WRAL WeatherCenter Blog

Warm Forecast Gone Cold...What's With That?

Posted April 28, 2005

Don't you just love the weather??? Always changing...Always challenging. Challenging is Friday's forecast!

Most of the week we expected Friday to be a warm bright day and then WHUMP!! We feel like the rug has been pulled out from underneath us. It looks like the warmth we previously expected may not be with all of us during the afternoon on Friday.

About 30,000 feet we have the jet stream running in a zonal flow(west to east flow). We saw one weak disturbance embedded in this flow cross our state Thursday evening which brought the clouds and spotty light rain and sprinkles. As I write this we have yet another disturbance embedded in the flow that is over Arkansas and Western Tennessee. Yesterday we thought the trajectory of the jet stream would have taken this more to the north of North Carolina into Virginia and points northward. Now the trajectory would have the disturbance crossing the northern part of North Carolina and southern Virginia. If this turns out to be correct we will see rain the first part of our day on Friday roughly from the Triangle northward.

Why would this rain keep us cool? Well the air across North Carolina is relatively dry at the surface. Dew points as I write this are in the low-mid 40s. If we get rain to fall into this dry airmass, evaporative cooling will take place. The dry air will cause the rain to evaporate as it falls to the ground releasing heat, hence cooling the air. Think of it like when you step out of a pool on a day when the humidity is low. The dry air will cause the water on your body to evaporate or dry. This leaves you feeling chilled. Evaporative cooling will occur in the lower part of the atmosphere where it rains in the morning.

The disturbance will depart in the early afternoon, but the effects of the rain may set the tone for the day and keep the air cool and the clouds in place for most of the area from the Triangle northward into Virginia. We may not see temperatures rise any higher than the mid 60s-low 70s in this area depending on sunshine. Now places in the south like Fayetteville, Southern Pines and Lumberton that may not see the rain will not have any cooling so once the disturbance departs, clouds will decrease and it is these areas that could come close to 80 for the high Friday.

Saturday we have the potential for severe weather and we will talk more about this Friday evening on the blog!
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