Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Feb 16 2024 00:57:16 FOUS30 KWBC 160057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 756 PM EST Thu Feb 15 2024 Day 1 Valid 01Z Fri Feb 16 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. Very low probability for flash flooding concerns remains for D2 across southeast TX, but will be monitoring the progression of a cold front interacting with elevated moisture and limited instability fields along coastal TX. GFS/ECMWF are indicating a local maximum near Corpus Christi, but to varying magnitudes with the NAM the most bullish with the areal extent of heavy rainfall approaching 2-3" within the coastal areas north of Brownsville. NBM probability for greater than 2" of precip remains very low with the 12z HREF also signifying a low-end potential for such totals, along with a flat 0% for the 1/3/6 hr FFG exceedance probabilities across the region. This will be mainly beneficial rainfall across the area with more ponding across impervious surfaces. Chenard/Kleebauer 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... ....20Z Update... No major changes to the previous forecast issuance of the SLGT/MRGL risk areas out west. Global deterministic remains in good agreement with regards to timing and total QPF for northern CA where the best moisture transport and low-level convergence pattern occurs. Expect a general 1-3" with as much as 4" max within the King Range south of Eureka. Florida remains a close call for a Marginal Risk at this time with the incoming rainfall expected Saturday afternoon into Sunday morning. Timing of the mid-level disturbance and frontal convergence along a progressing cold front will be key points to monitor in the coming days. The lean on maintaining the nil risk area over north-central FL was the the limited instability profile currently forecast with rates capped at 1-1.5"/hr, below the current threshold of 3-4"/1-hr necessary for FFG exceedance. If the cold front does speed up within the coming forecast cycles, that would lead to a bump in precip potential and better forcing correlated with the mid-level vorticity advection that could definitely trend us back towards a MRGL over the north-central portion of the peninsula. This will be monitored closely with hi-res deterministic and ensemble data coming into play the next few days, but to maintain continuity...have maintained the nil risk over the state. Kleebauer ....Previous Discussion... 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!_85XquH8jJ8zy5MqisSE_GZLBLPebROBN_y8enORYcZu= 1lTIs3deIv4aL10RbD1YSKbIAnJUzYa2Yh4b5uHrk-eWoHQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_85XquH8jJ8zy5MqisSE_GZLBLPebROBN_y8enORYcZu= 1lTIs3deIv4aL10RbD1YSKbIAnJUzYa2Yh4b5uHr-iYMXMQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_85XquH8jJ8zy5MqisSE_GZLBLPebROBN_y8enORYcZu= 1lTIs3deIv4aL10RbD1YSKbIAnJUzYa2Yh4b5uHrYtupBZg$=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) .