BLOG: SEVERE WEATHER THREAT SATURDAY
We are in the heart of severe weather season across the region but overall it has been a pretty quiet season so far. We did have severe thunderstorms Sunday and Monday in parts of the Valley, but events like that have been rare.This may change Saturday, but it is a difficult forecast.
THE SETUP
The dry, comfy air mass that we have now will quickly be replaced by a much hotter and more humid air mass Friday night into Saturday. Dewpoints will soar into the 60s and temperatures will reach the upper 80s. As you can see, this muggy air will be long gone again by Sunday.
Meanwhile, a cold front will approach from the north and west.
Good setup for severe weather, right? Yes....but will storms actually fire? That's the million dollar question. As you can see in the image above, the model we showed on TV tonight has almost no activity firing ahead of the front. Some other models show a similar lack of activity. Some of the modeling is more "robust".A model profile of the atmosphere Saturday afternoon shows plenty of instability and some decent wind shear. This graphic is very complicated but I have circled (in white) the part that shows the wind direction changing from west-southwest to northwest as you go UP in the atmosphere. This is what we call "wind shear" and is one of the more important ingredients for severe weather.
This much simpler graphic shows the "energy helicity index" which basically combines instability (ability for air parcels to rise) and wind shear. There is a bullseye near or just east of the Valley.
RISKS
The Storm Prediction Center places the Valley in the "Slight Risk" zone, which seems reasonable. I also think it is reasonable to have the heart of that zone centered to our east, as they do. I suspect the highest risk if east of I-79.Any storm that DOES manage to fire could bring down strong winds and hail. We cannot rule out a tornado somewhere in the region.
Remember that, severe storm or not, lightning is ALWAYS dangerous. If you hear thunder, you are close enough to a storm to be struck by lightning. Head inside.
TIMING
I think there can be a round of showers and storms EARLY Saturday along the warm front. After that, there is likely to be a long period of dry, hot and humid weather. Rain chances increase again toward evening.
By the way, that early Saturday activity may be the key to what happens later in the day. If debris clouds stick around for a while after the rain ends (or the rain simply lasts longer), the atmosphere may struggle to "recover" to the point that it can produce tall, heavy storms. That's one of the big forecast challenges.
STAY WEATHER AWARE
Have outdoor plans Saturday afternoon? I would not cancel them! Just be ready for some heat. Stay weather aware. Have multiple ways of getting warnings. SIRENS, in the majority of situations, should be toward the bottom of your list of ways to get warnings. There are much better, more reliable and efficient ways.
If you have not yet, please download the StormTracker 21 app in your app store of choice.More updates Friday.-Eric