Summary of High Wind Event of 7 March 2004

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Dylan Dennis
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Summary of High Wind Event of 7 March 2004 This event was characterized by a very strong jet streak that developed over North Carolina by 00 UTC 8 March, as seen in the Eta model analysis at 300 mb,

1 Summary of High Wind Event of 7 March 2004 This event was characterized by a very strong jet streak that developed over North Carolina by 00 UTC 8 March, as seen in the Eta model analysis at 300 mb, with wind speeds exceeding 150 kt. just downstream of the trough axis over central NC and SC. Upper-level divergence was rather weak at that time over much of the state. Eta model 300-mb analysis at 00 UTC 8 March.

2 The forecast of this feature from the 12Z Eta run on 7 March had the placement of the jet streak correct, as well as the weak divergence, but the strength of the jet streak was underestimated by kt: Eta model 12-h 300-mb forecast for 00 UTC 8 March.

Analyzed conditions in the mid-troposphere were rather dry at 12 Z on 7 March, as seen in the Eta model analysis of relative humidity showing RH values of 30% or less over much of the eastern

3 Analyzed conditions in the mid-troposphere were rather dry at 12 Z on 7 March, as seen in the Eta model analysis of relative humidity showing RH values of 30% or less over much of the eastern seaboard. The mb lapse rate at this time was between 9ºC and 12ºC over the region. Eta model analysis of 700 mb RH (shaded), winds (barb, kt.) and mb lapse rate (dashed black contours) for 12 UTC 7 March.

4 This lapse rate is supported by the 12Z GSO sounding on 7 March, which shows a surface based stable layer up to about 950 mb, with a saturated layer up to around 800 mb. The sounding actually is supportive of a weak CAD event, with shallow northeasterly flow just above the surface in addition to the surface inversion. Above the cloud layer the sounding is quite dry, with a subsidence inversion near 700 mb. GSO sounding for 12 UTC 7 March.

5 By 00 UTC 8 March, the situation had changed dramatically. The shallow stable layer has eroded, and the GSO sounding shows a very deep dry adiabatic mixed layer from the surface up to nearly 650 mb, with a very dry lower troposphere, a shallow cloud layer just below 500 mb. Jet level winds are 150 kt., supportive of the Eta 300 mb analysis shown earlier. The sounding is slightly unstable with CAPE of 225 J kg -1 and a lifted index of +1. GSO sounding for 00 UTC 8 March.

6 The changes in the sounding are even more dramatic when the 12Z and 00Z soundings are compared to each other. The deep mixed layer was created by significant warming in the column below about 775 mb, and dramatic cooling (> 10ºC) from 775 to above 500 mb. Drying also occurred through a large portion of the lower- and mid-troposphere during the period. GSO soundings to 500 mb from 12Z 7 March (solid) and 00Z 8 March (dashed). Winds from 12Z (00Z) sounding are in the left (right) column.

The deep mixed layer led to 1000-700 mb lapse rates increasing to over 24ºC over North Carolina by 00 UTC 8 March, just ahead of the region of higher 700 mb RH associated with the

7 The deep mixed layer led to mb lapse rates increasing to over 24ºC over North Carolina by 00 UTC 8 March, just ahead of the region of higher 700 mb RH associated with the rainband that was moving into the western Piedmont at that time. Eta model analysis of 700 mb RH (shaded), winds (barb, kt.) and mb lapse rate (dashed black contours) for 00 UTC 8 March.

8 So, the question now is what was responsible for producing the steep lapse rates seen in the 00 UTC GSO sounding? The warming in the portion of the sounding below 800 mb is likely due to solar heating and low-level warm advection occurring ahead of the surface front. However, that is only half the story, as the cooling above 800 mb that occurred is even greater in magnitude. What generated this cooling? Below are the Eta model analyzed sounding at GSO for 18Z 7 March and 00Z 8 March. By 18Z, the atmosphere has warmed considerably below 800 mb. However, there is still significant cooling seen above 800mb that occurs between 18Z and 00Z, suggesting that the cooling aloft took place later in the day on 7 March. Now, we will investigate what mechanisms may have produced the cooling in that layer. Eta model analyzed soundings for GSO at 18Z (solid) and 00Z dashed. Winds at 18Z (00Z) are in the left (right) column.

By 00 UTC 8 March surface analysis shows that the cold front has moved to the eastern slopes of the Appalachians with southwesterly flow ahead of the front and northwesterly flow behind the front.

9 By 00 UTC 8 March surface analysis shows that the cold front has moved to the eastern slopes of the Appalachians with southwesterly flow ahead of the front and northwesterly flow behind the front. Ahead of the front, temperatures are generally in the 60s and 70s, while in the mountains behind the boundary, temperatures have fallen into the 30s and 40s. Manual Surface analysis valid at 00 UTC 8 March showing isobars (solid lines every 2 mb) and isotherms (dashed lines every 5 F).

10 The next images are cross-sections from the Eta model along 36 N from 84 W to 73 W across northern North Carolina as seen in the image below:

11 The cross section shows temperature advection, vertical velocity, ageostrophic circulation and potential temperature. At 00Z, the surface front is evident with omega values of more than +20 µbar s -1 behind the strong cold advection extending along the steep frontal surface from the surface to 600 mb. Ahead of the surface front however, is an area of cold advection aloft in the 700 to 500 mb layer over northern North Carolina at this time, with low-level warm advection also seen from the surface to near 900 mb. Eta model cross section analysis at 00Z 8 March showing temperature advection with warm (cold) advection shaded in the warm (cool) colors, ageostrophic circulation (arrows), vertical motion with upward (downward) motion dashed (solid) and potential temperature (red contours).

By 03Z, the Eta forecasts the surface front to have passed the center of the section, which would be east of RDU. The front is quite strong, with very strong low-level cold advection occurring.

12 By 03Z, the Eta forecasts the surface front to have passed the center of the section, which would be east of RDU. The front is quite strong, with very strong low-level cold advection occurring. The cold advection just above the surface is somewhat forward tipped. The frontal surface is very steep, as the isentropes are nearly vertical from the surface to near 850 mb. The front slopes back to the west and upward, connecting to a second maximum of cold advection near 500 mb over the mountains. A pronounced vertical motion couplet is seen at the surface front, with upward motion exceeding 30 µbar s -1 ahead of the front and downward motion of +10 µbar s -1 behind the front. Eta model cross section 3-h forecast valid at 03Z 8 March showing temperature advection with warm (cold) advection shaded in the warm (cool) colors, ageostrophic circulation (arrows), vertical motion with upward (downward) motion dashed (solid) and potential temperature (red contours).

13 It seems likely that this high wind event was produced by a combination of a steep low level lapse rates and a strong cold front with a vigorous frontal circulation that served to produce ascent and precipitation ahead of the front which enhanced the downward motion behind the front by evaporative cooling. The steep lapse rates were likely generated by strong solar heating during the day and weak low-level warm advection ahead of the front. Some cold advection aloft ahead of the surface front completed the process by cooling the mid levels in advance of the strong forcing for ascent.

and 24 mm, hPa lapse rates between 3 and 4 K km 1, lifted index values

and 24 mm, hPa lapse rates between 3 and 4 K km 1, lifted index values 3.2 Composite analysis 3.2.1 Pure gradient composites The composite initial NE report in the pure gradient northwest composite (N = 32) occurs where the mean sea level pressure (MSLP) gradient is strongest