As expected, the Storm Prediction Center has nudged their slight risk of severe weather toward our area for Wednesday night and expanded their 5% chance of severe weather. In tonight's blog, I want to go through the SPC severe weather forecast, break down the setup, and give my assessment of the risk. Let's dive in.
Storm Prediction Center Severe Weather Risk For Wednesday / Wednesday Night
The Storm Prediction Center (SPC) continued to nudge their slight risk of severe weather eastward in their latest update. The slight risk extends to about 2 counties west of our area now.
SPC Categorical Risk Of Severe Weather Wednesday / Wednesday Night
Notice SPC now includes western IN/KY in their slight risk and has placed our area in their marginal category. Due to the confusion with the words "marginal" and "enhanced", we will not be using those in our forecast at this point. The bottom line is the marginal means a 5% chance of severe weather.
SPC Probabilistic Risk Of Severe Weather Wednesday / Wednesday Night
Notice SPC has a 5% of severe weather for virtually our entire area Wednesday night.
My Thoughts On Our Severe Weather Risk On Wednesday / Wednesday Night
Severe weather is really a delicate recipe that needs certain ingredients present if we are going to sustain organized severe weather. Since we haven't discussed severe weather in a while, I want to refresh you quickly on the ingredients I look for when assessing whether a severe weather event could occur.
The forcing Wednesday night continues to look good enough to support showers and t-storms, but not strong enough to overpower weaknesses in the other 2 severe weather fields. The front will be very strong, but it won't get here until Thursday and in a sense miss the window.
Notice the large but diffuse storm on Wednesday night in the mid levels.
The front looks solid, but you can see it is in northern Illinois when the window for stronger storms is open.
This means the forcing is easily sufficient to fire storms, but not strong enough to overcome weaknesses in the other 2 severe weather fields.
To support organized severe weather, we need to see a wind of at least 35 knot / 40 mph somewhere from the surface up to 6 miles above the ground. The wind energy on Wednesday is strong enough to support organized severe weather, but far from robust. The latest data shows much less "turning of the wind with height" but enough to still define this as slight directional wind shear. Remember that directional wind shear can enhance the severe threat and in this case give a slightly better chance of hail.
NAM Winds At The Surface Wednesday Night
The NAM shows the winds are from the south at the surface late Wednesday at about 10 mph. These are certainly not strong winds.
NAM Winds 1 Mile Above The Ground Late Wednesday
The NAM shows of about 40 knots / 45 mph just a mile above the ground. These are not overly strong winds, but they still meet minimum criteria (35 knots) for organized severe weather. The direction shifts to the southwest. This shift is smaller than the data showed yesterday, but still present.
The wind shear is directional wind shear, but much weaker than the data showed yesterday. This is a good thing.
Instability in the cooler seasons almost always tells the story about our severe weather potential. The data shows dew points in the 50s through Wednesday night and that is simply not very much moisture at the surface. Low moisture at the surface traditionally equals lower instability values and that has been the trend in the data. The NAM (which is normally bullish) shows the most instability with values peaking near 1,000 units Wednesday night.
The GFS shows almost no instability whatsoever on Wednesday. For what it is worth, the EURO computer model agrees.
My Thoughts On Severe Weather Chances Wednesday Night
The forcing late Wednesday continues to look somewhat "blah" for our area. It is enough to fire rain and t-storms, but not enough to overpower weaknesses in the wind or instability fields. The wind energy does meet the minimum threshold to support organized severe weather. There is directional wind shear but the most recent data downplays it quite a bit. The instability is definitely consistently showing up in the low category with the exception of the overly aggressive NAM compute model. The low dew points (low 50s) and the fact that the storms come through in the cold part of the data makes the low instability values believable. Wind energy is certainly supportive, but remember organized severe weather requires all three ingredients to be present and they just are not.
AdvanceTrak shows a wave of stronger storms preceding the line of severe storms to our west. This initial wave of storms moves across the area near 1 am Thursday in this run of AdvanceTrak. This appears to be an anomaly in the data, but a trend to watch. Notice the strong storms approaching our area at 4 am Thursday and collapsing when they hit our area. That is the weaker instability coming into play.
The bottom line is the instability is highly questionable on Wednesday night. There is enough wind energy to support a rogue severe storm, but the instability should limit the extent. I can see a isolated severe storm with the main threat as small hail or damaging winds, but nothing suggests an organized event. The Storm Prediction Center using the 5% chance seems justified in this situation. We will watch it, but an organized severe event is off the table at this point.
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