Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Feb 07 2024 09:08:00 FOUS30 KWBC 070807 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 307 AM EST Wed Feb 07 2024 Day 1 Valid 12Z Wed Feb 07 2024 - 12Z Thu Feb 08 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL AND SOUTHERN CALIFORNIA COAST, & CENTRAL ARIZONA... ....California... Upper trough axis is currently positioned over Southern California with lingering showers scattered across the southern half of the state. The first half of the period will be relatively benign as the primary forcing continues to shove eastward with the mean trough eventually situated over the Desert Southwest by tomorrow afternoon. Upstream, a swift-moving shortwave will dive down the the backside of the mean trough, closing off over Southern CA by the end of the period which will re-initiate a round of steady rainfall along the CA coastal plain from San Francisco all the way down towards San Diego by the end of the D1 forecast. Rainfall totals and rates are not anticipated to be nearly as prolific as the previous setup due to the progressive nature of the disturbance, but any additional rainfall will exacerbate the concerns of flooding due to the antecedent conditions being extremely wet thanks to a widespread 5-10" rainfall footprint in-of the aforementioned areas. 00z HREF blended mean and ensemble bias-corrected guidance is in lock-step with the terrain focused areas from Big Sur down through the Transverse/Peninsula Ranges seeing 1.5-2.5" which would cause some localized concerns considering that was the area that was impacted the most these past 72 hrs. Rates wise will be capped around 1"/hr with low probabilities showing up within the ensembles (10-15%), but any threat is warranted for at least a MRGL risk, so have maintained the previous forecast, and extended to the north along the coast to just south of Monterey, targeting the Big Sur area. The urban corridor is still forecast for 0.5-1.25" which would also be enough to cause some localized issues. ....Arizona... A low-end Marginal Risk has been maintained over AZ as the tail end of the precip caused by the deep-upper trough will linger through the first 3-6 hrs of the period before diminishing as the ascent pattern within the left exit region of the jet pushes eastward leading to limited impacts through the rest of Wednesday. The main areas of interest will be across the southern areas of the Mogollon Rim that fall below 5500ft MSL as snow levels continue to fall with the trough axis moving overhead. Rainfall rates between 0.5-1"/hr, at best is forecast within that time frame from 12-18z with some isolated convection lingering over southeast AZ after 18z, but low snow levels and better antecedent conditions will curb the threat for areas east of Phoenix towards Tucson. Kleebauer Day 2 Valid 12Z Thu Feb 08 2024 - 12Z Fri Feb 09 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR CENTRAL ARIZONA ... Trailing shortwave trough will pivot out of California with increased large scale forcing in-of central and western AZ by Thursday morning. Convective risks will be lower compared to the recent setup, however enough lingering moisture and ascent focused over the Mogollon Rim will initiate another round of moderate precip over the aforementioned area. A textbook area of mid-level difluence is depicted within all major guidance with the heaviest precip centered along the western Mogollon Rim to the west of Prescott, eventually across the central terrain to the north of Phoenix. Snow levels will be down to 5000ft MSL by Thursday thanks to the lower heights brought on by the mean trough that will encompass most of the country east of the Mississippi, so the main concern will lie within the lowlands and locations prone to flooding due to complexity of the terrain. Antecedent conditions will be primed due to the preceding 36 hrs of rainfall, so despite precip totals generally hovering between 0.75-1.25" for the event, that could very well be enough for some localized flooding concerns in those valley locations within the southern portions of the Mogollon Rim. This is a low-end MRGL, but the complexity of the terrain coupled with additional rainfall over an area with historically low FFG's will allow for the threat and the maintenance of the Marginal Risk area. Kleebauer Day 3 Valid 12Z Fri Feb 09 2024 - 12Z Sat Feb 10 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .