Tuesday Evening Update : Latest Storm Thoughts

The short and long of it…
Forecast remains intact and on track, no real need for any changes of this evening. Wanted to take a moment to explain the current set up and why the forecast is going to evolve the way that it is.
But first let’s get the details out of the way. (Images 1,2,3) are the current hires snowfall projections from three models, the HRRR,NAM and RDPS.
The data all paints a similar picture, a nose of warm air both aloft and at the surface pushing up the river valley and limiting snowfall amounts, more snow in the hill towns east and west of the valley and high elevation snowfall amounts exceeding six inches above 1500’.
Snow May break out as early at 9-11AM in the form of early frontogenesis as the warm front pushes north, this can be funny as they can leave some areas out of the action initially as a band of precip will lift north through the region and some southern zones may see a late morning lull in precipitation. You can kinda see this on (image four) which is the simulated radar for 9AM.
By 12-1PM overrunning precipitation has broken out region wide. This initial burst of precipitation is where the entire region will make or break the snowfall forecast, with the exception of the higher elevations.
Snow may fall heavily along and north of i84 between 11-6PM (image above is 5pm Thursday), all the while warmer air will be pushing north into the region, southern most zones may only see a very brief period of snow resulting in a trace to a coating. As you head further north the duration of snowfall before changeover increases, therefore so do the amounts. The range in our snow forecast for the valley locations (black) should be digested based on your location, further south (lower end), further north (higher end), locations on the valley floor nearest to the river will warm the fastest.
The 9AM initial burst of snow can impact the morning commute, that mid afternoon into late afternoon snow will impact the afternoon commute. Between 4PM and 8pm heavy rain will be pushing into the region from south to north adding to the mess and complexity and erasing earlier snowfall. This is shown in (Images 4,5,6) which is the the HRRR simulated radar for 9AM,4PM and 8PM.
Current Snowfall Forecast:
(Image 7) is our snow forecast which doesn’t differ much from our forecast last night, in a set up like this it’s best to aim low and keep snow amount expectations low, it will be a brief albeit potentially heavy period of snowfall before the change over.
Finally the reason why the forecast and many others have evolved in this manner. (Images 8 and 9.)
While we do have some cold air in place ahead of the system with lows tonight dropping below freezing in many locations, it’s a decaying airmass, and transient cold that is being evicted from the region by a warm front associated with a low pressure cutting to our west.
In the final image I have highlighted both the high pressure responsible for our cold air and marked the the flow around a high pressure is clockwise while the flow around low pressure is counterclockwise. The high pressure is retreating to the east, no longer supplying cold air from Canada into the region the question mark notes the missing high pressure from the previous image. Meanwhile the low pressure tracking to our east is pumping warmer air north into the region. Warm air is light and cold air is heavier and dense, when warm air is forced into cold air, it’s forced up and over it. This lift creates precipitation, otherwise known as overrunning precipitation.
When a cold air mass is not deep and well established the atmosphere eventually warms from the top down and the warm air scours out the colder air. This results in snow changing over to sleet and eventually rain. In the final image you can see that although a secondary low pressure does eventually form offshore it’s weaker than its parent low to our west. If the offshore low was the primary or energy was transferred to it, colder air would be able to rush back into the region, because our flow would no longer be out of the south. This has been a repetitive pattern this season and last, which is another topic for a less busy day!

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