Bonus Time

Posted Monday, February 19th, 2018 by Taylor McCorkle

I think it’s safe to say this storm has not disappointed in terms of producing copious snowfall across the Wasatch Front.
Storm Totals (so far):
Alta: 19″
Snowbird: 20″
PCMR: 14″
Solitude: 15″
PowMow: 12″
Brighton: 16″
While the main trough has moved off to the west and on to Colorado, we’re still in a position to get a few more inches, courtesy of that cranking northwest flow and our friend the Great Salt Lake.

Despite the KMTX radar taking a holiday right along with everyone in the state of Utah, the HRRR is still producing a nice lake band this evening. As the flow backs to westerly, the enhancement from the lake will move north.

HRRR simulated reflectivity.
HRRR simulated reflectivity.

For those of you who are unfamiliar with lake effect storms, many atmospheric ingredients must be just right for snowfall to be enhanced by a large body of water (GSL in our case). Here are a few things to consider:
1. Instability – The temperature difference (lapse rate) between the lake surface and 700 mb must be large enough to ensure that the boundary layer is unstable.
2. Moisture: Boundary layer must still have sufficient moisture in order to produce precipitation
3. Wind Direction: When the wind direction is oriented parallel to the lakes longest axis, the enhancement of precipitation will be greatest.

For lake enhancement in the Salt Lake Valley and Cottonwoods, we want that northwesterly wind to do its thing.  Northern valleys will fare better from lake effect when the wind shifts back to westerly.
So hows it looking for us?
Since this system was very cold, 700 mb temperatures are forecasted to be anywhere between -18 to -22 degrees C this evening into tomorrow morning.  This will create a ~25 degree C temperature difference between the lake surface and 700 mb. This means lapse rates will be around 7C/km, creating an unstable environment (warm surface air will rise and cool).
We also will be saturated through the depth of the boundary layer (air temperature ~= dew point temperature).
Finally, the flow will be out of the northwest with speeds of around 16-18 kt, persisting until around midnight when the flow will begin to turn to a more westerly component. This will mean that the Northern Wasatch will have their turn at some lake enhancement as well.

18 UTC NAM Forecast Sounding valid 10p -
18 UTC NAM Forecast Sounding valid 10p –

For the timing of the lake effect processes, we are looking at late evening into the overnight hours for NW flow and enhancement in the SL Valley/Cottonwoods. Early morning looks best for Ogden and the Northern Wasatch.  If all goes according to plan, I wouldn’t be surprised if the Cottonwoods got an extra 2-5″ and maybe a dusting in the valley.
Given the fickle nature of this atmospheric process, the stakes are higher and the possibility for a let down is definitely greater than with a typical forecast.   But this is why we call it “Bonus Time”! This weekend has already been awesome with all the snow thats fallen, this is just the icing on the cake!

Enjoy, friends!

Leave a Reply

Your email address will not be published. Required fields are marked *