The satellite image below was taken late Thursday evening. The obvious feature of interest is the thunderstorm cluster that's formed over South Dakota. Experience tells us that despite what models may show, what goes on within the convection structure of thunderstorms tonight can often dictate where tomorrow's storms form. A big factor in determining our weather the next 24 hours is tied to outflow boundaries, a complex and intricate part of forecasting.
So lets define outflow and why as as forecasters we should care about its existence
During the early stages of a thunderstorm's life cycle, its updraft (the warm, moist air flowing into the storm) feeds the structure and energy it needs to allow it to grow in size and strength.
Eventually, the weight of the water ingested into a growing thunderstorm will become too great for the updraft to sustain, causing precipitation to fall and a downdraft to develop. The downdraft, or the cooler air that sinks out of the base of the thunderstorm, will pool near the ground beneath the storm in what's known as a "cold pool." This cold pool as the thunderstorms matures will eventually begin spreading out away from its core like a ripple on a pond.
When the cold pool starts radiating out away from the parent thunderstorm, it's known as an "outflow boundary." The outflow boundary acts like a mini cold front, bringing with it cooler temperatures and gusty winds. Oftentimes, these outflow boundaries are able to force the warm, moist air ahead of them to rise, triggering even more thunderstorm activity. This is extremely common over the tropics and the Gulf Coast; a couple of storms will form and dissipate, sending out outflow boundaries that trigger a domino effect of thunderstorms that continues until the sun sets and the instability subsides.
While these pop-up storms are the most familiar, all types of thunderstorms, single cell, multicell (clusters or lines), and supercells — all produce outflow boundaries. While our models attempt to approximate where these mini cold fronts will be at any given time, it's a challenging variable to foresee.
That brings us to the situation at hand which is how Thursday night's outflow boundaries will impact storms on Friday.
Let me show you the cloud tops derived from Doppler radar. In this image I used a scale to see that the tops of the thunderstorm anvils were reaching as high as 45,000 feet above the earth. In other words, the storms you see in the radar image below are standing vertically 8 to 9 miles tall.
At that level they are poking into the atmosphere well above jet stream level which is approximately 30,000 ft. The air here is super-cooled and generally well below freezing. The downdraft and its rain cooled air originates at this level and when sufficiently strong. enough, the downdraft pulls the cold air to the surface and creates what's known as an outflow boundary. In the radar image of Thursday night's storms, you can see 2 outflow boundaries rippling out well ahead of the primary convection.
This "storm made" forcing is likely to drive convection, especially with strong warm advection feeding the cluster. However, as the night goes on the low level jet weakens and the days heating dissipates leading to lower levels of instability. and weakening or decaying storms. That in turn weakens the storms and potentially kills them. The question is, when? I have seen numerous examples of strong MCS clusters holding together well into the morning. The sustained cold pool allows them to get much further south than what models indicate. Each case is different.
One thing I can clearly see at 500mb is that the wind vectors Thursday night have a northwesterly component which is going to force the storms southeast into northern Iowa and NW Illinois through Friday. This is where the strength of the outflow boundaries becomes very important as it impacts how far south the storm clusters get before they weaken and dissipate. The GFS is still showing a healthy batch of storms over the northern third of Iowa at 9:00am Friday.
The EURO is less organized and that may very well be due to what it perceives as a weaker outflow boundary.
There's often no real way to know these intricate details which could mean the difference between a little rain or 1 to 2 inches. About the best a forecaster can do is indicate the chance is there for storms in the northern half of my area through morning.
By noon the storms should have dissipated but here's the kicker, there's abundant residual moisture still in place and later in the afternoon new storms are likely to pop as instability reaches its maximum and the convective temperature is attained. They will then create new outflow boundaries that dictate the position and movement of Friday nights storms. This will be important because moisture is going to be plentiful and instability high. That means any storms that form are going to be high precipitation producers.
Again, what happens with the remnants of Thursday nights storms will determine where new ones fire Friday afternoon and evening. The further south the first batch gets, the more likely the new ones today will form in the southern half of my later in the day. That seems to be the trend in recent models.
However it plays out, between the 2 rounds of potential convection Friday, the models show another healthy rain for parts of my area that really don't need it. The GFS shows these amounts through Saturday morning.
The EURO has this for the same period. You can see the important differences in amounts and location related to forcing that in some cases are outflow generated.
Aside from heavy rain the other concern for Friday afternoon is that some of the instability will be sufficient to produce strong enough updrafts for some high wind and perhaps a bit of hail. Shear is marginal so the tornado threat appears to be low. The Storm Prediction Center shows a slight to enhanced risk of severe storms in my local area.
All things considered it appears the weekend will get off to an unsettled start with scattered thunderstorms that have the potential of producing heavy rain and severe weather. Keep an eye to the sky Roll weather...TS