FGUS76 KMTR 181912 ESFMTR CAC041-055-097-190315- Hydrologic Outlook National Weather Service San Francisco CA 1112 AM PST Mon Nov 18 2024 HYDROLOGIC OUTLOOK NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA 1030 AM PST Mon Nov 18 2024 …PROLONGED MODERATE TO HEAVY RAINFALL WILL LEAD TO RAPID RIVER RISES AND INCREASED FLOODING POTENTIAL… * WHAT…The first significant atmospheric river of the season will impact the northern and central California region by midweek with impacts continuing into the weekend. Prolonged periods of moderate to heavy rainfall will lead to rapid rises on small creeks, streams and rivers as well as an increased flood risk. A significant and strong atmospheric river will impact the region by early Wednesday morning. Current model depictions show this system will stall over the North Bay bringing a prolonged period of moderate to heavy rain as well as strong winds to the North Bay. Precipitation totals of 3” to 7” are expected Wednesday through Sunday with up to 11” possible along the coastal range as this system sits north of the Golden Gate for the better part of Wednesday and Thursday before gradually dropping south across the San Francisco and Monterey Bay Area on Friday and Saturday. WPC (Weather Prediction Center) Day 3 Excessive Rainfall Outlook has the North Bay in a Marginal (at least 5%) to Slight (at least a 15%) chance of exceeding flash flood guidance for Wednesday into Thursday. This system is tapping into very moist subtropical moisture for an extended period of time which will allow for moderate to heavy periods of precipitation over several days. Moderate to heavy rain will lead to rapid rises of area rivers, streams, and creeks. Localized ponding of water in low-lying or poorly drained areas as well as localized flooding is possible due to blocked culverts, drainages, and storm drains. Main stem rivers are currently forecast to remain below flood stage, however the 10% probability of exceedance (very low chance) show GUEC1- Russian River at Guerneville may rise slightly above monitor stage over the weekend. A few other rivers to keep an eye on that show higher probability of reaching Action/Monitor or Minor flood stage according to the latest CNRFC forecast are… CTIC1 - Laguna De Santa Rosa at Stony Pt Rd near Cotati has a 75% probability of reaching Action/Monitor Stage and a 30% probability of reaching Minor flood stage by Thursday. MWEC1 - Mark West Creek at Mirabel Heights has a 50% probability of reaching Action/Monitor Stage and a 20% probability of reaching Minor flood stage Friday. GEYC1 - Russian River at Geyserville has a 25% probability of reaching Action/Monitor Stage and a 10% probability of reaching Minor flood stage Friday. * WHERE…Sonoma and Napa Counties. Latest model guidance suggests the North Bay will receive the greatest accumulation of precipitation with the heaviest rainfall expected over the North Bay coastal range and mountains. * WHEN…Wednesday through Saturday. Moderate to heavy rainfall expected over the North Bay Wednesday and Thursday. The heaviest rainfall accumulation and periods of potentially intense rain rates are expected Wednesday and Thursday. On Friday the system is expected to begin moving south and east across the region with rain showers lingering over the North Bay and spreading south through Saturday. * ADDITIONAL DETAILS...In addition to the heavy rainfall and localized ponding of water in low-lying or poorly drained areas, such as freeway offramps. Locally strong and gusty winds may result in downed trees, and power outages will be possible. Details regarding this storm will likely change as the system nears. Therefore, stay tuned to the latest forecast for the most up to date weather information. Confidence is high that we will receive rainfall from this system however, confidence is moderate with respect to the exact location and timing of the most intense rainfall. $$ CW