Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 22 2023 01:00:22 FOUS30 KWBC 220100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 759 PM EST Thu Dec 21 2023 Day 1 Valid 01Z Fri Dec 22 2023 - 12Z Fri Dec 22 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... The MDT risk over portions of Santa Barbara, Ventura and Las Angeles counties was generally left unchanged. The flood risk is ongoing across these areas, although should be on a downward trend as we head into the evening and overnight hours. The expectation is that the low/mid level easterly flow will push instability back offshore and also result in the rain bands shifting southwestward, and primarily offshore as well. With that said, the 18z HREF and recent HRRR runs are not really matching up all that well to current radar. Radar imagery shows activity persisting onshore more than the aforementioned guidance is forecasting. So while we still think the trend will be downward with a gradual offshore shift, QPF through the evening hours may be higher than these high res models are depicting. An additional 1-2" over the MDT risk area is possible. Given the further east current position of the rain shield, can not rule out the possibility that coastal portions of Ventura and Santa Barbara counties stay within the training axis longer than expected, which could result in localized 2-3" of additional rainfall. We are also noting rather robust convective development off the coast of southern CA. The model consensus is for this to move onshore between 06z-08z, but given the activity is currently closer to the coast than forecast, it may move onshore a couple hours earlier. Instability drops off rapidly onshore, and thus the model consensus is for this activity to weaken pretty rapidly as it moves inland. HREF 1"/hr neighborhood probabilities are as high as ~50% off the southern CA coast, and given recent radar/satellite trends, this seems reasonable. Probabilities drop off rapidly onshore, but we do see some 15-30% probabilities 1"/hr right along the coast, with more widespread 40-70% 0.5"/hr probabilities over much of southern CA. So while pretty rapid weakening is expected...it appears like some localized 1"/hr rates are possible along coastal areas, with 0.5"/hr totals more common. This activity should have rather quick eastward motions, which should generally keep total QPF under 1.5" through 12z Friday. With that said, rates may be high enough to still result in some flash flood impacts, especially within urban areas and any other susceptible locations...and this warrants the continuation of the Slight risk. Chenard Day 2 Valid 12Z Fri Dec 22 2023 - 12Z Sat Dec 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN PORTIONS OF CALIFORNIA AND ARIZONA... ....20Z Update... Models coming into an agreement on the location of the best mid-level forcing and orientation of the axis of instability ahead of our approaching Pacific trough. The evolution of the 500mb pattern still holds merit from the previous issuance, so the location of the SLGT and MRGL risks are well-established with only some minor adjustments on the northern edge to attribute for the lack of instability and potential transiton to winter precip a bit faster than what was progged. 12Z HREF continues to depict a wide swath of 1-1.5" QPF within the means with the blended mean field showing two axis' of 2+" potential within the corridor of highest instability (Colorado River Basin and south-central desert). NBM probability fields are also fairly prolific as of the latest runs with 30-40% chance of at least 1.5" of precip across south-central AZ from Pima county extending northeast into Pinal and Gila. This correlates well within the best instability forecast as southwest flow advects low-level moisture deep into AZ providing a coupled boundary layer and upper level destabilization regime within the difluent region of our upper trough. The SLGT risk will likely be the max potential given the setup, so expecting some continuity through the event. Further west over Southern CA, the closed reflection will pivot eastward and eventually open up as it approaches AZ. Ascent maximized under the upper low will allow for scattered moderate to locally heavy rainfall for a time Friday morning before moisture and ascent gets shunted eastward. The 0.75-1.25" QPF forecast with local maximum up to 1.75" was enough to maintain the previous SLGT expansion into the area, thus avoided any changes within the SoCal domain. Kleebauer ....Previous Discussion... The strong/deep closed upper level low is expected to be centered just offshore southern California early Friday morning, then begins to pick up speed as it moves eastward toward central Arizona by early Saturday morning, opening up as it does so. This will put much of southern California, far southern Nevada, and Arizona in a favored region of deep-layer forcing/lift while anomalous S-SSW low-level flow positions a plume of higher moisture initially into southern California then across Arizona during the afternoon/evening hours. Moisture anomalies with this system are expected to remain well above normal, near 3 standard deviations above normal for this time of year, and precipitable water values are forecast to reach or exceed 1" at times, especially across southwest to south-central Arizona. The GFS and ECMWF IVT forecasts from CW3E show an impressive surge approaching 500+ kg/m/s into central Arizona. During the afternoon, some limited instability may develop across the Desert regions of CA and eastward into southern Arizona, which may allow for some localized higher rain rates approaching 0.50-1.0"/hr at times per the 00Z CAM guidance. Overall, a broad area of 0.75-1.5" totals are expected for the 24-hr period ending 12Z Saturday, with some localized/embedded higher amounts up to 2.5" possible in any thunderstorms. Per collaboration with WFOs SGX, PSR, and TWC, we have bumped a large portion of the Marginal Risk to a Slight Risk across southern CA into much of southern AZ. This given the wetter QPF trends, along with a corresponding increase in HREF exceedance probabilities through 00Z 12/23 (particularly the probabilities of at least 0.50"/hr and 1.00"/3hr). Meanwhile, the latest HREF probs of 6hr QPF exceeding the 2 year ARI peaks between 60-80% across either side of the CA-AZ border between 12Z Fri-00Z Sat. Also supporting the upgrade to a Slight Risk is the recent CSU First-Guess Fields, including the UFVS-verified version. Hurley Day 3 Valid 12Z Sat Dec 23 2023 - 12Z Sun Dec 24 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE RED RIVER VALLEY SOUTH TO THE MIDDLE AND UPPER TEXAS COAST... ....20Z Update... Updated discussion within the second paragraph below... Full-latitude upper trough traverses the Rockies and moves into the High Plains late on day 3 (00-12Z Sun). During this time (late Day 3...Sat night), deep-layer ascent and south-to-north moisture flux will strengthen fairly rapidly along the Red River Valley south through North TX and much of the TX Triangle to the Middle-Upper TX Coast. This as the forcing becomes enhanced from both the subtropical upper level jet (left-exit region) and S-N oriented upper jet streak across the Central Plains (right-entrance region). PWs and 850 mb moisture flux anomalies in fact become 2 to 3 standard deviations above normal Sat night per the 00Z GEFS and SREF. The signal per the latest 12Z guidance is becoming increasingly bullish on the prospects of multiple areas of convection developing within the theta-E advection pattern extending from the Texas Gulf Coast into central Oklahoma ahead of the mean trough. Rich low-level moisture advecting north within the confines of a 30-50kt LLJ will allow for a focal point of convective development and maintenance as the broad ascent pattern shifts eastward through the southern plains. GFS deterministic has come around more to the idea of a more consolidated low over the plains, much sooner than the previous 06Z run that was way more progressive than any other deterministic and associated ensemble. This provides better continuity with the overall pattern progression and less spread in the means. Maintained the MRGL for the time being, but would not be surprised if this is bumped to a SLGT in later issuance as we get a better agreement on the placement of the heaviest QPF footprint. Hurley/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!97FJVJae0xHnpEqkJleWVWCJynnvrVnkoX_kihRa58f7= wAT50aqrq36uimGrXuzI3DhFhOHq7Ww3Y-k1pa82P8eHAcI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!97FJVJae0xHnpEqkJleWVWCJynnvrVnkoX_kihRa58f7= wAT50aqrq36uimGrXuzI3DhFhOHq7Ww3Y-k1pa82FlO4RPA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!97FJVJae0xHnpEqkJleWVWCJynnvrVnkoX_kihRa58f7= wAT50aqrq36uimGrXuzI3DhFhOHq7Ww3Y-k1pa82rc6oSwQ$=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) .