We are under a slight risk for severe weather this afternoon and need to evaluate this risk. With the sun popping out as expected temperatures have quickly risen. That adds some more fuel for the storms and it is time to evaluate this risk.
If you missed the SPC Severe Weather risk, please view my previous blog entry here...
We left things yesterday debating which computer model had the correct instability. The NAM kept the dewpoint at 67F or high and the GFS had a dewpoint of 63F. That led the GFS to have low instability, while the NAM had more robust instability. Let's start today by looking at the dewpoints across our area. My thoughts were the higher dewpoints were going to verify because the GFS was suffering from problems with it's physics as commonly is the case with cold fronts when there is resistance. Let's look at the numbers.
Clearly the higher dewpoints are verifying, so the better instability values of the NAM are verifying near 1500 units. We know today has sufficient wind energy for storms, so all that is left is the forcing to cause lift for the storms. We said yesterday that the lift would not be superb today, but sufficient for scattered storm development. The cold front is now slipping into the northwestern part of our area and we are seeing "bubbling" cumulus clouds already. These cumulus bubbles are normally the precursor to storms.
Indeed storms are starting to fire right now near English, IN so that cumulus field should be popping.
It looks like our forecast from yesterday is right on track. The higher dewpoints will lead to sufficient instability for scattered storms to fire. Some will likely go severe with the main threat as damaging wind and hail. Although tornadoes are not the primary threat in any way, shape, or form, one cannot be ruled out with slight turning of the winds. Timing should be near 5 pm - 7 pm for Louisville, but remember we are not talking about 100% coverage. About 50% of us should see some storms tonight.
This does not look like a monster severe weather outbreak.
Some strong storms continue to move across the area this Tuesday evening. After these storms end, the question of severe weather comes up again tomorrow. We have a very energetic storms system for this time of the year approaching the area and we need to discuss it's severe weather potential.
SPC Categorical Severe Weather Risk 7 AM Wednesday - 7 AM Thursday
Note the entire area is under a slight risk of severe weather on Wednedsay.
SPC Probabilistic Severe Weather Risk 7 AM Wednesday - 7 AM Thursday
Note the 15% chance covers our entire area.
Wednesday is an oddball day for so late in June because this storm is anomolously strong for this time of the year. It is one of the more powerful late June storms I have seen. What that means is we have pretty solid wind energy for this time of the year. Remember we look for 35 knot mid level winds to support severe weather and tomorrow clearly exceeds that. In fact at midday tomorrow, the winds in the mid levels of the atmosphere are near 50 knots above Louisville and 60 knots over SW Indiana.
NAM Mid Level Winds Midday Tomorrow
I can tell you wind energy like that is not common at all for late June. We also know that wind energy alone is not enough to support a severe weather threat. We still need an instability to fuel our thunderstorms and this is where the complexity comes with tomorrow. The NAM and GFS are very different in their instability forecast.
NAM Instability For Tomorrow Late
The NAM has been shifting between 1500 to 3000 units of instability tomorrow. That 3000 unit is very high.
GFS Instability For Tomorrow Late
The GFS has been consistently showing less than 1000 units of instability.
This major difference poses a great forecasting dilemma that we must address and we will in my forecast discussion below.
The last element we look for in severe weather is a forcing mechanism. The forcing will be present from a weak cold front that will become stationary in our area, but should be enough to trigger some storms.
WDRB's AdvanceTrak continues to show some cellular structure on the storms as the front approaches the area as you can see below.
We have marginal forcing, robust wind energy for this time of the year, and questionable instability... Let's put this all together now in my forecast thoughts.
Wind energy is going to be sufficient tomorrow to support a severe threat, but the question is will we have enough instability to support severe storms. That truly is a tough question. The GFS seems to be lowering the instability by dropping the dewpoint tomorrow to 63F late while the NAM keeps the dewpoint at 67F through late in the afternoon. Higher dewpoints with all else equal result in higher instability. I guess our question comes down to which is accurate? I like the NAM more because I believe it is slowing the front more appropriately and likely allow moisture to pool ahead of the front. The NAM physics tends to be superior with slowing fronts while the GFS seems to push fronts to far south and too fast. If that is the case, then the greater NAM instability would be realized.
I have seen these types of fronts a lot this time of year and traditionally what happens is they slow, stall, and models don't grab the thunderstorm development along them well. I like that AdvanceTrak is popping storms mid afternoon and allowing them to lag toward the Ohio river late in the afternoon. I think this is the most likely scenario. That would give an isolated severe threat south of the river where the forcing will be questionable and a little more coverage along and north of the river. The timing for Louisville would be mid to late afternoon. The main threats would be hail and damaging winds from these storms, but an isolated tornado cannot be ruled out. The wind shear is far more supportive of damaging winds than tornadoes, but the rare spin up is impossible to rule out.