Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Feb 16 2024 20:28:03 FOUS30 KWBC 162027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 PM EST Fri Feb 16 2024 Day 1 Valid 16Z Fri Feb 16 2024 - 12Z Sat Feb 17 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!582VFfiYvo1mw8K-5AjTgpJNDtB4EWqovWZChsrOXlC8= mvhgfy0Ht-hESYaR-LMFP3wmMktpVx6laKfSuhn3UCz1Y5k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!582VFfiYvo1mw8K-5AjTgpJNDtB4EWqovWZChsrOXlC8= mvhgfy0Ht-hESYaR-LMFP3wmMktpVx6laKfSuhn3FBMfxEw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!582VFfiYvo1mw8K-5AjTgpJNDtB4EWqovWZChsrOXlC8= mvhgfy0Ht-hESYaR-LMFP3wmMktpVx6laKfSuhn3IPLg1jw$=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) .