The return of spring-like temperatures this week has been swift and furious. Unexpected warmth, blue skies and blooming flowers definitely set the mood for spring – and maybe some yearning for a thunderstorm.

If you are one of them, this weekend is for you. But be careful, thunderstorms can be severe.

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Two separate shortwaves would radiate northeast through the Plains along the western periphery of a ridge. The first is scheduled for Saturday while the second is scheduled for late Sunday. Each type will affect a different area.

Saturday

This particular threat is probably a little further north than you might expect, at least when you think about the mid-severe spring event. And that fact may hinder its potential a bit.

Deeper lows will pull moisture northward into Iowa, Missouri, Illinois, and even Wisconsin. The dewpoints obtained should be sufficient to facilitate a serious threat – although perhaps just barely.

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I mentioned the oddity of the location for this event compared to the time of year. Using the map above, we see why. Dew points, although small for inclement weather, are forecast to be well (25 to 30 degrees+) above “normal” at this time.

Due to limited humidity/instability, this event can be difficult in production and will likely rely heavily on lift provided by the front.

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By examining the forecast sound, we can determine a number of things.

  1. Have the appropriate cutting force. If the storms could strengthen in the first place, there would be enough shear to sustain the supercells and/or create a tornado or two.
  2. A “skinny” (lack of stability) CAPE means that update streams can struggle to pick up speed, keeping storms from getting much stronger.
  3. There is a low or limited inversion. Here the temperature increases with height and we will need a lifting mechanism (front) or increase the instability to break it.
  4. There’s a decent amount of dry air in between. This supports the formation of degraded structures and poses the threat of damaging winds. The wind in this “dry layer” is ~55 kts. These are the speeds that can be teleported to the surface in an explosion.

Based on what the sound tells us, we certainly have a chance of severe storms, but limited humidity and therefore limited instability could mean that this event is a must. shuffle to create. We still have more than 48 hours left, so things could change – in favor of a stronger event or a weaker event. We’ll be monitoring trends and revisiting on Saturday morning.

Sunday

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Just looking at the location (closer to the Bay) for Sunday’s event, we can assume that conditions will be slightly more favorable for extreme weather, thermodynamically at least.

And it’s true, we are expecting more humidity/unsettledness in the form of higher dew point, however, this will be a nighttime event. If there’s no possibility of further turbulence through the daytime heating, we’ll rely on lifting and cutting to get things going, so to speak.

This then becomes the High Cut, Low CAPE (HSLC) event. These events can be complex. Sometimes they don’t produce much. Sometimes they produce more than expected. Consider a predicted sound:

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We have:

  1. Cut well with the order of about 45 knots. Sufficient to maintain supercells. Winds also turn with altitude, allowing tornadoes to be created.
  2. The “skinny” CAPE can get in the way of boosting as updates struggle to get up to speed.
  3. Light lid. It would take some extra lift or instability to break it.
  4. Dry air in between floors can create a threat of damaging winds
  5. Something we didn’t see on Saturday – low LCL. The parcels wouldn’t have to be lifted very far before being accelerated into the atmosphere. Low LCLs are common in moisture-rich environments near the Gulf
  6. Unlike Saturday’s event, there’s a lack of cold overhead and thus the lapse rate isn’t the best. Withdrawals can be difficult to boost.

Overall, this isn’t a perfect setup for inclement weather. Far from it, really. However, a lot of clipping can correct the instability.

We still have about 3 days left. If humidity increases in future projections, that could change the available volatility. The same can be said if it decreases. We will also revisit this setup on Saturday.

Since a lot can change in 2 to 3 days, especially when a lot of mid-level details are focused on, just know for now that there is the threat of severe weather on Saturday. and Sunday. Sunday is expected to pass overnight, continuing through Monday morning, so start preparing if you’re in the risk zone.

As I said above, we’ll review these threats in a blog Saturday morning and see what’s changed, if anything, and what we’re looking forward to.

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