Northwest Florida Weather (1-3-2017) - A 4-Letter Word, and A Word of Caution

After dealing with severe weather on Monday (which included 4 fatalities in Rehobeth, Alabama), we are looking ahead to the next system. Unfortunately, this system is incredibly complex, and brings with it the 4-letter "S"-word that causes uproar in the South and strikes fear and joy alike in the hearts of its residents.

That's right... SNOW.

12Z (6 AM CST) run of the GFS valid for 6 AM Saturday morning showing precipitation type and rate. Image from TropicalTidbits.com

12Z run of the GFS showing accumulated snowfall through 12Z (6 AM CST) Sunday. Image from TropicalTidbits.com

Now, if you've been browsing Twitter or Facebook, you may have seen an image similar to one of the images I posted above. Unfortunately, many see this map and take it at face value. However, there are several things you need to consider before taking this map for what it's worth. Don't get me wrong, maps like this are useful, but only if they're used correctly.

It's Not All Snow

One big problem with the GFS is that it cannot tell the difference between snow, sleet, freezing rain when totaling up snow accumulation. So, it's hard to know exactly how much snow will fall, how much sleet will fall, or how much ice will accumulate. Thus, totals are often inflated more in these messy mixed-precipitation events. To determine our precipitation type (abbreviated P-Type from here on), we have to look at one of my favorite things, a skew-T.

Skew-T taken from the GFS model near Montgomery, Alabama, for 6 AM Saturday. Thin red line is the temperature through the atmosphere, and the thin green line is the dew point profile. This skew-T (and the next 4) is from pivotalweather.com, with my annotations drawn.

So, allow me to explain this as briefly as I can. We have to start in the mid-levels of the atmosphere to see if there is enough moisture to produce snow, sleet, etc. We look for where the temperature is between -12 degrees Celsius and -17 degrees Celsius (snow growth region), since that is the optimal ice crystal growth area. If that has enough moisture, then we can assume that ice crystals can/will form. In Montgomery's case, it's completely saturated in the snow growth region. Proceeding downward in the atmosphere, we look for any layers that are above freezing (0 degrees Celsius) that could melt the ice crystal. In this case, there aren't any. Next, we look at the surface temperature to see if it's below/at/above freezing. For Montgomery, the GFS says that it is below freezing. Thus, the ice crystal never melts, and the P-Type is likely snow.

This skew-T is from Evergreen, Alabama, valid at 6 AM Saturday. As with Montgomery, the snow growth region is forecast to be completely saturated. Further down in the atmosphere, there is a slight warm layer that is above freezing. This will allow for some melting, but not enough to fully disrupt the ice crystal. It then passes through a sub-freezing layer, which refreezes the partially-melted crystal. The surface temperature is forecast to be below freezing. All things considered, the likely P-Type is sleet (ice pellets).

This skew-T is from Andalusia, Alabama, at 6 AM Saturday. As with the previous 2 soundings, the snow growth region is completely saturated. However, there is a deeper warm layer that warms to about 1 or 2 degrees Celsius. This may be enough to completely melt the ice crystal. It then passes through a shallow sub-freezing layer. If it only partially melts, it may refreeze and fall as sleet. If not, then it will either be freezing rain or just rain. The surface temperature is forecast to be at or just above freezing. If the surface temperature is at freezing, then the likely P-Type will likely be sleet (if it refreezes just above the surface) or freezing rain (if it melts completely). If the surface temperature is above freezing, then the P-Type will be plain old rain

Crestview and Destin are very similar to each other on the skew-T's. Both have lower moisture in the snow growth region. If anything forms, ice crystals in both locations pass through a substantial warm layer, which will melt any ice completely. Both soundings have little to no sub-freezing layer, so no refreezing will occur. Also, both locations have forecast temperatures above freezing. So, according to this, the P-Type for both locations would be (a very cold) rain.

So that covers precipitation type, but there are two more things that I want to cover.

Run-to-Run Inconsistency

An animation showing the last 12 runs of the GFS valid for 12Z (6 AM CST) Saturday. Animation from TropicalTidbits.com

One thing we meteorologists look for is some form of consistency in the models from run-to-run. The more it stays the same, the more confidence we can have in the forecast. Over the last 12 runs (3 days), the system that is forecast to impact the Southeast this weekend has jumped all over the place from run to run, which raised questions about what would actually happen. Only in the last few runs has it become more consistent, and confidence has increased a bit. Being 4 days out, changes are still possible, so we can't take any one run of any one model and run with it. We have to watch for consensus among the models and consistency within the models.

Know Your Sources

With the advent of social media, anyone (and I mean anyone) can get online and post a picture of a computer model. However, the difference is the explanation behind it. Now, I know some people who post these things are truly fascinated in the weather and possibly want to pursue their dreams. I know that they want to be like the meteorologist he/she admires. I think all of us meteorologists were there at some point, trying to mimic what our favorite meteorologist did. Others, however, do it for the hype behind it. They try to get as many likes or shares on Facebook and Twitter as they possibly can by using clickbait and over-exaggeration to get people to see it and believe them. I recommend that, if you see a post that you aren't sure about, ask a meteorologist who will have more insight on the situation. I can guarantee you we are looking at the same things you are and that they are. Just be careful who your sources are. I wrote a piece on this a while back when talking about the tropics, but it still applies. Read it here.

So, for the TL;DR (too long; didn't read). It looks increasingly possible that wintry weather could impact the Southeast. However, be wary of model snow maps as they are not always what they seem. Also, know your sources, and make sure you trust them enough to provide you with important information.

I do not think we will see much, if any, wintry precipitation in Northwest Florida at this time. If any were to occur, I think it would likely be in the form of freezing rain, especially closer to Century and Jay. The GFS Ensembles valid at Eglin AFB, Florida, only gives a 5% chance of measurable freezing rain (shown below). But, the forecast can (and probably will) change, and I will update my Facebook and Twitter to account for any changes.

from: http://www.emc.ncep.noaa.gov/mmb/cguastini/gefs/EMCGEFSplumes.html