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Michael D. Vescio and Richard L. Thompson

Abstract

An experiment was conducted at the Storm Prediction Center (SPC) to assess the accuracy of subjective probability forecasts for tornadoes within individual convective watch areas. Probability forecasts for one or more and three or more tornadoes were produced for 166 severe weather watches during 1997 and 1998. Categorical forecasts of maximum tornado intensity, as indicated by F-scale damage ratings, were also performed. The probability and intensity forecasts were made in an operational setting prior to the issuance of each watch to simulate the decision making process that might be employed if the SPC were to begin including probabilities in their watch products. Results indicate considerable skill in forecasting tornado probabilities, though the maximum intensity forecasts were not particularly accurate. It is hypothesized that accurate tornado intensity forecasts will be difficult to achieve until storm-scale processes are more fully understood.

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Michael D. Vescio and Richard H. Johnson

Abstract

The two most prominent surface pressure features associated with squall lines are 1) a surface mesohigh, centered within the heavy-rain region, and 2) a wake low, located at the back edge of the trailing area of light rainfall. The surface flow near these features is often highly unbalanced due to propagation and transience of the pressure fields. Centers of surface divergence and convergence are typically displaced rearward of the mesohigh and wake-low axes, respectively.

In an attempt to explain the basic mesoscale characteristics of the surface flow in the vicinity of squall lines, a simplified model of a propagating mesohigh-wake-low couplet is developed using a one-dimensional slab model of the boundary layer. The component of the flow normal to the squall line is predicted, with advective and frictional effects included but the Coriolis force neglected. Model results are compared to the observed airflow near the mesohigh and wake low associated with an intense squall line that moved through Oklahoma and Kansas on 10–11 June 1985.

The primary mesoscale features of the surface flow associated with squall lines are explained by the model. Displacement of the divergence and convergence axes to the rear of the mesohigh and wake-low axes is a result of propagation of the system. Strong winds are found out ahead of the mesohigh when the phase speed of the pressure wave matches the air-parcel velocities. This effect, along with the vertical transport of momentum by the convective line, can cause surging of the winds along the gust front. Strong surface convergence can occur behind the wake low even for weak pressure disturbances, which may account for the generation of new convection to the rear of some squall lines. In addition, as a result of reduced friction, strong and damaging winds may develop in the vicinity of wake lows that pass over open-water areas. Some squall lines possess very intense pressure gradients between the mesohigh and wake low (on occasion equivalent to that in the eyewall of a moderate hurricane); however, hurricane-force winds normally do not develop because air parcels do not stay in this gradient long enough to achieve extreme velocities.

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Richard H. Johnson, William A. Gallus Jr., and Michael D. Vescio

Abstract

Rawinsonde observations have been used to determine the flow structure in the vicinity of the tropopause atop the trailing stratiform precipitation region of an intense midlatitude squall line. Computations of vertical motion using the kinematic and thermodynamic methods show (i) upward motion in the mid- to upper troposphere within the stratiform cloud and (ii) downward motion in a thin layer 2–3 km deep centered near the tropopause at cloud top. The latter feature, which has received little attention and explanation, has recently been reported to exist above stratiform rain areas in the tropics, as measured directly by a wind profiler on the tropical western Pacific island Pohnpei.

The thin layer of sinking at the top of the trailing stratiform region occurs along the sloping upper cloud boundary and is associated with downward-sloping isentropes in the lower stratosphere to the rear of the convective line. The deformation of the isentropes is associated with an upward bulging of the tropopause, presumably caused by strong ascent in the convective line and/or mesoscale ascent aloft in the squall line system. In computations involving the thermodynamic method, the diagnosed sinking is greatest when cloud-top radiative cooling is included but it occurs even without it. Comparison of results from the various methods, however, suggests that radiative cooling at cloud top exists and is important, and it may have an amplitude on the order of 0.5°C h−1 as determined by Webster and Stephens. The strong cooling at the top of mesoscale convective systems such as these suggests a possible mechanism for dehydration of the lower tropical stratosphere that does not require a primary transport of water vapor from a particularly cold tropopause region such as the Indonesian maritime continent.

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Roger Edwards, Stephen F. Corfidi, Richard L. Thompson, Jeffry S. Evans, Jeffrey P. Craven, Jonathan P. Racy, Daniel W. McCarthy, and Michael D. Vescio

Abstract

Forecasters at the Storm Prediction Center (SPC) were faced with many challenges during the 3 May 1999 tornado outbreak. Operational numerical forecast models valid during the outbreak gave inaccurate, inconsistent, and/or ambiguous guidance to forecasters, most notably with varying convective precipitation forecasts and underforecast wind speeds in the middle and upper troposphere, which led forecasters (in the early convective outlooks) to expect a substantially reduced tornado threat as compared with what was observed. That, combined with relatively weak forecast and observed low-level convergence along a dryline, contributed to much uncertainty regarding timing and location of convective initiation. As a consequence, as the event approached, observational diagnosis and analysis became more important and were critical in identification of the evolution of the outbreak. Tornadic supercells ultimately developed earlier, were more numerous, and produced more significant tornadoes than anticipated. As forecasters addressed the meteorological facets of the tornadic storms on the evening of 3 May 1999, there were other areas of simultaneous severe-storm development, and one of the tornadoes posed a threat to the facility and family members of the forecast staff. These uncertainties and challenges are discussed in the context of SPC convective outlooks and watches for this outbreak. Recommendations are made for continued research aimed at improving forecasts of convective initiation and mode.

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