Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Feb 16 2024 08:03:52 FOUS30 KWBC 160803 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 302 AM EST Fri Feb 16 2024 Day 1 Valid 12Z Fri Feb 16 2024 - 12Z Sat Feb 17 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 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. An atmospheric river is expected to bring moderate to heavy rainfall to portions of central and northern CA on Saturday. The IVT plume should be weakening as it moves ashore, however IVT anomalies off the 12z ECMWF ensemble mean are still approaching the 97th percentile for February along portions of coastal central/northern CA. Strong low level convergence does stall out for a bit early Saturday over northern CA, which should extend the rainfall duration for a bit, before everything becomes more progressive off to the south and east by later in the day. The anomalous IVT, strong low level convergence, and initial slow movement of the rain shield will all combine to produce an area of moderate to heavy QPF values along/near the coast from just north of San Francisco to near Eureka...and thus we will maintain a Slight risk across this corridor. The NBM probabilities of exceeding 2" of rain are as high as 50-80% within this Slight risk area, with probabilities of exceeding 4" around 50% within the King Range south of Eureka. In addition to these totals, we expect at least some weak instability to work into the front Saturday as a strong mid level shortwave moves into northern CA. This increased forcing and associated steepening lapse rates should help drive some hourly totals up over 0.5". This combination of 1-4" of rain and localized rates over 0.5"/hr suggests some flooding is a possibility. Still think we are good without a Marginal risk over FL. A lack of instability suggests rainfall rates will stay below levels needed for flash flooding. Rainfall totals for this 24 hr period have also shown a gradual decline in the model guidance, with most solutions supporting widespread totals around 1", but keeping any 2"+ amounts fairly localized. Chenard Day 3 Valid 12Z Sun Feb 18 2024 - 12Z Mon Feb 19 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CALIFORNIA COAST... ....California... A second atmospheric river is expected to bring another round of heavy rainfall to portions of CA on Sunday. The heaviest rainfall amounts through 12z Monday are expected along coastal areas from near Monterey southward into Santa Barbara county, where a longer duration of elevated IVT into the terrain is expected. Rainfall amounts around 3" appear likely across these favored terrain areas, with a gradual uptick in rainfall rates expected Sunday night as IVT increases and some weak instability tries to push onshore. This rainfall should gradually result in an increased flood risk across the region, with that threat continuing into Monday. Further north along coastal CA rainfall totals should be lighter with 1-2" expected..however do note at least some probabilities of exceeding 3" within areas of favored terrain. Some briefly heavier rates will also be possible as we should see pretty strong low level convergence as a cold front and some weak instability moves ashore. This rainfall will also be falling in addition to the heavy rain amounts that are forecast the day prior (Saturday), so soil/stream conditions should be more sensitive by this time. Given this, we decided to expand the Slight risk to include most of the central to northern CA coastline. The other change made with this update was to expand the Marginal risk into the valley and Sierra foothills. This AR should have a decent inland push to it Sunday and thus do expect some heavier rain to push into the central valley possibly resulting in some localized flood concerns. ....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 2, instability will again be a limiting factor for flash flooding. Current global models really do not show much more than a couple hundred J/KG of CAPE over southeast FL...and it will likely be hard to get rainfall rates high enough for flash flooding with that. 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 instability ends up higher than currently progged. For these reasons think maintaining the Marginal risk along the urban corridor is the way to go at this time. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!55BHltcGnHSckkDzGcwbKZGov4zia0EeZP4tFoOS_W68= -crOAGNwar0oZ4iMsdlkwQgsRWfhkp4zwHq5Do-yFPo61xE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!55BHltcGnHSckkDzGcwbKZGov4zia0EeZP4tFoOS_W68= -crOAGNwar0oZ4iMsdlkwQgsRWfhkp4zwHq5Do-yk2NReyk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!55BHltcGnHSckkDzGcwbKZGov4zia0EeZP4tFoOS_W68= -crOAGNwar0oZ4iMsdlkwQgsRWfhkp4zwHq5Do-y2ASF9eM$=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) .