768 FOUS30 KWBC 121955 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Wed Mar 12 2025 Day 1 Valid 16Z Wed Mar 12 2025 - 12Z Thu Mar 13 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... 16Z Update: Current forecast SLGT risk across Southern CA remains on track as there has been little run-to-run variance of locally heavy rainfall totals in the targeted areas. The expectation is for the cold frontal rain band to make headway down the CA coast this period with the orientation of the low-level flow situated out of a southwesterly alignment generating better orographic enhancement to the precip field when acting against the coastal terrain and windward portions of the Sierra's. This is a pretty classic case for SoCal flash flood prospects, heightened more than usual due to the remnant burn scars prevalent within the San Gabriel's and Santa Monica Mtns. with emphasis on the Palisades and Eaton fire burn scars. Current FFG's within those smaller zones are ~0.5"/hr for runoff prospects and between 0.75-1" elsewhere within the coastal topography. 12z HREF prob fields are still generally elevated for hourly rates >0.5"/hr (60-80%) during the time frame between 00-12z in the D1 period which would correlate with better flash flood concerns, especially in the areas above. The prospects for >1"/hr rates are generally non-existent within the means with a very low prob output via the latest HREF output (<15%). This generally means there is a good handle on the max potential within this anticipated period of impact, and the relatively quicker forward propagation should limit the upper-end of any excessive rainfall thresholds. Considering the above variables and general continuity within the CAMs, global deterministic, and subsequent ensembles, the previous SLGT risk was generally unchanged with the primary focus on those more prone burn scar areas. Further north across Northwestern CA into the foothills of the Sierra's, a low-end probability for urban and small stream flash flood exists as the same frontal rain band discussed above progresses through the rest of the state with a short window for heavier rainfall as the frontal progression shifts south. Orographic enhancement within any coastal ranges and the Sierra's will be the primary driver for those elevated rain rates capable of producing some localized flooding concerns with complex topography. The San Francisco/San Jose/Oakland area will be one urbanized zone of interest through the course of the D1 given the local susceptibility for flash flooding to occur in a hillier, more urban zone. For more information on the flash flood threat across Northern California, please see MPD #058. The previous MRGL risk across the above zones was maintained with little to no change in the risk area. Further south into Los Angeles, a similar situation with the urban corridor generally in a better spot for flash flooding just due to the nature of lower FFG's and the IVT pulse protruding a bit further inland once south of Big Sur. Intra-hour rain rates will once again be the driver for the flash flood threat in the area as 0.5-0.75"/hr is forecast over the metro. Local 1-2+" of total rainfall is forecast within the LA domain, enough to warrant the area to be within the SLGT risk with MRGL's extended down the coast once you get south of Long Beach. Kleebauer ..Previous Discussion.. The forecast generally remains on track, so only minimal changes needed to the risk areas with this update. The greatest flood risk with this event remains over southern CA Wednesday evening into Thursday morning. The 00z HREF continues to indicate a high probability of hourly rainfall locally exceeding 0.5" moving into the Transverse Range of Santa Barbara and Ventura counties as early as ~04z, and then spreading into the San Gabriel Mountains of Los Angeles county closer to 08z. While the probability of 0.5"+ an hour hourly rainfall is high, the coverage of these more intense rates will likely be localized in nature, as shown by the HREF 0.5" per hour ensemble agreement scale probabilities (more of an indication of coverage) which peak in the 10-15% range over the aforementioned terrain areas. Most of the 00z CAMs indicate a narrow line of shallow convection moving south across these areas associated with strong cold frontal convergence and about 100-200 j/kg of CAPE. This feature, combined with orographic enhancement, should locally push hourly rainfall into the 0.5" to 0.75" range over the aforementioned terrain of Santa Barbara, Ventura and Los Angeles counties. Rates should tend to be a bit lower outside of the terrain, although the shallow convective line could locally push hourly rainfall over 0.5" even along lower elevation coastal areas. This axis of heavier rain will be quick moving and only last about 2-3 hours at any one location...which will limit total rainfall amounts and the magnitude/coverage of the flash flood threat. However, even with the quick movement, rainfall rates could be high enough to result in locally significant flood impacts near recent burn scars, and areas of minor flooding in other more flood prone urban or low lying areas. Elsewhere across the state of CA the deep mid level trough and cold front will bring widespread rain and mountain snow to the region. The IVT axis is rather narrow and quick moving and only forecast to peak around 400-600 kg/m/s. When combining this modest strength and lack of longevity of these higher values...this event will likely only reach weak atmospheric river levels. This limitation will be partially offset by the strength of the cold front and deep layer forcing associated with the synoptic scale trough and upper jet. Even so, rainfall totals over much of the central and northern CA are unlikely to be enough to cause much of a flood concern. Mainly looking at a beneficial rainfall, although this is a strong cold front with a well defined convergence axis along it, and this could drive some locally higher rainfall rates, both along the coast and within the Sacramento Valley where southerly flow could locally enhance convergence further. These higher rates could be enough to result in generally minor and localized flooding within areas of steep terrain along with urban and other prone low lying areas. Chenard Day 2 Valid 12Z Thu Mar 13 2025 - 12Z Fri Mar 14 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... 20Z Update: The primary factors for the continuation of the SLGT risk into D2 for the event over Southern CA is the persistence of the cold frontal rain band across the San Diego area extending into the southern edge of the Peninsular range. Current CAMs output is in agreement on timing of the main low-topped convective pattern persisting beyond the initial impacts of D1 into the early stages of D2 with a degrading signature not long after 18z Thursday. 12z HREF signals for >1" of precip (60-80%) within the short window remain very high, especially within the Foothills and atop the mountain clusters within that southern Peninsular Range. Hourly and intra-hourly rates will be the main cause of any flash flood concerns over the outlined area with CAMs forecast of 0.5-0.75"/hr at peak intensity within the zone of complex terrain with embedded burn scars. Urban factors closer to San Diego and the surrounding suburbs are also a factor, especially when assessing the stronger IVT signatures confined to the coastal sections of San Diego County early Thursday morning. Considering a general continuity from guidance this morning with relevant probabilities remaining elevated for heavy rain prospects, the previous SLGT risk was maintained with little to no adjustment necessary. Kleebauer Day 3 Valid 12Z Fri Mar 14 2025 - 12Z Sat Mar 15 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF LOWER TO MIDDLE MISSISSIPPI VALLEY INTO PORTIONS OF THE OHIO AND TENNESSEE VALLEY... 20Z Update: The general synoptic progression for the D3 setup remains consistent among guidance as a deepening surface low over the Central U.S. will generate a large scale response to regional precip development, much of which will maintain at least somewhat of a convective element. The fast propagation of any convective cells and/or precip clustering will limit the impacts overall, however some of the areas impacted over the Mississippi and Ohio Valley's are primed a bit more compared to other locations further south leading to a better risk for flash flood concerns as we move into Friday afternoon/evening. Recent trends in guidance have created somewhat of a discrepancy on where the main bulk of the heaviest precip will be located, mostly in part due to handling of the surface cold front that will be associated with intensifying cyclone. A general 1-2" with locally higher is expected within the scattered maxima depicted within the deterministic. Ensemble means are not as impressive due to some spread, however there's still plenty of coverage for at least moderate rainfall totals, namely within the zone of stronger theta_E advection accompanying the intensifying 850mb LLJ. Strong mid-level divergence is likely to occur downstream of the closed upper reflection ejecting into the plains leading to more widespread precip developing in-of the Mid- Mississippi Valley by Friday evening, quickly advancing eastward under the guide of the cold front. The southern edge of the precip field will likely be within the best instability axis, a textbook synoptic formula for some heavier echoes outside the primary precip shield. The MRGL risk from previous forecast issuance was most maintained, however some shifts further east and northeast were made to account for the latest trends within multiple deterministic. The western edges were sured up to account for perhaps a little more progressive push of the main convective line, especially when accounting for the forecast positioning of the LLJ. A small expansion south was generated due to typical bias of guidance being too far north with the convective precip output within this range and wanted to include the area of greatest instability ahead of the cold frontal progression. Kleebauer ..Previous Discussion.. A strong mid level low and shortwave energy ejecting east out of the Plains and into the MS Valley will likely trigger strong to severe convection by Friday afternoon across portions of MO, AR and IA. This activity will then spread eastward into portions of IL, IN, KY, TN and MS Friday evening into the overnight hours. Moisture is initially quite limited with this system although we eventually see a narrow corridor of higher PWs advect northward. Cell motions are also likely going to be quite fast off to the east northeast given the very strong wind fields in place. Thus this convection will likely pose more of a severe risk than it will a flash flood risk. However this is a very dynamic system with sufficient instability for deep convection, and so expect that rainfall rates will briefly be intense enough that we will see swaths of rainfall that exceed current model forecasts (maybe locally over 2"). Nonetheless, this still appears to be a lower end flood threat, and probably confined to localized impacts over any more sensitive urban or low lying areas that pick up a quick burst of intense rainfall rates. Chenard Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt