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Welcome to the Multimedia Impact Weather Briefing for Central Alabama. Today is Sunday, February
9th, 2014. This briefing will continue to focus on the potential for a wintry mix across
portions of north Central Alabama Monday evening into Tuesday morning.
As we discussed yesterday, precipitation will return to Central Alabama beginning on Monday
as yet another upper level disturbance tracks eastward across the Ohio/Tennessee Valleys.
Unfortunately, this will bring another opportunity for some wintry precipitation across the northern
portions of Central Alabama in the Monday evening into Tuesday morning timeframe. At
this point, our confidence is increasing in the chance for the wintry mix, but precipitation
type and accumulations are still uncertain. Based on forecast soundings for locations
generally along and north of I-20, there could be multiple transitions between precipitation
types. Therefore, some snow, sleet and freezing rain look possible for many areas in the north.
Although models seem to be coming into better agreement at the moment on the location of
the freezing line at the surface, there are other factors that have to be considered...for
instance, temperatures aloft. Therefore, please realize that the line between the wintry mix
and all rain could easily be shifted farther north OR south. It will be imperative that
you check the latest forecast and products frequently for any updates on what can be
expected in your particular location. With this event, we know that there will be
precipitation, but determining exactly which types will be present and which will be predominant
is quite difficult. I want to take a moment and try to describe some of the factors that
come into play when determining precipitation type. We'll take a look at a few forecast
soundings for the upcoming event to see what they show. If you're not familiar with soundings,
here is a brief overview. A sounding depicts the conditions of the atmosphere vertically
from the surface (bottom) up to the top of the troposphere (tropopause). The line farthest
to the right (red) indicates the temperature, while the line farthest to the left (green)
indicates the dew point. When both of these lines are very close together, it indicates
moist conditions. The wind barbs on the right of the image show the wind speed and direction.
I have the 0°C line highlighted in black. Looking at this first sounding, we want to
start by looking to see if there is any moisture in what we call the snow growth region (between
-10° and -30°), which is the area between the 2 brown lines I have drawn. As you can
see, the lines are right next to each other (very moist). This means that there is enough moisture for ice crystal growth.
Next, we have to see if there are any warm layers between here and the surface. Between
roughly 900mb and 700mb, the temperature line is well to right of 0°C (black line). I have
highlighted this warm layer in red. In this sounding, the warm layer is warm and deep
enough to melt the ice crystals. So, now we have rain. Now, is there a cold layer that
could refreeze the rain before it hits the surface? As you can see, the tempertaure line
goes right to or just below 0°C near the surface. I have this area highlighted in blue.
This cold layer isn't deep enough to refreeze the rain before it hits the ground, but with
temps at freezing at the surface, it will freeze on contact with the ground. Therefore,
this sounding
would indicate freezing rain. Let's take a look at another example. Here
we still have enough moisture for ice crystal growth, and there is still a warm layer. Notice
though that this warm layer isn't quite as deep and warm as the previous example. This
means that some of the ice crystals could melt while others only partially melt. The
temperatures near and just above the surface are a bit colder than before as well. This
sounding shows that precip at the surface could be in the form of freezing rain and/or
sleet. Finally, let's go over one more example. Again,
plenty of moisture for ice crystal growth. The big difference here is that there is no
longer a warm layer. The temperature line is now completely below freezing. In this
case, none of the ice crystals melt before reaching the ground. So what type of precip
would this be? You guessed it, snow. I know some of that may have been a bit confusing,
but the main point is that forecasting precipitation type is not as easy as we would like it to
be. All three of the soundings you have just seen are what the models are currently forecasting
for locations along and north of I-20 for Monday evening into Tuesday morning. Therefore,
at this time, be prepared for a possible mix between freezing rain/sleet/snow. We will
continually be analyzing the latest data to try and narrow down the most likely precipitation
types for each part of Central Alabama and any potential accumulations. We just want
everyone to keep in mind that this is a fluid situation, and updates/changes to the forecast
will be necessary. Look for more details over the next 24 hours.
Thanks for viewing this briefing. Just as a reminder, for rapidly changing situations
and while events are ongoing, please see the graphicasts on the front our website for the
latest information. If you have any additional questions, please give us a call at 205-664-3010.