it will be weakening, some heavier rains and flood threat may persist, with guidance supporting an additional 1-2" of rainfall. Meanwhile continued, albeit weaker, onshore upslope flow into the central and northern CA coastal ranges will support additional rainfall upwards of 1-3". Typically the lower PW and more showery nature of precip by this time would not warrant much of a flood risk along these coastal ranges that can handle heavy rainfall. However given the rainfall that will precede Tuesday, think this additional rain may still be able to cause some additional flooding...thus the Slight risk was expanded northward along the coastal ranges. Again leaning more towards the higher QPF values shown in the ECMWF/ECENS solutions over the drier GFS/GEFS. Also decided to expand the Slight risk into portions of the Sacramento Valley with this update. Again this is mainly driven by antecedent conditions and the expectation that enough weak instability will be present to support localized heavier convective cells into the day Tuesday. Overall expect rainfall coverage and magnitudes to be lower than day 2 (Monday), but the wet antecedent conditions will mean a lower bar will need to be reached for flooding concerns to arise...and the continued broad forcing/convergence and instability over the valley and foothill locations suggest this bar could very well be reached on an isolated to scattered basis. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6mPGh4Pro0ww3zkJ-LSMqupeML-9Tevfkv7n7hdereBj= kKRgY8jQGNEjEaLGWIsgv50EqzCJejckINTZble18VOu2x4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6mPGh4Pro0ww3zkJ-LSMqupeML-9Tevfkv7n7hdereBj= kKRgY8jQGNEjEaLGWIsgv50EqzCJejckINTZble1jN0oeo8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6mPGh4Pro0ww3zkJ-LSMqupeML-9Tevfkv7n7hdereBj= kKRgY8jQGNEjEaLGWIsgv50EqzCJejckINTZble1A80wrcg$=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) .