Occurring currently,will be in the near future (also the following day)
Occurring currently,will be in the near future (also the following day)
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.
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.
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.
Well here we are, the second week of August. What a change from a few weeks ago! Of course, I had already highlighted the shift in pattern back in July. Many signs pointed to at least an average or slightly below average August. Thus far that is on track. For more information check out http://moewx.com/?p=365 . Lets have a look at the bigger picture for the weekend. I will be away from the computer this weekend. In order keep you up to speed with the weather I am doing a weekend planner for you right here!
What changed? The easiest way to explain is by going up in the atmosphere and inspect the upper levels, the jet stream (Pictured below) has huge impact on short-term climate, and it has come to visit a bit early for fall. The Jet Stream as you remember is a discontinuous river of air high in the atmosphere, it is very important in forecasting especially looking at the synoptic (Regional) scale pattern.
Why is this important? When talking about precipitation, Meteorologist look for “lift”. What makes air rise. There are multiple ways for this to occur, however when looking for synoptic scale lift. One of the first place to look is the Jet Stream. There are others, but we’ll keep it simple. For this weekend you can cut out a ‘ridge’ (Arc) in the West and a ‘trough’ (U-shape) in the East. The trough jumps out because it is a Jet streak, which are areas of enhanced wind flow. They aid in strengthening a surface low pressure. It will also aid a Mid-level shortwave into the Midwest starting Friday, where there will be severe weather back to the west.
UPDATED 12:23PM 08/12/11
SPC has added a SLIGHT Risk in for Saturday. The set up remains largely the same. And my concern with limits remains as well. Indiana is not in the most favorable position in terms timing with daytime heating/instability. In addition there may be cloud debris or remnant ongoing showers by Saturday morning limiting the atmosphere ability to recover. However because of the larger scale lift mentioned above in association with the Jet Streak (pictured above) It may support an isolated chance of a few stronger or severe storms with damaging straight line winds and small hail being the main concerns. GFS continues to show a relatively stable boundary layer going into Saturday with ongoing convection and showers. In addition, the trend has been to slow the system down. For now better severe weather chances will extend from Central to Southern Indiana including the entire Tri-State area, where there will be a weak, but present low level jet able to fuel some instability during the late afternoon.
That said, rain totals will be the primary concern this weekend… or blessing. The entire state could benefit from a soaking
Here is the latest QPF Output from GFS 00z 08/12/2011
Notice most if not all of Indiana could see at least .75 to 1.00 inch of rain through 7:00 PM CDT Sunday. Locally there could be higher totals, if you get under one of the stronger thundershowers. Certainly looks promising for the mini drought. Lets keep out fingers cross this one works out.
SPC has a HIGH Risk for Severe Thunderstorms for Wednesday for the entire Tri-State and a good portion of Central and Southwest Indiana. Including Evansville, Terre Haute and Indianapolis Metro areas. We are especially concerned about pre-frontal convection and supercells in the late afternoon. These storms are monsters, and they will have a LOT to work with tomorrow. Damaging straight line winds, very large hail and strong tornadoes are all game Wednesday. Details and meteorology is all below…
Easiest way to do this is start from the top of the atmosphere and move down. I will start by analyzing the forcing at work. My intent is to completely diagnose the concern, and hopefully you learn a thing or two on the way. A much more simplified version will probably go up on www.fb.com/MoeWx , which by the way if you haven’t yet. Go and LIKE that page for updates on Facebook. And of course after 5:00 PM CDT. I will be on Twitter for coverage. Including LIVE coverage here on MoeWx.com. If the need arises…
The image to the left shows THREE different levels or heights in the atmosphere. The Jet Stream at 300 mb – It is represented by the blue and purple color fill. I will reference it as “300 mb”.
The Second level the mid-level in the atmosphere or 500 mb. It is represented by the yellow contour – I will reference it as shortwave energy or the main cold front.
and the wind barbs are the 850 mb level. It is located in the lower levels of the atmosphere. 1 tick = 10 knots.
Synopsis: There is a large upper level trough at 300 mb swinging across the Plains at this time of typing this that is punching across Oklahoma and Northern Texas. As a result a very strong low pressure has developed at the surface right along the nose of the feature… tomorrow as the energy rounds out the trough and shoots the main surface low NNE across Central Illinois. There will be a stark increase in low level moist flow via the low level jet. The image to the left shows 1:00 PM Wednesday. As this process occurs by early afternoon. 12z GFS has a robust 40 – 45 kt ( or 46 to 52 mph) – pretty strong flow. This will all be occurring over a steaming and very unstable boundary layer (below) here at the surface. the 300 mb upper level jet, though weakening compared to Tuesday night, will support a powerful cold front as much cooler air settling into the Plains with sinking heights and stable air surges east behind this whole complex. As a result the heights in front of this complex will rise substantially ahead of the trough in response.
Here is a shot at 1:00 PM 12z GFS for Skew-T. Basically this is a profile of the atmosphere, but gives a good picture of what condition could be like by early afternoon Wednesday. I’ll break it down.
Main things I want you to get from the right image is the amount of energy CAPE. That is available for the storms to work with… approaching 2,000 J/KG even higher in the Mixed Layer… Lifted Index ( basically the lower it is the less ‘work’ or forcing is needed to make air rise…) it is at -6. That is pretty low. On the right with the lines, notice there is a thin gray line at 5,000 ft. That is the ‘cap’ by Wednesday Afternoon. this will suppress any development up to that point, but it is not very strong. It will not hold. Basically by 1~2 PM I expect things to be buckling at the seams. Once it goes it will be explosive and things will turn bad in a hurry in the MODERATE zone or HIGH risk if one is issued, and I am pretty sure there may be one at this point. one thing that does look promising is that there is a lot of shear but not a lot of torque or turn to the wind fields ( on the far right) there is some and any supercell is capable of producing tornadoes. Don’t let your guard down.