« February 2011 | Main | April 2011 »

41 posts from March 2011

March 25, 2011

Late Afternoon Update... Snowfall Potential

This morning the big topic of discussion was the potential for snowfall Saturday night into Sunday and if there was a possibility of accumulation. With new data coming in all day long, I wanted to give you an update before you break for the weekend.


Snowfall Forecast Discussion


The NAM and GFS computer models continue to hang strong to their forecasts over the last 24 hours with little to no divergence. The NAM continues to pump out more snow than the GFS and I continue to believe it is just way over doing the snowfall totals. Here are the latest snowfall forecasts for each computer model.


NAM Snowfall Forecast Saturday Night to Sunday

Note the snowfall totals ramp up in southern Indiana according to the NAM.  I still think it is way over doing snowfall potential.


NAM Snowfall 


GFS Snowfall Totals for Saturday Night Through Sunday

Note the GFS shows much lower and more realistic snowfall totals for the area. Very little to no accumulation for Louisville.


GFS Snowfall 


HPC Forecast Possibility for 2" of More of Snow

Note the HPC agrees with my assessment and they have less than 10% chance of 2" or more snowfall for Louisville and ramping up to arond a 40% chance near Seymour, IN.


HPC Snowfall 


The bottom line is I think our forecast is on pace and I still like my snowfall forecast. Check my previous blog entry for my snowfall estimates for your area...




Are you kidding me? Snow Possible Again!

We have a storm system approaching the area on Saturday and Sunday and it is a very interesting storm. Our recent shift toward cold weather opens the door for some wintery precipitation with this storm system. Most would not see accumulation, but it is indeed possible parts of our area could get accumulations on some surfaces.


A Very Interesting Forecast Discussion


Starting off this morning we have a storm system west of Louisville that could bring some rain showers to the area. These showers will be light if you get one, but the storm is of significance. This storm will bring a renewed northerly wind to our are that will concrete some colder air for our next storm. Notice the winds already shifting to the north over southern Illinois and that means the colder air in central Indiana are going to be pushing into Kentucky.





Notice the 29 degree reading in Indianapolis, so we know more cold air will be pushing south and being really re-inforced over Indiana.




That cold air will become critical as our next storm approaches. We always say that for our area, we need to see a storm track south of the river for us to have any chance at wintery precipition. The NAM and GFS are in good agreement that this storm will track south of the river. Note the white arrow showing the low pressure tracking through Tennesee which is an ideal track for some form of wintery precipition in our colder months.

It is clear that our storm will have a storm track that can produce wintery precipitation and cold air in parts of the area, so now let's break down the precipitation type and time frames.



Who Could See Wintery Precipitation?


This is obviously the big question. Just a few things to discuss before jumping into this forecast. Remember the blue line you will see on the computer models is an approximate area that can be considered the rain-snow line. North of the blue line marked "0", we will see a mix to snow scenario.

The hard part in this forecast is not the storm track, but how much precipitation will linger after midnight Saturday because if a bulk of our area is to get snow or mix it will come after midnight. Let me first give you a look at the NAM and GFS.



GFS Winter Storm Prediction


GFS Computer Model Forecast for Saturday at 7 PM

Note the precip has not started for most of the area by 7 PM.




GFS Computer Model Forecast for Sunday at 1 AM

Note the rain-snow line approximately in south central Indiana. Rain in Louisville.




GFS Computer Model Forecast for Sunday at 7 AM

Note the GFS shutting down much of the precipitation after midnight when it appears cold enough to support snow in the Louisville area.




GFS Snowfall Totals

Note the red start indicating where Louisville is located. The GFS shows very little snowfall accumulation in Louisville, nearly 2" near Paoli IN to North Vernon IN, and possibly 3" near Bedford, IN.


GFS Snowfall Totals 



NAM Winter Storm Prediction


NAM Computer Model Forecast for Saturday at 7 PM

Note the NAM does not begin the precipitation by 7 PM on Saturday.




NAM Computer Model Forecast for Sunday at 1 AM

Note NAM is producing snow over south central Indiana and a wintery mix nearing the Ohio River.




NAM Computer Model Forecast for Sunday at 7 AM

Note the NAM is shutting down the precip during the morning on Sunday, but does end this as snow as far south as around Shepherdsville, KY to E-town, KY area.




NAM Snowfall Totals

Note the black arrow point to Louisville. The NAM is indeed showing light snowfall accumulations as far south as south central Kentucky. The NAM shows a couple of 1" - 2" of snow near the river and up to 4" near Bedford, IN to Seymour, IN to North Vernon, IN.




Clearly, the NAM is indeed the colder solution and hence has a bit more snow. The GFS is a bit faster in its movement and results in less precipition and resulting snow. Remember these are computer model forecasts and should be looked at with caution. I would not jump up and down based on the NAM solution, but please read my full forecast below.




My Forecast For Saturday through Sunday


There is plenty of evidence to support a late start of the precipitation Saturday. I think most of the rain/mix/snow will start in the evening on Saturday, not during the day. If you have plans Saturday, most of the area looks dry for the bulk of the day. Before we dive into the snowfall totals, I want to show you what the precipitation will be falling on... the temperature of the pavement. Remember for snow to stick, we need to see pavement temperatures near 32 degrees.




It is clear that the pavement temperatures are way above freezing, so this means sticking on the pavement will take more than just 1" of snow. Pavement temperatures as far north as Seymour are 41 degrees. This becomes critical in determining how much you will see because the computer models do not know the temperature of the pavement. I think this is good evidence that the NAM is over-forecasting snowfall totals.


Another critical point to note, the NAM was indicating a temperature of 31 at 7 am this morning while the GFS is indicating a 7 am temperature of 36 degrees. The actual temperature at 7 am this morning was 38 degrees, so the NAM has a huge 7 degree cold bias and the GFS is doing a very good job. That is really important to note when evaluating the computer model performance right now. It means to me the NAM is suffering from some issues with respect amount and depth of cold air in our area and this is not uncommon bias for the NAM. The GFS is doing a great job with the amount and depth of cold air and I like it's timeline.


Finally, the NAM shows more actual precipitation, whether rain or snow for the entire area. The NAM does handle this type of storm and the amount of precipition better. I like the liquid totals, but not the snowfall totals for the NAM.


The last point I want to note is freezing rain potential. Some have mentioned this storm could produce freezing rain. There is nothing in the temperature profile in any way that supports freezing rain right now. Any non-snow or non-rain should be a slushy sleet which has a much lower impact than freezing rain.


With those tidbits in mind, let's jump into the forecast.




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.


Midnight Saturday/Sunday


--Changing to a Rain/Snow slushy mix in Louisville. Still more Rain than anything else.

--South of River will still be dominated by Rain.

--Seymour has changed to all Snow.


After midnight through early Sunday morning


--Changing to light Snow then Snow showers in Louisville to E-town.

--Mix precipition as far south as Munfordville.

--All Snow north of the Ohio River.



Approximate Snowfall Totals - Note This Is An Early Estimate


--Near Munfordville will see no accumulation.

--Louisville will see snow, but the warmer pavement temperatures should do very well at melting any snow that does fall. A small accumulation on the grassy surfaces is possible.

--Near Scottsburg could see some lighter accumulation amounts up to around 1".

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



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



This is a fluid situation, so please check back to the blog and on Fox 41 for updates over the next 2 days. In addition, another storm late next week will need to be watched closely for winter precipition potential. We will dive into that storm after this one has exited.



March 24, 2011

Much Colder Temperatures Take Over...

After enjoying the luxury of 70s and 80s the last week, we had to wave good-bye to them as the storms moved across the area last night. The changes are significant as you step outside this morning. The big questions is how long will the cold last and that is something we will look at this morning.


Forecast Discussion


Looking outside this morning is a bit depressing if you enjoyed the warmer weather. The severe weather yesterday ushered in much colder air and as of early this morning we are cloudy, gloomy, and chilly.




You can see the winds are coming from the north, so let's take a look at what kind of air that wind will be blowing in.




Oh boy, 32 degrees in Indianapolis. I think you get the idea... much colder air is filtering into Kentucky and Indiana. Since cold air is probably the biggest weather topic today, I wanted to take a look at how long this cold will likely last.


Pattern Change


The jetstream is such a great indicator of weather patterns. When you have a ridge in your area, you get warmth. A ridge for the east coast is seen when the jet pushes way up into southern Canada. You can see that nicely with the jetstream from this last Monday. Notice how that ridge is controlling the eastern US.




When a ridge of high pressure builds, we see warm air build quickly underneath it. That was the source of the warm for the last week. We know that broke down yesterday with severe weather. Looking at the jetstream this morning we see a a big difference. Notice how a big dip is now placed over the eastern part of the US.




With that dip, the eastern part of the US will be dominated by colder weather. It takes quite a bit of energy and time to change jetstream orientations. This is why I refer to pattern changes as 10-14 day events. The first question has to be "Is this a temporary blip". Let me show you the jetstream orientation by Friday of next week.




You continue to see a dip in the eastern US, but now the pattern has amplified. When we look at some temperatures, you can see the dip in the jet continues to draw cold air right out of Canada into our area.




With that said, we now see this is not a 1 day change, but a real pattern change. To break this pattern will take time and it is probably not going to end before 14 days or so. The long range models have shown nice consistency well into April with a cool trend. Remember consistency on long range forecasting is a good indicator that the computer model's solution does have credibility. I will show you the last two runs of the GFS model for April 7th.


GFS Computer Model Run for April 6-7 (1 AM Computer Model Run)



GFS Computer Model Run for April 6-7 (7 PM Run)




Notice the consistency with that dip in the jetstream in the eastern US. 3 of the last 4 times the GFS was run, it shows a similar result.


The bottom line is a hard colder pattern will hold through late next week, then likely the colder days will still out number the warmer days through the first 1/3rd of April. Keep the winter gear handy for at least a few more weeks.



My Extended Forecst


For today we will see generally cloudy skies. Some breaks in the clouds may occur as we head toward sunset, but clouds will dominate. Temperatures will hover in the mid 40s all day long with the sprinkles ending around mid day.




Tonight we will get close to the freezing mark meaning a frost is indeed possible, especially outside of the city. Skies will turn partly cloudy.




Finally, as you expected the 7-day is cold. No days over the next 7 above 55 right now and all days have cold lows in the 30s. If you have weekend plans, the rain chance looks best late Saturday into early Sunday. Not the best weekend, especially when you compare it to last week. :(






For a recap of yesterday's severe weather, please check my last blog entry here...