Storms Likely On Sunday... Any Severe?
Yesterday I opened the discussion on the severe weather potential on Sunday. When we spoke yesterday, it looked like a few severe could occur, but it didn't look like an outbreak for us. I want to look at the latest data, so we can re-analyze that severe weather threat.
Storm Prediction Center Severe Weather Risk
Let's look at the severe weather risk for both Saturday and Sunday from SPC. Saturday looks like a very obvious severe threat and Sunday is a bit more difficult.
SPC Categorical Severe Weather Risk For 7 AM Saturday to 7 AM Sunday
Notice the large slight risk of severe weather to our west.
SPC Probabilistic Severe Weather Risk For 7 AM Saturday to 7 AM Sunday
Notice the huge 30% chance of severe weather over Iowa and Missouri. Large 30% probabilities on day 3 are normally a precursor to an upgrade to a moderate risk.
Now let's move on to Sunday...
SPC Probabilistic Severe Weather Risk For 7 AM Sunday to 7 AM Monday
Notice SPC doesn't have a slight risk for anyone on Sunday.
Severe Weather Discussion For The Weekend
Right now there is no data that suggest any severe weather threat for us on Saturday, but it will likely be an outbreak over Iowa and Missouri. Sunday is the timeframe when the cold front will be moving across our area, so that will be the focus of this blog. As we dive in, keep in mind the items we look for when assessing a severe weather threat.
While the forcing is not incredible on Sunday, it is enough to support a t-storm chance and this should be enough to fire storms. I have a 60% chance on Sunday.
The wind energy on Sunday is actually very strong. The latest GFS and NAM have come into better agreement on the system settling on the slower solution I suggested yesterday. Let's look at that wind profile from the ground up.
NAM Surface Winds Around 2 PM Sunday
Notice the surface winds are out of the southwest at about 10 knots or 10 - 15 mph.
NAM Winds 1 Mile Above The Ground 2 PM Sunday
Notice the winds at this height are out of the west-southwest and they are strong. The winds 1 mile above the ground are near 40 knots or 45 mph.
NAM Winds 6 Miles Above The Ground 2 PM Sunday
Notice the winds at this height are also out of the west-southwest at about 55 knots or 63 mph.
We don't see a lot of directional change in the winds with height, but the speed increases fast as you go upward in the atmosphere. This is called speed shear and normally supports damaging winds as the primary threat.
I have to say this will be the difficult area for Sunday. Since it appears the storms will come through our area early afternoon on Sunday, we won't have as much time to get warm as we would if the storms came through at 5 pm. The other complexity is that the NAM computer model seems to suggest a second line of storms could form behind the initial line Sunday evening. Since this is 4 days away, the small details simply cannot be resolved yet. With that said, let's look at 2 pm Sunday for instability values.
GFS Instability Values 2 PM Sunday
Notice the GFS shows about 500 - 750 units of instability in our area early Sunday afternoon.
NAM Instability Values 2 PM Sunday
Notice the NAM shows a very different picture with instability values up to 1,500 in our area. Notice the instability values are a bit further west than the GFS and this is why I think a second line could form along the front late Sunday.
My Thoughts On Severe Weather Sunday
The wind energy is more than supportive of a severe weather risk on Sunday. It is not what I would call bullish, but it is strong for this time of the year. The wind shear is not the more dangerous directional shear, but what we call speed shear. Speed shear is supportive of damaging winds in a setup like this. The forcing is not superb, but good enough to fire storms on Sunday. Finally our instability is lower tier, but the NAM does suggest it could peak over 1,000 units and with this wind energy that is more than enough to support severe weather.
In my opinion, I would place our area in a slight risk of severe weather. As I see it now, I think damaging winds would be the primary threat, but the magnitude of wind energy would support an isolated tornado threat. I feel comfortable saying I think we should be in a slight risk and this one needs to be watched closely. Since the low pressure is over Baja California right now, it is indeed possible that the low pressure could slow down. If it did, that would heighten the severe weather threat a little since the storms would come through during the heat of the afternoon. Right now the NAM shows the storms moving through around 2 pm, so this will be the item we watch very closely in the coming days.
I am starting to see evidence that a second line would form in the evening and that is going to be something to watch closely. A second line in the evening would have a good chance of producing severe weather. More details in the next few days.
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