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Lance F. Bosart

Abstract

During the period 26–29 May 1968 a shallow cyclonic circulation, known 1ocally as a Catalina eddy, developed in the offshore waters of southern California. A synoptic and mesoscale analysis of the event establishes the following: 1) the incipient circulation forms on the coast near Santa Barbara downwind of the coastal mountains, 2) cyclonic shear vorticity appears offshore in response to lee troughing downstream of the coastal mountains between Vandenberg and Pt. Mugu, California, 3) mountain wave activity may be aiding incipient eddy formation in association with synoptic-scale subsidence and the generation of a stable layer new the crest of the coastal mountains, 4) a southeastward displacement and offshore expansion of the circulation occurs following the passage of the synoptic-scale ridge line, and 5) dissipation of the eddy occurs with the onset of a broad onshore flow.

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Lance F. Bosart

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No abstract available.

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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

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

Abstract

On 18–19 February 1979 a major east coast cyclone deposited a record-breaking snowfall on the Middle Atlantic States. The storm is noteworthy because of the failure of the operational prediction models to signal the intensity of the event. The life cycle of the cyclone is reviewed with emphasis on the synoptic and mesoscale features and their possible linkage.

Prior to cyclogenesis the synoptic pattern features a massive cold anticyclone near the Great Lakes with a broad baroclinic zone extending from Texas eastward to the Atlantic coast. A region of enhanced lower tropospheric baroclinicity develops along the Carolina coastal strip in response to significant oceanic sensible and latent heat fluxes which warm, moisten and destabilize the boundary layer. Cyclogenesis is initiated along the coastal front as the result of lower tropospheric warm advection. The importance of the coastal front is that it effectively steers the cyclone north-northeastward parallel to the coast such that it eventually acquires a favorable phase relationship for deepening with respect to a vigorous short-wave trough moving eastward from the Ohio Valley by 1200 GMT 19 February.

Explosive deepening takes place in the ensuing 6 h coincident with the outbreak of convection near the storm center. By 1800 GMT, satellite pictures reveal a closed, clear storm eye while surface ship and drilling rig data disclose the presence of minimal hurricane force winds, primarily in the northern semicircle of the storm. Unlike a hurricane, however, the convection is asymmetric with respect to the vortex, being concentrated in the region of strongest surface winds.

The major operational model errors stem from poor sea level pressure and quantitative precipitation prognoses. Evidence is presented that initial analysis deficiencies coupled with inadequate boundary-layer and convective precipitation physics precluded a successful model forecast in this cam.

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Lance F. Bosart

Abstract

No abstract available.

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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

Abstract

An analysis of skill in predicting daily temperature and precipitation is presented for six years (1969–1975) of forecasts made for the Albany County Airport by students and faculty in the Department of Atmospheric Science of the State University of New York at Albany. The daily consensus forecast (made up by averaging the forecasts of all forecasters) shows no significant secular increase in skill for temperature. An apparent increase in the consensus skill in precipitation forecasting is noted with most of the increase occurring in the spring 1972 semester. Possible reasons for this increase are discussed. The skill (defined as the percentage improvement over a persistence climatological forecast) of the ensemble of forecasters over a persistence climatological control is near 50% for the first day decaying to 10% and near zero by the 3rd and 4th day for precipitation and to just under 10% for temperature by the 4th day. These results are consistent with the results presented by Sanders (1973).

Some relationship is found for skill to be a function of the variability of the daily temperature about the climatological mean. Skill, however, appears to be insensitive to the frequency of days with radiational cooling, a major local forecast problem. likewise skill appears to be independent of daily rainfall amount or frequency. These findings are consistent with those found for Boston by Sanders (1973).

Finally, the trend towards a plateau in skill noted by Sanders (1973) is confirmed for a different location.

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Lance F. Bosart

Abstract

No abstract available

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Lance F. Bosart

Abstract

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

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