MOUNTAIN WEATHER FORECAST

Stevens Pass Mountain Weather Forecast

Day and Time: Tuesday 1100

Thanks to the season’s first Puget Sound Convergence Zone snow event, Stevens Pass could have +6” of snow on the ground by the end of the day.  Likely more on the upper mountain.     

Current Observations:

24hr Precipitation: 1.65” water, 3” snow on the lower mountain 5” above 4500’  

Temperature profile: 31 base, 29 ridge      

Pass level winds:  Light W      

Barometer: 1008mb’s and holding             

Sky: OC              

Radar:  Snow showers in the Stevens Pass Convergence Zone with drying west of Everett.

The Forecast: The snow showers will slowly diminish this evening with temperatures falling into the lower 20’s overnight.   Drying and clearing with gradual warming can be expected for the remainder of the week.

Long Range:  The weekend will be warmer with some light rain showers possible.   

  

The Science:  The cool NW flow that filled in behind last night’s cold front brought us our first real dose of snow.  The snow will begin to taper off as the associated low pressure trough makes its way out of PNW.   High pressure developing off the PNW coast is already beginning to clear the sky over Puget Sound; this will eventually bring an end to the snow in the mountains as well.   A side effect: Cold air from the interior north will slide into eastern Washington tonight.  Watch for frost in the lower elevations as the freezing levels dip to or below 2000’.

 The approaching high pressure will slide into place tomorrow with dry and clear conditions setting up for the remainder of the week.

Confidence is strong for the rest of the week and moderate to poor for the weekend and beyond. 

Values from 4am today through

 4am tomorrow.

Precipitation: Today 24hr water: .5 snow: 3” Tomorrow’s 24hr water: 0  snow: 0 Trend and Timing: Snow ending after 10pm tonight.

Temperature: Low:  24  High: 34 Freezing level: 4000’ – 2500’ Trend and Timing:    Temperatures will drop to the lower 20’s tonight temperatures slowly recovering over the next 36-48hrs.

Winds Base: 0-5 W shifting E Ridge Top: 5-10 W shifting E Trend and Timing: Westerly flow will shift easterly overnight with east flow persisting through the end of the week.

Glossary of PNW Weather Terms: 

Storm Water Equivalent (SWE)…. how much water is in the snow or rain

Orographic lift…. an airmass that is lifted as it moves up and over higher terrain. This promotes cloud development, and enhances precipitation.

Dirty Ridge…. a weak, unorganized high pressure ridge.

Advection…. the transfer of heat by the flow of fluid.  For the PNW, this is usually from warm moist air entering the region from the south.

Flow…. wind direction.  In meteorology, the wind direction is always noted where the wind is coming from.   East flow is wind coming from the east; SW is from the south west…

The Inter-mountain West….. the region between the Rock Mountains and the Cascade Mountain ranges.  Mainly Idaho and Western Montana.

Puget Sound Convergence Zone….. This is formed when large scale westerly flow splits around the Olympic Mountains and then converges over Puget Sound.  The flow then accelerates up the river valley’s enhancing the orographic effect at the Passes. 

Occluded Front…..The boundary that separates the cold air mass behind a cold front and the warm air associated with a warm front. When the cold front overtakes the warm front it occludes.

High/Low Pressure…. High pressure is associated with sinking air that rotates clockwise. High pressure often results in dry and clear weather.  Low pressure is rising air that rotates counter clockwise and results in condensation, cloud formation and precipitation.

Atmospheric Stability….. A stable atmosphere is associate with high pressure or the inability for air to rise.  Instability or an unstable atmosphere forms under low pressure or rising air.

Thickness….. thickness is a function of the average virtual temperature between 1000 and 500 millibars.  The 5,400 (540 mm5) line generally divides polar air from mid-latitude air. The 5,400 line is also used as a guide to the rain/snow line. thickness is a function of the average temperature of a layer of air, then there will be a thickness gradient in the same region that the temperature gradient occurs.