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Lance F. Bosart

Abstract

Average kinematic vertical motion and relative vorticity profiles are presented for a long-lived midlatitude convective complex. A breakdown into active convective and stratiform precipitation regions shows very good agreement in the vertical motion profiles with published results for tropical convective systems.

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Lance F. Bosart

Abstract

A case study is made of the Texas coastal rainstorm of 17–21 September 1979 in which upward of 50 cm of rain inundated the area. The precipitation developed along a weak baroclinic zone left in place by a trough passage at higher latitudes. A cold upper tropospheric vortex over the southwestern United States enabled relatively cooler and drier air to flow southward over the warmer waters of the western Gulf of Mexico. Differential heating and moistening along a Texas coastal front slowly destabilized the atmosphere and set the stage for a convective scale response.

A mesoscale cyclonic circulation formed near the southwestern end of the coastal front and along the western edge of a convective cloud cluster. Embedded within this circulation was a short-lived mesocyclone which achieved tropical storm strength for 12 h. The case is a specific example of a mesoscale circulation in which origin and evolution is controlled by synoptic scale patterns. The mesocale disturbance, once formed, moves northeastward parallel to the coast. It gradually moves into an environment more favorable for quasi-geostrophic intensification as the circulation expands in area.

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Lance F. Bosart

Abstract

Correction to Volume 5, Issue 3, pages 527-528.

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Lance F. Bosart

Abstract

A case study is made of a period of exceptional warmth in the mid and upper troposphere over the Carib-bean from 19–21 February 1964. The case is a good example of intense winter interaction of mid-latitude and tropical circulations during which lateral forcing from higher latitudes appears to play a prominent role. For example, at 500 mb, temperatures of −1 to −2C which exhibited spacial and temporal continuity on the synoptic scale were noted. These values were more than three standard deviations warmer than monthly mean values and occurred in association with a strong low-latitude polar jet along the Gull Coast of the United States.

Isentropic trajectories document average 12-hr descent rates of up to 2 cm sec−1 in the vicinity of 15N in association with the warmth area. Criteria for inertial instability is satisfied over a portion of the anti-cyclonic flank of the strong jet stream leading to a speculative case for a transverse thermally indirect circulation such that a persistent anticyclone north of Puerto Rico is strengthened. Reed and Vleck's hypothesis on the annual temperature cycle of the tropical lower stratosphere is examined on a limited basis with daily data both directly and indirectly with rather inconclusive results.

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Lance F. Bosart

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Lance F. Bosart

Abstract

The results of a near real time experiment designed to assess the state of the art of quantitative precipitation forecasting skill of the operational NMC LFM-2 are described. All available LFM-2 quantitative precipitation forecasts were verified on an area-averaged basis for southern and central New England for the period 0000 GMT 3 January through 0000 GMT 14 May 1979. Individual point verifications were also made for Albany, Boston, Concord, New York and Portland.

On an area-averaged basis the LFM-2 beat (lost to) the climatological control by +18.5% (−7.4%) for the 12–24 h (24–36 h) forecast projection. On a point basis the overall LFM-2 forecasts lost to climatology by −7.2 and −21.9% for these forecast projections.

Close examination of the results suggests that much of the loss of predictive skill in the model forecasts is the result of systematic overprediction of precipitation accompanying major cyclonic events. Possible reasons for this behavior are examined through a discussion of individual synoptic cases.

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Lance F. Bosart
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Lance F. Bosart

Abstract

No Abstract available.

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LANCE F. BOSART

Abstract

Detailed hourly precipitation patterns are analyzed for two major winter U.S. east coast storms that exhibited considerable mesoscale features. Pronounced spatial and temporal continuity is noted for individual convective rainfalls within the cold air. Such features can also be tracked in the wind and pressure fields. Finally, some thoughts are offered on the possible dynamic significance of organized mesoscale precipitation patterns, along with comments relevant to the forecasting of such patterns.

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Lance F. Bosart

Consensus (the average of all forecasts) skill levels in forecasting daily maximum and minimum temperature, precipitation probability across six class intervals, and precipitation amount at the State University of New York at Albany are reviewed for the period 1977–82. Skill is measured relative to a climatological control. Forecasts are made for four consecutive 24 h periods for Albany, N.Y., beginning at 1800 GMT of the current day.

For minimum temperature, the skill levels average 57%, 41%, 26%, and 15%, respectively, for 24, 48, 72, and 96 h in advance. For maximum temperature, a more limited sample yields corresponding skill levels of 84%, 49%, 34%, and 19% for 12, 36, 60, 84 h ahead. Linear regression analysis yields little in the way of a definitive trend, given the smallness of the explained variance. Comparison with other readily available objective and subjective operational guidance establishes the credibility of the consensus forecast.

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