Most weather events need a storm system, or some sort of forcing, for the event to actually unfold, such as a deep low pressure for a snowstorm or west northwest winds hitting the Appalachians for upslope precipitation. Severe weather becomes very interesting to predict because sometimes it can happen without any forcing mechanism.
So how do we know? Meteorologists use (sometimes on a daily basis, especially during winter storm events and during the summer) a handy dandy product called the skew-t diagram. I'll post a picture of the 0z skew-t from KIAD (Dulles) from 8pm Tuesday night below.
This diagram tells us the information we need to know about the atmosphere and its structure. The data is retrieved via weather balloons and gives us temperature and dewpoint data from the surface to upwards of 9 miles into in the atmosphere (and sometimes beyond that!). On the left-most graphic, the red line is the temperature line and the green line is for dewpoint. The dashed blue lines are critical temperatures, which are -20 degrees C and 0 degrees C (freezing). We use this for winter storms to detect any layers of warm air above the surface. This would cause areas of snow to mix with or change to sleet or freezing rain.
It would be almost impossible to accurately forecast severe weather without using a skew-t diagram. We can see the structure of the atmosphere and determine numerous stability indicies using the diagram. The one draw back is that weather balloons are usually only sent up twice per day. However, when large severe weather outbreaks are forecast, weather balloons will be sent up as needed. Weather balloons are sent up mainly near large airport sites - therefore the closest ones to the Valley are KIAD (Dulles) and KROA (Roanoke). We can view forecast skew-t for KCHO (Charlottesville), but those are just forecast, not actual data.
So what can these magical diagrams tell us? Luckily, the products put out by the Storm Prediction Center calculate many of the variables that we are looking for. Under the graphic with the green and red lines, you'll notice a bunch of numbers and parameters (who would want to calculate all that by hand?!) and their representative values. We're going to talk about many of these parameters throughout the upcoming severe weather season but today we're going to focus on what meteorologists use to see if there will be thunderstorms without any forcing.
Above the big bold word "Supercell" on the lower portion of the graphic, we see two variables: MaxT and ConvT. MaxT is the maximum possible temperature if everything was aligned perfectly for maximum possible heating. The reading of 95 degrees was the maximum possible temperature for Dulles Airport yesterday, whereas the actual high temperature was 93 degrees. ConvT is the Convective Temperature.
Ding ding ding! This is the parameter we look at for the possibility of thunderstorms without forcing. We can go into the whole nitty gritty science of it (if someone asks) but that's the minimum temperature the surface would have to be for there to be the potential for storm initiation. Notice it was 108 degrees F for KIAD at 8pm last night.
One caveat, if you so choose to look at skew-t diagrams on a regular basis, is that those variables are valid at the time of the sounding - 8pm. Temperatures and parameters may be different at 5pm and 11pm. This is where using forecast skew-t diagrams come into play.
To wrap this all up, I used forecast skew-t diagrams to find out our convective temperature today at our max daytime heating. The forecast skew-t use the same models that I've talked about in the past. Looking at three different models, the average convective temperature for SHD is 97 degrees today. Therefore it looks like the possibility of storms is unlikely.
If you want to check out these diagrams on your own, you can visit the Storm Prediction Center's website - this one will take you straight to the sounding page ---> http://w1.spc.woc.noaa.gov/exper/soundings/
We may have some severe weather tomorrow (with forcing from our cold front) so stay tuned to the blog for what we can expect!