Things Are About to Get Interesting

I like our chances of picking up several inches of mountain snow in the Cottonwoods later today, tonight, and early tomorrow as a cold, "juicy" northwesterly flow develops over northern Utah.  Perhaps we'll do better if the lake effect can kick in with the right orientation.  If we can get high precipitation rates, you might even see some of the white stuff on the benches.

Here are some thoughts on lake effect that are relevant for tonight's forecast.  First, lake effect is relatively rare in September.  During the latter half of the month, you can expect an event on average every 4-5 years.  Events are more common later in the fall when there is a higher frequency of trough passages and cold air intrusions.

Source: Alcott et al. (2012)

Although events are scarce this time of year they do happen.  In addition, if we look at the environmental conditions in which lake effect occurs throughout the cool season, we find that during the fall (and spring), one generally needs a much larger temperature difference between the lake  and the overlying airmass [we usually use the 700-mb (about 10,000 ft) temperature as indicative of the overlying airmass] to generate lake effect.  The graph below is fairly busy, but the red box-and-whiskers show the range of lake-to-700-mb temperature differences during lake-effect periods and illustrate fall and spring events generally require larger differences than events in the spring and fall winter.

Source: Alcott et al. (2012)

The dashed red line reflects our best guess of the minimum lake-to-700-mb temperature difference needed to generate lake-effect in any given month.  In September, it's about 21ºC.  The good news is that we'll be beyond that tonight.  Lake temperatures as of yesterday were around 20ºC, whereas the 700-mb temperature is forecast to be -5 to -7ºC (depending on what model you look at) late tonight and early tomorrow morning.  Even though the lake will cool some over the next several hours, we should be above the necessary (but not sufficient) threshold.

The problem is that that threshold has been met before in September without producing lake-effect (the black box and whiskers show differentials without lake-effect, confirming this).  This is because there are a number of additional factors at play, one of which is the relative humidity.  It's simply harder (or impossible) to get lake effect from a dry low-level environment.  If we look at all lake-effect periods, we find that the likelihood of lake-effect increases with the size of the lake-to-700-mb temperature difference (relative to the dashed red line above, something we call ΔT excess, or how far you are above the magic threshold) and the low-level (850-700 mb) relative humidity.

Source: Alcott et al. (2012)

Of course, there are still other variables at play, such the time of day.  There is a strong diurnal modulation of lake-effect with events more common at night and especially just before and around sunrise and less common in the afternoon.  This effect is more pronounced in the Fall and Spring when solar heating is stronger than it is in the winter.

Source: Alcott et al. (2012)

So if you put all this scentific mumble jumble together, what do you have.  Well, we have several key ingredients coming together at that optimal time (late night and early morning): Big lake-to-700-mb temperature difference, high relative humidities, and flow from the west-northwest to northwest.  We can extract information about the environment forecast by the computer models and estimate the likelihood of lake-effect and you get the numbers below from the GFS and NAM.

So, the NAM is positively giddy, the GFS a bit more subdued.  In the case of the GFS, it's simply a little warmer and drier, so the odds are reduced a bit.  Note that those are instantaneous probabilities.  If you were to consider the odds of lake effect at any time during the late night or early morning, it would be higher than the maximum values provided in the tables above.

Put all this together, and you have a good chance of lake effect.  Whether or not it goes and goes big is something we really can't forecast reliably.  In addition, even if it goes, there's the issue of where and how much.   This is why I don't bet the house on the Dreaded Lake Effect.