Poster announcing a West Coast Precipitation COMET training course for river forecasters.
OAR Hot Items Bulletin

Orographic precipitation enhancement

  • rain rate is most strongly controlled by upslope winds at 1 km altitude, especially in a LLJ
  • blocking increases rain upstream of terrain
  • precipitation efficiency is 50% higher when a LLJ is present than when it is not:
    0.9 (mm h-1) (m s-1)-1 with a LLJ vs. 0.6 (mm h-1) (m s-1)-1 without a LLJ

Warm rain

  • rain rates in west coast storms can exceed 20 mm/h in conditions without a bright band
  • for cases with the same rain rate, reflectivities were roughly 8 dBZ lower in warm rain cases
  • 35% of rain in the 1997/98 winter occurred without a bright band, < 5% did in 2000/01
  • Significant rain can occur without a bright band, but not every year.

Bright-band detection

  • an automated algorithm using wind profiler observations was developed and is being tested
  • real time output are on the web and deduced altitudes are a better estimate of snow level than is the 0°C level
  • rapid 2000 ft increases in melting level are often observed and can triple runoff in CA watersheds

Sea-surface fluxes and coastal rainfall

  • high surface winds combined with anomalously warm coast SSTs can increase coastal rain
  • observations in a flooding event showed a 27% increase in CAPE due to these fluxes
  • this flooding event was capped off by convective rain rates that reached 80 mm/h

Flooding in adjacent watersheds

  • wind direction in the warm sector of a major storm determined the location of a rainshadow
  • the rain shadow resided partially over one watershed but not over an adjacent watershed
  • small (± 10°) wind direction variations can strongly affect winter flooding in complex terrain

Narrow moisture plumes

  • P-3 soundings offshore with roughly 80 km spacing documented a narrow moisture plume
  • the core of the plume was roughly 100 km wide, with IWV > 3.0 cm
  • the core values were 40% greater than the maximum IWV derived from GOES due to clouds
  • Key aspects of a narrow moist plume responsible for a 4" rain event were missed by GOES and numerical models.