IMproving Prediction of Heavy Rainfall with Elevated ConvectionLaurel McCoy, University of Missouri and NOAA/NWS, Portland, OR
Chad Gravelle, CIMSS/SSEC University of Wisconsin, Madison, WI
Charles Graves, Saint Louis University, St. Louis, MO
Presented to the National Weather Association Annual Meeting
21 October 2014, Salt Lake City, UT
Acknowledgements
elevated thunderstorms will occur
http://weather.missouri.edu/PRECIP or
studies
convection in US
northeast of a
determined that
the “…division
between elevated
and surface-based
[convective] activity
Pure: τ > f -1 ex: wraparound
Surface influences on mid-level parcels reduced or
eliminated because of their 1) vertical location and/or
2) temporal history
Surface influences on mid-level parcels (if any)
mitigated by their arrival over frontal inversion
Mixed: τ < f -1 ex: warm sector castellanus
Surface influences on mid-level parcels unrestricted
A Hierarchy of Elevated Convection
Pure
Mixed
Objectives
Objectives
producing elevated thunderstorms in this region
Methodology
Methodology
Weather Service County Warning Areas (CWAs):
Kansas City/Pleasant Hill (EAX)
Local rainfall maximum
within CWA boundary
data to find event times
Event time defined as NARR time-step with heaviest
rainfall occurring over next 3 hours
Used NARR data to evaluate if event was elevated
2-meter θe and precipitation maximum
NARR sounding from rainfall max
Creating Composites
Coordinates for local rainfall max
Composited using software from SLU
NARR grid layers overlaid with coordinates centered on centroid of CWA
Parameters averaged over 207 x 207 grid
Grid squares = 32 km2
for elevated thunderstorm events
6-hours prior (t-6)
12-hours prior (t-12)
250-mb Wind and Divergence (Upper-Level Jet)
500-mb Absolute Vorticity
850-mb Wind (Low-Level Jet)
850-mb θe Advection and 2-meter θe (Surface Boundary Location and Transport Maximum)
1000-500-mb Thickness
Mean-Sea-Level Pressure
Precipitable Water
Most-Unstable CAPE
divergence (dashed)
T=00
Results- Kansas City
Results- Kansas City
filled)
Results- Kansas City
Results- Kansas City
Cross-Section Kansas City
Unique patterns to look for when forecasting heavy- rainfall-producing elevated thunderstorms:
Strong signal; strong variability
Divergence > 3 x 10-5 s-1 (lift)
Event located within or just south of 850-mb θe advection maximum (convergence max)
Signals LLJ from the SSW (moisture; lift; instability)
Strong signal; small variability
>30 K-index values (instability)
2-m θe pattern (confirms elevated convection)
Conclusions
X “marks the spot” in cross sections
Interquartile ranges - enhance confidence in forecasting
heavy rain events with elevated convection
Conclusions
Lifting – 250-mb DIV > 3 x 10-5 s-1
Instability – K Index > 32
PRECIP Project
elevated thunderstorms will occur
http://weather.missouri.edu/PRECIP or