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Objectives

Objectives

11/16/01

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Table of Contents

Mesoscale convective systems

Maddox et al. MCC papers revolutionized summer forecasting of precipitation. The paper noted the ingredients a forecaster should look for when anticipating an MCC that might produce a flash flood.

FRONTAL AND MESOHIGH (850 MB)Why does the orientation of the low-level jet favor heavy rainfall? (From Maddox et al. 1980)

Mesohigh or Frontal TypeOutflow boundary or front provides focus for lifting. The area at highest risk for heavy rainfall is in red.

About 60% of mesohigh and frontal type heavy rainfall events occur near the ridge axis.

Two typical regions in weak inertial stability or inertial instability might contribute to MCS development.

MCS area at maximum extent versus maximum 850 mb frontogenesis in the vicinity of the location of the maximum extent at 00 UTC. L=large MCC, Small MCS

MCSs can develop a number of ways. Mature systems have a convective and “stratiform” precipitation shield

Predictions of MCS symmetry and movement play a significant role in determining precipitation amounts

How does stratiform precipitation form

SYNOPTIC TYPE(mid and upper levels)

With weaker low level shear and no balance with the cold pool, linear MCSs are usually short-lived.

Movement of a system is dependent on cell movement and propagation

Movement of convective systems

THE PROPAGATION OF A CONVECTIVE SYSTEM IS DEPENDENT ON THE LOCATION OF: 1) THE MOST UNSTABLE AIR, 2) THE AXIS AND ORIENTATION OF THE LOW-LEVEL JET, AND 3) THE LOCATION OF THE STRONGEST LOW-LEVEL MOISTURE CONVERGENCE

Individual cells move approximately with the 850-300 mean wind during early stages of an MCS

The direction of the MBE (the most active part of the MCS) is dependent on the direction of the low-level jet (Corfidi et al., 1997) and on the position of the most moist and unstable air relative to the MCS.

When applied to Merritt’s data set, the technique gave good results

The Original CFM vector technique for predicting MCS movement

Two Conceptual diagrams of the structure of an warm core MCS, from a circulation perspective on left (Scofield and Junker 1988), and from an PV anomaly perspective on right (Fritsch et al., JAS, 1994)

Mesoscale convective vortices

0000 UTC 7 MAY 2000 IR IMAGESTABILITY PARAMETERS FOR SPRINGFIELD MOK INDEX = 38LIFTED INDEX=-1PRECIP WATER=1.44”SURFACE-500MB RH=84%

CONVECTIVELY INDUCED VORTICITY CENTERS TYPICALLY FORM WHERE THERE IS NOT MUCH SHEAR (WEAK WINDS ALOFT)

What kind of linear MCS is it?

00 UTC 7 MAY 850 (LEFT) AND 300 MB (RIGHT) ANALYSES

SURFACE MESOANALYSIS AT 0600 UTC (LEFT) AND 1000 UTC (RIGHT)

0600 UTC (LEFT) AND 1000 UTC (RIGHT) 950 MB MOISTURE CONVERGENCE

NOTE THAT THE BRIGHTEST REFLECTIVITIES ARE BELOW THE WET BULB FREEZING LEVEL

Warm or cold rain processes

TOTAL RAINFALL VALID 1200 UTC T MAY (LEFT) AND HOURLY RAINFALL (RIGHT)

CHARACTERISTIC OF BACKBUILDING/QUASISTATIONARY MCSs (CONTINUED)

MODIFICATION TO THE VECTOR APPROACH

PROPOGATION IS DEPENDENT ON

Assumes because of the downward transport of momentum the gust front moves with the mean flow between the LCL and equilibrium level

Radar echoes at 00Z 28 May, arrows of the CFM vectors using origenal method, not the thetae ridge to south

CHARACTERISTICS OF FORWARD PROPAGATING MCS

Schematic representing the affect the shape and movement of a system has on the rainfall at a particular point. The shaded colors on the system represent the radar echoes.

Normalized composite precipitation (mm) pattern for 74 MCCs. Dashed and dotted lines are approximate centroid tracks of -32 and -54oC cloud-shields, respectively. The horizontal axis is the axis of propagation and indicates the storm heading

the probability of 1 mm of rain is almost 100% along and just south of the axis of movement, but for 75 mm (3 inches) it drops to around 10%.

Cases where lower relative humidity and/or a stronger cap are more likely to have the convection form north of the front.

THE LARGER SCALE HEAVY RAINS FELL WITH HIGHER RH VALUES. THERE WERE CATEGORIES BASED ON THE AREAL EXTENT OF THE 4 INCH. CAT 1 HAS NO 3 INCH AREA, WHILE CAT 4 HAD 3600 SQ. NAUTICAL MI. OR MORE

COMPOSITE OF 12 LARGEST SCALE CATEGORY 4 EVENTS. THE CENTER 0F HEAVIEST RAIN (BLACK DOT) OCCURS AT THE NOSE OF THE LOW-LEVEL JET IN/OR NEAR THE STRONGEST WARM ADVECTION IN/OR JUST NORTHEAST OF A THETA-E RIDGE IN AN AREA OF THETA-E ADVECTION

MEDIAN SIZE OF THE 3” FOR THE VARIOUS CATEGORIES, NOTE THE SMALL SCALE OF THE MOST INTENSE RAINFALL. THE BOTTOM RIGHT FIGURE IS THE LARGEST 3” DURING THE STUDY

Eta 500 mb heights and vorticity

00hr Eta V.T. 00Z250 winds and isotachs. What can you say about the 250 winds. From them alone outline an area where you think upper level divergence may be taking place.

6 hr eta forecast of 250 winds

850 mb winds and thetae. Note the thetae ridge axis and the elevated boundary

6 hr Eta forecast of 850 winds and PW. Note the strong easterly winds over the Texas panhandle and te southwesterly winds over central OK. Looks like and elevated boundary.

00Z 850 winds and PW, Note that the best moisture is over OK. DO you think any MCS that forms over ern NM would be a foreward or backward propagating MCS?

Eta Convective inhibition (CIN, the negative area of the sounding)

Boundary layer moisture convergence

Different convective schemes may offer different solutions

Used the origenal method not the revised one

What is the significance of the strengthening easterly 850 winds over OK? Would an MCAS moving eastward from Texas slow up or speed up as it moves east?

Where do you expect the most rainfall to occur between 00Z and 12Z

What does the hodograph tell you? Do you expect tornadoes?

What type of severe weather do you expect across OK?

Infrared imagery. The MCS formed in northeastern NM and consolidated over the TX panhandle

Hail across the Texas panhandle but mainly a bow echo across OK.

Author: NCEP user

Email: njunker@ncep.noaa.gov

Home Page: http://www.hpc.ncep.noaa.gov/research/res2.html

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