cdcollura
EF5
Good day,
A negative tilt can be a good or bad thing, as too much of ANYTHING can have bad points.
We have to bear in mind something important that tends to happen if low-level backed winds, developing low-pressure, moisture convergence AND a negatively tilted trough being not "quite" in the right places above and below each other.
In other words, having a strong difluent upper jet exit region, with H5 (500 MB / 18,000') winds WSW at 90 Kts and H3 (300 MB / 30,000') winds west at 120 Kts+ OVER the portion of ths surface low where the winds are SE / ESE and warm front + dryline intersection would be NICE.
However, if the negative tilt is extreme, it becomes like a tilted "hairpin" where the NW flow aloft over the Rockies suddenly bends around the end of the "hairpin" and flows back around, from the SE (yes, I have seen this with such troughs, SE at 18,000 AND 30,000 feet).
What does this do? It puts a UNIDIRECTIONAL but highly sheared (speed shear, not the favored directional shear / helicity) flow at nearly ALL levels, with SE surface winds, SE winds at 10,000 feet, all the way up to 30,000 and higher, maybe verring to SSE if your lucky.
With such a setup, a fast-moving SQUALL line will be the order of the day, with chasers not understanding the setup well becoming baffled with "Why is this so outflowish, LOL, the winds are SE going right into if for cryin' out loud??"
Examples of such disappointing setups are outlined below that I was on and you can probably look up what the upper winds were doing over that area...
1). Eastern WY near Cheyenne on May 21, 2002.
2). Southeastern SD last May, May 23, 2006 to be exact.
3). Central Iowa, March 31 (most recently) this year 2007.
A GOOD example of a "hairpin" trough is at the link below. This one for #1 (Eastern WY on May 21, 2002) above. That setup would be MUCH better for tornadoes had the surface low / backed flow been about 300 miles farther SW (look at the 850 and 300 MB layers) ...
http://weather.unisys.com/archive/eta_init/0205/02052200.gif
An example (#2 above) of May 23, 2006 in SE SD can be seen at the link below. Also, notice the winds at 850 MB and 300 MB - NO veering ...
http://weather.unisys.com/archive/eta_init/0605/06052400.gif
Yet another example, for #3 above, for Iowa on 3-31-2007. Again, look at the SSE aurface flow, and SE aloft (over Iowa) ...
http://weather.unisys.com/archive/eta_init/0704/07040100.gif
All these cases had a "hairpin" type negatively tilted trough and SE surface winds ahead of low pressure with plenty of moisture and boundaries and produced a line, little or nothing else outside of gustnadoes / isolated RITOR (radar indicated) storms within the line.
All these examples also had a MODERDATE risk as per SPC on them as well, two of which having 10% tornado probs or higher!
Such wind patterns also encourage line-parallel flow, where winds blow parallel to a boundary, such as a front, causing a line of storms (storms "compete" with one another).
Now, if the end of the "hairpin" - the portion of where the strong WSW winds are, just before snapping around and coming from the SE farther ahead - Then THAT region hopefully can stay over the SE backed surface winds - Much better ;-)
A negative tilt can be a good or bad thing, as too much of ANYTHING can have bad points.
We have to bear in mind something important that tends to happen if low-level backed winds, developing low-pressure, moisture convergence AND a negatively tilted trough being not "quite" in the right places above and below each other.
In other words, having a strong difluent upper jet exit region, with H5 (500 MB / 18,000') winds WSW at 90 Kts and H3 (300 MB / 30,000') winds west at 120 Kts+ OVER the portion of ths surface low where the winds are SE / ESE and warm front + dryline intersection would be NICE.
However, if the negative tilt is extreme, it becomes like a tilted "hairpin" where the NW flow aloft over the Rockies suddenly bends around the end of the "hairpin" and flows back around, from the SE (yes, I have seen this with such troughs, SE at 18,000 AND 30,000 feet).
What does this do? It puts a UNIDIRECTIONAL but highly sheared (speed shear, not the favored directional shear / helicity) flow at nearly ALL levels, with SE surface winds, SE winds at 10,000 feet, all the way up to 30,000 and higher, maybe verring to SSE if your lucky.
With such a setup, a fast-moving SQUALL line will be the order of the day, with chasers not understanding the setup well becoming baffled with "Why is this so outflowish, LOL, the winds are SE going right into if for cryin' out loud??"
Examples of such disappointing setups are outlined below that I was on and you can probably look up what the upper winds were doing over that area...
1). Eastern WY near Cheyenne on May 21, 2002.
2). Southeastern SD last May, May 23, 2006 to be exact.
3). Central Iowa, March 31 (most recently) this year 2007.
A GOOD example of a "hairpin" trough is at the link below. This one for #1 (Eastern WY on May 21, 2002) above. That setup would be MUCH better for tornadoes had the surface low / backed flow been about 300 miles farther SW (look at the 850 and 300 MB layers) ...
http://weather.unisys.com/archive/eta_init/0205/02052200.gif
An example (#2 above) of May 23, 2006 in SE SD can be seen at the link below. Also, notice the winds at 850 MB and 300 MB - NO veering ...
http://weather.unisys.com/archive/eta_init/0605/06052400.gif
Yet another example, for #3 above, for Iowa on 3-31-2007. Again, look at the SSE aurface flow, and SE aloft (over Iowa) ...
http://weather.unisys.com/archive/eta_init/0704/07040100.gif
All these cases had a "hairpin" type negatively tilted trough and SE surface winds ahead of low pressure with plenty of moisture and boundaries and produced a line, little or nothing else outside of gustnadoes / isolated RITOR (radar indicated) storms within the line.
All these examples also had a MODERDATE risk as per SPC on them as well, two of which having 10% tornado probs or higher!
Such wind patterns also encourage line-parallel flow, where winds blow parallel to a boundary, such as a front, causing a line of storms (storms "compete" with one another).
Now, if the end of the "hairpin" - the portion of where the strong WSW winds are, just before snapping around and coming from the SE farther ahead - Then THAT region hopefully can stay over the SE backed surface winds - Much better ;-)
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