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   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   1003 AM EST Thu Jan 1 2026   
      
   Day 1   
   Valid 16Z Thu Jan 01 2026 - 12Z Fri Jan 02 2026   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHERN CALIFORNIA...   
      
   ...16Z Outlook Update...   
   Changes to the Slight Risk area in southern California currently   
   reflect convective trends, wtih deep convection persisting along   
   and ahead of a surface cold front over Pacific waters approaching   
   that area. Mesoanalyses depict a pool of weak surface-based=20   
   instability just offshore of San Diego County that has enabled=20   
   deeper convection to materialize and repeat/train into portions of=20   
   San Diego Metro. This regime, along with persistent orographic lift   
   against the Transverse and Peninsular coastal ranges, will=20   
   continue to support flash flooding in addition to mud/rock slides=20   
   and occassional near-stream flooding for at least another 6 hours=20   
   or so. The Slight Risk has been expanded southward to the=20   
   international border and trimmed on its westward extent across=20   
   Santa Barbara and Ventura Counties based on aforementioned trends.=20   
   The Marginal Risk areas remain in tact as a large upper low=20   
   approaches the state of California, spreading areas of moderate to=20   
   heavy rainfall especially across coastal/Sierra front ranges=20   
   through the evening.   
      
   See the previous discussion for additional information.   
      
   Cook   
      
   ...Previous Discussion...   
      
   A 500mb trough with a small, yet potent, closed circulation will   
   become part of a negatively tilted 200-500mb upper-trough axis   
   that is partially responsible for the slug of highly anomalous   
   moisture that will engulf much of the West today. The focus remains   
   on Southern California where the Transverse Ranges soils remain   
   quite sensitive in wake of last week's significant flooding. IVT   
   values are encroaching upon 500 kg/m/s and embedded within SSWrly   
   flow that is aligned orthogonally to the Transverse Ranges.   
   Additional forcing will be located along the frontal boundary that   
   will trigger additional convection over the East Pacific upon   
   approach. PWATs are forecast to be just over 1.25" off the Southern   
   California coast through this morning, but around 18Z, a dryslot   
   in the 700-300mb layer should result in the conclusion of today's   
   heaviest rainfall. Some lingering heavy rainfall may be possible in   
   the Peninsular Range east of San Diego beyond 18Z, but most   
   rainfall around the L.A. region should be just about over (aside   
   from lingering upslope into the Transverse Ranges).   
      
   Latest HREF guidance does show minor instability present (<250   
   J/kg on average) between 09-18Z, which may be just enough when   
   taking into account sensitive soils to result in additional flash   
   flooding in not just the Transverse Ranges, but also the greater   
   Los Angeles metro area as well. The new 00Z HREF 24-hour QPF   
   probabilities for surpassing 3" of rainfall peaks around 50% over   
   the San Bernadino and low chances (15-25) for localized totals   
   eclipsing 5". Snow levels remain very high, making just about all   
   of the expected QPF to be in the form of rain. The lack of higher-   
   end instability is what is keeping the Slight Risk in place, but   
   make no mistake-- there remains a heightened risk for flash   
   flooding this morning in Southern CA with rock slides and debris   
   flows in/around burn scars possible, including urbanized flash   
   flooding.   
      
   Farther north, most area have recovered a bit better from last   
   week's significant rainfall, but the copious amounts of moisture   
   combined with sufficient synopitc-scale forcing aloft and   
   supportive upslope flow along the Coastal Range and as far inland   
   as the Sierra Nevada does suggest a Marginal Risk for flash   
   flooding is still warranted. The 00Z HREF does show a small area   
   between Santa Maria and Big Sur where MUCAPE >250 J/kg could   
   materialize between 15-18Z and foster more potent instantaneous   
   rainfall rates as convection moves through. Still, the line of   
   showers and thunderstorms would be progressive and keep the flash   
   flooding potential to a very localized area.   
      
   Mullinax   
      
   Day 2   
   Valid 12Z Fri Jan 02 2026 - 12Z Sat Jan 03 2026   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   CALIFORNIA...   
      
   An expansive upper level cutoff low over the Pacific well west of   
   the northern California coast will approach the northern California   
   coast Friday night. The occluded front wrapping around a surface   
   low will be the primary forcing for heavy rainfall into northern   
   California Friday night. The occluded front will turn the flow a   
   bit more onshore (southeast before the front to south-southwest   
   behind it). This will allow the coastal ranges to upslope some of   
   that moisture, locally enhancing rainfall rates, particularly into   
   the northern Coastal Ranges and the Klamath Mountains. Some heavy   
   rain may impact the Bay area and the coastal ranges south from   
   there in the waning couple hours of the period, assuming the timing   
   of the front moving into the coast remains unchanged. IVT values   
   between 500 and 750 kg/ms are far from impressive, and moisture   
   anomalies rise to around 2 sigma above normal, a common occurrence   
   in this part of the world, especially the past few weeks. Thus,   
   with the rainfall event just getting going, coming in in the middle   
   of the night, and with less-than-impressive dynamics, the Marginal   
   Risk for much of the central and northern California coast remains   
   largely unchanged with this update, and the Marginal looks on   
   track.   
      
   Wegman   
      
   Day 3   
   Valid 12Z Sat Jan 03 2026 - 12Z Sun Jan 04 2026   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   CALIFORNIA...   
      
   A series of upper level disturbances rotating around a large upper   
   level low will impact much of California and the West Coast this   
   weekend. With each upper level disturbance, there will be a   
   commensurate surface front that also makes its way ashore into   
   northern California. The result will be an almost non-stop rain,   
   especially for upslope areas on southerly flow, as well as for all   
   of the coastal ranges. That said, each disturbance will briefly   
   increase rainfall rates, with those rates decreasing back down in   
   between disturbances. Thus, Saturday's rain looks to be a long-   
   duration relatively light (for most areas) rain, rather than there   
   being big concerns on any one area seeing prolonged heavy rain.   
   Given the area's good ability to drain rainwater quickly, a   
   Marginal Risk remains in effect for northern California. The area   
   will continue to be monitored for a possible Slight Risk upgrade   
   with future updates.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!5ohuCikjrKAwZAFRpsatOeIB9cgsRlzzU_30OLjzZccq=   
   oyu_Bb9OjVOeaihELQFY74ilH-zYdxFic9EyYCLWlv8LMxQ$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!5ohuCikjrKAwZAFRpsatOeIB9cgsRlzzU_30OLjzZccq=   
   oyu_Bb9OjVOeaihELQFY74ilH-zYdxFic9EyYCLWaDaGg68$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!5ohuCikjrKAwZAFRpsatOeIB9cgsRlzzU_30OLjzZccq=   
   oyu_Bb9OjVOeaihELQFY74ilH-zYdxFic9EyYCLWp8TtQ8I$=20   
      
   =3D =3D =3D   
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