Updated 800am CT – Thursday / 18 September 2014
…. Flash Flood Watch Remains in Effect ….
…. Abundant Moisture and Passing Upper Air Low Pressure Disturbances ….
…. Rain Chances Decrease by Friday Night into the Weekend ….
On this Thursday morning…. a weakening stationary front extends from south central Mississippi northwestward into southwestern Arkansas and eastern Oklahoma and then into Kansas. A surface low pressure trough extends south southwestward from western Kansas into far west Texas and eastern New Mexico. As a result of this surface weather pattern, our local surface winds are southeasterly.
In the upper levels of the atmosphere… a broad trough.. or line.. of upper level low pressure is moving eastward through the eastern third of the USA with another trough of low pressure moving ashore onto the west coast. The upper air reflection of Pacific tropical cyclone Odile.. with abundant high and mid level Pacific moisture.. is located over southwestern New Mexico west of El Paso. Weakened upper level high pressure.. acting like a lid on the atmosphere.. as been shoved southward into northeastern Mexico and deep south Texas. As a result of this pattern, the upper level winds over south central Texas and the Austin metro area, about 18,000 feet above the ground, are west northwesterly at 20 to 25 mph.
After overnight flooding in the Austin metro area.. with rainfall amounts of up to 6 inches and continuous lightning (in the 2 to 3 o’clock hour this morning – in an hours’ time – I actually saw, through our lightning data recording equipment, almost 5000 distinctive strokes of lightning in west Austin!!), things have now become a bit more quiet.
However, with the upper level trough to our northeast and the advancement eastward of the remnants of Odile, the upper level high pressure area, or the “lid” on the atmosphere, has been pushed southward in Mexico. At the same time.. abundant low level moisture continues, along with an increasingly unstable air mass, continues over the area. As the upper air disturbances.. from the west.. traverse the area and creates atmospheric lift, we’ll continue to see much better rain and thunderstorm chances through tomorrow. The latest computer and NWS manual guidance suggests that we could see another 1 to 3 inches of rain with isolated heavier amounts. While the rainfall will certainly be beneficial in nature overall, where rainfall occurs in areas that were flooded earlier today, we could see rapid onset of flooding. At the least, localized urban and small stream/low water crossing flooding is possible. In addition, with cloud cover and precipitation chances over the next few days, afternoon high temperatures remaining slightly below seasonal averages.
By late Friday into the weekend, the upper level high pressure ridge (the “lid” on the atmosphere) will strengthen a bit with rain chances coming down and clouds thinning out a bit.
Guidance suggests that another cold front will move southeastward across the area by late Sunday into early Monday with a renewed chance of rain showers and thunderstorms.
Regarding my forecast, my local forecast confidence: My forecast confidence over the next two days is high to very high (still concerned with the exact rain timing and amounts). My confidence is very high Friday night through much of the upcoming weekend. My forecast confidence falls back to medium to high for late Sunday night into Monday in reference to precipitation and cold frontal timing.
Have a good Thursday….
Meteorologist Troy Kimmel