Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 06 2024 15:57:37 FOUS30 KWBC 061557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1056 AM EST Tue Feb 06 2024 Day 1 Valid 16Z 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... 16Z update... Rain along the Golden Coast has decreased significantly as it advanced further inland this morning. Lingering showers throughout the rest of the morning and into afternoon may yield additional light accumulations, however the overall threat for excessive rainfall has lowered for a stretch of the coastline. As such portions of the Marginal Risk area was trimmed to the south/east. Scattered to widespread rain will persist across southern California and the western two-thirds of Arizona through this period. The Slight Risk areas that are currently in effect still cover the locations with elevated runoff and flooding risk and were maintained. Campbell ....Southern California... Two main areas of interest today with regards to flash flooding concerns stemming from a continued impact of our deep, upper trough and prevalent jet dynamics associated with our disturbance. The first highlight is across Southern CA where persistent onshore flow will reside through the area the first half of the forecast period with steady light to moderate rainfall oriented from southwest to northeast within the confines of eastern Ventura county all the way down through San Diego county with the heaviest rains focused in the Peninsula Range to the northeast of LA and east of San Diego. Additional totals of 2-3" will be common within the zones highlighted in the terrain with 0.75-1.5" for lower elevation areas, including the urban centers. The persistent flow orthogonal to the terrain has been the cause for such significant totals across the region and any additional rainfall will allow for a continued impact until we see the primary plume vacate to the east. This will eventually occur by later this afternoon, but lingering showers will still be forecast in-of the aforementioned areas through the remainder of the period, especially as multiple smaller shortwaves embedded in the flow traverse overhead, spurring additional development near the Pacific coast. This area has been highlighted with a SLGT risk with an extension on the northern edge of the risk area to encompass Los Angeles due to the recent trends in guidance showing additional rains that could exacerbate ongoing flooding within the urban corridor. ....Desert Southwest... The second area of focus will turn towards the Colorado River basin into the AZ terrain with an axis of heavy precip slated for the Mogollon Rim. As the mean trough pivots eastward, a strong upper jet will orient perfectly over the southwest US to enhance large scale forcing within the left exit region of the jet streak. Impressive height falls are forecast from west to east through the period with 00z deterministic soundings from all hi-res and global guidance signaling steep low to mid-level lapse rates with a tongue of elevated MUCAPE (500-1000 J/kg) focused over the Lower Deserts in Yuma/La Paz counties into the western sections of Maricopa. Flow will also become favored for a prolonged upslope component in-of the Mogollon Rim starting near Mojave county all the way across the bordering areas of both Phoenix and Flagstaff's respective CWA's. This points at a wide spread area of 1-2" with locally as much as 3" within that corridor with the highest totals over the terrain below 6000ft MSL where snow levels will take a while to fall during the period. A SLGT risk was maintained from previous forecast and the spatial coverage correlates well with where convective probabilities are highest, and/or rainfall totals are above 1.5" as a marker. In-between the two Slight Risk areas and points north, elevated convective potential is still plausible with additional rainfall anticipated in the low-lands of southern NV and southwest UT, especially within Zion National park where slot canyon locations will be the primary area of interest with any convective development during the course of this evening. Kleebauer Day 2 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 ... ....Southern California... Persistent onshore flow will remain across southern CA into the southern portion of the Peninsula range within Orange and San Diego counties with rainfall totals up to 1" possible during the morning and afternoon on Wednesday. This is just additional rainfall compared to the previous period which provides a compounding effect with flooding potential continuing through Wednesday. There is a break anticipated in the heavier precip, but a trailing shortwave on the backside of the mean trough will cut down the CA coast with a secondary burst of rainfall anticipated after 00z Thursday through the end of the D2 period. This is a period of interest as model consensus is converging on the stronger mid-level vorticity maxima moving overhead with a rise in rainfall coverage and rates within an area that will have already been hit fairly significantly prior to any additional rainfall. 00z HREF mean and blended mean places a maxima over the southern Peninsula Range to the east of San Diego with forecast of 1-1.5" of rain on top of what falls earlier. Considering the saturated soils across the entire region from the primary AR, have maintained the focused MRGL across the area with a potential for a targeted SLGT risk within the terrain east of San Diego if preceding rainfall primes the soils as expected, or further than forecast. ....Arizona... Lingering rainfall across the Mogollon Rim from the previous forecast period will maintain an elevated flood concern within areas below the forecast snow level of 5000ft MSL. The heaviest rains will occur in the morning on Wednesday when jet dynamics are still at play and lingering moisture anomalies remain high before a slow degradation of mid-level moisture as the mean trough pivots to the east. Total rainfall between this evening through Wednesday morning will be around 1.5-2.5" with locally as high as 4" within the terrain to the north of Phoenix with areas like Prescott and the mountains/hills further east as the main focus points across AZ. Convective signatures will be waning through the course of the period and a break in heavier precip is expected during the afternoon and evening hours on Wednesday, so the MRGL was maintained with no necessary upgrades.=20 Kleebauer Day 3 Valid 12Z Thu Feb 08 2024 - 12Z Fri Feb 09 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL ARIZONA... Primary longwave trough axis will continue to press eastward into D3, but increasing consensus on a digging shortwave on the backside of the mean trough will re-enhance an axis of precip over CA into AZ by the middle of the period. Model consensus is split on the amplitude of the trough which will play a significant role in where the heaviest precip will fall. Ensemble means have leaned closer to more precip into the central portion of AZ with a location along the Mogollon Rim with a stripe of heavier precip over areas like Prescott through northern Gila county. The GFS is the most bullish in the precip representation thanks to a highly amplified 5H connotation with large scale forcing plentiful in-of the terrain north of Phoenix. PWAT anomalies are on the way down during that time, but considering the previous day rainfall, soils will be saturated and primed for any additional rain that develops overhead. Soundings for the time frame signal some weak instability signatures that would allow for shallow convection, but this will be depend on the overall trough evolution. As of now, decided to add the MRGL risk to the area north of Phoenix, running across the Mogollon Rim between the northern edge of the Phoenix CWA and the southern edge of Flagstaff's CWA. Precip totals on the order of 0.75-1.5" are possible in this scenario with the WPC forecast closer to the 60-70th percentile from the latest NBM. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5JrtS3HlL1GDOof8R3pp18-cbOrPySEigSpZS4zsmTs7= auKRXfg7HLv4Aitw5GW_VCrxlnZ9Eybe-XKviUzmxYacKW4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5JrtS3HlL1GDOof8R3pp18-cbOrPySEigSpZS4zsmTs7= auKRXfg7HLv4Aitw5GW_VCrxlnZ9Eybe-XKviUzmWU2TOR4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5JrtS3HlL1GDOof8R3pp18-cbOrPySEigSpZS4zsmTs7= auKRXfg7HLv4Aitw5GW_VCrxlnZ9Eybe-XKviUzmmeZtqeY$=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) .