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Organized Convection
M.I.S.T. mechanism
Moisture
 Fuels thunderstorms through latent heat release (needs to be present at base of
unstable layer to achieve free convection)
 The buoyancy of a parcel is directly affected by moisture
 Form precipitation (possible heavy, even flooding); evaporation cooling and
precipitation loading enhance downdraft and downbursts
 Operational assessment:
1) 700mb chart: (T-Td), moist area (mid cloud) and dry area
 High mid level cloud cover hampers insolation for SFC based convection
 Dry mid level enhances downdrafts/microbursts
2) 850mb chart:
 Moisture axis: if over SFC moist axis  deep layer moisture; important for
elevated or nocturnal convection
 Moist Tongue: see area of strong moisture advection (Td > 10C, 12C …)
 Adding moisture increases CAPE (updraft)
3) sfc map:
 Moisture axis: SFC Td not always representative of moisture in BL
 Moist Tongue: 12˚C or so … case dependent, look for collocation with
convergence (mfc)
 highlights moisture gradients
4) Tephi: (mix-ratio, depth of moisture); can parcel reach LFC?
5) Sfc obs (metar): moisture in BL
6) Satellite (morning ST/FG): evidence of significant moisture in PBL
Instability
 Convective storm need instability to induce updraft and release latent energy
 It characterizes updraft strength and modulates storm intensity.
Operational assessment:
1) Tephi/Prog Tephi:
 lift parcel (modify tephi)  CAPE or static instability
 Tw lapse rate >= pseudoadiabat rate  PI
 Tw > LVWA  LI
2) 500mb chart:
 Thermal Trough: leading edge cold advection; cold advection destabilize
the column
3) 700mb chart:
 Thermal
Ridge: cold advection behind erodes capping lid; ahead
destabilizes wrt elevated convection (e.g., Acc along TROWAL)
 Thermal Trough: trailing edge cold advection  beginning stabilization
(SFC based)
 Shade areas of warm/cold advection
 Differential temp. adv. 700-500mb (T75 = T700 – T500)
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Dry intrusion: Steeper lapse rates; enhance microbursts and RFD in
supercells through entrainment in downdraft
850mb chart:
 LL moist advection increases CAPE and updraft strength (latent heat release)
 LL warm advection destabilizes profile (veering) but can strengthen cap
 Thermal Ridge: capping lid and temperature advection; elevated convection
ahead of ridge
Surface chart:
 Thermal Ridge: less important than moisture but highlights warm tongues
Satellite/radar/lightning: evidence of Acc / TCu
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Wind shear

Low-level shear important for cold pool interactions and new cell generation
(multicells)
 Deep shear important for tilting updraft prolonging storm longevity
 For supercells deep shear necessary for generation of mid-level rotation and
vertical perturbation pressure gradients
Operational assessment:
1) Plot a hodograph using winds at sfc, 850, 700, 500 and 250mb
2) Estimate mean wind and shear at sfc-500mb (0-6km) and sfc-700mb (0-3km) and
SRH
3) Jet in 500mb (80kt):

indicate deep shear (0-6km), For severe TS (esp. supercells) look for > 30kt

updraft tilt
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storm motion
4) Jet in 700 mb (30kt):
 indicate low level shear (0-3km)
 updraft tit
 storm motion
 increase Storm Relative Helicity (SRH), if the winds veer through the inflow
depth
5) Jet in 850mb:
 Increase low-level shear (e.g., cold pool interactions, tornadoes)
 Increase SRH if winds veer in BL
 cyclonic side favors MCCs
Vertical motion (trigger):
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For convection to occur, air must be lifted to LFC
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Vertical motion aids in destabilize, erode the capping lid through ascent, enhance
updraft strength
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Mechanisms: isolation, topographic lift, low level convergence, vorticity
advection aloft, along frontal boundary and drylines, gravity waves etc.
Operational assessment:
1) 250 mb:
 ageostrophic circulations (right entrance - left exit)
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cyclonic side of jet
 quantitatively use divergence at 250mb or PVA at 250mb on GRIB viewers
500 mb:
 PVA  associated with divergence aloft
 can trigger convection or intensify existing convection
700 mb:
 vertical velocity chart; descent can inhibit convection
850mb:
 deep layer of convergence favorable for both SFC and elevated (nocturnal)
convection
 trough on LL Max. Wind chart from CMC
Surface chart (sfc convergence):
 sfc trofs, wind shift line, front, dry lines
 isallobaric field: increase wind convergence
Satellite/Radar: look Jet stream and vertical motion (intensity and propagation)
Topographic lift
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Diagnose Elevated ACC
• ascent associated with warm advection (eastward and poleward of lows)
• positive vorticity advection(downstream of shortwave troughs)
• cooling aloft associated with upper trough
• zones of rising air in frontal circulations
• upward motions along drylines
• upward motion in the left-front and right rear quadrants of jet streaks
• upward motions induced by gravity waves
• low level jet
• enhanced lift due to intersecting boundaries
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Heavy Convective Rainfall
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strong and sustained updrafts
high moisture content with high surface dewpoints
high relative humidity through a deep tropospheric layer.
slow moving system or system repeatedly move over the same area.
weak to moderate vertical wind shear is present through the cloud layer.
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stationary re-development along the cold pool due to interaction with either lowlevel shear, low-level boundaries, or orography.
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Tornadic supercells
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high RH near the ground (>70%) - resulting in lower cloud base
inflow highly streamwise - supercell track along pre-existing low-level boundary or
convergence axis (e.g., outflow, front, dryline)
baroclinic generation of horizontal vorticity along forward flank downdraft
ry air at mid-levels to increase RFD strength
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strongly curved hodograph to enhance SRH
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Large Hail in TS
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strong updraft
large and ‘fat’ CAPE
Cool environment with steep lapse
rate in mid-level
Deep vertical wind shear
Low wet-bulb freezing level
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Low mean T of downdraft air
Large hail size aloft
Reflectivity over 45 dBZ at midlevels.
The formation of the hook echo at
low levels.
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Forecasting Dry Microburst
• Inverted V sounding
• 700mb-500mb lapse rate > 8 oC/km
• 850mb-500mb T (lr85): > 33 oC
• 700mb-500mb T (lr75): > 24 oC)
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High cloud base
Weak wind shear
Moist layer above 700 mb
(RH>60%)
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Forecasting Wet Microburst
 dry mid-level unstable air (RH<30%)
 warm moist (RH>70%) boundary layer
 significant CAPE (updraft) and DCAPE (downdraft)
 Weak wind shear
 700-500 temp difference > 18 oC
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