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41 posts from March 2011

March 29, 2011

Warmth back in the Extended, but Severe Weather might be too..

In the short term, we have to talk about rain that will be moving into the area tonight through tomorrow. Some locations may change over to some light snow and that will be our first point of our morning discussion. In the longer range, there are some signals that may point toward a severe weather chance as a strong storm may develop late in the weekend and early next week. Let's take a look at both stories today.


Tuesday/Wednesday Storm Discussion


Rain is far to our west this morning and really of very little concern for us today. Very, very low humidities should be a great defender today from us getting much other than a light shower.




The latest track on this storm looks like it will move a little further south than when we talked yesterday. In fact, I think the chance of any light snow accumulating on this storm now are pretty small. I still think a small dusting could occur from Madison, IN, to North Vernon, IN to Seymour, IN, to Bedford, IN. All of our southern Indiana counties could end the system with some snow mixing in, but I don't think this system amounts to much. GFS is in full agreement with my thoughts here as well.


GFS Snowfall Totals through late Wednesday


GFS Snowfall Totals 


Rather than overhyping a storm that doesn't deserve that, let's just move on and look at this system as primarily a rain event. Louisville will see mainly rain tonight and it may mix with a couple of wet flakes tomorrow. This is absolutely nothing to be worried about at all for Louisville.



Monday Severe Weather Threat???


There is a lot of flexibility in the long range forecasting right now. Keep in mind this is a number of days out, so we are still talking potential only. The longer range models are starting to repetitively indicate a very powerful low passing through Illinois into the Great Lakes region. That is a preferred track for severe weather in the Ohio Valley. Take a look at a very intense low pressure by early Monday morning over the Great Lakes.


GFS Forecast For Monday at 7 AM

Note a very, very intense 988mb low pressure over the Lakes.





GFS Forecast For Monday at 1 PM

Note the GFS shows the storms arriving early on Monday.





The GFS is indicating remarkable wind energy with this system. This is basically 3 times stronger than the wind energy with the storm system that produced severe weather across our area last week.



GFS Winds About 1 Mile Off the Ground for Monday at 7 AM

Note the incredibly robust winds near 65 mph - 75 mph over much of the area.



The GFS is also indicating sufficient instability to match this wind energy. Remember we like to see 1000 units of instability of more to support wind energy like this.


GFS Instability Forecast For Monday Morning

Note the 1000 - 1500 units of instability appraching the Ohio Valley Early Monday.


Gfs CAPE 1 



My Severe Weather Thoughts


I make no secrets that we have anything close to a firm grasp on a storm this far out. Judging solely on pattern recognition, I do think this storm does show a signal that can indeed produce severe weather in the Ohio Valley. The track this storm takes across the US is one the GFS tends to do very well with in terms of speed and timing. The larger pattern does seem to support the GFS solution of a very intense low pressure over the upper midwest as well. The GFS does show a morning time for the storms, which is not normally when we severe major severe events, but this far out that time frame is highly unreliable. What I do think will happen is a severe weather situation will occur in the region, but the exact location will need to be monitored closely. This does have the potential to be a significant storm system in a time of the year where the severe weather is climotologically favored in the Ohio Valley. I will be watching...



My Extended Forecast


Today we may enjoy some breaks in the clouds during the late morning or early afternoon, but still this should be a mainly cloudy day. Temperatures will top in the mid 50s and I will need to hold a slim chance of a shower during the day.




The rain will move into the area in the evening and continue for the night time hours. The heavier rain should fall south of the river. North of the river, in particular near Seymour, you will probably mix with snow by daybreak.




Tomorrow our northern counties will end with light snow while Louisville may mix with a few flakes as the storm departs. For our southern and south central Indiana counties, little to no accumulation is expected on this storm. Thursday or Friday could have a light shower or two, so I will keep a slim chance. Saturday looks like the best day of the weekend, but Sunday does not look like a wash out. The early part of Sunday may have some rain and thunderstorms then turning partly cloudy and warmer in the afternoon. Finally Monday is a day we need to watch for any severe weather potential.






March 28, 2011

Can the Tuesday/Wednesday Storm Produce Snow???

The big topic over the weekend had the be the "white stuff" falling from the sky. What a week we had last week... highs in the 80s, severe weather, then ending the week officially with 0.3" of snow. To say the last week of March was wild would be an understatement. One more storm to end March, let's talk about whether it is a rain or snow scenario.


Forecast Discussion


We are watching a weak storm move across the area today. The humidy is very low, so I am not concerned about too many problems from this storm. We will go between storms later today, then wait for our next storm late tomorrow. Ahead of the storm, we will see pretty warm conditions tomorrow, but like our last storm the dry atmosphere will allow temperatures to drop quickly once the precipitation begins. The question is can they drop low enough. In this scenario, it would take about 35 degrees to 37 to support snow, so let's begin our look.  

The NAM and GFS look very similar in their setup for this system with very similar amounts of precipitation. Note where the blue line is located on the following maps. Keep in mind that is a "rough and dirty" rain/snow line on the computer models. We will look at the computer models from 1 am Wednesday through 7 am Wednesday.


NAM Forecast for 7 AM on Wednesday

Note the NAM holds things a bit warmer on this next storm. Seymour would see a quick transition to snow or go all snow late Tuesday to early Wednesday..


Nam precip totals 1 


NAM Forecast for 7 am Wednesday

Note by the time the cold air arrives in Louisville, the precipitation is shutting down.


Nam precip totals 2 


GFS Forecast for 1 am Wednesday

Note the GFS says it is way too warm to support snow in Louisville by 1 am on Wednesday, but shows a near or all snow scenario for Seymour, IN.



GFS Forecast for 7am Wednesday

Note, the GFS (like the NAM) is shutting down the precip by the time it gets cold enough to support much snow in Louisville.




HPC Forecast for 2"+ of Snow

Note the negligable numbers near the river up to 40% near Seymour, IN.


HPC 1 



My Snowfall Forecast


The GFS is again my forecast model of choice. It has done very well with the precipitations amounts and temperature profile of the atmoshere. I like that it is showing on a few flakes as the storm ends Wednesday morning from Louisville to E-town. Clearly this is primarily a rain event for the Louisville metro and points south.


The only area that I feel could pick up an accumulation would be near Seymour, IN to North Vernon, IN to Bedford and maybe as far south as Scottsburg, IN. The last storm cooled the pavement temperatures considerably near Seymour, IN. Take a look at the 35-37 degree pavement temperatures near Seymour, IN.




I think those colder pavement temperatures could support up to a 1" accumulation from Bedford, IN, to Seymour, IN, to North Vernon, IN on the pavement and 2" on the grassy surface over this area.



Full Forecast


Today we could see some flurries early near Louisville and snow showers are likely south of E-town, KY early. We will all see decreasing clouds late this afternoon with a high near 50.




With our next storm approaching very quickly, we will see a return to mostly cloudy skies tonight with a low near the freezing mark.




Finally your 7-day. Rain appears likely late tomorrow through early Wednesday and will possibly mix with a couple of wet flakes as it ends in Louisville. Stray showers are possible on Thursday and Friday, but temperatures will start to moderate. By the weekend, the weather pattern will start to change to a "high amplitude" jetstream. This should allow significantly warmer weather with the occasional "2" day spurt of colder air, but the warm should out number the colder days starting late this week.



March 26, 2011

Saturday Update for Snowfall Potential Tonight

I just wanted give a quick Saturday update to those curious about how things will unfold tonight. Snow potential is possible for part of the area tonight, so I felt a Saturday update was warranted.


Forecast Discussion


Looking at all the data over the last few days has been quite interesting. The NAM continues to pace this storm too cold. The NAM does not take todays high above 44 degrees and we are currently at 46 degrees.  The GFS is nailing our afternoon temperatures much better and I feel is a better representation of the snow potential from this storm. The NAM is obviously producing more snow since it is a colder solution, but as stated above I don't feel it is accurate, so I will leave it out of the discussion.


Here are the GFS snowfall totals and I think a good representation of what will fall from the sky. Remember falling from the sky and sticking on the pavement are very different things.


GFS Snowfall 


Pavement temperatures remain very warm with many pavement temperatures in the 50s. That takes a ton of snow to cool the pavement enough to support road accumulations.


Regardless of what model you like, the atmosphere is too warm to support snow during the onset of the precipitation. We are running in the mid and upper 40s, so we have some cooling to do. That makes accumulation on roadways much more difficult. I have emphacized through this event, that it will be very difficult to get much accumulation of snow on the pavement in the Louisville area.


HPC seems to agree with my thoughts as well. Notice their low probability under around 30% of 2" of snow or more for our area.


HPC Snowfall 



My Snowfall Forecast


Other than some minor tweaks, I like the forecast I had yesterday for much of the area, but let's look that over again.




Saturday evening...


--Rain in the Louisville area. Way to warm to support frozen precip as the system starts.

--All areas south of the river start as Rain.

--Seymour starts as a Rain/Snow mix, but changes quickly to snow.


10 PM Saturday Night...


--Changing to quickly to Rain/Snow in Louisville. No accumulation at this point.

--South of River will see mainly Rain.

--Seymour has all Snow.


After midnight through early Sunday morning


--Light Snow then Snow showers continuing Louisville to E-town. Slick spots are possible, especially on elevated surfaces during snow bursts.

--Mix precipition as far south as Munfordville.

--All Snow north of the Ohio River. Again, slick spots are likely on elevated surfaces with a greater impact the further north you go.



Approximate Snowfall Totals 

--Near Munfordville should see no accumulation.

--Louisville will see snow, but the warmer pavement temperatures should do very well at melting most snow that does fall. An accumulation of up to 1" on the grassy surfaces is possible.

--Near Scottsburg could see some accumulation amounts near 1" - 2", but mainly on the grassy surfaces.

--Near Seymour could see a 1" to 3" snowfall out of this system and the highest impact.



Note all areas will have more accumulation on the grass than the pavement because the roads are so warm.