Archive for December 18th, 2011

Severe Weather & Tornado Threat on Monday

Published on Sunday, 18 December 2011 21:24

David Reimer

0 Comments

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_5004" align="aligncenter" width="550" <strong class='StrictlyAutoTagBold'>captionstrong>="Day 2 Severe Weather Outlook"][/<strong class='StrictlyAutoTagBold'>captionstrong>]

Lets start off by taking a look at the latest <strong class=’StrictlyAutoTagBold’>severestrong> weather outlook from the Storm Prediction Center. This outlook was issued at 11:30 AM on Sunday. The next outlook will be issued around Midnight tonight and we’ll bring it to you then. At this time, a standard risk of <strong class=’StrictlyAutoTagBold’>severestrong> weather has been issued for much of Central and Eastern Texas. Major cities in the <strong class=’StrictlyAutoTagBold’>severestrong> weather risk include Victoria, San Antonio, Austin, Waco, D/FW, and Houston. The standard risk of <strong class=’StrictlyAutoTagBold’>severestrong> weather can be identified by the orange <strong class=’StrictlyAutoTagBold’>linestrong>. Areas within this orange <strong class=’StrictlyAutoTagBold’>linestrong> have a 15% chance of experiencing <strong class=’StrictlyAutoTagBold’>severestrong> weather within 25 miles of their location <strong class=’StrictlyAutoTagBold’>on Mondaystrong>. Outside the standard risk of <strong class=’StrictlyAutoTagBold’>severestrong> weather is a <strong class=’StrictlyAutoTagBold’>lowstrong> chance of <strong class=’StrictlyAutoTagBold’>severestrong> weather, identified by the dotted brown <strong class=’StrictlyAutoTagBold’>linestrong>. This includes much of the state of Texas. Severe weather is defined as tornadoes, hail larger then one inch in diameter, and <strong class=’StrictlyAutoTagBold’>windsstrong> in excess of 58 MPH. Lightning is not included in the <strong class=’StrictlyAutoTagBold’>severestrong> weather criteria. I wouldn’t be surprised to see this outlook shifted a bit with the new issuance at midnight.

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_5005" align="aligncenter" width="550" <strong class='StrictlyAutoTagBold'>captionstrong>="0Z NAM: 500 Millibar <strong class='StrictlyAutoTagBold'>windsstrong> at 6 <strong class='StrictlyAutoTagBold'>PMstrong> Monday"][/<strong class='StrictlyAutoTagBold'>captionstrong>]

Without adequate mid to upper <strong class=’StrictlyAutoTagBold’>levelstrong> <strong class=’StrictlyAutoTagBold’>supportstrong> and associated <strong class=’StrictlyAutoTagBold’>windstrong> shear, storms really aren’t able to get past the multicellular phase. That’s why we always start off by looking at 500 millibars, which is at about 18,500 feet above sea <strong class=’StrictlyAutoTagBold’>levelstrong>. When looking for the possibility of organized convection, we like to see <strong class=’StrictlyAutoTagBold’>windsstrong> out of the west or southwest with speeds at or above about 40 knots. In the spring and summer time, we normally have issues with upper <strong class=’StrictlyAutoTagBold’>levelstrong> <strong class=’StrictlyAutoTagBold’>supportstrong>. We do not normally have that issue in the cool season. Tomorrow’s system is quite strong and should have more then enough upper <strong class=’StrictlyAutoTagBold’>levelstrong> <strong class=’StrictlyAutoTagBold’>supportstrong> for organized thunderstorms. Winds should be at or above 60 knots, approaching 80 knots in some locations.

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_5006" align="aligncenter" width="550" <strong class='StrictlyAutoTagBold'>captionstrong>="0Z NAM: 850 Millibar <strong class='StrictlyAutoTagBold'>windsstrong> at 6 <strong class='StrictlyAutoTagBold'>PMstrong> Monday"][/<strong class='StrictlyAutoTagBold'>captionstrong>]

Descending down to 850 millibars, which is around 5,000 feet above sea <strong class=’StrictlyAutoTagBold’>levelstrong>, we analyze the lower parts of the atmosphere. Wind direction makes a world of difference in the <strong class=’StrictlyAutoTagBold’>lowstrong> levels as it helps determine the amount of turning in the atmosphere. When looking for the potential of <strong class=’StrictlyAutoTagBold’>lowstrong> <strong class=’StrictlyAutoTagBold’>levelstrong> rotation and tornadoes, we want to see a sustained <strong class=’StrictlyAutoTagBold’>lowstrong> <strong class=’StrictlyAutoTagBold’>levelstrong> jet. Wind speeds normally need to be at or above 30 knots to <strong class=’StrictlyAutoTagBold’>supportstrong> sustained supercells and their attendant <strong class=’StrictlyAutoTagBold’>lowstrong> <strong class=’StrictlyAutoTagBold’>levelstrong> mesocyclones. Wind direction is also important. If <strong class=’StrictlyAutoTagBold’>windsstrong> are out of the southwest, that normally supports a more linear mode (squall <strong class=’StrictlyAutoTagBold’>linestrong>) and lessens the potential tornado risk. If <strong class=’StrictlyAutoTagBold’>windsstrong> are out of southeast, that creates turning with height and thus the potential of <strong class=’StrictlyAutoTagBold’>lowstrong> <strong class=’StrictlyAutoTagBold’>levelstrong> circulations. Looking at tomorrow, we have a very strong <strong class=’StrictlyAutoTagBold’>lowstrong> <strong class=’StrictlyAutoTagBold’>levelstrong> jet that should be out of the south/southeast. Winds will be in excess of 50 to 60 knots. In terms of <strong class=’StrictlyAutoTagBold’>windstrong> shear, this system is very strong. We do see systems like this during the winter time, but we don’t normally have the <strong class=’StrictlyAutoTagBold’>instabilitystrong> necessary to <strong class=’StrictlyAutoTagBold’>supportstrong> organized thunderstorms. In fact, we had a setup like this about two weeks ago when parts of North Texas had snow. It all depends on the positioning of the weather features. If this storm was about two hundred miles further south, we would end up be dealing with a snow storm in North Texas most likely, but since we’re on the southeastern edge of the system, we’re in the warm sector.

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_5007" align="aligncenter" width="550" <strong class='StrictlyAutoTagBold'>captionstrong>="0Z NAM: Surface dewpoint values at 6 AM Monday"][/<strong class='StrictlyAutoTagBold'>captionstrong>]

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_5010" align="aligncenter" width="550" <strong class='StrictlyAutoTagBold'>captionstrong>="0Z NAM: Surface dewpoint values at 12 <strong class='StrictlyAutoTagBold'>PMstrong> Monday"][/<strong class='StrictlyAutoTagBold'>captionstrong>]

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_5009" align="aligncenter" width="550" <strong class='StrictlyAutoTagBold'>captionstrong>="0Z NAM: Surface dewpoint values at 6 <strong class='StrictlyAutoTagBold'>PMstrong> Monday"][/<strong class='StrictlyAutoTagBold'>captionstrong>]

Right now, dewpoint values across most of the state are around 40 degrees with higher values closer to the Gulf Coast as moisture return has already begun early this evening. As the storm system responsible for all this crazy weather across the state continues to approach, a very strong <strong class=’StrictlyAutoTagBold’>lowstrong> <strong class=’StrictlyAutoTagBold’>levelstrong> jet will develop. This <strong class=’StrictlyAutoTagBold’>lowstrong> <strong class=’StrictlyAutoTagBold’>levelstrong> jet will rapidly move moisture northward across the state. In fact, this may be one of the fastest moisture return scenarios I’ve seen. Depending on how high the dewpoints get will determine how significant Monday’s <strong class=’StrictlyAutoTagBold’>severestrong> weather threat will end up being. Current data suggests that dewpoints will be sufficient to <strong class=’StrictlyAutoTagBold’>supportstrong> organized <strong class=’StrictlyAutoTagBold’>severestrong> weather, including surface based thunderstorms. The primary ingredient that is still in question are the <strong class=’StrictlyAutoTagBold’>instabilitystrong> amounts. These three graphics show how quickly the higher dewpoint values will be moving across the state. Each of these graphics are six hours apart, beginning at 6 AM and ending at 6 <strong class=’StrictlyAutoTagBold’>PMstrong>. Unless something significant changes which prevents the moisture from moving northward, I’m not concerned about a lack of moisture.

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_5011" align="aligncenter" width="550" <strong class='StrictlyAutoTagBold'>captionstrong>="0Z NAM: Surface Based Instability values at 6 <strong class='StrictlyAutoTagBold'>PMstrong> Monday"][/<strong class='StrictlyAutoTagBold'>captionstrong>]

As is often the case with cool season events, determining how much <strong class=’StrictlyAutoTagBold’>instabilitystrong> will be present during an event is the primary complication to a forecast. Several factors affect <strong class=’StrictlyAutoTagBold’>instabilitystrong> amounts. My concerns for tomorrow‘s <strong class=’StrictlyAutoTagBold’>instabilitystrong> values are going to be how far north the 60 degree dewpoint <strong class=’StrictlyAutoTagBold’>linestrong> gets and if we have any breaks in the cloud cover tomorrow. A few days ago, it looked like <strong class=’StrictlyAutoTagBold’>instabilitystrong> values would be below 500 joules per kilogram, thus limiting the <strong class=’StrictlyAutoTagBold’>severestrong> weather threat. Models have increased the expected <strong class=’StrictlyAutoTagBold’>instabilitystrong> values and are now showing between 750 and 1,000 joules per kilogram across portions of North and Central Texas. At 6 <strong class=’StrictlyAutoTagBold’>PMstrong>, the NAM (North American Model) has the primary <strong class=’StrictlyAutoTagBold’>instabilitystrong> axis along the Interstate 35 corridor from San Antonio north to Fort Worth. With the strong upper <strong class=’StrictlyAutoTagBold’>levelstrong> disturbance moving in and a very favorable <strong class=’StrictlyAutoTagBold’>windstrong> field, it will not take a lot of <strong class=’StrictlyAutoTagBold’>instabilitystrong> to <strong class=’StrictlyAutoTagBold’>supportstrong> organized <strong class=’StrictlyAutoTagBold’>severestrong> weather. It’s always difficult to determine the exact ratio needed in terms of <strong class=’StrictlyAutoTagBold’>windstrong> shear and <strong class=’StrictlyAutoTagBold’>instabilitystrong>, but 750 joules per kilogram is definitely enough to <strong class=’StrictlyAutoTagBold’>supportstrong> <strong class=’StrictlyAutoTagBold’>severestrong> weather <strong class=’StrictlyAutoTagBold’>on Mondaystrong>.

Lets talk about specifics. The overall pattern is supportive of a linear complex of thunderstorms, known as a squall <strong class=’StrictlyAutoTagBold’>linestrong>. If we didn’t have sufficient <strong class=’StrictlyAutoTagBold’>instabilitystrong> in place, this <strong class=’StrictlyAutoTagBold’>linestrong> would likely only be your normal <strong class=’StrictlyAutoTagBold’>linestrong> of thunderstorms with lightning and thunder, nothing else. However, it appears we will have the <strong class=’StrictlyAutoTagBold’>instabilitystrong> needed to <strong class=’StrictlyAutoTagBold’>supportstrong> <strong class=’StrictlyAutoTagBold’>severestrong> weather across Central and North Texas <strong class=’StrictlyAutoTagBold’>on Mondaystrong>. Around 12 <strong class=’StrictlyAutoTagBold’>PMstrong>, the <strong class=’StrictlyAutoTagBold’>linestrong> will likely be located from Wichita Falls south to Abilene, maybe a bit west of there. By 6 <strong class=’StrictlyAutoTagBold’>PMstrong>, this <strong class=’StrictlyAutoTagBold’>linestrong> will be on or approaching the I-35 corridor. The <strong class=’StrictlyAutoTagBold’>instabilitystrong> axis will be moving east along the <strong class=’StrictlyAutoTagBold’>linestrong> of storms. Because of the moist atmosphere and the strong <strong class=’StrictlyAutoTagBold’>windsstrong> aloft, it won’t take much to transport down the strong <strong class=’StrictlyAutoTagBold’>windsstrong> located just a few thousand feet above the ground down to the surface. Thus, I do expect damaging <strong class=’StrictlyAutoTagBold’>windsstrong> to be the primary threat <strong class=’StrictlyAutoTagBold’>on Mondaystrong>. How widespread the damaging <strong class=’StrictlyAutoTagBold’>windstrong> threat will be is going to depend on the <strong class=’StrictlyAutoTagBold’>instabilitystrong> amounts, which will be affected by any breaks in the clouds plus mesoscale features that we can’t determine until a few hours out. In the area of highest <strong class=’StrictlyAutoTagBold’>instabilitystrong>, generally between Austin and D/FW, is an area which I’m concerned about in regards to a tornado threat. If we we’re dealing with discrete supercells tomorrow instead of a linear squall <strong class=’StrictlyAutoTagBold’>linestrong>, it would be a major tornado outbreak across the area. At this point, it appears that will not be the case. However, any storms that form ahead of the squall <strong class=’StrictlyAutoTagBold’>linestrong> will have the potential to become supercellular and produce a tornado. Brief tornadoes are also possible within the squall <strong class=’StrictlyAutoTagBold’>linestrong>.

Honestly, I’ll be able to be much more specific about the threats in the morning. At the least, we should have a threat of damaging <strong class=’StrictlyAutoTagBold’>windsstrong> with the squall <strong class=’StrictlyAutoTagBold’>linestrong> along with a tornado risk across Central Texas. We’re going to be cloudy most of the day. It may not feel like a <strong class=’StrictlyAutoTagBold’>severestrong> weather day when you wake up in the morning, but trust me, that is going to quickly change. One final thing to note about tomorrow‘s setup. Storms will be moving very quickly, especially out ahead of the squall <strong class=’StrictlyAutoTagBold’>linestrong>. We may be dealing with 60 to 65 MPH storm motions, so if you go under a warning, take action immediately because you will not have much time. We’ll be posting plenty of updates tonight and tomorrow regarding any developments on the <strong class=’StrictlyAutoTagBold’>severestrong> weather threat. I know you normally expect <strong class=’StrictlyAutoTagBold’>severestrong> weather a week before Christmas, but it can happen. Don’t lower your guard because we’re in December.

In terms of my chasing plans, I am planning on chasing tomorrow. My current thinking is that I’ll head south on Interstate 35 towards Waco, but I’ll likely refine my plans when I wake up in the morning. I’ll have my live streaming video up and running so be sure to stop by! Those who have watched before know I can be entertaining with the audio running.