Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 19 2024 01:19:20 FOUS30 KWBC 190119 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 818 PM EST Sun Feb 18 2024 Day 1 Valid 01Z Mon Feb 19 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... ....01Z Update... Only minor tweaks were made to the outlooks across CA (mainly the Slight Risk across parts of the eastern San Joaquin Valley and western Sierra), as current forecast thinking remains on track. Most recent GEFS output shows 850-700 mb moisture flux anomalies reaching +4 by 12Z along the Central CA Coast from Point Conception and Santa Barbara and the western portions of the Transverse Ranges. This as IVT values reach 500+ kg/m/s per the latest CW3E output (both GFS and ECMWF). The latest (18Z) HREF shows probabilities of 0.50+ inch/hr increasing to above 60% across the Central CA Coast after 03Z...then to 80%+ east of Point Conception after 08Z. Hurley ....16Z Update... There is a break ongoing presently in any widespread or significant rainfall across California this morning. That is expected to change starting as early as 21Z/1pm Pacific as a strong front associated with a well-developed low out over the eastern Pacific becomes vertically stacked, while a 170 kt subtropical jet streak rounds the base of the upper level trough/vertically stacked low, and is directed into southern CA. The result of this pattern will be plenty of lift in the atmosphere in the left exit region of that jet, combined with 45 kt of onshore flow at 850 directed into the 7,000 foot coastal ranges of southern/central CA. Soils in this area remain well saturated from yesterdays' and previous days' rains, so the 3-5 inches of rain with locally higher amounts directed at the coastal ranges will essentially all be converted into runoff. Thus, expect numerous instances of flash flooding in the Moderate Risk area, some of which may be significant/considerable. The Moderate risk area remains unchanged. The highest totals through 12Z/4am Monday will be through the Santa Lucia Range of Monterey and SLO Counties, with another precipitation maximum further south into Santa Barbara County and the western Transverse Ranges. The surrounding Slight Risk area remains for all of the Sacramento Valley and northern portions of the San Joaquin Valley, as well as the lee/eastward facing slopes of the Santa Lucia Range. Despite a maximum of precipitation along the coast, these more inland areas will themselves pick up 2-4 inches of rain on also saturated ground, with 1-3 inches of rain for much of the Sacramento Valley. As northern California has seen heavier rains more recently, less rain is needed to result in flash flooding. The Slight Risk area remains for the west facing slopes of the Sierras below the snow level, where topography will amplify the impacts from any flooding. Finally, the Marginal Risk area was extended a bit northward on the Nevada side of the CA border, as southerly flow into the Sierras should allow a long-duration light rainfall to occur into the Lake Tahoe/Reno/Carson City areas of far western NV. Snow levels will be plenty high enough for the valleys to pick up up to an inch of rain with locally higher amounts, which could result in isolated flash flooding. Wegman ....Previous Discussion... ....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. 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... ....2030Z Update... No major changes were made to the ERO risk areas in California. The Moderate Risk for the Transverse Ranges remains in place, as the largest rainfall amounts of this heavy rain event are expected on Monday. The low over the Pacific will be vertically stacked by Monday, resulting in the low stalling out. The associated fronts therefore will also stall out, which looks most likely to occur over much of CA, though the heaviest amounts will be when the initial slug of Pacific moisture impacts the coast...which will be into the Transverse Ranges. No changes were made to the inherited Moderate Risk area. For the Slight, some modest expansions were made through the Sacramento and San Joaquin Valleys to account for both continued rainfall and runoff concerns into areas getting less rainfall. For those latter locations, meaning areas getting much less rain (due to lack of upslope support), but the runoff from the nearby mountains greatly raises river levels, such that the amount of rain locally that bumps the area into "excessive" territory is much less. This is expected to become an increasingly common occurrence all throughout the Central Valley over the next few days. The area considered at the higher end of the Slight Risk category includes much of the coastal ranges north of the Bay Area through to about Crescent City, as well as downtown L.A. The Slight Risk includes much of the L.A. suburbs east/inland of downtown, including Hollywood, San Bernardino, and Riverside as well as all of the west-facing foothills of the Sierras. The biggest change from inherited was a larger expansion of the Marginal Risk area to include areas to the east/lee of the Sierras, as well as the urban areas of Reno and Carson City, NV. "Slopover" precipitation is expected to continue across all of northern Nevada, with the valleys likely staying plenty warm enough for all rain. Since Monday will continue the rainfall event starting tonight, the same flooding concerns depicted in the Day 1 Marginal Risk area will continue through Day 2/Monday. There is some uncertainty at the tail end of Monday night as to how far south/east the rain gets to southeast of L.A., so the Marginal Risk area was tweaked southward there as well. Wegman ....Previous Discussion... 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... ....2030Z Update... A few more substantive changes were made to the inherited forecast for the Day 3/Tuesday time period. In northern CA, while large-scale rainfall amounts will be coming down, what rain does fall on Tuesday will be occurring in an area that simply will not be able to handle any more rain, so small amounts of rain will continue to have large/outsized impacts. Thus, while the Slight Risk area was expanded to include much of the Sacramento Valley and northern parts of the San Joaquin Valley, it's a lower end Slight as any significant rainfall will be just along the foothills of the Sierras, with most locations remaining under an inch of new rain for the day. The Santa Lucia Range south of Monterey may also pick up 1-2 inches of new rain, locally enhancing the flooding threat there as well. The heaviest rainfall totals in CA will be further south into the L.A. Basin. The Slight Risk for this area was expanded considerably to include all of the surrounding suburbs, with San Bernardino area perhaps in the bullseye for most rain expected on Tuesday. The cause of the rain remains the same...a stalled out front/A.R. will remain stuck over this same area, with the axis of heaviest rain perhaps having moved 25 to 50 miles east since Monday. This will put 1-3 inches of rain right over the highly urbanized L.A. Basin, which alone would likely cause some flooding, but at the tail end of a multiple day rain event would likely see at least a few instances of significant flooding. Thus, the L.A. Basin broadly is considered on the higher end of the Slight Risk category, and will be considered for a Moderate Risk upgrade with future shifts. The Slight Risk area was also expanded south down the coast to include San Diego and all the way to the Mexican border as the slow shift in the primary frontal position should lead to steadier rains further south down the coast. The surrounding Marginal Risk area includes much of the lee side of the Transverse Ranges and the southern San Joaquin Valley, which despite forecasts of much less rainfall, should still see enough rain to cause isolated flash flooding. Wegman ....Previous Discussion... 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!8_ZqsQBNiiqbXBHdyJfVedpZXBnAmdGoTO369ZVT3dKc= 7KSrC4ZzehXmh-JHX_4Jx8jzGvjzc5uZNWYAHPQqTgypyyg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8_ZqsQBNiiqbXBHdyJfVedpZXBnAmdGoTO369ZVT3dKc= 7KSrC4ZzehXmh-JHX_4Jx8jzGvjzc5uZNWYAHPQqd7sngA4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8_ZqsQBNiiqbXBHdyJfVedpZXBnAmdGoTO369ZVT3dKc= 7KSrC4ZzehXmh-JHX_4Jx8jzGvjzc5uZNWYAHPQqybgKP88$=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) .