April 4th Severe Weather

An area of low pressure is going to be moving across into the Great Lakes. Associated with this area of low pressure will be a very strong cold front. Some of the model runs of the ECM for example how been showing a very steep drop in temperatures in a relatively short amount of time.

For example on the 31st of March the ECM  on its one run had at 18 Z Wed the surface temperatures in the lower 70s. In a 12 hr span these temperatures drop to the upper 30s..This would be a 34 degree drop in temperatures in a 12 hr period.
On top of that the 850s are in the 15 degree C range..to drop below 0 over the next 12 hrs.

Now since that point and time the timing of the front has changed and instead of coming thru on wednesday afternoon it appears as though the front is actually going to arrive between 00Z Tues and 12 Z tues. So this front should cross the area during the overnight areas.

Temperatures out ahead of the front are not progged to be quite as high as what they originally were but they are still going to be quite warm ahead of this front and cool off rapidly behind it. Temperatures should still reach the 60-70 range and perhaps slightly higher all depending on how far north the warm front makes progress.

Today the 12 Z GFS still had temperatures around 60 degrees at 6 Z or 2 AM tuesday morning!

12 Z GFS takes the region at 6 Z tuesday from 60 degrees to 6 hrs later a 23 degree temp drop to 37 degrees @ 12 Z tuesday.
850s go from a +13.2 to a -3.3 or in other words from 56 degrees to 26 degrees in 6 hrs..

The above again is using ABE as a base for the discussion.

You can see the potency to this cold front by looking at the shear that is associated with that cold front. The shear itself involved with this system would be capable of producing wind damage across the area.

Now, Dr Greg Forbes of the weather channel has what he calls a TorCon # . What is a TorCon #? We will let him explain it:

The TOR:CON values range from 0 to 10. A value of 4 means that there is about a 40% chance of a tornado within 50 miles of a location in the specified area of severe thunderstorm activity. This also means that there is a 60% chance that a tornado will NOT occur.
Higher values mean there is a greater likelihood for tornadoes. But for many parts of the U.S., the average daily chance of a tornado within 50 miles is 4% or less even at the peak time of year, so even low TOR:CON values are meant to indicate and elevated - but not guaranteed - threat of a tornado nearby.

That scale is as follows:

Description of TOR:CON values:
0 - near-zero chance of a tornado or severe thunderstorm nearby
2 - very low chance of a tornado nearby, but hail or strong wind gusts possible
4 - low chance of a tornado nearby, but hail and/or strong wind gusts possible
6 - moderate possibility of a tornado in the area of concern
8 - high probability of a tornado in the area of concern

Using this chart puts the general area at a 3-5 range with the higher range to the south in the southern states.

Lift index on both the 12 Z GFS and the 18 Z GFS continues to range from –2 to –4. Lapse rates are between 6.0 & 7.0. So we have steep enough lapse rates , strong enough lift index..and more then ample enough shear. However..the two factors working against this are :

1. The timing of the front. It would be much better for the front to arrive between 18 Z and 00z because of the daytime heating that would be involved. However..the drop in temperatures should be a sufficient enough spark even with the time the front arrives .

2. Limited CAPE involved. However..we feel the other parameters should make up for this limiting factor.

During the day the severe weather will be to the west…this however is just a forecast for those that are further east. Isolated tornadoes can not be ruled out but the main threat from these storms should be damaging winds in excess of 60 MPH and hail!