Wednesday, March 14, 2018

Expect the Unexpected from This Spring Storm

It's worth a look at the combined cloud/radar and NAM cloud/precipitation forecast loop below to get an idea of the lack of organization of precipitation systems forecast to impact the weather of northern Utah over the next 2-3 days.  Note their banded or "blobular" structures.  Blobular is of course a highly scientific word (ha ha) used here to describe cellular features produced by a model that due to it's sparse grid spacing (12-km) is incapable of producing convective storms that look like those of the real world. 

The chaotic nature of those precipitation features means if you are looking for a precise forecast of when and how much it is going to rain or snow over the next couple of days, you've come to the wrong place. 

Let's start with perhaps the easy part: Today.  A combination of instability, strong flow, and vertical wind shear means we will see some showers and thunderstorms this afternoon.  The NAM forecast sounding for 2200 UTC (4 PM MDT) shows 320 Joules/kg of surface Convective Available Potential Energy (CAPE), a measure of how much energy a surface parcel of air would gain if it were lifted vertically through the atmosphere.  Locally, values may be higher.  Although such CAPE values are pretty pathetic for those looking for midwest-type severe storms, but are enough to make things interesting for Utahns.  Strong flow and vertical shear is also indicated in the wind profile. 
As such, the Storm Prediction Center has us in marginal risk of severe thunderstorms in the mid to late afternoon when "thunderstorms will offer the potential for damaging gusts and hail near severe limits." 

Source: NWS
Beyond showers and thunderstorms, expect some gusty south winds today, with the possibility of some blowing dust.  There is no longer snow cover over valleys and basins to our south and west, so dust emissions are possible if the land-surface conditions are favorable and flows are sufficiently strong.  Temperatures today will remain mild, although snow levels may drop locally during stronger showers and thunderstorms and may include large graupel or hail. 

After today, the pattern might best be described as unsettled, which is a nice way of saying there will be precipitation, but where, when, and how much is unclear.  Snow levels will fall overnight and probably be near bench level early tomorrow morning. 

The now somewhat old 0300 UTC initialized SHREF shows a mean of about 1 inch of water total at Alta-Collins by 0000 UTC (6 PM MDT) tomorrow afternoon, but the range is colossal, spanning from about 0.1 to 2 inches. 

Everything will depend on the position and intensity of precipitation features accompanying the system as it swings through. 

Stuff that falls today will likely be of the wet, high-density variety at high elevations, possibly including some big graupel or hail.  A garbage bag might be required at times, especially at mid and lower elevations, which will probably see rain that could turn temporarily frozen precipitation of a "variety of forms" during periods with higher precipitation rates.  Snow levels and densities will drop later tonight. 

A reasonable guess for Alta-Collins would be 7-14 inches from today through 6 PM tomorrow, with more possible if they are lucky enough to get a pounding from one or more of these precipitation features.  Note that I use the scientific term "guess."  Expect the snow to come in fits and starts at times. 

Welcome to spring!

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