Post on 19-Jan-2016
Title card
A Look at Environments Associated with Nighttime Supercell Tornadoes in the
Central Plains
Meteorologist
Jon DaviesPrivate
© Dick McGowan & Darin Brunin
ProcessesBasic parameters that suggest potential for strong tornadic supercell storms:
CAPE – buoyancy/instability for thunderstorm updrafts
SRH (storm-relative helicity) – change in wind direction and increase in wind speed not far above ground as a source of “spin” or rotation in low-levels
Deep layer shear – increasing winds with height through a deep layer to organize and strengthen updrafts
CAPE in low-levels near the ground (0-3 km CAPE) and low cloud bases (LCL heights) for storms that are strongly surface-based without excessive cold outflow
If air bubbles nearground don’t have to be lifted very far tobecome buoyant and rise on their own, developing low-level circulations can stretch and spin faster
Low-levelCAPE and astronglysurface-basedenvironmentprobably help with stretching at the ground.
stretchingat ground
If air bubbles nearground are too cool,it’s like the updrafthas to lift bricksinstead of ping-pong balls near the ground
Cool, stableair in low-levelstends to reduce stretching atthe ground,though theremay be CAPEfurther above.
stretching aloft
Layer of cool,
stable air
Stats
noon 6 pm mid mid 6 am noon 6 pm mid mid 6 am
North Dakota tornadoes 1950-1989 Mississippi tornadoes 1950-1989
Tornadoes by time of day (from Grazulis)
100
80
60
40
20
75
50
25
MLCAPE 2050 J/kg0-1k SRH 143 m /s 0-1k EHI 1.80-6k shear 58 ktsMLLCL 1200 mMLCIN –55 J/kg
2 2
weak inversion
RUC analysis
MLCAPE 2165 J/kg0-1k SRH 357 m /s 0-1k EHI 4.80-6k shear 55 ktsMLLCL 500 mMLCIN –150 J/kg
2 2
strong inversiononly 2 hours later!
Tornado warned supercell near Dodge City after dark,
no tornadoes
surface cooling
RUC analysis
MLCAPE 960 J/kg0-1k SRH 370 m /s 0-1k EHI 2.20-6k shear 52 ktsMLLCL 700 mMLCIN –15 J/kg
2 2
little or no inversion
Nighttime killer tornado at Evansville IN
22 dead
middle of the night!
RUC analysis
Key characteristics – plains vs. eastern U.S.
A key difference between environments in the Great Plains and areas farther east (Gulf Coast-Ohio Valley) that impactsthe occurrence of nighttime tornadoes:
An elevated mixed layer from the dry desert southwest is often present in the plains. Cooling at nightfall beneath this layer can rapidly change a strongly surface-based environment to one that is more elevated and not reallysurface-based, reducing the chance of nighttime tornadoes.
In the eastern U. S., the absence of this elevated mixed layer when strong low-level moist advection is takingplace often makes for a better chance of tornadoes atnight.
A typical nighttime evolution of parameters in the plains…
Typical nighttime evolution of parameters in the plains…
SRH0-1 km SRH
NAM/WRF model
0-1 km SRH
Low-level shear increases with nocturnalbacking of winds andthe nocturnal increase inthe low-level jet
NAM/WRF model
0-1 km SRH
Low-level shear increases with nocturnalbacking of winds andthe nocturnal increase inthe low-level jet
NAM/WRF model
EHI0-1 km EHI
NAM/WRF model
0-1 km EHI
The resulting rise in SRH increases nighttime CAPE-shearcombinations
NAM/WRF model
0-1 km EHI
The resulting rise in SRH increases nighttime CAPE-shearcombinations
NAM/WRF model
Low-level CAPE0-3 km CAPE
NAM/WRF model
0-3 km CAPE
BUT…nocturnal cooling decreases low-level CAPE, causing nighttime environmentsto become increasingly“elevated”
NAM/WRF model
0-3 km CAPE
BUT…nocturnal cooling decreases low-level CAPE, causing nighttime environmentsto become increasingly“elevated”
NAM/WRF model
Characteristics to watch for…
Nighttime situations to watch for in the Central Plains are those where the elevated mixed layer and accompanying inversionare not particularly strong, and where strong warm-moist advection is taking place to reduce the impact of nighttime cooling.
Model forecast characteristics that can help with assessing nighttime environments more conducive to tornadoes after dark in the Central Plains:
• Strong CAPE-shear combinations at night
• The presence of significant low-level CAPE(CAPE below 3 km) at night, suggestingstrong warm-moist advection and a moresurface-based environment
Some case studies…
Case studies…
28 Feb 2007 KS-MO tornadic supercell
28 February 2007
Tornadic supercelleast-central Kansas 7:30-8:00 p.m. CST(in late February!)
© Andy Fischer
© courtesy KMBC-TV
EF-4 tornado after dark in
Linn County, KS2/28/07
Tornadoes continuedinto Missouri
after 10 p.m. CST
0245 UTC
MLCAPE 870 J/kg0-1k SRH 460 m /s 0-1k EHI 2.50-6k shear 56 ktsMLLCL 930 mMLCIN –55 J/kg
2 2
weak inversion
RUC analysis
MLCAPE 620 J/kg0-1k SRH 280 m /s 0-1k EHI 1.10-6k shear 53 ktsMLLCL 1050 mMLCIN –30 J/kg
2 2
little or no inversion
WRF 3 hr forecast for CNU valid 03 UTC
17 April 2004 N Iowa nontornadic supercells
17 April 2004
Poor low-level CAPEindicated
Strong supercellsafter dark in
northern Iowa
Isolated wind damage,but no tornadoes
0245 UTC
MLCAPE 1030 J/kg0-1k SRH 480 m /s 0-1k EHI 3.10-6k shear 60 ktsMLLCL 970 mMLCIN –210 J/kg
2 2
strong inversion
RUC analysis
4 May 2007 Greensburg tornado
4 May 2007Greensburg event
0145 UTC
© Dick McGowan & Darin Brunin
© Courtesy Wichita Eagle
EF-5 tornado strikesGreensburg KS
before 10 p.m. CDT5/4/07
Tornadoes continuedin centralKansas
well after midnight!
MLCAPE 1600 J/kg0-1k SRH 205 m /s 0-1k EHI 2.00-6k shear 46 ktsMLLCL 1640 mMLCIN –190 J/kg
2 2
strong inversion
RUC analysis
Erroneous moisture! (too dry in low-levels)
MLCAPE 3800 J/kg0-1k SRH 240 m /s 0-1k EHI 5.60-6k shear 47 ktsMLLCL 710 mMLCIN –10 J/kg
2 2
weak inversion
WRF 3 hr forecast for PTT valid 03 UTC
Much better depictionof low-level moisture!
MLCAPE 3800 J/kg0-1k SRH 240 m /s 0-1k EHI 5.60-6k shear 47 ktsMLLCL 710 mMLCIN –10 J/kg
2 2
WRF 3 hr forecast for PTT valid 03 UTC
Much better depictionof low-level moisture!
hodograph
Greensburg KS(EF-5)
Moore OK (F5)
fromJohns & Davies
1993
Quick review of the cases we just looked at…
Review
28 Feb 2007
28 Feb 2007
28 Feb 2007significant tornadoes
well after dark(significant low-level CAPE)
17 April 2004
17 April 2004
no tornadoesafter dark
(no low-level CAPE –elevated storms)
17 April 2004
4 May 2007
4 May 2007
4 May 2007strong-violent tornadoes
well into the night(plentiful low-level CAPE)
SummaryRegarding potential for strong or violent tornadoes after dark, on forecast models such as the WRF, watch for:
• Strong CAPE-shear combinations after dark
• Significant low-level CAPE continuing after dark
RUC caveats Beware the RUC! It is often too dry in low-levels with strong moisture advection situations such as:
• Retreating (westward moving) drylines
• Advancing warm fronts
The WRF seems to handle these situations better.
Close© Jon Davies
My site (Greensburg tornado case study):
http://members.cox.net/jdavies1
Earl Barker’s site: www.wxcaster.com
© Jon Davies
My site (Greensburg tornado case study):
http://members.cox.net/jdavies1
Earl Barker’s site: www.wxcaster.com
I’ve moved!
Jon Davies9101 Alpha Ridge Rd.Trimble MO 64492(north of Kansas City)
e-mail remains for now:jdavies1@cox.net
Book
F-4
Available from www.amazon.com
(search on “Jon Davies”)
andwww.farcountrypress.com
Close© Jon Davies
My site (Greensburg tornado case study):
http://members.cox.net/jdavies1
Earl Barker’s site: www.wxcaster.com
New address:
Jon Davies9101 Alpha Ridge Rd.Trimble MO 64492(north of Kansas City)
e-mail remains for now:jdavies1@cox.net