Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 05 2024 20:23:50 FOUS30 KWBC 052023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 323 PM EST Mon Feb 05 2024 Day 1 Valid 16Z Mon Feb 05 2024 - 12Z Tue Feb 06 2024 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... 1600Z Update... The big picture of the ongoing/persistent atmospheric river setup over areas of the Southwest remains unchanged, but given the latest radar trends and considerations of the 12Z HREF and recent HRRR trends, some expansion of the Marginal and Slight Risks were done for areas a tad south and east of the southern Sierra Nevada where areas of moderate to heavy rain have been materializing and are expected to locally persist today given the quasi-stationary front and axis of Pacific moisture transport draped over the region. Some very minor expansions of the Moderate and High Risk areas were also done for areas of Ventura and Los Angeles Counties in southern CA given extreme sensitivities on the ground right now from last night's rains, and additional shower activity that is expected to persist over these areas today. Orrison Previous discussion... A persistent plume of elevated low to mid-level moisture will remain almost stationary across southern CA with flow orthogonal to the terrain in-of the Transverse Range within San Barbara/Ventura counties, as well as the Peninsula Range within Los Angeles/San Bernardino counties as the plume shifts slightly eastward on Monday. Rates within the heaviest bands of rainfall will reach between 0.5-1"/hr with average rates still running up near 0.25"/hr for several hours. Totals across the region have already breached 2" over an expansive area from the central CA coast down into southern LA county with embedded totals exceeding 4" now a common occurrence over the most consistent waves of heavy precip over Santa Barbara and Ventura counties. This has already triggered multiple flash flood warnings across the area with copious flooding reports popping up over the population centers along the coastal plain. Given the incessant nature of the precip over the complex terrain north of the coast and the urbanization factors within the zone being impacted, there will be a continued higher risk for flooding potential through the course of Monday. Storm totals over 6" will be common over a large portion of Southern California with local 8-10" amounts nestled within the eastern half of the Transverse Range north of Santa Barbara. Intensity of rainfall will wind down the back half of the forecast period, but the persistence over Los Angeles/Orange counties will maintain a high-end threat through the entirety of D1. This setup is a textbook case for widespread flooding that will induce road closures and affect travel prospects across a densely populated area of the CONUS. A High Risk was maintained across Southern California with lower risk areas extending well inland and further north along the coastal plain. Kleebauer Day 2 Valid 12Z Tue Feb 06 2024 - 12Z Wed Feb 07 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA & THE DESERT SOUTHWEST... 2030Z Update... Generally modest changes were made to continuity, which included some tweaks made to the Slight Risk areas over southern CA, southern NV, and western/central AZ. Synoptically, there is little in the way of change to the setup, although there are some detail differences noted in the 12Z HREF guidance for QPF. This is especially the case over western AZ with respect to the potential for at least small-scale areas of linearly oriented convection. The multi-model hires consensus still supports some heavy rainfall potential here, and there are some low-end probabilities from the HREF of seeing rainfall rates reach or locally exceed 1"/hour over the deserts of southeast CA and western AZ as the axis of deeper moisture and a nose of some instability crosses these areas in conjunction with the arrival of stronger DPVA with the upstream upper trough/shortwave approaching. The Slight Risk area was adjusted to locally expand the Slight Risk area into southeast CA, but to trim the portion of it over southern NV since the consensus of the 12Z HREF QPF, and better axis of instability/rainfall rates is focused generally farther down to the southeast. Elsewhere, the Marginal Risk area was reintroduced over a small area of southwest UT (to near the Zion NP vicinity) given rather favorably moist southwest flow into some of the terrain here. For areas below the snow line, there may be enough intensity to some of the orographically enhanced shower activity to foster an isolated concern for runoff problems which involve the normally sensitive slot canyons. Orrison Previous discussion... Deep moisture plume will begin migrating eastward as the closed low off CA opens and pivots inland over the course of Tuesday. Deep moisture advection regime will shift from SoCal and NV into AZ by the second half of the period with increasing rainfall threat over the Colorado River basin, eastward into the Lower Deserts of AZ within Maricopa/Yuma counties and adjacent higher deserts within La Paz/Pima counties. Highest totals of 1-2" with locally up to 3" over the Mogollon Rim are forecast with areas above 6-7k ft receiving snowfall, limiting the coverage for flooding concerns on the northern periphery of the risk areas. Limited convective potential will stunt the ability for this to be a bigger event over the area, but enough precip across the deserts will generate some flooding concerns. Have maintained the SLGT risk for the area from previous forecasts with a tightening of the northern extent of the risk area in AZ and removal of the MRGL into southwest UT due to trends in guidance shifting the event to more snow than rainfall. Persistent onshore component over southern CA will allow for an additional 1-2" of rainfall from the eastern Transverse Range down through LA, Orange, and San Diego counties with the max of 2-3" focused over the Peninsula Range to the northeast of Los Angeles proper. Totals in the lower elevations outside the ranges will be around 3-6" for the whole event with terrain focused areas into the 7-10" mark with the highest totals somewhere in the Peninsula Range. Despite rates decreasing and the main plume shifting eastward, the compounding effect of significant rainfall over the span of 48-60 hrs will allow for heightened flooding concerns despite the improvements. The worst impacts will be geared towards the front half of the period when the onshore flow will be strongest and mid-level vorticity advection is most prevalent. Kleebauer Day 3 Valid 12Z Wed Feb 07 2024 - 12Z Thu Feb 08 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA AND ARIZONA... 2030Z Update... A Marginal Risk area was added in across the Peninsular Range of southern CA where the arrival of additional shortwave energy and steepening mid-level lapse rates will work in tandem with relatively moist low-level upslope flow to maintain a threat of some locally heavy shower activity. Given the extreme sensitivities locally from the most recent atmospheric river event, these additional rains may promote some additional runoff concerns. Elsewhere, the Marginal Risk area over AZ was adjusted to be focused mainly near and just south of the Mogollon Rim area where the latest 12Z model consensus supports the greatest concentration of rainfall. Overall, the additional rainfall totals should be rather modest, but at least a low-end threat for some isolated runoff problems cannot be ruled out. Orrison Previous discussion... Residual moisture stemming from the current atmospheric river will still be present across the eastern half of AZ with ongoing precip forecast within the terrain (Mogollon Rim). The primary time frame of impact will be on Wednesday morning when the trailing energy from the trough associated with the event will pull through along with some support meandering within the left-exit region of the jet streak on the lee of the trough. PWAT anomalies will be still be hovering between 1-1.5 deviations above normal within that time of 12-18z on Wednesday before drier air protrudes over the region and we see a degrading threat of precip for the back half of the period. Considering the compounding effect of precip from the day before continuing into D3, decided to maintain the forecast MRGL to account for ongoing flood potential in-of the terrain across southeast AZ up through central AZ within the confines of the Mogollon Rim. It is possible the MRGL could be vacated if the disturbance exits faster than currently modeled, or trends in guidance dwindle for lingering precip that could exacerbate any flood prospects/ongoing flooding within the confines of the current MRGL risk. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7KLJAYlCJ1migAnbLMtMxv3DcnqAt7jyByjov9mdnUb6= tUCrdmEKCOHsl7S03n1V7wSO9xAIiuOu8yyN8F7hsCnLMZ8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7KLJAYlCJ1migAnbLMtMxv3DcnqAt7jyByjov9mdnUb6= tUCrdmEKCOHsl7S03n1V7wSO9xAIiuOu8yyN8F7hrK97lx0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7KLJAYlCJ1migAnbLMtMxv3DcnqAt7jyByjov9mdnUb6= tUCrdmEKCOHsl7S03n1V7wSO9xAIiuOu8yyN8F7hC0FQ7QE$=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) .