GROUP 4 : UPDATED 22 Feb 2007 Huigang Yang Wendi Kaufeld Matt Sienko

1) TUESDAY MESOSCALE DISCUSSION A) At 12z Thursday, a cold front coming in from the Pacific will be positioned over north-central California through the San Francisco Bay. The cold front quickly progresses through northern California, passing through the Tahoe area and into western Nevada by 00z Friday. B) As for the frontal forcing, the cold front will force the warm and moist air ahead of the front to lift and will be one of the causes of the precipitation for this forecast period. Upslope flow over the Sierras enhances the rising motion, which also contribute to the total amount of snow fall. There are two other sources of lift involved over this time frame as well. At least one positive vorticity maximum will approach the area Thursday and the 18z and 00z panels of both the NAM and GFS place the Lake Tahoe area in a region of upper-level divergence. C) Temperature would be low enough for snow for the duration of the forecast period. The NAM predicts 16 mm of liquid precipitation, which would result in 128 mm of snow using an 8:1 ratio. The abundance of moisture in the area as seen by high 700mb RH values as well as high low level moisture seen in the cross-sectional plots suggests that with so many lifting mechanisms in place that the total snowfall has potential to be much higher than simply the equivalent the NAM gives. A rough estimate of snowfall expected within the 12z-00z time frame might wind up being at least twice the amount calculated. With strong winds, travel would be significantly hazardous with blowing and drifting snow. D) Both the GFS and the NAM have been consistent over the past 3 runs (06Z, 12z, and 18z Feb 20)and both models are in fairly good agreement regarding timing and location of heavy precipitation Thursday as well as regarding the position of the cold front. Temperatures should remain cold enough for snow through this forecast period but the GFS indicates a slightly stronger cold pool behind the front with tighter packing of mb thicknesses at 00z Feb 23. Precipitation amounts vary a little with the NAM dropping higher liquid equivalents than the GFS, perhaps due to the grid spacing differences between the models. One significant difference between the models is the small vorticity maximum the NAM brings into the South Lake Tahoe area early Thursday morning. If it verifies, the snow may fall in heavy bursts in the morning as this feature slides by, something the GFS would have missed.

2) TUESDAY PUBLIC FORECAST A cold front coming from the northwest off the Pacific will begin to influence weather over northern and central California overnight Wednesday. Light snow can be expected overnight, accumulating to around 1-2 inches by early morning. Snow showers may pick up in intensity at times during the early to mid morning hours. As the cold front approaches and passes through our area Thursday afternoon, snowfall will become heavy at times while winds become strong out of the southwest. Winds will become west-northwesterly and temperatures will begin to drop significantly by late afternoon after the front passes through. Snow plows will have to be deployed throughout the day Thursday and especially in the afternoon and evening as snow becomes heavy and strong winds create hazardous driving conditions with blowing and drifting snow. Total snow accumulations for Thursday morning through late Thursday afternoon will likely fall into the 6-10” range for the Southern Lake Tahoe area and western-facing slopes as well as ski areas may see more than a foot of snow from this event as snowfall continues into Friday. Snow will cease Friday afternoon/evening. (Snow accumulations and timings based mainly on 12z NAM precipitation liquid equivalents and influence of lift at certain times/moisture availability, with a hedging on the high side of precipitation simply because it’s difficult to judge snowfall totals with model resolution and influence of orographic lift.)

3) TUESDAY FIGURE 1

4) TUESDAY FIGURE 2 At 700 mb

5) TUESDAY FIGURE 3

6) WEDNESDAY MESOSCALE DISCUSSION

7) WEDNESDAY PUBLIC FORECAST

8) WEDNESDAY FIGURE 1

9) WEDNESDAY FIGURE 2

10) WEDNESDAY FIGURE 3

11) NOWCAST AND WARNINGS

12) CRITIQUE (1)

13) CRITIQUE (2)