Team 1

Meteorologists

Hilary Minor & Matt Sanders

Mesoscale Discussion

A strengthening Aleutian Low will bring together several synoptic ingredients to induce snowfall over the Cascades, specifically Snoqualmie Pass, Washington. There is a trough located over Washington during the forecast period. A 250mb jetstreak will propagate over the forecast region during this time. Snoqualmie Pass will be under the favorable left front region of this jetstreak. This will transport moisture into the area and provide lift. A low level jet of 65-70 knots from the SW will also aid in transporting moisture to the area. The topography in this region will result in the uplift of the strong flow off the Pacific and produce precipitation. Additionally, a cold front will come through the forecast region around 00Z Thursday providing additional lift and bringing cooler temperatures with it.

Initially a marginally cold air mass is in place over the NW. SSW winds will scour this cold air out of the region replacing it with temperatures just above freezing. Later, a cold front will sweep through the area bringing in cold air from WNW. The changing temperatures will affect the precipitation type during our forecast period. Initially the temperature structure will be marginal for snow in the area of Snoqualmie Pass, with temperatures at or just below the freezing mark. This means that precipitation at the beginning of the forecast period will likely be snow. However, at that time the forcing mechanisms will be weak and confined to areas with the greatest topographic relief. The pass is situated at a slightly lower elevation than the surrounding mountains where the greatest forcing will occur. This will limit precipitation until about 20z when the front moves through the area and enhances lift over the pass. Thus expecting very light snow and snow showers with little accumulation during this period. After 20z the enhanced lifting will produce a steadier snowfall. There may be some mixing with rain just prior to the fronts passage due to the warm SW flow ahead of the front. This means that the bulk of snow accumulation will be confined to the period after 20Z Wednesday. After 20z the 4 inch snowfall accumulation threshold is reached by 00Z when the MM5 indicates .43 inches of liquid. 10 inches of snowfall is achieved by 08Z. By the end of the forecast period, this region should expect 13 to 14 inches of snow.

When the front passes through the area sustained winds of 25 MPH with slightly higher gusts may occur between 00Z and 06Z. Because the winds are initially coming out of the SW, the orientation of the pass may protect this area from the highest winds. However, when the wind direction switches to the east the wind will be flowing parallel to the pass and wind speeds through it should increase to near 25mph with slightly higher gusts.

Forecast for Snoqualmie Washington: 12Z Wednesday – 12 Z Thursday

Moisture coming in off the Pacific will allow snow to develop Wednesday in the Cascades.

Although snowfall will initially be light Wednesday morning, it will pick up in intensity after the noon hour. Light snow and snow showers will produce little accumulation before 12 P.M. Expect snow accumulation to reach 4 inches between 1 and 4 P.M. Snow will continue to fall steadily and is expected to reach 10 inches between 11 P.M. and 2 A.M Thursday. By 4 A.M. on Thursday Morning, snow accumulation may reach 13 to 14 inches.

Wind should not be much of a concern as this system moves in to the coast. Sustained winds may briefly approach 25 MPH after 2 P.M Wednesday afternoon. It will also be hard realize gusts above 35 MPH.

MM5 24 Hr Precip

(12Z Wed – 12Z Thurs)

WRF 24 Hr Precip

(12Z Wed – 12Z Thurs)

MM5 Surface Winds

Stampede Pass,WA Soundings

Stampede Pass,WA X-Section

Note that it is generally AOB freezing

during the forecast period

Verification For Snoqualmie Pass

Our forecast for Snoqualmie pass between the hours of 12Z Wednesday March 8 th and 12Z Thursday March 9th was verified on some of the points and failed on others.

We predicted that 4 inches of snow would fall between 21Z and 0Z and this was verified because 4 inches of snow occurred by 21Z.

Our prediction for the time when 10 inches of snow would have fallen was between 7Z and 10Z but 10 inches of snow actually fell by 3Z.

The METARS showed that winds gusted above 35 MPH at 2Z. We predicted that winds would gust above 35 MPH after 2Z.

Our predictions for Snoqualmie Pass were off because we predicted the 10 inches too late. This could have happened because of the map resolution of the MM5. The precipitation gradient was very tight and a slight difference in the location of the precipitation would have thrown our forecast off.