Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Dec 21 2023 19:23:00 FOUS30 KWBC 211958 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 257 PM EST Thu Dec 21 2023 Day 1 Valid 16Z Thu Dec 21 2023 - 12Z Fri Dec 22 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ....16Z Update... Previous forecast remains on track with the evolution of the second wave of heavy rainfall moving into portions of Southern CA. Recent radar trends align well with the thinking of prevailing S/SE flow moving into the terrain areas northwest of Oxnard/Los Angeles with local maximums located within the south facing hills in the Santa Ynez. The occluded low acting as the main driver continues to slowly meander to the south-southeast off the CA coast with the occluded front located just off the Southern CA coast. Primary impacts will be over the terrain where the secondary punch of heavy rain will become a compounding threat for localized flooding with mudslides and debris flow potential within any flood areas. 12Z HREF probabilities remain elevated for at least 2" of rainfall within the comprised Moderate risk area, as well as 40-45% probability of at least 1"/hr rates at times from now through 00z this evening until the main wave shifts southeast off the coast. Oxnard area was impacted fairly significantly last evening with rainfall totals already between 3-5" there as well with more of any urban flood threat located in the population center just northwest of LA. This area will be the other area of interest as one bout of heavy rain could spur issues once again after taking on so much rainfall prior to the this next wave. By this evening, focus shifts to the southern most areas of CA with the San Diego coastal areas down to the border of Baja, but the expectation is for a less robust QPF footprint, thus no need for extension of the Moderate and maintenance of the current Slight Risk. LA proper will be on the eastern fringe of the heaviest rainfall, so the Moderate was generally kept out of precaution as it lies within the precip axis margin of error. A very slight nudge was taken for the east side of LA, but a majority of the Los Angeles area remains within the highest forecasted category. Kleebauer ....Previous Discussion... A strong, anomalously deep closed upper low will slowly meander off the southern California coast through Thursday night, setting up a prolonged and potentially significant rainfall event for portions of Southern California including the Transverse Ranges and portions of the Los Angeles metro. The position of the upper low will put the area in the favored area of forcing for deep-layer ascent (maximizing upper difluence/divergence), while embedded vort maxes rotating around the low will work to provide extra/enhanced lift at times. Most recent model guidance continues to depict two main waves -- one coming through this morning and afternoon, followed by another surge overnight and into Friday morning. Most of the guidance has the second wave coming in a bit farther downstream, i.e. east of the LA-SD metros and across the deserts. In any case, the ingredients will feature fairly anomalous moisture, with PW values between 3-4 standard deviations above normal and IVT values 400-500 kg/m/s. Some instability (MUCAPEs 250-500 J/kg) is forecast to develop as well along the immediate coast, which will help to realize the higher rain rate potential, where hourly totals may exceed 1" at times, particularly for the favored south facing slopes of the Transverse Ranges. The other factor that increases the QPF and flood threat is the relatively slow motion of the storm, which may allow for some prolonged rain bands to set up. The favorable southerly flow nearly perpendicular to the terrain will enhance the orographic ascent and should enhance rainfall totals, which may reach 3-5" for the 24-hr period ending 12Z Friday. 48-hr totals (including rain from Wednesday-Wed night) may exceed 6-8" (localized 10") in the areas northwest of Los Angeles in the Transverse Ranges, and this is likely to lead to significant flooding, including flash and urban flooding as well as potential for rock slides, mud slides, and debris flows over sensitive burn scars like the South Fire burn scar in Ventura County. Very few changes were made to the existing Moderate Risk area, which covers both the terrain flood risk as well as the potential for flash flooding for western LA metro areas. Hurley/Taylor 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 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .