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The Why Behind The Forecast
Dangerous Severe Weather Threat Friday
0I have tried to remain optimistic models would trend away from a nightmare scenario, but the continue to point towards a very grim reality starting first thing on Friday morning.
Synopsis Part 1: There is a large scale upper level trough currently over the rocky mountains there will be a shortwave trough rotating around the base of the mean trough. As it drops into the Plains early this evening it will aid in forming a surface low in East Central Oklahoma. Cyclogenesis will be rapid. As it develops, it will send a surge of warm moist air north along a warm front which is currently stalled out from yesterday’s cold front in northern Mississippi/Alabama. This warm frontal boundary will rapidly surge north as a surface low rapidly deepens starting in NE Oklahoma and races to the NNE towards Chicago. 12z NAM and 00z GFS depicts thunderstorms erupting along and north of this warm frontal boundary by 6:00 AM Friday morning due to the rapidly evolving dynamics in the morning. These storms now also have the potential to be strong to severe Friday morning. While we are dealing with potentially severe storms early, there will be stout SW low level jet regime developing and rapidly surging the boundary NNE into Central Indiana. (pictured above)
Synopsis Part 2: This appears to be a two part event for the Tri-State one early morning with the warm front (above) and then another event from the trailing cold front in the afternoon before quickly transitioning east into SE Indiana and Central Kentucky. This is where it gets tricky and the models diverge. As the Surface and 850mb low continue to deepen (get stronger) and move NNE. The Tri-State will be placed firmly in the warm sector by 12:00 PM in the afternoon. At this point we will be capped. That will shut off any remaining convection. Strong SW wind will kick in and skies may even clear out, which is bad… the sun will add ample support to an already very discerningly violent set up. We measure this energy with a parameter called CAPE - Convective Available Potential Energy. That simply means IF storms develop, what will they have to work with. It is very important to understand that just because there is high CAPE does NOT mean there is a guarantee for severe weather! This IMPORTANT to understand. It is however a red flag in the correct set up. That said lets take a look at these values as this is what will make or break the magnitude of the event. Models disagree here. NAM model has been consistently off the charts in regards to instability. To save multiple maps and pictures, and your time I will post one image that has everything we need it is called the Skew-T: 
WARNING: This is valuable information, but not non-meteorologist friendly. But a necessary tool to assess the situation.
The first thing that should jump out id all the ‘red’ on the left. These are severe weather parameters. CAPE we discussed earlier is the top value. This screenshot is taken just before storms erupt on NAM 12z model in Evansville, IN at 3:00 PM CST. NAM is showing a robust and extremely unstable airmass in this image. 1800 J/kg is a scary number to see in early March. Its common in the summer is not higher. Moving moving down, EHI: Energy Helicity Index. used to measure the potential for tornadoes. It is a normalized value, anytime this goes above 1 we get concerned. Its at 2.99. LI - Lifted Index: this is a measure of how buoyant a parcel of air will be if there was no forcing (a front) involved. In other words if I have a section of air and I released it randomly in the atmosphere will it sink or float. The lower this value the more concerning it is. -6 is significant. the rest for the purposes for this outlook will be omitted. The other thing on this image that should jump out is the pink on the right. This is a measure of wind speed as you go UP in the atmosphere. This is what we call extreme speed shear Winds are blowing at 60 kts or ~70 mph at ~6,000 ft off the ground… on top of that we also have winds blowing at 127kt or ~150 mph at 30,000 ft. This creates a vacuum environment. Then air is moved out of the way aloft the air form the surface has to rush in to replace it, thus ENHANCING the already very unstable lift described above. This again is a recipe for disaster. You will also notice that as you get toward the bottom of the image you can see where the lines seem to move (and change color)? These are measure the direction the wind is coming FROM. We don’t like to see this change with height (as you go UP). Because that will support rotating updrafts or what we call Mesocyclones,(strong rotation aloft) which, if brought near surface can spawn large and very dangerous tornadoes… One light note though, the winds are not as veered as we need to see for the more significant threat for the long track tornadoes, but keep in mind this is just a model and that is something on a mesoscale… may not pick up reality.
Summary: In the interest of time. I will leave this here and summarize. I may ADD to this post later tonight after work, so look for an update. But I think the tone is clear. We are very concerned about am extreme severe weather event tomorrow. I am personally hoping and praying for a bust and this does not happen, but the data suggest we better get you the public prepared. Please if you do not already and I cannot stress this enough. GET A NOAA WEATHER RADIO! This is not a negotiation, this is not up for discussion. It will save your life. I am begging you I don’t want to see more lives lost. Be weather aware. Small updates will be on the fan page: if you have not LIKE it www.facebook.com/MoeWx Right now I will be at work during this catastrophic event I am trying to change that. For now best you can do is be prepared for the worse and and respond quickly if a Tornado WARNING is issued. It is likely we will see a PDS – Tornado WATCH by tomorrow. Particularly Dangerous Situation. Maybe even a HIGH risk for severe weather would not shock me Not sure where yet, but its possible Probably the eastern half of the Tri-State if anywhere.
Possible wintry surprise for Western Kentucky
0NAM 12z likes Kentucky this morning. *This is not a forecast* (yet), but we’re watching it closely. Western KY could steal the show again this year with snow! It happened last year, its possible. Last night the forecast models ‘corrected’ themselves with the additional data that was added to them from the data dead zone in the eastern Pacific Ocean. There is a post on the fan page pertaining to the importance of the data obtained in the additional flights sent out by NOAA yesterday I encourage you to watch it. One of the corrections took a strong coastal low and shifted approximately 300 miles NW which places a surface low in northern Alabama, Mississippi and Georgia. This places a swath of heavy wet snow across the Tennessee Valley and lower Ohio River Valley on the southern portions of the Tri-State. 
Why do I like this more than the other set ups, this one is actually unique, no two storms are really the same. The upper air characteristics and enhanced support has my attention. As I have said many time before I never tell you anything without looking at multiple things to back it up. While I typed this the 12z GFS model run was rolling. It looks similar, and lines up with the previous mentioned model runs but has a different opinion on the snowfall aspect, but worth watching. It has been the warmer solution of the bunch. This will be a tricky set up in that the cold air looks to be in place this weekend mainly because of the storm that is coming through today. Profiles support the idea. I took the Skew-T from Bowling Green, KY which appears to be the bullseye for now. This has the potential to be a big one. But as I always say, it has to hold! (And personally, I am ok with a slight NW shift)
The above diagram shows a moist column through 15,000 feet in addition temperatures at or below freezing through the entire column. The position of the low will support all snow as cold air advection will be taking place at this time period. Further making the set up legit. Question is, where? is this done shifting NW ? Keep it here we’ll keep you posted. I should note, if this shifts at all to the NW over the next 3 days most of Tri-State will get nailed with this. We’re watching it closely.
One other thing that has our attention with this storm as I mentioned above is the mesoscale convective banding. The feature showed up on ECMWF model last night. and now on NAM and GFS. though it is important to note GFS is still a few miles SE of the NAM/ECMWF/CMC (Canadian) solutions. This definitely has our attention. We’ll have more details as they become available keep your fingers crossed!
Short-Term Update
0102 days
by moewx
in Uncategorized
Good Evening all. I first want to apologize for the lack of updates! The weather has just been that quiet lately. If this blog is silent you can consider that a GOOD thing, if you do not enjoy active weather that is. Nevertheless, I wanted to put about a quick short-term update for the overnight into Wednesday. I’ve been discussing this vigorous upper level low pressure for a while now. It is located just west of the Mississippi River Valley and will rapidly progress east over the next few hours. Below is a radar snap shot of 10:00PM CST. It looks more deceiving than it is much of what is over Southern Illinois and Southwest Indiana is virga (not reaching the ground).
It will take a bit of time to moisten up the entire column I took a screen shot of the Skew-T from 02z RUC KEVV Evansville (below), which shows a 3,000 -4,000 foot dry layer which will inhibit much in the way of precipitation initially. But shows a much more moist layer by about 5:00 AM. By then 0°C will be just off the surface, just enough to help with some snow by the morning hour. Looking for generally under an inch, but it will be enough to cause some traffic problems. I think .25 – .40″ inch of snow will be common. I think the models are just a hair behind on the temperatures, we’ll be right around freezing here at the surface, maybe slightly above. For now we wait and see what this little storm has in store for us. Enjoy 
Wintry Weather to start the weekend.
0120 days
by moewx
in Short-Term
Good Morning all, I thought I’d go ahead and throw up a short-term update over the next 24 hours as location will be very important in terms of weather and precipitation type and placement. The event we are dealing with will be called overrunning, or the term isentropic lift. And to quickly define that all that really says is how the moisture is being lifted into the atmosphere. Main principles you need to understand about today will be that cold air is more dense(heavier) than warm air. Thus, if warm air is forced into an area of dense cold air it will first lift up and over as if there is a “ramp” in place. That is what we will have over Indiana going into this evening. Cold dense air is already in place. Go outside or take a look at the thermometer if you don’t believe me! Anyways, on to the forecast!
Lets take a look first at the pieces to this puzzle:
The above map shows the mean(average) upper level show, and for the most part it’s actually fairly zonal (west to east) with a very strong and fast flow going from west to east, however within this flow are waves of energy (vorticity) Red is positive and Blue is negative. We’re specifically focusing on the red approaching the Midwest. And that is because this will be associated with the life that will ultimately occur over the next 24 hours. Before spreading East, where I think things eventually turn for the worse for the Upper Ohio Valley. The map I got from Oklahoma State University: http://hoot.metr.ou.edu/upperair/vadvec/305 It was the least ‘busy’ map I could find to describe this event. It is a very good resource if you want another tool to assist in weather forecasting! Next factor will be that stationary front that will be across the upper Tennessee Valley that is the separation between the warmer moist air to the south and cold dense air to the north. It will meander a bit north over the course of today as we get the warm return (red arrows) as the disturbance (dashed line) moves in from the west. As the warm air moves in this evening it will run right into this cold air that is in place, and it will move up and over the cold air, thus the cloud cover this afternoon, and when the disturbance arrives it will become precipitation. Here is the tricky part.
Easiest way to describe this is by use of a “Skew-T” below I will highlight what you need to know.
This is a snap shot of 00z Model run last night for the GFS model. Data is from KIND in Indianapolis for about 6:00 PM EST. Today. And shows by this point the overrunning will be underway by this point. The SURFACE temperature will remain below freezing through the entire duration of this event. However, if you look on the chart you’ll notice I pointed out “Warm Air Aloft”, when you go up to about 2 -3,000 ft in the air it is ABOVE FREEZING! How is this possible. Remember, the first paragraph, I mentioned the one principle you will need to understand for this whole event. This is what it looks like on a Skew-T diagram. This set up will either produce Sleet or Freezing rain. The depth of the cold layer in this case suggests Sleet across The I-70 corridor.
Here is a horizontal map view of today’s distribution.
Keep up with Interactive Radar for the latest advisories and forecast if you’re in between the black lines going into the evening hours it will be very dangerous and slick this evening. Do not travel unless you absolutely need to.
Short-Term Snow Update
0128 days
The arctic front has cleared most of the region, if it hasn’t it will over the next hour or so. Temperatures will rapidly decline over the rest of the afternoon and early evening and the winds will increase with time. As those temperatures fall, there will be an increased snow:liquid ratio. If you remember the last two events had wet snow with them with typical 10:1 which simply means, for ever 1 inch of rain that falls you will get 10 inches of snow. The colder the atmosphere is, the drier the snow, the easier it is to accumulate. We are expecting the “fluffy” snow for this event due to the arctic air moving in. In terms of timing, I think this will be a bigger factor to the North across Central and Northern Indiana. But It will squeeze out an inch maybe up to 2 inches across the Northern tier of the Tri-State, generally north of I-64.
Just a little current analysis: Front has passed and we have a cold air mass moving in via a potent shortwave trough (Indicated by the color fill) That is what we call positive vorticity advection. This is a clockwise spin in the atmosphere measured at 500mb, or 17,000 ft in the air, that aids lift mainly to the east. To measure that lift, I also plotted the 700 mb(~9,000 ft up) Omega values. Negative Omega means air is rising, the lower that value the stronger the lift thus precipitation. Conversely, the positive values mean air is sinking. Things will quickly stabilize as this feature moves over head and it will. Its coming NE towards that Tri-State. Unfortunately I will be at work. I will not be available to do much in the way of nowcast. Either way, wheels are turning for this event. It won’t amount too much down here in the Tri-State, but we’ll take it! Generally 1-2 inches expected here. And 2-4 inches across most of Central Indiana and 3-6″ across Northern Indiana especially in Lake Effect zones.
