Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 15 2024 15:54:06 FOUS30 KWBC 151554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1053 AM EST Thu Feb 15 2024 Day 1 Valid 16Z Thu Feb 15 2024 - 12Z Fri Feb 16 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....16Z Update... The nil D1 ERO was maintained from the previous forecast with very low probability for flash flood concerns focused along the southern Rio Grande, mainly within the confines of the developing surface trough in-of Starr and Hidalgo counties. Recent HREF blended mean did come up within the vicinity of the neighboring county lines with most of the hi-res suite indicating a max within a narrow region of higher theta-E's, likely situated along the developing surface trough this afternoon and evening. 12z BRO sounding was meager with observed MUCAPE, but it was non-zero and is forecast to increase a touch this afternoon over the southern portions of TX thanks to increasing boundary layer moisture and better mid-level moisture advection from a weak disturbance moving into the area out of MX. A low probability of thunder will exist within the zone closest to the surface trough, and considering the near-record airmass (PWATs) for mid-February in place, some modest rainfall over the course of the evening could cause a local flooding risk where heavy rainfall is persistent. As of now, the 1/3/6 hr FFG exceedance probabilities remain at 0% within the 12z HREF fields and are certainly at 0% for the NBM considering a more conservative forecast regarding the respective 1/3/6 hourly totals. In summary...the threat is non-zero but still below the 5% threshold with any flooding likely relegated to a triangulated area between McAllen-Rio Grande City-McCook in Deep South Texas. Kleebauer ....Previous Discussion... Over south TX 1-3" of rain is expected today into tonight. HREF neighborhood probabilities of exceeding 3" are as high as ~40%, suggesting localized 3-5" totals are a possibility. However, limited instability is expected to keep rainfall rates in check...generally peaking at 1"/hr on a localized basis. Dry antecedent conditions is resulting in high FFG over the area...thus despite some of the high QPF coming out of various HREF members...the probability of exceeding 1,3, or 6 hr FFG is 0% in the 00z HREF. Thus while areas of heavy rain are probable, and some localized ponding of water is possible...the chance of flash flooding still appears to be below 5%. Thus will continue with no risk area in the ERO. Chenard Day 2 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 3 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... 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 99th 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 01Z NBM probabilities of exceeding 2" of rain are generally 50-80% within this Slight risk area, with probabilities of exceeding 3" 30-50% within some of the favored terrain areas. 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. Some model spread remains with this system. The 00z GFS/NAM are a bit weaker with the AR and forcing, resulting in slightly lower QPF. The 00z ECMWF/CMC/UKMET are more aggressive with this system and QPF amounts. A GFS/NAM outcome would result in a lowered flood threat...however will note that the ECMWF and UKMET have been the more consistent models thus far with this system. From a probabilistic sense a Slight risk remains warranted, and is supported by the aforementioned NBM probabilities and potential for localized heavy rainfall rates. The Marginal risk over central FL was removed with this update. Moderate to heavy rainfall still appears probable...with guidance generally showing anywhere between 1 and 3 inches of rain. However instability in the guidance is minimal, suggesting this will be more of a widespread stratiform rain with generally low rainfall rates. This more prolonged steady rain is unlikely to result in flash flooding. Will continue to monitor instability trends on future forecasts, and if higher rates end up appearing more likely we can always bring back a focused Marginal risk on future updates. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4vR06ldcBFfuJL1tCEP9dl8IDYTqQJawrwFoDL5sdZ6P= dTq-M3cjmIvUZY02IRecLeHN8A3L_g1aeUK495n1nmo58AE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4vR06ldcBFfuJL1tCEP9dl8IDYTqQJawrwFoDL5sdZ6P= dTq-M3cjmIvUZY02IRecLeHN8A3L_g1aeUK495n1n4JZyjM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4vR06ldcBFfuJL1tCEP9dl8IDYTqQJawrwFoDL5sdZ6P= dTq-M3cjmIvUZY02IRecLeHN8A3L_g1aeUK495n1JQz55JE$=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) .