Mesoscale Convective Systems: A Review
What impact do MCSs have on my life? While MCSs are most common in the Plains and upper Midwest, they can develop and move into the mid-Atlantic region, resulting in very heavy rains and, often, squall line and bow echo formation (see "Example" section, below). At the very least, the resultant cloud shield from a decaying Midwest MCS can pass over North Carolina & affect sky condition and temperatures. Also, outflow boundaries from decaying MCSs can act as a focuses for new convection, especially when they intersect other boundaries [this occurred in the Jarrell, TX, tornado (May 1997)]. Finally, mesoscale vorticity centers (MVCs) created by nighttime MCSs can spawn convection downstream the following day. (An MVC will appear as a "spinning" in the residual cloudiness that was once the MCS.) An MVC moving into a very unstable air mass was the cause of the derecho event of May 25, 2000.
PUBLIC INFORMATION STATEMENT|
NATIONAL WEATHER SERVICE RALEIGH, NC
530 PM EDT FRI MAY 26 2000
TODAY THE NATIONAL WEATHER SERVICE IN RALEIGH SURVEYED THE DAMAGE PRODUCED BY THURSDAY'S DAMAGING WIND EVENT OVER THE NORTHERN PIEDMONT OF NORTH CAROLINA. CONCENTRATING ON GUILFORD AND ALAMANCE COUNTIES... WIDESPREAD STRAIGHT LINE WIND DAMAGE TO TREES AND POWER LINES WAS OBSERVED.
HARDEST HIT AREAS WERE WESTERN GREENSBORO... GIBSONVILLE... ELON COLLEGE AND BURLINGTON. OTHER SIGNIFICANT DAMAGE IS KNOWN TO HAVE OCCURRED IN MANY LOCATIONS EXTENDING FROM WINSTON-SALEM TO NORTHEAST OF DURHAM.
THE AREA OF WIND DAMAGE WAS UP TO SIX MILES WIDE IN THE CITY OF GREENSBORO. WELL OVER ONE HUNDRED SQUARE MILES OF AREA RECEIVED SOME DAMAGE FROM THE EVENT.
THERE WAS NO EVIDENCE OF DAMAGE CAUSED BY TORNADOES. ALL OF THE DAMAGE WAS BLOWN IN ONE DIRECTION WHICH IS INDICATIVE OF STRAIGHT LINE DOWNBURST WINDS PRODUCED BY POWERFUL SEVERE THUNDERSTORMS. ESTIMATED MAXIMUM WIND SPEEDS FOR MOST OF THE DAMAGE AREA WAS 60 TO 80 MPH. HOWEVER THERE WERE SEVERAL SMALL AREAS WHERE THE DAMAGE PATTERN INDICATED THAT STRONGER WINDS OCCURRED. MOST LIKELY STRONGER MICROBURST WINDS WERE EMBEDDED WITHIN THE LARGER WIND FIELDS WHICH LED TO THE AREAS OF GREATER DAMAGE. WINDS IN THE MICROBURST AREAS WERE ESTIMATED TO HAVE BEEN 90 MPH OR MORE.
METEOROLOGIST IN CHARGE
SPECI KGSO 251405Z 26008KT 1 1/2SM +TSRA BR BKN015CB OVC055 18/17 A2981 RMK AO2 PRESRR FRQ LTGICCCCG ALQDS TS ALQDS MOV E P0008 |
SPECI KGSO 251356Z COR 30013G60KT 1/4SM +TSRA FG BKN015CB OVC055 17/16 A2985 RMK AO2 PRESRR CONS LTGICCCCG ALQDS TS ALQDS MOV E P0001
METAR KGSO 251354Z COR 32015G71KT M1/4SM +TSRA SQ FG FEW001 SCT055CB BKN070 17/16 A2984 RMK AO2 PK WND 26071/1346 TSB45RAB47 PRESFR SLP097 CONS LTGICCCCG ALQDS TS ALQDS MOV E P0056 T01670156
SPECI KGSO 251346Z COR 26042G71KT 1SM TS SQ HZ SCT055CB BKN070 23/19 A2986 RMK AO2 PK WND 26071/1346 TSB45 PRESRR FRQ LTGICCG ALQDS TS ALQDS MOV E P0008
METAR KGSO 251254Z 26011KT 8SM FEW065 23/19 A2977 RMK AO2 SLP072 T02330194
Above: Timeline, damage PNS, and GSO METARs for May 25, 2000, derecho event.
Because of their small size and the nature of model initialization, MCSs and resultant MVCs are often not depicted very well by the models. Therefore, forecasters should understand and be able to quickly recognize the conditions which favor MCS development (see below). If the 500 mb ridge axis is located to our west and MCSs develop over the upper Midwest and Great Lakes, they should be closely monitored for potential effects on our weather.
What are the distinguishing characteristics of an MCS? An MCS may evolve from an isolated cell or small group of cells, or may be triggered as a large convective system from the onset. MCSs typically form in the late afternoon or evening, and often last 3 hours or more. They are often marked by an extensive (up to several hundred km wide) anvil cloud that represents the anticyclonic outflow near the tropopause. An MCS typically has a warm-core low aloft, and a mesoscale downdraft at low levels. (An MCC, a form of MCS, possesses a nearly circular cirrus canopy, and persists for 6 hours or more.)
Where do they form? MCSs often develop near a roughly east-west oriented surface frontal boundary (they may also be focused along an outflow/mesohigh boundary), underneath the anticyclonic-shear side of the upper jet, along the northern or northwest edge of the 500 mb ridge (see figure below). This frontal boundary will be slow-moving and usually almost parallel to the mean steering flow. MCS development frequently coincides with a zone of strong low level warm air advection and wind convergence, associated with a strong south or southwesterly low level jet (see figure below).
What are some favorable factors for MCS development and maintenance? The reference notes (located on the AWIPS Intranet) for the Rain_MCS AWIPS procedure detail many of these factors. They include:
Deep warm layer. This will increase precipitation efficiency. On the sounding, the depth from the LCL to the 0° isotherm should be 3-4 km.
Instability. Look for high CAPEs (at least 1500 J/kg, and preferably > 2000), low lifted indices (<-2) and a lapse rate (850-500 mb) of 6.5 or greater.
Strong low level convergence. This is often found at the nose of the low level wind max (check 925-850 mb, or better yet, an appropriate nearby isentropic surface).
Strong persistent upper level divergence. The heaviest rain will occur on the southern edge of the divergence area at 250 mb (or, again, a corresponding isentropic surface). This 250 mb jet is often located just east of a slow-moving 500 mb trough.
850 mb frontogenesis. While not a necessary condition, the largest and most long-lived MCSs are associated with strong frontogenetical forcing.
How will an MCS move? An MCS will generally move with the mean cloud layer (850-300 mb) wind. However, the propagation vector (along which new cells form upstream; also call backbuilding), which is approximately in the opposite direction of the low level jet and proportional to it, should also be considered. The stronger the low level jet (compared to the mean wind), the more the MCS will deviate from the mean wind (turn right, or backbuild). Also, MCSs and associated heavy rain favor areas of thickness diffluence. They often track along or just right of the thickness lines (typically 1000-500 mb thickness), and may backbuild into an area of diffluent thickness. The graphic below gives an overview of the propagation based on the position of the low level jet and the most unstable air. (Click here for details of the Corfidi vector method of predicting MCS movement)
Are there any limiting factors to watch out for? Check the soundings for a cap (e.g. 700 mb temps >12°C), which may limit the size of the convective system. (A 5° mid level cap in the Plains on April 6, 2001, kept the forecasted severe storm outbreak from being as bad as anticipated.) Also, beware of the limitations of the models to accurately depict mesoscale structures such as outflow boundaries and MVCs. Use the models to identify a possible MCS pattern, not necessarily the MCS itself. Finally, an extensive cloud shield during the day may limit instability.
To review: A favorable region for MCS generation will have deep moisture, a deep warm layer, instability, strong low level convergence (associated with the nose of a low level jet), and strong upper level divergence. MCSs are not just a Midwest phenomenon; they can and do affect the mid-Atlantic region. Since MVCs produced by decaying MCSs can instigate or increase convection downstream, forecasters must keep close track of MCS remnants using satellite imagery. Keep an eye on residual MCS cloudiness that is spinning, especially if it is moving into unstable air.
This review simply covers the basics and is just the tip of the iceberg. If you are interested in additional information, COMET's METED training site offers an excellent online module on MCSs. Additionally, very informative slides from an MCS VISIT session, including more details on predicting MCS movement/evolution and the importance of the resulting cold pool, can be found here.
-- Gail Hartfield with Scott Sharp -- 4/01; updated 7/03