Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 17 2024 00:47:08 FOUS30 KWBC 170047 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 746 PM EST Fri Feb 16 2024 Day 1 Valid 01Z Sat Feb 17 2024 - 12Z Sat Feb 17 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley 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... ....2030Z Update... ....California... Almost no changes were needed with this afternoon's update. The Slight Risk for a portion of the northern CA coast north of the Bay Area is unchanged. A strong front moving into the coast will interact with the coastal mountains Saturday morning, which will drop most of the day's forecast rain. As the front pushes inland, rain along the coastal mountains in the Slight risk area will temporarily weaken. Many (though notably, not all) of the guidance suggest that a secondary weak low will form off the Oregon coast, which will put northern CA in the onshore flow side. The low will take advantage of 200-500 J/kg of MUCAPE to cause renewed convection Saturday afternoon and evening, which will move into the coast. The convection will individually be stronger than any embedded convection with the earlier front, which could cause widely scattered flash flooding in the Slight Risk area. The inherited Slight depicts the area of greatest concern well, so no changes were needed. Further inland, southerly flow into the mountains of the northern Sacramento Valley will locally enhance precipitation rates, but low snow levels should allow most of the precipitation to fall as snow into the higher elevations. However, the snow levels will still be high enough that snowmelt may play a role in enhancing local flooding potential, so the Marginal Risk for the area remains in place with no changes. One small change added was to the Marginal Risk area around Point Conception. Between 00Z and 06Z Sunday, as the leading front moves south along the coast, rainfall rates should still be high enough (up to 1/2 inch per hour) that combined with locally wet soils, isolated flash flooding will be possible into the western portions of the Transverse Ranges. Thus, the Marginal Risk area was expanded south to include Point Conception and the adjacent mountains. ....Florida... A slowing front currently over the northern Gulf Coast will stall out over the Florida Peninsula on Saturday. There is good agreement in the guidance that instability will be lacking except at the very southern edge of the frontal precipitation shield, and even there it will only amount to a couple hundred J/kg. Thus, convection in general will be lacking with the front, meaning the multiple inches of rain expected across portions of the state will be in the form of a long-duration light rain. Adding that soil moisture levels in this area are well below normal for this time of year, it's highly unlikely that this pattern will result in flash flooding, so a Marginal Risk was not introduced. Wegman ....Previous Discussion... 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 FOR MUCH OF THE CALIFORNIA COAST... ....2030Z Update... ....California... A second frontal system associated with a stronger low and upper level trough will impact the California Coast on Sunday, specifically from Sunday afternoon through the overnight period. While there may be as much as a 24 hour break in rainfall from the end of the Day 2 period rain on Saturday afternoon until the start of the Day 3 frontal system on Sunday afternoon, the heavy rainfall from Saturday is not expected to be fully run off before the start of Sunday's rain. Thus, saturated soils and more heavy rain Sunday should re-raise stream, creek, and river levels across northern California. Expect an additional 2-3 inches of rain through Sunday night for most low-elevation areas, which because it will be falling on already saturated ground will result in more widely scattered flash flooding into the northern coastal ranges and the northern Sacramento Valley. Any runoff may be further enhanced by snowmelt as well. In coordination with STO/Sacramento, CA forecast office, the Slight Risk area was expanded to include much of the Sacramento Valley, while the Slight continues for the coastal mountains. The Slight Risk area continues through the Bay area and all the way down the coast to around Santa Barbara. For this portion of the California coast through the Transverse Ranges, the front may get hung up for longer late Sunday night, which will locally enhance the flash flooding threat, especially when added to any rainfall that will occur Saturday evening. The surrounding Marginal Risk area was also expanded east to include some of the valleys to the lee of the Sierras, including the Lake Tahoe area with this update. The latest guidance suggests that some precipitation will get over the summits of the Sierras, resulting in some rainfall into the usually much drier valleys east of the ranges. As these areas of eastern CA and extreme western NV have soils that are more hydrophobic, resulting in less rainfall needed for flooding concerns to arise, the Marginal was expanded to include them. ....Florida... The stalled out front from Saturday over northern Florida will slowly drift southward, impacting more of central and southern Florida on Sunday. As on Day 2/Saturday, instability will still be lacking across most of the precipitation shield. Despite the approach of the right entrance region of a 160 kt jet locally enhancing the lift and resulting in more rainfall broadly across the peninsula, it still appears that most of the rainfall will be a stratiform variety, keeping rates light enough that flash flooding is not expected in most areas. The Marginal Risk was expanded up the east coast all the way to Daytona Beach, mostly to account for uncertainties in where the heaviest rainfall is expected, but the Marginal Risk largely highlights the urban I-95 corridor where FFGs are locally lower. For much of the Marginal Risk area, this second consecutive day of steady rain may be enough rainfall over the combined 2 days to result in isolated flooding in more flood-prone areas. Wegman ....Previous Discussion... ....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!-_K-4PueHimXUDaHibkAqzxDcG59PH-GUx7kZSLN8jDK= fRFtbqxjYFc4CU4-0Paqq7JWJiBeJ9E6BK_pZjbskHg3hXY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-_K-4PueHimXUDaHibkAqzxDcG59PH-GUx7kZSLN8jDK= fRFtbqxjYFc4CU4-0Paqq7JWJiBeJ9E6BK_pZjbs4p5DBCk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-_K-4PueHimXUDaHibkAqzxDcG59PH-GUx7kZSLN8jDK= fRFtbqxjYFc4CU4-0Paqq7JWJiBeJ9E6BK_pZjbsDsgv3LI$=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) .