Hydrometeorological Discussion
Issued by NWS Tuscaloosa, AL
Issued by NWS Tuscaloosa, AL
606 AGUS74 KWCO 091518 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT SUN MAR 9 2025 .Synopsis... Moderate to locally heavy rainfall in Florida...Flash flooding possible in American Samoa...Additional rainfall beginning mid-week in California...Flooding impacts possible late this week through the weekend in the Lower Mississippi Valley into the Ohio Valley... .Discussion... Florida and South Georgia... Moderate to locally heavy rainfall may bring the potential for localized flash and urban flooding to areas of the Panhandle and northeast FL through today. Forecast rainfall amounts average 1 - 3" with locally higher amounts, especially if training occurs (WPC). Top soils have become more wet in areas along the Gulf (60 - 75%, 0 - 10cm RSM, NASA SPoRT) indicating a diminished infiltration capacity. The National Water Model (NWM) Short Range Forecast (SRF), forced by the HRRR, suggests some potential for rapid onset flooding (ROF) across the area of concern. Probabilities are generally between 25 - 50% from Panama City, FL to Tallahassee, FL eastward into Valdosta, GA and Jacksonville, FL. Latest RFC forecasts indicate minor river flooding with most rivers cresting mid to late week. Overall, flash and urban flooding are the primary concerns, and localized small stream flooding cannot be ruled out. American Samoa... Heavy rainfall may bring flash flooding through the weekend to all islands with ponding of water and flooding of low-lying areas possible. Landslides are also possible in areas of steep terrain. .California... Additional rounds of rainfall are forecast to impact the region on days 4 - 7 (Wed - Sat), with the heaviest rainfall expected on days 4 and 7 (Wed/Sat). Top soils are wet (55 - 70%, 0 - 10cm RSM, NASA SPoRT) in areas along the coast, north of Santa Maria to Eureka, as well as the Sierra Nevada, with most streamflows above normal (USGS) suggesting that rainfall could quickly be converted to runoff into streams that are already running high. The NWM Medium Range Forecast (MRF) is signaling for rapid-onset flooding probabilities generally near 25% for areas surrounding Fresno, San Francisco, and northern portions of Sacramento. While associated annual-exceedance probabilities are around 50%, there are localized signals near San Francisco, less than 10%, suggesting there could be some localized small stream responses. Most peak flow arrival times on days 4 - 5 (Wed - Thu). Despite models coming into better agreement as far as timing, confidence still remains low in the exact magnitude of flooding impacts later this week. Lower Mississippi Valley into the Ohio Valley... Heavy rainfall may bring the potential for flooding impacts starting day 6 (Fri) and through the weekend. Currently, ensemble river forecasts show action to minor river flooding throughout the area of concern. This may increase as the event approaches. The NWM is signaling peak flow arrival times on days 6 - 10 (Fri - Tue). Recent rainfall has wetted soils (60 - 80%, 0 - 10 cm RSM, NASA SPoRT) and streamflows are normal to much above normal indicating a diminished infiltration capacity. With lighter rainfall forecast throughout the week up to this event in areas of the Lower Mississippi Valley and Southeast, there are many areas where soils will not have time to recover and moisture will likely increase with persistent rainfall.This could lead to rapid runoff into already elevated streams, creeks, and rivers. //TMK $$