Day 1 Severe Storms Forecasting

Jim LaDue – WDTB

08 June, 2005

Topics

• Use today’s example to discuss 2-8 hour severe storms forecast strategies– Convective forcing – lines vs. isolated – Convective mode

• If isolated, what type?• If a line, what type? • Hazards type?

– Timing – • When is initiation likely?

General philosophy

1. Diagnose the current weather with real observations first

2. Then compare reality to what the model analyzed

3. Then use the model and your understanding of its errors to make a prognosis.

Which model run to believe?

A simple methodology for convective initiation

After that is figuring what stormtype there’ll be.

Analyze regions of potential

Convective instability

look for forcing mechanisms to destabilize the

atmosphere

Next look for low-level triggering

mechanisms

General regions of potential convective instability

• What fits your conceptual model best?

Forcing mechanisms

Short-wave forcing

• 500 mb Q-vectors

Synoptic forcing mechanisms

• Complications of playing the jetstream game

Cyclonic curved jet anticyclonic curved jet

Ascent only where the red dots are located

• Playing the jet game isn’t as critical with a bowling ball low

Forcing contd

• Vertical motion from the RUC

• Vertical motion acts to remove the CAP

• Not the only type of CAP remover

Differential thermal advection

• Warm advection down below or• Cold advection up above• Bottom line, the sounding destabilizes

• Differential thermal advection

• 700mb cooling

• 850mb

• Ascent cooling

• Compensated by solar heating

Differential thermal advection/heating

• Bottom line?

• Note lifting of the CAP at FWD

Low-level forcing mechanisms• Low-level

frontogenesis– How deep is it?

• Dryline – Poor at forcing CI– That’s good

• Any other troughs?

Deep frontogenesis

• Shows up from surface past 850 mb

Surface boundaries

Convective forcing summ

front

ogen

esis

DPVA

WAA

Forcing, now coverage

• CI is underway. The strongest forcing areas will most likely generate linear modes

• But geometry of forcing, especially with boundaries is important

Boundary geometry

• Things to consider– Boundary-relative steering layer flow– Boundary-relative anvil-layer flow– Shear relative to lines of forcing

Boundary-relative steering layer flow

• Promotes more initiation if this value is small

• Can be good if CIN is a problem

• Too little CIN with forcing and this can be a problem

Boundary-relative Flow Parameters

Boundary-relative flow

Boundary-relative storm motion

Storm motion

Flow

Boundary motion

Effects of Boundary-relative kinematics on storm morphology

• Steering flow

Is the storm going to remain on, fall behind or overtake a boundary?

This may affect storm type beyond CAPE and shear

stable unstable

Boundary-relative anvil-layer flow

• If parallel to a line of forcing:– This can promote interstorm seeding and cold

pool development

• If directed ahead of a the forcing line– Limits cold pool development greater chance

of isolated modes

• If directed behind line of forcing– Depends but it can promote a rear inflow jet

Effects of Boundary-relative kinematics on storm morphology

• Shear

Shear vector

RM

LM

Now given your expected coverage are likely, what are the threats?

• Severe winds,

• Severe hail

• Tornadoes

• Heavy rain

Ingredients for supercells and severe squall lines/bow echoes

• Deep moist convection (CAPE> a few hundred j/kg)

• Strong vertical wind shear– Best represented by 0-6 km bulk shear– Subtract the winds at 6 km from the boundary layer– Can be represented by Bulk Richardson Number

Shear or BRN shear = 0.5 (Uavg)2

where Uavg is the difference from the mean 0-6 km wind and the mean wind in the lowest 500 m.

Do I have enough shear?

If I have around 18 m/s (35 kt) of shear between 500 mb and close to the ground. Just eyeballing 500mb, look for at least 30kt in the lower plains and 20 kt in the high plains. I personally look for that 40kt of shear

supercell motion

sfc

6 km

1. Draw the shear vector from the surface to about 6km (in red here)

2. Plot the mean 0-6km wind if it isn’t there already (green dot)

3. Plot a line perpendicular to the shear vector that passes through mean wind (thin line)

R

L

4. The right (left) mover is about 8 m/s right (left) of the mean wind along the thin line.

Horizontal cross sections of supercell motion

Make sure you are aware of ordinary and supercell motion before leaving.

Multicell Motion

If a multicell backbuilds, heavy rain is a potential threat Use original MBE Vector (“Corfidi”) Technique

After Corfidi et al. (1996)

Vcl = 0-6 km mean wind

Vllj = direction of 0-1.5 km wind

Vmbe = multicell motion

Vcl

Vprop = -VLLJ

VMBE

Multicell Motion

4) Boundary interactions• Modulates/enhances development of new

convection

Blue = steering layer flowGreen=triple pt motionRed = multicell motion(Weaver, 1979)

Supercell tornado threat• We don’t know the ingredients and this

is still frontier science• But here are some parameters to look

for deciding whether to chase or not.– High storm relative helicity (SRH),

especially in the lowest 1km– A strong sustained updraft, preferably

one that begins close to ground, strong buoyancy in low levels

– Warm moist rear flank downdraft, low LCL is a good starting proxy parameter

Storm Relative Helicity

• Air that is spinning around on its axis in the direction of motion (a thrown football)

• It is storm relative, therefore one must anticipate storm motion prior to storms

• Best visualized on a hodograph • Also can be represented as a number in units of

m2/s2 and contoured• Usually measured in the lowest 3 km but now

measured also in the lowest 1 km.

SRH• Recent research shows better discrimination

between tornadic and nontornadic supercells with 0-1km SRH. Most sounding programs and maps use 0 – 3 km SRH.

• Look at soundings for evidence of high 0 – 1 km SRH.

Edwards and Thompson, 2000

Simple and perhaps better:0-1km shear

• Look for 20 kts or more for most significant mesocyclonic tornadoes

SRH contd.SRH can be enhanced by supercells themselves, especially supercells utilizing high CAPE and shear.

Estimated hodograph within 20 km of the storm in following page.

Warm moist RFD• This cannot be so

easily anticipated and every storm can have different RFD temperatures

• But high RH boundary layers with low cloud bases (LCL) seem to have some relation

Rasmussen and Blanchard, 1998

Estimating LCL heights

• Look at surface obs in an unstable airmass – LCL = 222 (T – Td) LCL in feet, Temps in F

• LCLs should be less than 1500 m for best tornado threat

– LCL height also displayed from soundings

LCL height on the SPC product

This storm is creating its own SRH

CAPE = 4800 j/kgSRH initially at zero0-6km shear = 60kt

Strong low-level buoyancy

• Recent research courtesy of Jon Davies, Bill McCaul, suggest strong low-level buoyancy is associated with most significant tornadoes.

http://home.kscable.com/davies1/LLbuoyprimer/LLbuoy_background.htm

Strong low-level buoyancy

• There also is a lower Level of Free Convection (LFC) with most significant tornadoes.

http://members.cox.net/jdavies1/waf796/waf796.htm

Strong low-level buoyancy

• There also is a lower Convective Inhibition (CIN) with most significant tornadoes.

http://members.cox.net/jdavies1/waf796/waf796.htm

LFC height example

• A little marginal in SE OK. Good in NC KS

Nonmesocyclonic tornadoes

• Prefer strong low-level vertical vorticity and good low-level lapse rates/buoyancy

How to forecast HP, CL, LP supercells

• Storm-relative anvil layer winds likely affect the storm type– LPs more common with SR anvil winds > 30 m/s – Classics: SR anvil winds 18 – 30 m/s – HPs: SR anvil winds < 18 m/s.

• Storm-to-storm seeding – Several storms in close proximity seed each other

increasing rain potential and HPs

• Moisture– This is a distant third but very dry atmospheres may keep

storms LP

Storm-relative anvil layer winds

SR winds in range for classics. Isolated storm becomes long-lasting Hoisington, KS storm. Photo by Corey Mead

Interstorm seeding

Storm on flanking line merges with target storm.Result was possibly a complicated storm structure during initial stages and possible interruption in tornadogenesis.

Forecast methodology

As you approach initiation time, concentrate more on satellite, surface, profilers, radars to update your analysis

– If the mesoscale models look good, use them for your supercell, tornado parameters.

Summary

1. Determine expected convective coveragea. Low coverage implies updraft shear dynamicsb. High coverage implies organized multicells, cold

pool/shear dynamics in addition to updraft/shear dyamics

2. Then determine your storm type and main hazards

a. Many of the parameters can be used for multicell and isolated cell convection

b. Hodograph/Skewt analysis is important!

3. Do not trust the models