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April 01, 2011

Improved Weekend, but Severe Weather Threat Looming...

A weak system will be slipping across the area today that could cause a few showers and thunderstorms, but this system is not the "show". The big story will be a huge warm-up for Sunday and possibly a severe weather outbreak on Monday. Our discussion will heavily focus on the weekend and impending severe weather situation for Monday.


Today's Clipper Style System


We have a weak clipper system that will be moving across the area late this afternoon. That system has the chance to produce some showers and possibly a clap or two of thunder. AdvanceTrak is doing a good job showing some of the showers and storms late in the afternoon. Here is a look at 7PM this evening from AdvanceTrak where you can see some of the scattered storms.




This is a quick moving system and by 10 pm most of the rain chances will be done. If you have early evening plans, then an umbrella may be a good idea just in case.


Severe Weather Discussion


As we start to look into this next storm, we will have to deal with both the warm front and cold front on this storm. The warm front will be pressing across the area on Sunday and usher in much, much warmer air to Kentuckiana. With that warm front, some isolated showers or thunderstorms could fire, but again I want to emphacize it does not look like an all day rain in any way on Sunday. Actually Sunday will be partly cloudy, but you just may have to dodge a storm or two during the day and it will be warm as temperatures soar in to the 70s.


Monday is the day we have been talking about for 5 days now and will be the main focus on this discussion. Watching the computer models for a number of days now has been interesting. We have seen a few changes in some of the fine details from the computer models over the last 24 hours, and we have seen some incredible consistency within the computer models as well. Severe weather is delicate, so the small details become incredibly important. As we close in on the event, I will be able to indentify more and more of those details as we refine our forecast within the coming days. The GFS has been shifting the time of the storms arriving in our area as it has been creating a stronger and deeper upper level low. This could put the storm time frame in our area anywhere from late afternoon to early overnight on Monday. For the most intense severe threat, we would like to see those storms move through late afternoon/early evening when the most heat is present. With that said, let's look at some of the details, but first a reminder of the severe weather ingredients.


Severe Ingredients 



The Forcing Mechanism for this storm system remains very strong. We will have a monster upper low moving across the area late Monday/Tuesday with a strong cold front. That will be more than sufficient to fire thunderstorms.



The Wind Shear remains nothing less than incredibly strong. The winds about 2000 feet off the ground during this storm are unbelievably strong.


GFS Winds About 2000 Feet Above the Ground Late Monday

Note the winds near the Ohio River are a robust 70 mph! Transporting winds from 2000 feet to the ground during a thunderstorm is a very easy thing.


GFS Winds About 1 Mile Above the Ground Late Monday

Note the winds 1 mile off the ground are gigantic 103 mph over Lousville! That is astonishingly high!


To be honest, these are incredibly high wind values and almost a guarantee that this event will have damaging winds associated with it as it moves across the area. If coupled with moderate instability then we will have some real problems and that is where our discussion moves.



The Instability is the area where the computer models are having some issues. Remember we like to see instability values above 1000 units to match large wind energy like we will have in this storm. The latest model trends have been slowing the storms time frame for the Ohio Valley shifting between late afternoon / early night for the our area. I don't think the exact time frame is overly important yet because such a windy day should be able support warm temperatures well into the night. Here is a look at the latest GFS instability forecast.


GFS Instability Forecast Late Monday/Early Tuesday

Note the GFS has lowered instability values considerably since yesterday. Basically the GFS has cut the amount of instability in half to around 750 units across the Ohio Valley.


Gfs CAPE 1 



Now my blog is about forecasting and not reciting what a computer model says, so I am not satisfied just taking a computer model's word and moving on. Instability is created in a number of ways, but in this situation warm temperatures and high dewpoints are the key to creating larger instability values. To evaluate the GFS solution, we must determine if the temperatures and dewpoints look legit. If I think the temperature or dewpoint will be higher than what the GFS is forecasting, then that would mean higher instability values will be present than what the GFS is currently forecasting.


GFS Temperatures Late Monday

The GFS is showing a surface temperature of around 64 on Monday and to be completely honest I don't think that number is right. We now know the source of the lower instability on the GFS, so to me I feel it is safe to throw that number out right now. I am forecasting a high of 75 on Monday and I feel the instability will be considerably higher than the GFS is forecasting.



Gfs surface temp 1 


Since I do believe instability will be in the moderate category and potentially considerably higher than the GFS is forecasting, I think we may have a problem. The time the GFS is bringing the storms through has been shifting between late afternoon Monday and around midnight Monday/Tuesday. We need to be cautious about indicating a specific time frame now because there is still 96 hours until the storm arrives. With that said, let me show you the last two runs of the GFS.


GFS Precipition Forecast for Very Late Monday/Early Tuesday (7 PM Run)

Note this run of the GFS shows an explosion of storms across our area. The timing is near midnight Monday/Tuesday on this run of the GFS. Flash Flooding could be a concern also as the GFS is indicating a legit 2" of rain across our area.




GFS Precipition Forecast for Late Afternoon / Early Evening Monday (1 AM Run)

Note, this run of the GFS still shows an explosion of storms across our area. The timing is late afternoon on this run of the GFS. You can also see the GFS is showing very heavy rain with the storms on the order of 2".




Finally, the Storm Prediction Center is still highlighting our area in a slight risk of severe weather and they appear to be as concerned as I am. Day 4 shown below is Monday.


SPC 4-8 


Here is the Forecast Discussion from the Storm Prediction Center.


Models maintain likelihood for widespread severe storms from the mid-lower Mississippi Valley into the Ohio/Tennessee Valleys and Central Gulf Coast States on Day 4. Gulf Moisture will advect northward across this region in association with very strong eastward migrating southerly low level jet. T-Storm activity will be supported as forcing for deep ascent attending the upper trough shifts east of stronger elevated mixed layer. This and strong wind fields will support threat for supercells and line segments with tornadoes... large hail and damaging winds.




My Severe Weather Forecast


The storm system moving in on Monday will indeed have remarkably strong wind energy with it. Some of the winds 1 mile off the ground may approach an astonishingly high 100 mph which gives high credibility toward at the bare minimum a high damaging wind threat. I do not agree with the lower instability forecast from the GFS on the last run. I think it is showing errors and I think the instability will be higher than the GFS forecast and I think more than sufficient to support severe weather. The system remains very concerning with all modes of severe weather on the table. Damaging winds, large hail, and tornadoes all remain possible for Monday based on the current data I see. Some of the latest computer model runs are also starting to show a wind profile that can enhance the tornado threat. The bottom line... be ready for Monday. Severe weather looks very likely and a major severe weather outbreak is still a real possibility.


The secondary threat will be flash flooding. There is a real possibility that some locations could get 2" of rain out of this system over a short period of time, so flooding and flash flooding issues will need to be monitored as well.




My Extended Forecast


Today we will see increasing clouds as the day progresses. Showers or thunderstorms could form today with the best chance late afternoon and early evening. Highs will top near 57.




We will keep a 30% chance of showers through 10 pm - 11 pm tonight and then decreasing clouds after midnight. The low will drop to around 39.




On the extended, I think we will have a pretty nice weekend. A breezy and partly cloudy Saturday will top near 59. On Sunday, I cannot rule out a few scattered thunderstorms otherwise partly cloudy with superb high near 73! Monday is our severe weather day, but showers will linger through Tuesday. After a brief cool down on Wednesday, we will pop back into the 70s by Thursday and Friday. The pattern is really looking much more spring like.





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Hi. You r the ONLY meterologist i listen to, watch & 100% believe in! Im TERRIFIED of storms! i live in a camper in Lexington Indiana: will the storm Monday come my way?

Janie, thanks for your trust! :) The entire area has a threat of severe weather on Monday and there is not exception. We can pin down the greatest threat as we close in.

I just got done reading your blog and watching the weather. I will keep my eyes to the sky Monday because, I am also scared of storms. I lost my home and a lot of things I held dear to my heart in the 2004 Marengo, IN tornadoe. I don't think I will ever get over the fear of storms and I now try to have everything in order when I know when we are having bad storms. Thank you for being so on top of things and letting people know what to expect so many days ahead!!

Beth, thanks for checking out the blog and the note. :) Knowing what to expect is half the battle and I am dedicated to trying my hardest to help. Monday has a lot of potential and over the next couple of days we will be dialing in what specific threats we can expect.

Marc, great blog, very informative. I think it is interesting on how some of the other networks this morning said the system will not be near as strong here. You have not gone that route yet. What is interesting to note, do you think the energy of the storms in Bama, Miss, and Tenn could zap our area? Not that we won't have some Warnings but I remember a similar situation that happened last April the infamous Yazoo City Twister and we were under a PDS, moderate risk all that and we hardly had anything here, even though all the variables were there. Everything to the South zapped the Ohio Valley. Seems very similar. Just my arm chair forecasting thought

John, we are glad to have you on the blog. I enjoy our emails, so it is nice to see you enjoying the blog now.

There is a difference if you are reading a computer model and accepting the output versus *forecasting* the weather. The GFS shows much lower instability and I suspect that is the source of those statements. However for the GFS instability to verify, the temperature will have to be around 64 on Monday. The GFS is still forecasting 750 units of instability with a nominal high temperature of 64. This is something that is critical to this forecast. You know me pretty well from our emails and I am not one to follow just because a computer model says something will happen.

As far storms to our south cutting off our energy, this is not the type of system that normally has that issue. The concern for us would be rain specifically in our area during the day on Monday. That is still a real possibility. Those are some of the small details we will need to watch.

Marc, I am extremely impressed with your blog. The amount of detail, as well as your personal interpretation of the models is awesome. Thank god Fox41 finally let you get a blog going. You are clearly the #1 meteorologist in the city, with Milosevic a distant second!

Marc, when it comes to storm forecasting you are tops in the Louisville area, hands down. That said, I will be carrying my weather radio with me to work Monday. Thank you for the heads up early.

Marc, I do remember on that day, we had a lot more cloud cover and cooler temps than forecasted. Still 4 days out, will be very interestin ghow the weekend plays out and what happens Monday

I'm so glad you forcast, and don't just take the computers models as what will happen. It shows me that you actually do the work a meterologist is supposed to do, and not just rely on computers. Thanks for being so good at your job.

Thanks, Marc, for the detail you provide in forecasting the weather. It's very reassuring to know you always provide us with much more information than we can get from any other source. Kudos to you!

Marc, Can we count on you to give thorough updates saturday and sunday like you have today to let us know how we are progressing? Graet Job on the Blog!

Hi, Marc. This is great. I was involved in one of the tornados April 3, 1974, and I still get very nervous when these outbreaks are forecast. You're doing a great job. Thank you.


I am suprised the GFS Instability values dropped, but then again I am not. I believe the real key here for us experiencing damaging weather will be what you hit on early in your blog. This is wind speed at the 2,000 feet elevation mark. Most of that energy can make it to the ground in a hurry and then cause...... damaging straight line winds and possibly worse.... There seems to be some inconsistency with GFS and projected forecast highs. If the actual high does peak 10 degrees warmer than projection then I believe there is more fuel in the tank than most are anticipating. I believe there is true validity there as well in that expectation. In my opinion soccer and baseball practices just might be canceled Monday evening.... Hopefully, most get on board with this scenario. If all the small details and variables line up, we just might be entering the stormy season with some power.

I really enjoy your blog! Great info! Will you be updating over the weekend? We all appreciate your hard work.

Big Blue fans, may not see much of the ballgame Monday night. WINK WINK!

Ryan, thanks for the kind words. :) It is exciting to me to see so many getting involved now. This is for all of us to chat about weather, so please feel free to make it your own too. :)

Bill W, thank you for the confidence!

John, you are right that this far out there are still details that remain open. We will dial this in over the next couple of days.

Deanna, thanks! :) Accuracy is what ultimately is most important to me. There are days I agree with the computer models and days I think they are out to lunch.

Ray, I am glad the blog is helpful. I just want to make sure you are prepared for whatever Mother Nature offers. Knowing what to expect is half the battle.

Judy, I am glad you enjoy the extra details. This is a lot of fun for me to unload all my thoughts rather than the Cliff notes.

chrisb, I will try my best to keep you updated through the weekend.

Derek, I think we are on the same page. The CAPE differences are absolutely surface temp based. Those highs will be critical on Monday.

Ronda, thanks for dropping in and posting. I will try my best to have updates for the weekend.

John, don't even talk like that!

Everyone, thanks for the comments! This discussion is for all of us, so please feel free to leave comments for me and each other.

Marc, great blog you and the fox 41 weather team is the only 1 I trust the other station is town here lately seem to scare not inform keep up the GREAT WORK!!

This is why we watch Fox41 for all of our local news and especially the weather. My family and I feel that you are the only true Meterologist in Louisville, not just a "weatherman". You and the Fox41 weather team always do a fantastic job of keeping our area informed and giving the most accurate and reliable weather information. We'll be watching on Monday as always and thanks for doing such a fantastic job!

Latest statement from NWS. Now they are talking about Heavy Rain (which we don't need) being the primary issue. I am really beginning to think we may dodge the worst and this will be much worse down in Dixie. Still listen to Marc, I am no expert just an armchair guy.



Chad, thanks for the trust. :) We have drawn a line in the sand on this one. We are simply trying to give accurate information based on the facts that so you can be prepared.

Don, thanks for the comments. :) Please let everyone know that I appreciate their trust.

Nothing is set in stone for Monday, but with such strong winds we have to watch it very closely.

John, I am not sure that is what I am getting out of that discussion. I think they are simply stating each threat the storm may have.

The latest suite of data is now moving the storm faster, but the back and forth is common this far out from a storm. I don't want to string everyone along with unreliable time frames, so let's continue to eye the threat understanding it has potential.

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