Where was the Snow?

The snow forecast last night for Lexington had meteorologist predicting anywhere between 1-3 inches to some meteorologistpredicting 4-8 inches. The differing was due to a complicated setup that lead to a difficult forecast. The West side of Lexington got less than half an inch while the East side of Lexington has reports of a little over an inch. We didn’t get as much snow as was expected because the surface temperature wasn’t able to drop below freezing long enough for the snow to accumulate. There was not cold air advection (the process in which the wind blows from a region of cold air to a region of warmer air) with this storm, if temperatures were going to dropbelow freezing it was going to be because of dynamics. More specifically a process we call dynamic cooling, which is a positive feedback mechanism. Dynamic cooling occurs from rising air and heavy precipitation, which works to cool the air from the top down. As air rises it cools, which lowers the freezing level and causes snow to form aloft. That snow falls into warmer air below and melts. Melting is a cooling process because it cools the air around it by absorbing heat. When there is heavy precipitation more melting occurs aloft, which results in more cooling and causes the freezing layer to continue to drop and can reach the surface.

 

There are many factors that go into forecasting a nighttime low. Cold air advection (CAA), cloud cover, and dew point are a few. With this system CAA wasn’t present and there was heavy cloud cover associated with the rain, which traps outgoing radiation and keeps the surface warmer. For the low to dip below freezingdynamic cooling was going to be the driving mechanism. Temperature cannot fall below the dewpoint, when the temperature equals the dewpoint the atmosphere is saturated. For most of the night the dewpoint was above freezing, it wasn’t until the dewpoint fell under 32 degrees that the temperature was able to fall under freezing. The air temperature did not fall under freezing until 5:05am when it was measured at 31.9 at the Fayette co Mesonet site. At this time the soil temperature was 32.9 degrees and the wind was from the SW at 3mph. This tells me CAA was not the reason for the temperature drop because wind from the south is warm or warm air advection (WAA). Dynamic cooling dropped the temperature of the column of air below freezing, but was unable to cool the surface enough for snow to accumulate. 

 

As we can see from the Kentucky Mesonet, the low stayed above freezing or right below it for most of the state.

 

Dynamic cooling started cooling the atmosphere below freezing around 10pm and it can be seen on radar. 

 

 

Radar was showing snow but people would look out there window and see all rain and wonder where the snow was. Radar is not able to see at the surface, the farther away from the radar site a specific location is, the higher up in the atmosphere the radar is sampling. For example in Maysville the radar is showing snow, but at the time there were reports of all rain. The NWS radar in Wilmington samples the atmosphere in Maysville ¾ of a mile above the surface. There was snow aloft overMaysville, but it was melting back to rain before it reached the surface. Lexington Airport received 1.05 inches of rain from the system yesterday. If it would have been all snow using the 10:1 ration we would have gotten 10.5 inches of snow from this event but that was not the case we only ended up with a trace on the West side of Lexington.

 

 

Temperature Inversions

The plume from Buffalo Trace Distillery can’t rise vertically because of the inversion layer

Driving into work this morning, the plume rise from the Buffalo Trace Distillery caught my attention because it was being trapped by an inversion layer in the atmosphere. Usually the atmosphere cools with increasing height, but a temperature inversion, or inversion layer, is when the air gets warmer with increasing height. Typically the sun heats the Earth’s surface and air around it, warm air is less dense than cold air, so that air rises until it cools to its surrounding temperature. This rising motion is called convection. With a surface inversion, the air at the surface is cooler than the air above it so it cannot rise. The consequence of this is that air pollution can get trapped near the surface, and have a serious heath effect on people. For example in London in 1952 a significant amount of air pollution got trapped near the Earth’s surface and caused 4,000 premature deaths and made up to 100,000 people sick. Luckily because of the EPA and Clean Air Act we do not see events like this in the US, but China and India have major air pollution problems that cause health issues.

 How do Inversions Form?

There are different types of inversions but this is an example of a radiation inversion. Radiation temperature inversions are typically at night when there are clear skies and calm winds. The Earth cools by putting off terrestrial radiation. To form an inversion there needs to be clear skies because clouds reflect a portion of the terrestrial radiation back to the surface which keeps the boundary layer warm. Calm winds are important so there isn’t mixing. When you combine clear skies, calm winds, and the outgoing radiation you get maximum cooling in the boundary layer so the air there is cooler than the air higher up in the atmosphere and an inversion forms.

 This morning is an example of a strong inversion. We use a thermodynamic diagram, called a sounding, to look at the vertical profile of the atmosphere. The red line is the temperature and green line is the dew point. When the temperature line moves right it indicates warming. As we can see at the surface the red line sharply goes right, that indicates the inversion layer.  

Inversions and Thunderstorm development 

Capping inversions are important in the development of severe thunderstorms. A capping inversion is when there is a layer of warm air in the atmosphere that acts to shut off convection (upward movement). When the cap is broken, either by extreme convection or a lifting mechanism like a front, the sudden release of the built up convection under the cap causes rapid, and usually severe thunderstorm to development. 

Pictured above is an example of a capping inversion. The red line is the temperature as you rise up in the atmosphere. The blue shaded area is the stable layer, The green area is the capping inversion. Notice how the red line turns right, that is how you identify an inversion.

How to forecast Snow/Sleet/Freezing Rain

The approaching winter storm will likely start as snow and transition to freezing rain and then to all rain. So how do meteorologist forecast precipitation type?  The main thing that determines the precipitation type is the temperature aloft.

Snow is just ice crystals aloft that fall and never unfreeze because the temperature at different levels it falls through is always below freezing, this includes the surface being at or below freezing.

Rain starts as ice crystals aloft but when it falls into warmer air a loft that is above freezing it unfreezes and turns into rain. The rain never falls into temperatures below freezing again so it stays in liquid form until it reaches the surface.

Sleet starts as ice crystals aloft like snow, but then falls into warmer air that is above freezing and melts into rain. The liquid then falls into a deep cold pool, where temperatures are below freezing and refreeze into ice crystals before falling to the surface which is also at or below freezing. (Sleet is NOT hail. The difference is how they are made a loft)

Freezing rain, like sleet, starts as ice crystals aloft and falls into warm air and unfreezes. The difference between the two is that the freezing rain doesn't fall until a deep cold pool of air below freezing, so it does not get the chance to refreeze. So the liquid reaches the surface, which is at or below freezing, as rain and freezes on contact.

                           Above is a diagram to show how each form of precipitation occurs.

So we now know warm air a loft is important in forecasting between snow, sleet, and freezing rain but how do we determine what the temperatures up in the sky will be? For this we look at a thermodynamic diagram, which we call a skew-T or a sounding.

Pictured above is a skew-t diagram which shows us altitude, temperature, dew point, wind speed and wind direction. We get these diagrams from launching weather balloons twice a day and then inputting the data from the balloons into the weather models that are run. The models then make their own forecast soundings, which are graphs like above for how they think the atmosphere will look like vertically at a particular place and time in the future.

The sounding above is a freezing rain sounding. The blue line is the 0 degree Celsius line, everything to the left of it is below freezing and everything to the right is above freezing. The red line is the temperature line and the green is the dew point line. Start at the top of the graph and pretend you're an ice crystal and follow the red line down. You will see you are above freezing until about 800 mbs up in the atmosphere. Then the red line goes to the right of the freezing line which means the air temperature is above freezing and you would melt into liquid. As you continue following the red line you go back to the left of the freezing line but do not have a chance to refreeze before you hit the surface which is also below freezing so you hit the ground as rain but freeze on contact.

Hopefully this gives you a little insight on how we forecast for the different types of winter weather!

Another Round of Severe Weather for the Area 5/21/14

The Storm Prediction Center has us in a high-end 30% slight risk for severe weather for tomorrow (5/21/14). Remember this means there is a 30% chance of severe weather within 25 miles of a point.

For severe weather we look for 4 main ingredients or SLIM. Shear, Lift, Instability, and Moisture. The setup for tomorrow will not be lacking moisture. The models are forecasting dew points (amount of moisture in the air) into the 70s. Honestly that is probably too high because dew points in the 70s are REALLY high even for summer (which we haven't hit yet) but dew points in the lower to mid 60s are likely and will provide ample energy for the storms.

The map above is a GFS run that shows instability (CAPE) for our area tomorrow. CAPE is often over estimated in the models. This run is showing values of 3000 J/kg for us, which is more than enough for some strong updrafts in the storms tomorrow.

Finally for shear. The forecasted skew-T seen above shows some good speed shear (changes in wind speed with height) but we don't see the directional shear (changes in wind direction with height) that we would need for supercell and tornado development. Also the freezing level (level where temperature = 0 degrees C) is around 600 mbs. Because of the high CAPE values and semi-low freezing layer we could see some hail tomorrow from these storms.

To sum it up. This will be a Wednesday evening, to late evening event. Strong winds and hail are the main threats with the tornado threat looking small.

Tornado Watch for the Area

The Storm Prediction Center has a SLIGHT RISK of severe weather for our area today. For this particular case there is a 15% chance of damaging winds and damaging hail occurring within 25 miles of any point within the slight risk area. Also there is a 5% chance of a tornado occurring within 25 miles in the risk area. Two different Tornado Watches have been issued for the Ohio Valley. The first watch, for central Kentucky, is until 8pm. The second watch, for Northern Ky extending into most of Ohio is until 9pm. A major severe weather outbreak IS NOT expected today but conditions are right for strong to severe storms to form with damaging winds, large hail, and ISOLATED tornados. Meteorologist tend to want to "cry wolf" or overstate the threat of severe weather which I try not to do. So to sum it up, slight risk of severe weather today with the main threat being damaging winds but an isolated tornado is possible. We have two different watches over our area until 8pm and 9pm and if you are put under a warning be smart and take appropriate action.

Watch 1

Watch 2 

Why is There a Temperature Difference Across the State?

        So far this week Maysville has been experiencing mild temperatures with highs in the low to mid 70s, while most of Kentucky has seen highs in the 80s. So why is that? The answer to that is actually quite simple, there is a stationary front located across Northern Kentucky and Southern Ohio/Indiana. A front is nothing more than a boundary between two different air masses. To the north of the stationary front we see cooler temperatures, while to the south of it we see warmer, and typically more humid conditions (but that isn't the case with this front). I got the picture below from the Kentucky Mesonet yesterday and nerded out because of how clear the front was from the surface observations.
         Two main ways to locate a front are by finding the temperature gradient and the change of wind speed and direction. The top part of the image shows the surface temperatures across the state, while the bottom shows the wind speed and direction. We can see temperatures in the high 70s above the line and temperatures in the low to mid 80s below it. The front is even more recognizable by the change in wind direction and speed. To the south of the front, where there is warm air advection, we see southerly flow while to the north we see northerly flow.

So what does this stationary front mean for our weather over the next few days? For Tuesday we will see the same sunny and mild conditions we saw on Monday. The front will lift north as a warm front Wednesday and temperatures will climb into the mid 80s for Wednesday and Thursday. These warmer temperatures wont last for long as a cold front sweeps through the area this weekend, which means the return of rain and temperatures back down into the 70s. 

First Moderate Risk in 320 days

The Storm Prediction Center has placed the Ohio Valley under a moderate risk for severe weather Wednesday evening into Thursday.

  

 We are under a 45% hatched area. So what does this all mean?

A moderate risk is not rare for the Ohio Valley, but it is not common. This is the first moderate risk for our area in 320 days. The storm prediction center's probability is the probability of one or more severe weather events occurring within 25 miles of any point. The hatched area means a 10% chance or greater of significant severe weather occurring. Significant severe weather is defined as hail 2 inches in diameter or larger, winds 75 MPH (65 knots) or stronger, a tornado of strength EF2 or stronger. 

I'll try to not get too technical here. Low pressure is going to move out of the Mississippi Valley tomorrow. A warm front will move north into northern Ohio and some storm might pop up along that. We will be in the warm sector and there is a slight chance for a pop up storms but we should remained capped. The strong storms are going to form in front of a cold front and will likely turn into a squall line. Damaging winds will be the main threat but a tornado can not be ruled out. I expect to see a few tornado warnings issued tomorrow from rotation on radar, but not a tornado breakout.

The models are in pretty good agreement that the dynamics are there for strong storms. What they don't agree on, is timing. This should be a Wednesday evening into Thursday morning event. We should get a better idea of timing tomorrow. 

To summarize, keep an eye on the weather tomorrow but it is nothing to be scared about. It should be an evening into early morning event with damaging winds being the main threat. I'll try to give some updates tomorrow.

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