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Temperatures slowly warm, showers return, and a quick lesson in severe weather

Scattered showers return late Sunday.
Credit: WHAS
Showers increase in coverage Sunday night.

LOUISVILLE, Ky. — We’ve had an up-and-down ride of weather the past few days with Friday and Saturday being the most extreme. Friday saw a high temperature of 76° that crashed to a high of only 44° Saturday! Brrr. The good news is we begin a gradual warming trend Sunday, but could more thunderstorms and severe weather be in our future?

Forecast at-a-glance:

  • Scattered showers return late Sunday into early Monday with temps recovering to the 50s
  • Temps continue to warm into the 60s for the first part of the week
  • 70s are possible too, especially Thursday and possibly Friday
  • Rain and thunderstorms are expected Tuesday, but what about severe weather?

Weaher setup: A powerful cold front Friday sent temps crashing from the middle 70s Friday into only the middle 40s (and even upper 30s for some!) Saturday. Stubborn clouds and a north wind worked against us. It was certainly a dreary day and fitting for the general mood across the country.

Clouds will dissipate some Saturday night thanks to some dry air moving in, but it doesn’t last long. Any sunshine we might see Sunday morning will be short-lived as clouds build back in for the afternoon ahead of an area of rain showers. Temperatures will only be in the lower 50s with an east wind. Models aren’t quite handling the shower activity as well as I’d like, but we’ll be dry for the morning with the likelihood of rain increasing through the afternoon and especially the evening. No thunder is expected, but a few downpours could be possible.

Credit: WHAS
Pockets of showers arrive Sunday afternoon.
Credit: WHAS
Showers increase in coverage Sunday night.

Although futurecast might look impressive, rain totals will generally be less than half an inch across the area. The scattered shower activity will end by mid-morning Monday after which clouds will again try to disperse. Should they, expect temperatures to climb into the upper 50s Monday afternoon. Should they not, which is at all possible, we’ll likely be stuck in the lower-to-middle 50s. Not bad, but still below our normal high which is now 60°!

Credit: WHAS
Showers will be ending around mid-morning Monday.
Credit: WHAS

Our attention then turns to the west as a storm will be developing in the lee of the Rockies late Monday. An area of low pressure will form near southern Kansas Tuesday and strengthen as it approaches Louisville. A warm front and cold front will form as well, and this sets the stage for some potential severe weather.

Tuesday’s severe weather potential: Tuesday is still three days away, so a lot will change between now and then, but there are early signs that strong thunderstorms are possible. As with the past few events, western and southern Kentucky are better situated to see these thunderstorms. The warm front I mentioned above will lift north through Tuesday and bring warmer, more moist air with it. That’s what thunderstorms need in order to be healthy. There are uncertainties, however:

  • How much sunshine will there be? Sunshine is critical for storm intensity as it helps fuel them.
  • What time will the front arrive? The later the front, the less likely our chances for bad weather.

Models currently time storms for Tuesday afternoon and evening, but we’ll have to monitor how this changes.

Credit: WHAS
This particular model (NAM) shows rain approaching Tuesday afternoon as a warm front lifts northward.
Credit: WHAS
The location of where severe storms may form Tuesday afternoon still needs to be worked out among the models.

What makes for severe weather? Let’s take a very simple look at what is needed for severe weather. There are four key ingredients:

  • Moisture
  • Instability
  • Lift
  • Wind shear

Moisture: When thunderstorms (or any rain for that matter) occur, it’s because the atmosphere is considered unstable. Moist air is more unstable than dry air and promotes lift, especially in the lower levels of the atmosphere. The higher the moisture content, the higher the potential for heavy rain as well.

Instability: Instability in the atmosphere is measured by CAPE. CAPE stands for Convective Available Potential Energy. In other words, CAPE is the amount of fuel a thunderstorm hypothetically has at its disposal to feed from. Higher CAPE = more fuel. The atmosphere is unstable if a parcel of lifted air is warmer than the air surrounding it. Think of a hot air balloon; it only rises because the air inside the balloon is much warmer than the air outside of it. If it was cooler than the outside air, it would sink. The same applies for these hypothetical invisible bubbles of air in the atmosphere! In order to get higher instability, you not only need moisture, but ample sunshine to warm the surface and help get those parcels moving.

Credit: WHAS
NAM forecast CAPE for Tuesday evening. There isn't much, but it's enough! The bullseye (for now) is western Kentucky and Tennessee.

Lift: Just what it sounds like, air doesn’t move of its own free will, something has to make it move. There are many things that can encourage lift such as a front, outflow boundary, topography, or upper level divergence. What’s important is that there’s enough umf behind that lift to get that parcel of air high enough in the atmosphere.

Wind shear: Wind shear is the difference in wind speed and direction with height. Wind shear is important because it can separate updrafts and downdrafts in thunderstorms. It’s also critical in how strong a thunderstorm can get. If wind shear is low, rain will fall through the column that air is rising in and quickly kill the storm.

If you’d like to read more about thunderstorm and severe weather development, here’s a link to a great resource.

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