GROUP # 5UPDATED 02/20/07

Faye Barthold

Michelle Benny

Ting Sun

1) TUESDAY MESOSCALE DISCUSSION

• Fronts– By 12Z February 22 the low pressure system that has been sitting off the Pacific coast has begun to move onshore into

the Pacific northwest. The cold front will cross the South Lake Tahoe region during the day on Thursday, resulting in colder temperatures and the chance for significant precipitation, as flow around the low will bring in moisture from the Pacific. 700mb temperatures will remain well below freezing throughout the period. 850mb temperatures (~surface), however, will hover around freezing for much of the day on Thursday before dropping below freezing by 0Z February 23, potentially leading to changes in precipitation type.

• Forcing Mechanism for Precipitation– The upper level trough is located west of California, off the Pacific coast. At 300mb, a jet streak flowing around the base

of the trough will result in upper level divergence to the east, over the Tahoe region. Upper level convergence is associated with low level convergence, resulting in rising motion downwind of the upper level trough. Additionally, positive vorticity located in the base of the upper level trough will be advected into the Lake Tahoe region throughout the day, further enhancing rising motion. In addition to these favorable upper level dynamics, a surface front crossing through California combined with the variable terrain of the Sierra Nevada mountains will create significant convergence and lift, enhancing precipitation throughout this region.

• Potential Winter Weather– With cold temperatures aloft, precipitation should be in the form of snow throughout the period. Warm temperatures early

in the week, resulting in above freezing ground temperatures, will limit snow accumulations initially. With forecasted liquid equivalent precipitation ranging from 0.75-1.25in, a 10:1 snow to liquid ratio would approximately result in about 9-14 inches of snow in South Lake Tahoe. Since surface temperatures will be near freezing throughout, a lower snow to liquid ratio may be more realistic. The colder mountain peaks will likely receive more snow. With a significant amount of snow falling during the day, slick mountain roads and reduced visibilities due to blowing and falling snow are likely.

• Uncertainty based upon NAM and GFS models– At 12Z 22 February, the GFS is forecasting the center of the low to be farther north and to move on shore faster than the

NAM. By moving the low on shore sooner, the GFS brings more precipitation to Lake Tahoe in the 6 hours prior to 12Z February 22 than the NAM does. Additionally, the GFS is slightly warmer than the NAM, keeping the 5400m line along and to the west of the Lake Tahoe region. By 18Z the GFS weakens the low slightly to 1004mb, and most of the precipitation is west and south of Lake Tahoe. The NAM, on the other hand, places the maximum in precipitation just north of the region. By 0Z February 23, the GFS has moved the low on shore in southern Canada and features a more NE to SW tilted trough. The NAM has a slightly stronger low pressure system, still located off the Oregon/Washington coast, and a neutrally tilted trough. The NAM at this time is also forecasting a higher maximum in precipitation. The rain/snow line is located in approximately the same location in both models. The upper level features are similar in both models. Overall, the storm intensity and track are similar in both models, with the biggest area of uncertainty seems to be with the timing of the heaviest precipitation.

2) TUESDAY PUBLIC FORECAST

• ...WINTER STORM WATCH IN EFFECT FROM LATE WEDNESDAY NIGHT THROUGH LATE THURSDAY NIGHT…• A TOTAL OF 1 TO 2 FEET OF SNOW IS POSSIBLE ABOVE 7000 FEET...WITH UP TO 12 INCHES AT LAKE LEVEL. • Time that winter weather conditions will onset

– During the time period of 12z February 22 through 00z February 23, winter precipitation should be relatively constant in the Lake Tahoe region due to the system currently moving into the area. This system should be in place prior to the forecast period.

• The type of winter weather hazard to expect (in one of two categories: freezing rain (ice on road – deploy salt trucks), or snow (deploy plows))

– Winter weather will be in the from of snow and blowing snow from 12z February 22 through 00z February 23 in the Lake Tahoe region. Due to the fact that temperatures have been relatively warm and above freezing the past few days, icing on the roadways may be an issue in the mountains. This is due to warmer ground temperatures allowing for the falling snow to melt as it makes contact with the ground. This melted snow may refreeze as ice temperatures drop and snow continues to fall in the area. Additionally, high winds may produce considerable drifting and blowing snow, which may drop visibilities and make travel hazardous. It is recommended that salt trucks be deployed as soon as snow begins to fall to help prevent any icing on the roadways. Once snow has started to fall and accumulate, deploy snow plow trucks to ensure that snowfall does not prove detrimental to motorists. This will most likely begin to occur prior to the forecast period.

• Expected accumulations– Accumulations have to potential to be rather heavy. In the forecast period, the possibility of 9-14 inches of snow in South

Lake Tahoe is possible. It is also possible that the higher elevations may see accumulations near 2 feet. Blowing and drifting snow in this storm may alter storm totals, creating large drifts.

• Time when precipitation transitions will occur – All precipitation should be in the form of snow during the time period of 12z February 22 through 00z February 23. It is

highly unlikely to see other forms of precipitation in the Lake Tahoe region during the time period. • Time when will precipitation end

– Precipitation should be relatively constant, with few breaks in the snowfall. However, snowfall will be highest through Thursday evening. Snow will begin to slow down towards the end of the day on Thursday but scattered snow showers and flurries are still expected.

• This forecast was based upon data from the following sources:– WRF model– GFS model– NWS in Reno, NV

3) TUESDAY FIGURE 1

• Surface Analysis

NAM forecast for 12Z February 22

4) TUESDAY FIGURE 2

• Upper Level—850mb Temperatures

NAM forecast for 0Z February 23

5) TUESDAY FIGURE 3

• Cross section (Monterey, CA to Elko, NV)

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)