24 November 1997 -- Precursors of a West Coast Precipitation Event

GOES-9 water vapor / MRF winds

GOES-9 water vapor image with

300 hPa streamlines/wind speeds

GOES-9 water vapor

GOES-9 water vapor image

Animations: Java | QuickTime

GOES-9 infrared channel 3 (water vapor) imagery (above) revealed the precursors of a significant precipitation event which developed across parts of California and the western U.S.. A trough of low pressure was intensifying in the Gulf of Alaska on 24 November 1997. Significant jet energy could be seen diving southward along the western portion of the developing cyclone, signalling a slow eastward movement of the system until this jet rounded the base of the low. South of this developing cyclone, two distinct plumes of upper-tropospheric moisture and clouds could be seen streaming northeastward from Hawaii toward the California and Oregon coast. As the intensifying low moved closer to the West Coast and continued to tap moisture with a tropical connection, moderate to heavy precipitation fell across parts of the western U.S.

Daily radar-estimated precipitation totals [courtesy of NASA/MSFC Global Hydrology Resource Center]:
25 November | 26 November | 27 November

GOES sounder as well as DMSP SSM/I total precipitable water (below) confirmed the presence of increasing tropospheric moisture within the long fetch of these subtropical plumes.[DMSP SSM/I image courtesy of the NOAA/NESDIS/ORA Ocean Physical Processes Team]

GOES-9 total precipitable water

GOES-8/GOES-9
total precipitable water

DMSP SSM/I total precipitable water

DMSP SSM/I
total precipitable water


Another feature of interest was the appearance of mountain waves as the strong southwesterly flow of the southern branch of the subtropical jet impinged upon the Sierra Nevada mountain range. The GOES-9 water vapor and visible imagery below shows these waves as they develop during the day, just to the lee of the snow-capped peaks in eastern California. Note how a dark cloud-free band (a "foehn gap") becomes better-defined on the water vapor images (19:00-19:15 UTC), about 30 minutes before the first appearance of visible wave cloud billows (about 19:30-19:45 UTC). This water vapor signature of rapid subsidence can be used for short-term forecasting of mountain wave clouds and their associated turbulence.

GOES-9 water vapor

GOES-9 water vapor image

(Java animation)

GOES-9 visible

GOES-9 visible image

(Java animation)


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