Thursday, October 8, 2009
Heavy Rain, High Winds, and Isolated Tornadoes Possible!
A strong cold front will be moving into East Texas Early Friday morning and along and ahead of this front, there is a chance of severe storms, including isolated tornadoes. The image above is of a forecast radar image for 7AM tomorrow morning. Instead of seeing a straight line of storms we see more of a wavy pattern. This is called a line echo wave pattern and in many cases this type of storm system will produce very gusty winds and isolated tornadoes. Many East Texas tornadoes occur in patterns such as the one moving through East Texas tomorrow morning.
In addition to a slight risk of severe weather, heavy rain will be a concern. The amount of moisture available for thunderstorms is unusually high for this time of the year thanks to a strong southerly flow off the Gulf this afternoon. As the cold front moves into East Texas, it will force this warm moist air up and over the front producing widespread heavy rain behind the main line of storms. Some places, mainly north of I-20, could see 2 to 4 inches of rain tomorrow. The image below shows the storm total precipitation that is forecast. Notice the heavy rain forecast across the northwest counties of East Texas, where flooding was a problem in September. We also see a bulls eye of heavy rain across our southeastern counties however, we have not seen as much rain here over the past 30 days so flooding is not as great of a concern.
Now most of the dynamic energy needed for severe weather will move north of our area but there are a few features that jump out at me that could lead to gusty winds and isolated tornadoes. The image below is from a short range high resolution forecast model which shows the amount of instability available as the line of storms enters East Texas tomorrow. Most of East Texas is colored in pink which indicates surface based lifted index values less than -6°C. This is more than enough instability for intense thunderstorm updrafts.
The next image shows there will be lots of moisture available as the dark blue colors indicates dew point temperatures in the low 70. You will also see a couple of small low pressure areas, or mesoscale lows, indicting areas of backed winds leading to low level rotation. With the amount of moisture available, cloud bases will be very low increasing the threat of isolated tornadoes.
There will also be moderate amounts of energy to help with thunderstorm development. The image below shows the amount of surface based CAPE. A large area of East Texas has CAPE values over 2000 J/kg out ahead of the storms, providing additional energy for thunderstorm updraft. What is not shown is much of that energy is confined to the lowest levels of the atmosphere, where there is more spin available. This could help in developing low level mesocyclones which could produce an isolated tornado.
In the final image, the amount of 0-1km storm relative helicity, or the amount of low level spin in the atmosphere, is shown. Most of East Texas is at or above 160 which is enough when you combine the amount of CAPE to produce an isolated tornado. You can also see areas of enhanced 0-1km helicity near the mesoscale low pressure areas. This could really enhance the possibility of isolated tornadoes as the line moves through.
So as this line of storms moves through tomorrow morning, we could see some wind damage and isolated tornadoes. Overall I believe the main threat will be flooding rains but we will be monitoring the situation very closely.