CENTRAL REGION TECHNICAL ATTACHMENT 97-03
The Utility of MAPS/RUC in Forecasting Two Midwest Severe Weather Events
National Weather Service
Davenport (Quad Cities), Iowa
The Mesoscale Analysis and Prediction System (MAPS) (Bleck and Benjamin 1993), which is run as the Rapid Update Cycle (RUC) at the National Center for Environmental Prediction (NCEP), provides forecasters with a powerful tool for the diagnosis and short-term forecasting of parameters important in the development of severe deep moist convection. The ability to diagnose derived fields based on surface observational data every hour and to forecast critical meteorological features with a three-hour time resolution are two items that MAPS gridded data facilitates.
Two severe weather events that occurred in Iowa and Illinois are examined from the perspective of evaluating the utility of MAPS as a diagnostic and forecast tool. These events occurred on May 9 and 13, 1995. The contrasting cases, the first in which upper-tropospheric features played a key role and the second in which surface features played a key role, will show how MAPS can effectively be used for both strongly and weakly synoptically-forced events. The focus in this paper will be on the three- hour forecasts and surface analyses valid at or near the time of convective initiation, or about one to three hours before the first occurrence of a severe local storms.
The synoptic pattern in the May 9, 1995 event consisted of a closed surface and mid-tropospheric low pressure system associated with strong dynamic forcing moving across Iowa and northern Illinois. Thunderstorms developed along a warm frontal boundary in eastern Iowa and northwest Illinois, and showed radar characteristics of classic and HP supercells. Subsequent squall line development along a trailing cold front in Missouri contained several bowing segments that eventually moved northeast and merged with several supercells. Thirteen tornadoes, four rated F3, and numerous large hail reports (up to three inches) occurred in eastern Iowa and northwest Illinois during a 3.5 hour period (Figure 1).
Figure 1. Severe weather reports for May 9, 1995. Hail reports are dots and tornado tracks are indicated by a line. The four F3 tornadoes are labeled
At 1800 UTC, the three-hour forecast from the 1500 UTC MAPS run indicated a jet streak at 250 mb over northern Arkansas and southern Missouri (Figure 2). The secondary circulation implied by the jet streak was well diagnosed by the Q-vector and Q-vector divergence fields. In Figure 3, the dashed area extending from southeast Iowa into Missouri is a reflection of the dynamic forcing for upward vertical motion implied by the thermally indirect circulation in the jet exit region. Convergence at 850 mb at the nose of a modest low-level jet is indicative of the lower branch of the circulation (Figure 4). Also, Figure 4 depicts two distinct wind regimes at 850 mb; strong southwest flow over Missouri where shear was unidirectional with height, and weaker south to southeast flow over eastern Iowa and northern Illinois where the wind was veering with height. The squall line developed in the area with an unidirectional wind shear profile while the supercells developed in the area where low-level winds were veering with height.
Figure 2. RUC three-hour forecast valid 1800 UTC 9 May 1995 of 250 mb wind (kts) and isotachs (kts).
Figure 3. RUC three-hour forecast valid 1800 UTC 9 May 1995 of 500 mb heights (gpdm), 500-300 mb layer Q-vectors and Q-vector divergence (10-19m Pa-1m-2s-1). Negative values (dashed) indicate convergence and forcing for ascent.
Figure 4. RUC three-hour forecast valid 1800 UTC 9 May 1995 of 850 mb wind (kts) and divergence (10-5s-1). Negative values (dashed) indicate convergence.
The 700-500 mb layer lapse rate is a useful tool for forecasting convection (Doswell et al. 1985, Wolf and Thaler 1995). In this case, an instability axis was indicated roughly along the Mississippi River from southern Missouri to southern Wisconsin (Figure 5) where lapse rates ranged from 7.0 to over 7.5 -°C/km (conditionally unstable for that layer). Interestingly, the development severe local storms occurred within this instability axis and primarily where it intersected the surface warm front (Figure 6) in eastern Iowa and northern Illinois. Also at the surface, an area of moisture flux convergence was evident in the MAPS analysis at 1800 UTC across northern Missouri and southern Iowa (Figure 6), coincident in time and space with the location where both the supercell and squall-line storms initiated.
Figure 5. RUC three-hour forecast valid 1800 UTC 9 May 1995 of 700-500 mb lapse rate (-°C km-1).
Figure 6. RUC surface analysis valid 1800 UTC 9 May 1995 of temperature (dashed °C) and moisture flux convergence (g kg-1hr-1).
||CASE II - MAY 13, 1995
The synoptic pattern in the May 13, 1995 event consisted of a closed surface and mid-tropospheric low pressure system over southeast South Dakota, well northwest of the main area of severe local storm development across southeast Iowa, northeast Missouri and western Illinois. Thunderstorms developed along a warm frontal boundary in northeast Missouri, and showed radar characteristics of classic supercells. The storms resulted in numerous reports of very large hail (up to 4.5 inches) and several tornadoes, two of which were long-tracked and rated F4 (Figure 7).
Figure 7. Severe weather reports for 13 May 1995. Hail reports are dots and tornado tracks are indicated by a line. The two F4 tornadoes are labeled.
At 1800 UTC, the three-hour forecast from the 1500 UTC MAPS run indicated a jet streak at 250 mb over southeast Nebraska (Figure 8). As in the previous case, the secondary circulation implied by the jet streak was well diagnosed by the Q-vector and Q-vector divergence fields. Note in Figure 9 that the dashed area, which indicates dynamic forcing for rising motion, was coincident with the area of early morning hail reports in north central Iowa (Figure 7). If one applies the conceptual model of the four-cell circulation about a jet streak that would indicate downward vertical motion in the area where the most widespread severe weather occurred. The Q-vector diagnosis implies that the secondary circulation fits more of a two-cell model likely due to the subtle curvature of the wind field (Moore and VanKnowe 1992). Thus, based on the Q-vector diagnosis, it appeared that vertical motion related to the secondary circulation of the jet streak was neither a positive nor a negative contributor to the severe weather activity in eastern Iowa and western Illinois that afternoon.
Figure 8. RUC 3-hr forecast valid 1800 UTC 13 May 1995 of 250 mb wind (kts) and isotachs (kts).
Figure 9. RUC 3-hr forecast valid 1800 UTC 13 May 1995 of 500 mb heights (gpdm), 500-300 mb layer Q-vectors and Q-vector divergence (10-19m Pa-1m-2s-1). Negative values (dashed) indicate convergence and forcing for ascent.
Figure 10. RUC 3-hr forecast valid 1800 UTC 13 May 1995 of 700-500 mb lapse rate (-°C km-1).
At 850 mb, convergence was most pronounced over north central Iowa (not shown), perhaps in association with the upward branch of the thermally indirect circulation in the jet exit region. However, there is a hint of a weak convergence axis in southeast Iowa and west central Illinois. An examination of 700-500 mb lapse rates shows an instability axis arcing from south central Missouri up the Mississippi River Valley (Figure 11).
Figure 11. RUC surface analysis valid 1800 UTC 13 May 1995 of moisture flux convergence
The MAPS surface analysis at 1800 UTC (not shown) depicted a closed surface circulation over southeast South Dakota with a warm frontal boundary extending southeast into northern and eastern Missouri. Strong moisture flux convergence (Figure 11) and theta-e advection (Figure 12) were occurring in north-central and northeast Missouri along the warm front. It is in this area that convection first initiated. Surface winds there were southeast, so the wind profile was characterized by both strong speed and directional shear, supportive of supercell storms.
Figure 12. RUC surface analysis valid 1800 UTC 13 May 1995 of theta-e advection (K hr-1). Positive values are solid lines, negative are dashed.
In each of these cases, a limited suite of MAPS surface analyses and three-hour forecasts provided useful guidance capable of helping a forecaster focus on both the timing and location of convective initiation and the synoptic-scale mechanisms supportive of the subsequent development of severe local storms. In the May 9 case, the environment was shown to be strongly forced on the synoptic-scale with the secondary vertical circulation from a jet streak perhaps playing a key role in developing an environment favorable for severe storms. In contrast, lower tropospheric features played a key role in the development of the May 13 case.
A wealth of data from satellites, wind profilers, ACARS, ASOS, etc. are generated on an hourly (or even more frequent) basis. It can be difficult for a forecaster under the constraints of a heavy workload during complex weather to take advantage of that data and the crucial information contained therein. The RUC, through its powerful data assimilation system and three-hour analyses and forecasts, provides a tool that integrates the technological benefits of much of the National Weather Service's Modernization into a package that is easy for forecasters to use. This requires a change in mind set from the past when numerical guidance was available only every 12 hours rather than every three hours. With the upgrades planned for the RUC in 1997 (Benjamin et al. 1996), model analyses and forecasts will be produced every hour. In other words, a "new" model run will always be available for short term forecasting.
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