Severe Weather Threat Increasing...

Model Discussion for 2/27/07 8pm

Good evening once again everyone!

Before we get to tonight's model discussion, I'd like to care of a little business first. You will see many changes soon folks. Pay attention to my blog for further details! This is just the beginning... I am also a Moderator of my Buddy Twister's website, which again is at http://www.geocities.com/cincyforecast/ there you will also find his weather forum that I am a part of. To access the forum, you can also click here: http://www.createforum.com/usaweather/ Please check it out, and sign up... it's free! It'll be great to hear from all of you. Become one of my weather spotters and post your weather observations there! Anything weather related can be discussed! There's even an off-topic section as well, for non-weather related items. I also send my model discussions and forecasts out to about 40 people via email! Thanks again to everyone who has supported me in this weather adventure!!! It's been a fun ride, and it's only just begun!

Alrighty... now on to the weather and tonight's models. I'd like to start off by discussing our severe weather chances for Thurs. To see the latest from the SPC, click here: http://spc.noaa.gov/products/outlook/day3otlk.html On this graphic for Day 3 which is Thurs., you can see that we are in a slight risk for Severe weather, with a 15% probability of getting a severe T-storm. Now, the best chance as you can see, is going to be off to our South. I imagine that we'll see some of those slight risk areas be upgraded to the Moderate Category by tomorrow. The best chance to me looks like Southern KY southward thru the TN Valley and on into MS, AL, and GA.

The 12Z GFS Calls for a half inch to an inch of rainfall with this system. But, I am still downplaying the idea of a severe outbreak here... no..no...no. I am, however, increasing the threat of isolated Severe Storms. I do think we'll see a handful of severe weather reports across the region on Thurs, especially in the afternoon/evening, when the cold front comes thru. Wind shear profiles are excellent, however, instablility is lacking with capes of only 500J/KG expected. The front will be moving too quickly, as the NAM is about 4 hours faster with bringing the front thru than the GFS today. Also, deep, juicy moisture is lacking as dewpoints should only rise into the low to mid 50s. Therefore, the threat is increased somewhat today based on today's models, however, only isolated storms should develop, especially embedded within the cold frontal passage Thurs evening. The severe threat should be over with after 10pm Thurs. evening. We'll just see some leftover rain showers during the overnight.

As we approach Fri. and into Sat., we'll see highs early on Friday in the lower 40s then falling thru the 30s throughout the day. Rain showers will changeover to flurries and snow showers and linger on into the day on Saturday. No accumulation is expected. However, check out the 18Z NAM today. Now it looks rather interesting. http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/nam/18/images/nam_slp_084m.gif
When looking at this model, let me point out a few things. First of all, the blue line marked 540. The 540 thickness line is a good indicator of where the rain/snow line is. Look at how far South it is. It runs from Northern TX to Middle TN thru the Virginia's. We are well North of that line so any precip. should be all snow, according to this model. Now, look at the dark green over our area. According to the key on the right, it would be close to .25" of liquid precip. Now, okay, how much snow would that be Les? Well, it's simple.

Well, let's take a look here:

Snow to Water ratio... Normally, when temps. are in the upper 20s to the lower 30s. the snow to water ratio would be 1" of rain = 10" of snow, or a 10:1 ratio of snow to water. Below, you will see a common list of surface temps. and there corresponding ratios:

Temp. Snow : Water Ratio
=========================================================
29-32 10:1, sometimes as low as 8:1 with temps of 32 or greater.
24-28 15:1
19-23 20:1
Below 19 25:1 to as low as 30:1, and in rare
cases like the Rocky Mts., where a lot of moisture
falls into very dry air, you can get 40:1 ratios too!!!

So, with that in mind, .25" of precip, with temps. in the 30s would probably yield an 8:1 Ratio for Saturday. Also, some melting may occur due to the warmer ground and surface temps now that we start to see in March, leads me to this conclusion at this point in time:

1" rain = 8" of snow (8:1 Ratio)
So, .25" of rain = 2" maybe 3" of snow.
Then take off for any melting that may occur and I'd say 1" or less at this time. That's how you can figure out snowfall forecasts, folks. Very easy!!!

Now, as we slip into the 2nd half of the weekend and on into the beginning of next week, dry, but chilly conditions continue, with a NW Flow in place. Now, the 12Z GFS shows something interesting for Tuesday, March 6th in the late afternoon and during the overnight hours of Tues. A clipper-type system could bring snow to the region. Look at where the 540 thickness line is at: http://www.weatherunderground.com/modelmaps/maps.asp?model=NAM&domain=US You can look at the GFS, and NAM on this site. Check it out! At 180 hours, 7pm on Tues., the GFS model shows a nice band of snow developing in Central Indy thru Central OH, with the 540 thickness line just to the North of Cincy, South-side of Dayton. Then watch what happens by 192 hours (7am Wed. morning)... It shows some nice precip. developing over Central OH, with the 540 Thickless line dropping thru the Cincy Metro and setting up somewhere in or South of the Owen, Grant, Pendelton Co. line in Northern KY. Then, if you look at the next frame, 204 hours, it's outta here!

Longer Term:

Well, for tonight's long term discussion, take a look at the 12Z GFS again, all the way out to 384 hours... click on each frame and check it out! You'll notice a colder weather pattern, as several times, the 540 Thickness line drops way South of here into the Lower TN Valley. Also, you'll see quite a bit of storminess too! Could my 4-6" snowfall prediction for the month of March come true? (You can read that entire write-up on the Weather Forum. See link at beginning of this blog).

Stay tuned to the emails, weather forum, and this blog, for further updates!