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.