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Runnin’ Against the Wind Making Your Own Forecast. Justin Turcotte. Case: July 17, 2010. What originally looked like a potential tornado day largely busted.

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case july 17 2010
Case: July 17, 2010
  • What originally looked like a potential tornado day largely busted.
  • I was working the graveyard and got some brief rest before heading south. In the interest of time I did not look closely at the morning model guidance
  • I should have done so as there was a significant change between the 00 UTC and 12 UTC NAM output. This could have saved me a an afternoon and tank of gas….
slide11

00z NAM QPF for 18-21z

Storm initiation and potential chase target area

forecast discussion
Forecast Discussion
  • Scattered convection associated with the weak shortwave was expected during the early morning hours in western and central North Dakota.
  • This convection was forecast to fizzle in the east.
  • LLJ not very focused. Perhaps a stray storm cluster in west central MN around daybreak on edge of cap.
  • Cap forecast to hold off surface based convection in ND/SD/MN border area until ~5pm. Ample shear and instability expected for supercells and potential tornadoes.
  • Storms expected congeal into linear MCS across Minnesota and South Dakota through the evening.
slide15

12z NAM 200 hPa @ 18z

00z NAM 200 hPa @ 18z

slide16

12z NAM 500 hPa @ 18z

00z NAM 500 hPa @ 18z

slide17

12z NAM 700 hPa @ 18z

More amplified trough

00z NAM 700 hPa @ 18z

slide18

12z NAM 850 hPa @ 18z

The 1470 line much farther south

More convergence

00z NAM 850 hPa @ 18z

slide19

12z NAM 700 hPa @ 18z

Slightly more cool aloft

00z NAM 700 hPa @ 18z

slide20

12z NAM 850 hPa @ 18z

More cool at this level too.

00z NAM 850 hPa @ 18z

slide21

12z NAM 310K Isentropic Surface @ 18z

Enhanced region of upglide

00z NAM 310K Isentropic Surface @ 18z

slide22

12z NAM QPF for 15-18z

00z NAM QPF for 15-18z

Dude, where\'s my QPF?

slide23

12z NAM QPF for 18-21z

BOOM

00z NAM QPF for 18-21z

forecast discussion1
Forecast Discussion
  • Stronger more convergent LLJ suggests storms more likely to develop along edge of cap in western Minnesota and reflected in NAM QPF.
  • More prominent shortwave in North Dakota => less cap => earlier storm initiation in ND/SD/MN border area and potential for initial discrete cells to be less isolated.
  • Stronger forcing suggests transition to linear mode will be faster (already expected to be fast based on 00z model guidance).
  • Very narrow storm chase window.
slide25

1800 UTC

Storm initiation

slide30

2300 UTC

“He gone.”

slide31

12z NAM @ 18z

MPX Radar @ 19z

The NAM isn\'t always worthless.

slide34

2200 UTC

BLAH!

A mess. North Dakota chase target FAIL.

slide36

12z NAM 700 hPa RH @ 03z

Sometimes model forecast moisture fields are as good or better indicator of precipitation placement than the QPF.

Radar composite @ 03z

slide37

12z NAM @ 03z

Even the losers get lucky sometimes.

slide38

There were several tornado reports from the supecell striking central Minnesota. SBCIN and LFC from SPC mesoanalysis graphics suggested the cell was elevated reducing the likelyhood of tornadoes. Numerous credible chaser accounts suggest tornadoes did not occur (or were very brief spin-ups).

There were no tornado reports in the 10% tornado probability area and only a very brief tornado in the 5% area.

summary
Summary
  • The 00z NAM ultimately was junk but the later 12z NAM offered redemption.
  • A forecaster should look at multiple runs of a model in order to track changes with time. Usually, the “trend is your friend”.
  • Comparing multiple elements from separate model runs will allow you to better diagnose why the QPF changed between these runs.
  • Small changes in the atmosphere can have a significant impact on timing, placement, and severity of thunderstorms.
summary1
Summary
  • Model QPF is perhaps best utilized as a quick situational awareness tool. If the output was perfect many metorologists would get the pink slip.
  • Model moisture field forecasts are often quite useful for determining instantaneous precipitation placement (and in some cases, summer or winter, where model QPF is not event present.
  • Consider looking at more obscure or less utilized foecast variables (In this case an isentropic surface pointed right to where a supercell developed).
slide41

END

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