Weather experts: can you explain this Area Forecast Discussion?

Question

Over the radio, we heard the civil defense sirens, and rushed inside to check the weather. This is what we found for the area of Peoria, IL.

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE LINCOLN IL

1056 AM CDT MON MAY 26 2008

.DISCUSSION...

ISSUED 1051 AM CDT MON MAY 26 2008

RATHER MESSY ATMOSPHERE THIS MORNING. CONVECTION OVERNIGHT AND

PATCHY ACTIVITY FROM THE PLAINS TO THE MIDWEST HAVE TURNED THE

ATMOSPHERE OVER QUITE A BIT. PLENTY OF MID AND HIGH LEVEL CLOUDS

IN PLACE TO KEEP DESTABILIZATION TO A MINIMUM. PROBLEM MAY BE THE

LIMITED CLEARING OUT IN THE QUAD CITIES AREA. ACTIVITY SHOULD BE

LATER IN THE AFTERNOON. WITH THE VERY SKETCHY CLOUD

SITUATION...TEMPS ARE ALSO A BIT OF A PROBLEM FOR THE FORECAST.

NOT PLANNING ANY IMMEDIATE UPDATES TO THE FORECAST. WILL TAKE

ANOTHER LOOK CLOSER TO NOON...AND TWEAK THE TEMPS AND POPS.

Answer 1

it means there uncertain if there will be a tornado so they will tune back in later to see what will happen

Answer 2

I am Norwegian and we don't have tornadoes as you have in the US. But the principle is the same if it is tornado, hurricane, thyphoon, cyclone, thunderstorm, or simply storm.

You need a convection: i.e. rising air. That creates the low pressure and the wind as the pressures tries to even out.

Then you need colder air in altitude. Because it is the difference of temperature that will make the air to rise; just like in your fireplace's chimney.

So, the colder there is, up there, the more chances to have a really big 'chimney' and the stronger the weather will be.

From the DISCUSSION, I read that the spotted convection. So, there is a danger. They read the temperatures. Yes, a diagram showing different temperatures at different altitudes is the best tool to predict the weather.

Mid and high level clouds keep destabilization to a minimum. Yes, mid level (above 6,500 feet) and high level clouds will prevent to creation of a big 'chimney' going from the surface to the upper atmosphere. The thing is: when humid and warm air rises, after reaching the dew point, the humidity condenses as water droplets, forming clouds. But the adiabatic cooling of saturated air is only half of that of dry air. Hence the air continues climbing. But if there are already intermediate layers of clouds, it will even out.

So, this 'discussion' seems to be a possibility of convection (tornado, thunderstorm, etc.) that doesn't really seems to happen.

This being said, the semantic used by those people is really not what we expect from a European weather station. I guess it's their local lingo.

Answer 3

Sorry, my answer is a little outdated.  You probably wanted this answered earlier.  But if you still wanted to know:

First, keep this in mind:  A hotter surface temperature generally means more unstable, and better chance of thunderstorms.  (Cold temps in the higher atmosphere also make it more unstable, but just keep in mind what I said about hot ground = unstable)

"RATHER MESSY ATMOSPHERE THIS MORNING. CONVECTION OVERNIGHT AND

PATCHY ACTIVITY FROM THE PLAINS TO THE MIDWEST HAVE TURNED THE

ATMOSPHERE OVER QUITE A BIT"

(by messy they mean unclear/unsure of their forecast.  Then, the 'turned atmosphere' means that the earlier thunderstorms in the area have cooled the surface temps and warmed the atmosphere slightly.  So it has made the atmosphere more stable, and so less tendency for more storms.)

"PLENTY OF MID AND HIGH LEVEL CLOUDS

IN PLACE TO KEEP DESTABILIZATION TO A MINIMUM. PROBLEM MAY BE THE

LIMITED CLEARING OUT IN THE QUAD CITIES AREA"

(means lots of clouds are helping to keep the instability at bay, except near the Quad cities where some breaks in the clouds are letting the ground heat up and atmosphere get more unstable again....so they are hinting that quad cities area is more likely than other places nearby to have a new round of t-storms)

ACTIVITY SHOULD BE

LATER IN THE AFTERNOON. WITH THE VERY SKETCHY CLOUD

SITUATION...TEMPS ARE ALSO A BIT OF A PROBLEM FOR THE FORECAST.

NOT PLANNING ANY IMMEDIATE UPDATES TO THE FORECAST. WILL TAKE

ANOTHER LOOK CLOSER TO NOON...AND TWEAK THE TEMPS AND POPS.

(This part is saying that since the the amount of cloud is more/less than they thought, it's messing up their temperature forecast too.  Also, if t-storms are going to happen near quad cities, they'll see signs (like some cumulus forming), and then they'll be more sure of t-storms in that area.  In any event they'll update the temperature forecast, and the POPS (probability of precip. i.e. 20% or 50% or whatever) just after noon.)

Hope that helped.