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   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   752 PM EST Sat Nov 15 2025   
      
   Day 1   
   Valid 01Z Sun Nov 16 2025 - 12Z Sun Nov 16 2025   
      
   ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHERN CALIFORNIA...   
      
   ...16Z update...   
      
   No significant changes were made to the outlook given 18Z HREF=20   
   guidance, including exceedance probabilities, which were fairly=20   
   similar to the 12Z ensemble suite.=20   
      
   Hurley   
      
   ...previous discussion follows...   
      
   A large and energetic upper level low centered off the coast of   
   southern California will approach and make its way inland during   
   this period. As the low approaches this morning, it will draw a   
   very moisture-rich moisture plume into southern California, due to   
   its connection with the tropics. This will be in the form of an   
   atmospheric river. While it has been raining in light to moderate   
   intensity over much of the overnight period, when this atmospheric   
   river approaches, it will cause a notable increase in rainfall   
   intensity. This is due not only to the moisture content of the   
   atmosphere increasing to nearly 1.5 inches PWAT, but also the   
   advection of instability. 400 J/kg of MUCAPE is expected at the   
   peak of the event late this morning. This will support scattered   
   elevated convection. Thunderstorms with maximum rainfall rates to 1   
   inch per hour are likely across much of coastal southern   
   California today.   
      
   Rainfall totals yesterday broadly ranged from between 1/2 and 1   
   inch across the L.A. basin to 1.5 to 2 inches from Santa Barbara   
   west to Point Conception. Today with multiple rounds of showers and   
   thunderstorms capable of heavy rain, expect those amounts to at   
   least double from San Diego north to Point Conception. As usual in   
   these strong southerly flow setups, the heaviest rains will be in   
   the Transverse Ranges north of L.A., with likely runoff impacting   
   the adjacent urban areas. Given the area's sensitivity to heavy   
   rain, the Moderate Risk was largely left the same with this update,   
   albeit with a small southward expansion over all of Orange County   
   and far northern San Diego County. Otherwise there were no major   
   changes as the event remains on track with good agreement in the   
   guidance.   
      
   Around midday, the back edge of the heaviest rains will move   
   across the coast, reducing further rainfall to scattered off-and-on   
   showers for much of the rest of the forecast period. The vigorous   
   shortwave forcing the heaviest rain will push north into the   
   interior of the Southwest, ending the heavy rain threat at the   
   coast. Thus, the heavy rain threat will continue further inland   
   across the southeastern California deserts, as well as into   
   southern Nevada and western Arizona. The greatest forcing inland   
   will be across the southern Sierra Nevada from Bakersfield north   
   and east to southwestern Nevada. It's in this region that the   
   greatest threat for inland flooding will be present, and where a   
   higher end Slight Risk remains in effect. Given it's November, the   
   higher elevations of the southern Sierra Nevada will see multiple   
   feet of snow from this event.   
      
   For Las Vegas, expect light rain to impact the city and points west   
   for much of the morning, but the heaviest rains will move through   
   from midday into the evening. Being further inland with a bit less   
   moisture to work with, a Slight Risk remains in effect for that   
   area with few changes to the overall forecast.   
      
   Finally, the upper low itself will move ashore in the late   
   afternoon, then track northeast to near the Lake Tahoe region by   
   12Z. Wraparound rainfall, while much lighter, will persist from the   
   Bay Area south to Santa Barbara, which could worsen any ongoing   
   flooding occurring there late tonight.   
      
   Wegman   
      
      
   Day 2   
   Valid 12Z Sun Nov 16 2025 - 12Z Mon Nov 17 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF   
   CENTRAL AND NORTHERN CALIFORNIA AS WELL AS FOR SOUTHWEST UTAH...   
      
   ...20Z Update...   
      
   Expanded the Marginal Risk a bit across SoCal (to include more of   
   the Transverse Ranges along with the LA and SD metro areas), based   
   on the current event (ribbon of deeper moisture, areas of heavier   
   rain rates) lingering Sunday morning (after 12Z).   
      
   Hurley   
      
   ...Previous Discussion issued at 0830Z...   
      
   ...Central and Northern California...   
      
   The upper level low and its associated energy that brought the   
   heavy rainfall Day 1/today will rapidly lift northeastward into the   
   Intermountain West and weaken through the day, meaning its   
   influence on the sensible weather across California will be limited   
   to the very early morning. Much of the rainfall associated with   
   this Marginal Risk will be associated with another deep trough that   
   will dig southward and cut off by 12Z Monday. As the low tracks   
   roughly parallel to the coast, additional shortwave energy embedded   
   within the low will force widespread light to moderate rainfall   
   across much of northern California on Sunday. The low's trailing   
   cold front will cause the precipitation shield associated therewith   
   spread southward down the coast into late Sunday night. The   
   Marginal for points south of San Luis Obispo will be mostly due to   
   heavy rainfall in the area from Saturday, since the duration and   
   intensity of rainfall during this Day 2/Sunday period will be   
   rather minimal. Regardless, rainfall amounts overall across all of   
   northern California will be significantly lower than the heavy rain   
   expected across southern California today. Since most of the area   
   can handle the inch or less of rain expected in most areas away   
   from the favored upslope areas of the northern Sierra Nevada and   
   the Klamath mountains.   
      
   ...Southwest Utah...   
      
   As the shortwave from today lifts north across Nevada and into   
   Idaho, there will be a prolonged period of SSW flow into   
   southwestern Utah on Sunday. Further, weakening but still potent   
   secondary shortwave energy riding south and east of the first upper   
   shortwave will cause both shortwaves to turn negatively tilted   
   during the day. This will increase the lift favorable for heavy   
   rain. Across the Pine Valley Mountains area of southwest Utah, the   
   mountains there are the first a south-southwest flow up the   
   Colorado River Valley encounter. Thus, some guidance is hinting at   
   outsized rainfall amounts that could impact the area due to the   
   added upslope and locally greater moisture moving into those   
   mountains. Flooding potential will also be increased in this area   
   due to the presence of a burn scar with a history of flash flooding   
   resulting from heavy rain. A Marginal Risk was added to this area   
   due to the multiple pieces of guidance showing higher heavy rain   
   potential.   
      
   Wegman   
      
      
   Day 3   
   Valid 12Z Mon Nov 17 2025 - 12Z Tue Nov 18 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE LOS   
   ANGELES BASIN OF SOUTHERN CALIFORNIA...   
      
   ...20Z Update...   
      
   No major changes were made for this update to the Day 3/Monday ERO.   
   Just some minor adjustments based on the latest QPF, but capping   
   the ERO at a slight risk for now still seems reasonable, especially   
   considering antecedent conditions after today's (Saturday)   
   rainfall in this same region. See previous discussion for more   
   details about the overall set up.   
      
   Santorelli   
      
   ...Previous Discussion issued at 0830Z...   
      
   A cutoff low tracking south down the coast of California will move   
   across the hard-hit areas of the Los Angeles Basin on Monday. As it   
   moves through, the upper level cold air associated with the low   
   will once again increase instability around the low. Thus, expect   
   another 1-3 rounds of showers and embedded elevated thunderstorms   
   to move across the L.A. Basin through the day and into Monday   
   night.   
      
   A few things to note about this round of heavy rain: 1) Unlike   
   today's low, this low will be moving south-south-east with time. It   
   should have somewhat less tropical moisture to work with as   
   compared with today's low. PWATs will struggle to approach 1 inch.   
   This is because the southward movement will draw cooler and drier   
   air from the north Pacific with it, instead of having a fetch from   
   the tropics. However, the cooler air with it would mean a bit more   
   instability. Thus, while the storms may be a bit stronger than   
   today in some areas and at times, the much lower moisture levels   
   will more than offset that. 2) The low will be a bit faster mover,   
   towards the SSE, parallel to the coast. The faster movement should   
   allow any storms to also be faster movers, limiting the time any   
   one area is in heavy-rain-producing storms. 3) The storms will be   
   moving SE with time on Monday as contrasts the northward storm   
   movement of the storms today. This too should mean the storms will   
   have less upslope into the Transverse Ranges, as they'll be moving   
   more parallel to the mountains. All of these factors should limit   
   the rainfall.   
      
   Meanwhile, the one thing that Monday's storms have going for them   
   above the storms that today will have is antecedent conditions, as   
   the heavy rain from today will have risen local stream and creek   
   levels such that the lesser amounts of rain expected Monday will be   
   starting at a higher starting point than the rain expected today.   
   This is the rationale for the Slight Risk upgrade.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!60mWHBc8rXGVInxjO_JxEzjWm8mdDV4Aipes79TpzcyL=   
   mD2UCkAycz9lcIvV_r_tsZL4C5vWG9o69bNz2KQKSK0lfN4$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!60mWHBc8rXGVInxjO_JxEzjWm8mdDV4Aipes79TpzcyL=   
   mD2UCkAycz9lcIvV_r_tsZL4C5vWG9o69bNz2KQKLp2wkbA$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!60mWHBc8rXGVInxjO_JxEzjWm8mdDV4Aipes79TpzcyL=   
   mD2UCkAycz9lcIvV_r_tsZL4C5vWG9o69bNz2KQK1_4asR8$=20   
      
   =3D =3D =3D   
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