From Mike Powell@454:3/105 to All on Thu Jan 1 09:23:10 2026
AWUS01 KWNH 010958
FFGMPD
CAZ000-011800-
Mesoscale Precipitation Discussion 0001
NWS Weather Prediction Center College Park MD
457 AM EST Thu Jan 01 2026
Areas affected...Southern CA
Concerning...Heavy rainfall...Flash flooding possible
Valid 011000Z - 011800Z
SUMMARY...Flash flooding is possible from the Coastal Range to the
Transverse Ranges where soils remain sensitive and saturated.
Localized urban flash flooding around the greater Los Angeles
metro area is also possible this morning.
DISCUSSION...GOES-19 satellite imagery shows a compact 500mb low
embedded within an increasingly negative 200-500mb mean trough
located west of CA. This upper trough is working in concert with a
700mb ridge near Cabo San Lucas to direct a highly anomalous plume
of moisture into not only CA but much of the western U.S.. PWATs
along the CA coast this morning are topping 1.25" and are above
the 99th climatological percentile. IVT is also topping 500 kg/m/s
and winds are out of the SW, which is supportive of some upslope
enhancement along orthogonally-oriented terrain from the Coastal
Range on south and east through the Transverse Ranges. As the
triple point of an occluded low approaches, so will the nose of a
500mb jet streak and low-level winds will strengthen off the
coast. Steepening mid-level lapse rates are expected as the
upper-low approaches and 00Z HREF guidance does show as much as
250 J/kg of MUCAPE potentially available as far north as Big Sur
this morning as a result.
06Z HREF guidance shows high chance probabilities (>70%) for 6-hr
rainfall totals >2" between 12-18Z along the San Bernadino
Mountains and low-to-moderate chances (30-50%) along the San
Gabriel. These are the mountain ranges most susceptible to
potential flash flooding in wake of last week's significant flood
event and lingering burn scars. As the the dry slot quickly moves
in by 18Z, the core of the heaviest rainfall should move north and
east with only lingering upslope flow keeping periods of rain
around the Transverse and Peninsular Ranges. Given the low FFG's
and sensitive soils in the higher terrain, as well as locally
heavy rainfall rates around the highly urbanized Los Angeles metro
area, flash flooding is possible this New Year's Day morning. Note
that rock slides and debris flows are also possible in the
Transverse Ranges where soils and burn scars are highly sensitive.