Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sat Feb 17 2024 09:05:00 FOUS30 KWBC 170842 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 341 AM EST Sat Feb 17 2024 Day 1 Valid 12Z Sat Feb 17 2024 - 12Z Sun Feb 18 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND NORTHERN CALIFORNIA COAST... No major changes to the previous forecast issuance of the Slight and Marginal risk areas in CA. The forecast generally remains on track with an atmospheric river expected to bring moderate to heavy rainfall to portions of central and northern CA today. A steady rain will overspread northern CA this morning, although initially the IVT axis is not all that impressive...weakening as it tries to get inland and oriented in a less favorable south/north way. So initially rainfall rates will be relatively low, although the persistent nature of the rainfall will still gradually increase rain totals. By ~18z a stronger shortwave will begin to approach the coast, helping increase forcing and finally push the front and IVT axis inland and off to the southeast. This shortwave will also steepen lapse rates and allow for some weak instability to work into the front...thus expect we will see some 0.5"/hr rainfall after 18z along portions of the central and northern CA coast between San Francisco and Eureka. A Slight risk remains for this area, with the 00 HREF supporting 1-3" across this corridor, with totals locally exceeding 3" within the favored terrain. Overall this is not expected to result in considerable or widespread flooding, but as the higher rates move onshore some localized flooding issues could arise. Chenard Day 2 Valid 12Z Sun Feb 18 2024 - 12Z Mon Feb 19 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SANTA BARBARA COUNTY IN SOUTHERN CALIFORNIA... ....California... A second stronger system is expected to bring another round of heavy rainfall to portions of CA on Sunday. 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. Overall not much change to the Slight risk covering a good portion of the CA coast into the Sacramento Valley and the western foothills of the Sierra. The northern portion of the Slight risk will have seen heavy rainfall on Saturday as well...so this additional rain will be falling on what should be increasingly saturated ground conditions. The IVT axis is expected to make good inland progress on Sunday, 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 0.5"/hr rainfall on a localized basis. This should be enough to result in scattered flooding from the coast across the valley and into the Sierra foothills. 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. A second area of heavy rainfall will exist along coastal areas from near Monterey southward into Santa Barbara county, where a longer duration of elevated IVT into the terrain is expected. We have upgraded portions of Santa Barbara county to a MDT risk with this update. The persistence and orientation of the IVT plume into the Transverse Range supports a developing significant flood risk Sunday night into Monday. Rainfall intensity should pick up by ~06z Monday as the magnitude of IVT increases and some weak instability tries to work into the area from offshore. The general consensus amongst the downscaled global guidance is for 3-4" of rain through 12z Monday, with the rain and flood risk continuing past 12z and into the day 3 period. ....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. As was the case on day 1, instability will again be a limiting factor for flash flooding. However will note that a warm or stationary front should be over southeast FL with a wave of low pressure riding along this front. This should result in some added low level convergence, and the possibility that enough instability can pool along and south of this boundary to support rates high enough for flash flooding. Overall continue to think most of the rainfall depicted by the guidance is on the cool side of the front within the more stable airmass...and thus would mainly be stratiform in nature and not produce rainfall rates high enough for flash flooding. However if convection can organize near/south of the warm front then an isolated flash flood risk could evolve. The best threat of this happening is over southeast FL where a Marginal risk was maintained. Chenard Day 3 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 3 forecast, with areas of significant flooding expected to continue. The 00z downscaled GFS/ECMWF/CMC were all in good agreement indicating event total rainfall of 4-8" within the favored upslope areas of these counties. 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 18z Monday. Elsewhere we will maintain a Slight risk along much of coastal CA and a Marginal risk into the valley and foothill locations. The approach of the large scale trough and low will support continued elevated IVT values into CA through Monday. with that said, both IVT and PW magnitudes will be lower than what we will see on days 2 and 3. Thus in general would expect the coverage of heavier rains to not be as large as previous days...with the nature of rainfall more showery than previous days. However persistent onshore upslope flow will really favor the coastal ranges, where several more inches of rain may fall. Given we will likely have elevated streamflows and saturated conditions from the previous days rains, additional flooding is probable. In addition, the lower PW but more unstable airmass will favor some localized heavier convective cells, which given the increasingly saturated conditions, could result in at least localized flood concerns within both the Slight and Marginal risk areas. There are some pretty big QPF differences...with the ECMWF/ECENS generally much wetter than the GFS/GEFS over central and northern CA. While reality may be somewhere in between, the ECMWF output does appear a bit more realistic given the continued forcing/upslope flow...and thus would lean more in that direction at this time. 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