No abstract

This note describes changes that have been made to the National Centers for Environmental Prediction (NCEP) operational ''early'' eta model. The changes are 1) an decrease in horizontal grid spacing from 80 to 48 km, 2) incorporation of a cloud prediction scheme, 3) replacement of the original static analysis system with a 12-h intermittent data assimilation system using the eta model, and 4) the use of satellite-sensed total column water data in the eta optimum interpolation analysis. When tested separately, each of the four changes improved model performance. A quantitative and subjective evaluation of the full upgrade package during March and April 1995 indicated that the 48-km eta model was more skillful than the operational 80-km model in predicting the intensity and movement of large-scale weather systems. In addition, the 48-km eta model was more skillful in predicting severe mesoscale precipitation events than either the 80-km eta model, the nested grid model, or the NCEP global spectral model during the March-April 1995 period. The implementation of this new version of the operational early eta system was performed in October 1995.

A synoptic-dynamic climatology was constructed using all 24-h 2-in. (50.8 mm) or greater rainfall events in nine states affected by heavy rains and flooding from June through September 1993 using 6-or 12-h gridded analyses from the Regional Data Assimilation System and geostationary satellite imagery. Each of the 85 events was assigned a category (0-4) based on the areal coverage of the 3-in. (76.2 mm) or greater observed precipitation isohyet. A variety of meteorological fields and rules of thumb used by forecasters at the Hydrometeorological Prediction Center are investigated that may help identify the most likely location and scale of a convective precipitation event.The heaviest rain usually fell to the north (downwind) of the axis of highest 850-mb winds and moisture flux in an area of 850-mb warm temperature and equivalent potential temperature advection. The rainfall maximum also usually occurred to the north or northeast of the axis of highest 850-mb equivalent potential temperature. The scale and intensity of the rainfall appeared to be related to 1) the magnitude of the warm advection, 2) the 1000-500-mb mean relative humidity, 3) the breadth of the axis of stronger values of moisture transport feeding northward into a surface boundary, 4) the strength of low-level moisture flux convergence, and 5) the length of the low-level moisture flux convergence that was aligned along the mean flow upstream from the location of the rainfall maximum. The latter finding suggests that propagation plays an important role in modulating the scale and intensity of rainfall events.

Extreme rainfall events contribute a large portion of wintertime precipitation to northern California. The motivations of this paper were to study the observed differences in the patterns between extreme and more commonly occurring lighter rainfall events, and to study whether anomaly fields might be used to discriminate between them. Daily (1200-1200 UTC) precipitation amounts were binned into three progressively heavier categories (12.5-50.0 mm, light; 50-100 mm, moderate; and Ͼ100 mm, heavy) in order to help identify the physical processes responsible for extreme precipitation in the Sierra Nevada range between 37.5°and 41.0°N.The composite fields revealed marked differences between the synoptic patterns associated with the three different groups. The heavy composites showed a much stronger, larger-scale, and slower-moving negative geopotential height anomaly off the Pacific coast of Oregon and Washington than was revealed in either of the other two composites. The heavy rainfall events were also typically associated with an atmospheric river with anomalously high precipitable water (PW) and 850-hPa moisture flux (MF) within it. The standardized PW and MF anomalies associated with the heavy grouping were higher and were slower moving than in either of the lighter bins.Three multiday heavy rainfall events were closely examined in order to ascertain whether anomaly patterns could provide forecast utility. Each of the multiday extreme rainfall events investigated was associated with atmospheric rivers that contained highly anomalous 850-hPa MF and PW within it. Each case was also associated with an unusually intense negative geopotential height anomaly that was similarly located off of the west coast of the United States. The similarities in the anomaly pattern among the three multiday extreme events suggest that standardized anomalies might be useful in predicting extreme multiday rainfall events in the northern Sierra range.

No abstract