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GROUP # 2 UPDATED 02/20/07. NAMES Amanda Edwards Joe Clark Dong Wang. 1) TUESDAY MESOSCALE DISCUSSION.
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GROUP # 2UPDATED 02/20/07 NAMES Amanda Edwards Joe Clark Dong Wang
1) TUESDAY MESOSCALE DISCUSSION • TLV will be downstream of a mid and upper-level trough early Thursday morning as a large system begins crashing onshore. The GFS and NAM handle this system comparably, with the GFS being slightly stronger and faster. We will use the WRF and favor the timing of the global GFS. • Curvature of the 300-mb flow will be enough to produce favorable upper-level forcing (Fig 1) while location downstream of a deep 500-mb trough will provide additional forcing from PVA. At the lower levels, an 800-mb upper-level front, basically the surface front at TLV, is progged to be to the west at 12Z Thurs and enter the area by 10 a.m. (18Z) (Fig 2). The primary forcing mechanisms before 18Z will be the aforementioned 300-mb forcing for ascent as well as weak to moderate orographic forcing. However, as surface and low-level winds veering to be from the west as the system comes onshore, and the best upper-level dynamics shift to our south, passing overhead during the early afternoon, precipitation intensity should increase rapidly. The WRF precip fields are in agreement with this as the bullseye of heaviest precipitation shifts overhead during the day. Precipitation should be underway by 12Z and approaching its peak in intensity around 18Z, after adjusting for timing. Light snow will continue to fall through 00Z before tapering off sometime afterwards. • The thermal profile becomes increasingly cold, but not initially saturated. This lends itself to lower ratios (8:1) at the onset and higher ratios as the event reaches its peak (15:1). Given this, expect 1-2’’ before 18Z with up to five more by 00Z Fri, given 0.5’’ liquid equivalent output by the WRF.
2) TUESDAY PUBLIC FORECAST • A weather system will move into South Lake Tahoe in the early morning hours and snow will be present by 4am Thursday morning. Snow will be light in the early morning with little accumulation. As the system develops and continues to move westward, the snow will continue and intensify through the late morning and afternoon. A total accumulation of 4 to 6 inches is expected, with snow being heaviest between 10am and noon local time. The snow will continue past 4pm PST, and will taper off through the evening. Gusty winds will combine with the snow to produced reduced visibilities as low as ¼ mile during the peak of the storm, leading to potentially hazardous conditions on local roadways.
