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03/13/2013

Light Snow On Thursday ... Let's Talk About Whether It Could Accumulate!

We have a quick moving storm that will press across the area tomorrow. It will increase clouds quickly and ultimately produce a rain / snow mix late tomorrow. Let's jump into the discussion about whether you need to expect any accumulations from this storm.

 

Thursday Storm System -- Discussion

 

The storm for Thursday is a very fast moving clipper style system coming out of Canada. We have seen better agreement about this storm coming in late afternoon on Thursday as the low goes right overhead. Notice the mid level low is right on top of us by around 4 pm Thursday.

 

Nam_500_1

 

The key to the forecast on Thursday is the very dry air that is over our area right now. When we have very low dewpoints, it creates a situation called Evaporative Cooling. The precipitation falls into the dry atmosphere, evaporates and literally draws heat out of the atmosphere creating impressive cooling in the lower levels on the atmosphere. Notice the very low dewpoints in our area right now.

 

Temps - Heat Index - DMA

 

Now it is time to look at the atmospheric sounding to see whether the atmosphere supports snow late on Thursday. Remember when looking at the sounding, the bottom is the ground and the top of the sounding is the top of the weather part of the atmosphere. The red line is the temperature, the green line is the dewpoint, and the blue line is the freezing line. It is critical to note when the green and red lines are far apart representing a dry atmosphere. The other important thing to note is when the red line is left of the blue line then that part of the atmosphere is below freezing.

 

NAM Forecast Sounding At 4 PM Thursday

Notice the NAM shows a very dry lower part of the atmosphere at 4 pm supporting a ton of evaporative cooling. That spread of the green and red lines show a very dry atmosphere.

 

Sounding 1

 

 

NAM Forecast Sounding At 6 PM Thursday

Notice the NAM cools the lower part of the atmosphere dramatically by 6 pm and the NAM shows an atmosphere that supports snow.

 

Sounding 2

 

As we look at AdvanceTrak, we see that it really agrees that we will see some rain mixing in with primarily snow. Notice it increases the snow in the metro around 5:30 pm and this sticks around through around 10 pm to midnight. The surface temperatures generally appear to stay above freezing during this entire storm. Note, the timestamp is on the top right part of each image.

 

AdvanceTrak 1

 

AdvanceTrak 2

 

AdvanceTrak 3

 

AdvanceTrak 4

 

AdvanceTrak 5

 

AdvanceTrak 6

 

 

Snowfall Totals

 

The NAM, GFS, and AdvanceTrak are generally similar but the EURO is the odd one out on this storm. To be honest, the GFS style solution seems to be the most reliable thus far and it has shown a good track record with snowfall totals this year. Take a look at the totals noting the name of the computer model is on the top right part of each image.

 

EURO

 

GFS

 

NAM

 

AdvanceTrak Totals

 

 

My Thoughts On Snowfall Totals On Thursday

 

One of the biggest complexities with snowfall accumulation forecasts is how the snow will stick or melt on pavement / grass / elevated surfaces. There is no conceptual model to allow us to scientifically determine the characteristics of how snow will accumulate on the ground. We have very warm pavement temperatures area wide right now and that will undoubtedly erase much of the snow that falls on the pavements. Notice all the upper 40s - 50 degree pavement temperatures area-wide.

 

Pavetemp

 

This creates the complexity everytime we have to forecast snow. A couple of things stand out on this storm... we will have temperatures above freezing during this storm and there is very warm pavement temperatures. In a storm like this, it means accumulations would primarily occur on elevated surfaces or on the grassy surfaces. The atmosphere will be the coolest along and east of I-65 and to me that area looks like the best candidate for up to  1" on the grassy surfaces Thursday night. It is possible a few roads could have snow on them outside of this city but it would have to occur in a snow band. That is something we will watch for.

 

Timing is from around 5 pm - 11 pm for the metro with about 1 hour earlier in the northwest part of our area and as much as two hours later in the southeast part of the area.

 

 

 

 

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Comments

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I really wish we would get one big snow and get it over with,,,,

Barbara, that is not easy this late in the year.

Marc,

Do you see any sign of Spring temps rolling in consistantly in the next few weeks around 60s-70s without it having a cold front moving in behind it (like last weekend). Looks like the Groundhog was right so far.

Thanks!

Jason, honestly no. The signs show some real swings in the next couple of weeks.

Groundhog right? He was WAY wrong. After his prediction we saw huge snow storms in the US. We have been at or below normal for 22 of the last 28 days. I think that may have been the worst groundhog forecast ever.

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