1. INTRODUCTION
Warm-season
mesoscale convective systems (MCSs) impact much of the United States with heavy
rains and often severe weather, primarily during nighttime hours. These systems typically dissipate or decline
in intensity during the four hours or so before local noon. However, some are maintained or regenerate
and continue into the afternoon. The
Morning Convection Project (MCP) was begun for the purposes of better
understanding the factors that influence late-morning MCS evolution, and to
provide tools that can be used to improve short-term forecasting of these
systems. The MCP is a collaborative
effort among the National Severe Storms Laboratory (NSSL), the University of
Oklahoma, and the National Weather Service Forecast Offices in Norman, Oklahoma,
and Dodge City, Kansas. The MCP is supported
by a grant from the Cooperative Program for Operational Meteorology, Education
and Training (COMET).
In succeeding
sections, input from Norman and Dodge City forecasters will be discussed. A climatology of MCSs from 1996-2000 as well
as an overview of the study of RUC2 model analyses will be included. Finally, future work of the MCP will be
outlined.
2. FORECASTER
INPUT
An effort has been
made to assess what factors forecasters believe are most important to
short-term forecasts of MCS evolution.
National Weather Service forecasters at the two offices provided
real-time input (via an NSSL web site) on a subset of those convective systems
that occurred within the period of interest during the summers of
1997-2000. Comments were received on 37
systems during the four summers. The
primary input items provided for each system were: (1) the forecast evolution
made at 0900 UTC for the 1200 UTC to 1600 UTC period for systems that were
affecting or expected to affect the county warning area of either office, and
(2) the major factors considered in formulating the forecast MCS
evolution. A total of 95 responses were
received listing major factors considered in the forecasts. Not surprisingly, for systems for which
comments were recorded, both radar and satellite trends were mentioned roughly
half the time as important factors in short-term evolution of systems. However, environmental characteristics (e.g.
stability, storm-relative flow, and upper-level features) were also listed
often. “Climatology”, referring to the
forecasters’ experience (which suggests the tendency for MCSs to weaken or
dissipate during the late morning), was mentioned 22 percent of the time. This factor is likely often taken as a starting
point in an MCS forecast.
3.
CLIMATOLOGY
A climatological study of 145 individual MCSs that occurred during the summers of 1996-2000 within the County Warning Areas (CWAs) of Norman and Dodge City was undertaken. In addition to occurrence within the CWAs in the 0900-1700 UTC period, included systems had to meet the following criteria: (1) movement greater than or equal to 5 m/s, (2) size greater than or equal to 100 km in longest dimension, and (3) duration of at least three hours with an intensity of at least 40 dBZ for one hour within area and time period of interest. Systems were chosen based upon examination of hourly mosaic images from the National Climatic Data Center (NCDC) archive of NEXRAD national reflectivity and upon the NCDC archive of individual station WSR-88D Level II data. Surface and upper air charts were also examined, as well as maps indicating cloud-to-ground lightning within specified periods. The climatology for each system included the track, initiation mechanism, character of evolution, occurrences of severe weather, and occurrences of cloud-to-ground lightning.
Figure 1 shows the number of MCSs per month and year. Each year had a relatively constant number
of systems (ranging from 26 to 35), however the majority of systems occurred
during the months of June and July (53 and 56 MCSs respectively). A study of the number of hours in which each
CWA was affected by MCSs showed that Norman’s area was impacted twice as often
as Dodge City’s area. This was expected
due to the fact that Norman’s CWA (123320 km2) is about twice the
size of Dodge City’s (58130 km2).
However, during the month of August, Dodge City’s area was affected more
frequently. This is due to the
climatological conditions of August in which strong upper-level northwesterly
flow resides further north than during the rest of the summer. Also, the upper-level ridge and cap are
strengthened over the Plains in August and fewer fronts are able to penetrate
into the Norman CWA.
Figure
2 shows the evolution tendencies of the systems between 1300 UTC and 1700
UTC. As expected, a
Figure 1: Number of MCSs
affecting OUN and DDC CWAs by month and year.
majority
of the systems decreased or dissipated during the late-morning hours. However, a significant percentage (26%)
remained steady in intensity. This
subset of systems is very significant, since a short-term forecast of these
MCSs based on climatology alone would be inaccurate. Figure 3 shows the motion by month for the systems. Direction, in this case, is defined as the
45 degree sector centered on a specific direction. Overall, the largest number of systems affecting the southern
Plains shows movement from west to east (47%).
The number of systems moving from northwest to southeast falls off
sharply in August as the upper-level ridge strengthens over the southern Plains
and strong upper-level northwesterly flow resides farther north over Nebraska
and the Dakotas. A breakdown of
evolution of the MCSs by movement shows, once again, that a majority of systems
in each movement category decreases or dissipates, while a significant
percentage remains steady in intensity (especially for systems moving into the
area from the west). Systems moving
from southwest to northeast appear to have the highest likelihood to increase
in intensity out of all the movement categories.
Figure 4 shows
a breakdown of severe weather reports per month, year and CWA. Severe weather reports were collected for
the entire lifetime of the systems. Two
interesting anomalies appear in the data.
One is the absence of severe weather reported in the Norman CWA during
1998. The other is the substantial
increase in severe weather reports during July 2000 resulting from two
extremely strong MCSs that pushed across the area. One of these systems caused wind gusts of over 90 knots in
northern Oklahoma City. Figure 5 breaks
down severe weather reports by type for the entire climatology. Wind damage represents the majority of all
severe
weather reports every year except 1999
when hail reports were the plurality.
Hail reports represent the second most frequent type of severe weather
in all other years.
4. RUC2 MODEL DATA / FUTURE WORK
RUC2 model
data were obtained for summers 1999 and 2000.
Model data for these two years only will be analyzed due to the fact
that prior to April 1999, the RUC2 missed significant level temperature and
dewpoint data from rawinsondes. RUC2
data are being used instead of actual rawinsonde observations due to the
superior time and spatial resolution of the model data.
Using RUC2 sounding data, one objective of our analysis will
be to determine the shear and stability impacting each MCS and how the
evolution of the system is affected by changing amounts of shear and
stability. RUC2 model soundings will be
graphically reproduced using the GEMPAK utility NSHARP. In addition to the sounding, NSHARP
calculates several stability parameters.
It also yields a hodograph for the chosen location. Based on storm motion, NSHARP calculates
storm-relative winds at several levels.
Based on the model sounding, NSHARP calculates mean wind at three levels
and environmental shear at four levels.
Using this data, line-normal shear for each MCS under study will be
calculated. How the line-normal shear
changes over time will be compared to the evolution of the system and its
impact evaluated. Once this is
determined, it is hoped that it will be possible to ascertain which squall-line
theory(s) (i.e. Thorpe-Miller-Moncreiff theory, Rotunno-Klemp-Weisman theory,
or Xu-Xue-Droegemeier theory) best fits the morning MCSs of the southern
Plains.
There are
several items being planned for future research as part of this
project-in-progress. This includes an
assessment of an operational model in its ability to predict environmental
fields that are significant factors in the evolution of these systems.
5. CONCLUSIONS
As revealed by
forecasters at the NWS offices in Norman and Dodge City, climatology is a major
factor in the short-term forecasting of MCSs.
Climatology indicates, and our study supports, the notion that a
majority of MCSs decrease in intensity or dissipate during the late-morning
hours. However, our study also shows
that more than a quarter of all MCSs actually retain or increase their
intensity during this time period. Any
short-term forecast for these systems based on the climatic probability of
their dissipation would be inaccurate.
Therefore, through study of shear and stability using RUC2 model archive
data, factors will be evaluated that might be used in an operational setting to determine which systems will continue into the
afternoon. This knowledge could aid
greatly in short-term forecasting of these MCSs across the southern Plains
during summer afternoons.
