During the 13 month period of this study a variety of cases was observed. Five different cases were chosen as examples of what parameters have the greatest influence in creating the various strength winds over Washington County. The following case studies include two Wind Advisories, two High Wind Warnings, and one Non-event.
The values generated in AWIPS are in degrees Celsius/12 hrs. To convert AWIPS values to NTRANS divide by 4.32 (which is derived by converting C/12hr to C/s x 10-4). Although cold advection is calculated as a negative term, for brevity and simplicity, thermal advection in these cases will be referred to an absolute value without units.
The October 27 event occurred with a closed low located over southwest Arizona. The driving force for this event was created by the surface pressure gradient (which peaked at 6 mb at 09Z between CDC-LAS, not shown). Despite NE oriented 700 mb winds at 40-45 kts at 06Z (Fig 1A), support from aloft was minimal, if any, since the thermal advection was near zero (as with any closed low). Note in this case that the surface pressure gradient was oriented more parallel than perpendicular to the 700 mb wind direction. Comparing the forecasted gradients between CDC-LAS and CDC-GCN the stronger of the two was CDC-GCN (a moderate 6 mb) (Fig 1B). This is consistent with several other cases, in that as long as a general N-S surface pressure gradient is present, it does not matter whether the orientation is NW-SE or NE-SW. The main contribution is the strength and not the orientation.
| FIGURE 1A - 00Z Oct 27, 1996, 6 hr forecast of 700 mb thermal advection (valid 06Z) | FIGURE 1B - 00Z Oct 27, 1996, 6 hr forecast surface pressure gradient (valid 06Z) |
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The strongest gust of 50 mph occurred at 08Z. Winds at White Reef remained between 45-50 mph through 11Z then dropped off to between 35-45 mph through 22Z. If the ETA 00Z 12 hr forecast (Fig 2A) had been correct, surface winds would have dropped off to below 25 mph, due to a rather weak surface pressure gradient spread out equally across the state. However, the ETA 12Z analysis (Fig 2B) indicates that a more tightly packed gradient prevailed over southern Utah with a moderate 5 mb surface pressure gradient between CDC-LAS, sufficient enough to maintain wind gusts in the 35-45 mph range, but just below Wind Advisory criteria as was observed at White Reef.
| FIGURE 2A - 00Z Oct 27, 1996, 12 hr forecast of surface pressure gradient (valid 12Z) | FIGURE 2B - 12Z Oct 27, 1996, analysis of surface pressure gradient (valid 12Z) |
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At 700 mb, the thermal advection value was similar between the ETA 00Z 12 hr forecast (Fig 3A) and the 12Z analysis (Fig 3B), both of which were near zero. The position of the 700 mb circulation was nearly identical over western Arizona, but a noticeable change had occurred in the wind speed over Nevada and Utah, specifically over Washington County. What initially was forecast to be 30-40 kt winds by 12Z, were now analyzed at 20-25 kts. Obviously, winds of this magnitude would not and did not support Wind Advisory criteria.
| FIGURE 3A - 00Z Oct 27, 1996, 12 hr forecast of 700 mb thermal advection (valid 12Z) | FIGURE 3B - 12Z Oct 27, 1996, analysis of 700 mb thermal advection (valid 12Z) |
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White Reef reported only 4 hours of sustained winds barely exceeding 31 mph (07-10Z) and 5 hours worth of 46-50 mph gusts (07-11Z).
CASE SUMMARY - This is a good example showing the importance of thermal advection at 700 mb. When similar moderate surface pressure gradients of 6 mb and moderate-strong 700 mb winds of 40 kts were present in two other cases studied, HWW criteria winds were produced. The difference was the absence of thermal advection in this case versus thermal advection values of greater than 3 in the two HWW cases. Therefore, without any or very minimal thermal advection, as in any closed low, winds typically will barely reach WA criteria.
This case also showed how the orientation of the surface pressure gradient to that of the canyons is not that important, as long as there is a basic N-S gradient. The main contribution is the strength and not the orientation.
The December 14-15 event was produced by the combination of a moderate surface pressure gradient and moderate-to-strong 700 mb support. Typically, a strong or moderate surface pressure gradient causes the onset of the gap winds, which is then frequently followed with support aloft. However, the onset of this episode was largely due to momentum aloft in the form of moderate 700 mb winds being mixed down to the surface by moderate thermal advection at 700 mb. Figures 1A and 1B, ETA's 6 hr forecasts valid at 18Z, indicate that the 700 mb cold advection was already moderate(7-8) over central Washington County while at the same time the CDC-GCN surface pressure gradient was moderate (6 mb). (Note: the actual difference in surface pressure between CDC-GCN was 5 mb.)
| FIGURE 1A - Dec 14, 1996, 6 hr forecast of 700 mb thermal advection (valid 18Z) | FIGURE 1B - Dec 14, 1996, 6 hr forecast surface pressure gradient (valid 18Z) |
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At 00Z, the 700 mb winds increased to 40 kts and the thermal advection continued to intensify (8-10)(Fig 2A), while the surface pressure gradient remained nearly unchanged between 5-6 mb(Fig 2B).
| FIGURE 2A - Dec 15, 1996, 00Z analysis of 700 mb thermal advection | FIGURE 2B - Dec 15, 1996, 00Z analysis of surface pressure gradient |
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| FIGURE 3A - Dec 15, 1996, 6 hr forecast of 700 mb thermal advection (valid 06Z) | FIGURE 3B - Dec 15, 1996, 6 hr forecast of surface pressure gradient (valid 06Z) |
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CASE SUMMARY - This event was close to reaching HWW criteria with the combination of a moderate surface pressure gradient, moderate-strong 700 mb winds, and moderate thermal advection. At White Reef, gusts maxed between 52-56 mph for 7 hours. A slight increase to the any of these three parameters would likely have nudged this event to a HWW. The onset of this event was attributed to moderate thermal advection at 700 mb causing a downward transport of momentum. The moderate thermal advection at 700 mb continued past sunset (til nearly 06Z), with winds continuing at WA levels. As the combination of support aloft and at the surface weakened, winds fell below WA criteria. This occurred more rapidly than the ETA 6 hr forecast, which had smoothed the surface pressure gradient incorrectly between CDC-GCN, showing the importance of following the position of the tightest surface pressure gradients by hourly observations, hand drawn surface analyses, and/or by the RSA script (NWSFO SLC NTRANS local program).
In 1996-97, there were five "long-fused" wind events which met High Wind Warning (HWW) criteria. The combination of strong thermal advection and strong pressure gradient was a common denominator in three-of-five of these cases. In the other two cases, just a strong pressure gradient was present without additional support from strong thermal advection. These type of occurrences are associated with closed lows. One of these closed low events was January 6-7, 1997, which will be discussed due to its unusual longevity of HWW type winds and various driving forces. The other HWW case that will be discussed is an open trough case from December 17-18, 1996, which produced strong thermal advection for an extended period of time over the area of concern.
The January 6-7, 1997, was a High Wind Warning event associated with a closed low initiallydeveloping over southeast Utah and then maturing over Arizona. Once formed, this closed low moved very slowly from southwest Arizona to southeast Arizona during a 12 hr period. In its forming stage, Jan 6 (00-06Z), 700 mb thermal advection and wind speed were weak (-2.0 to -2.5 and <25 kts, respectively), but the surface pressure gradient was moderate (7 mb) (Fig 1A-B). WA type winds were occurring at this time at both White Reef and Badger Spring and reached HWW one hour later at 07Z in response to a strengthening surface pressure gradient.
| FIGURE 1A - 00Z Jan 6, 1997, 6 hr forecast of 700 mb thermal advection (valid 06Z) | FIGURE 1B - 00Z Jan 6, 1997, 6 hr forecast surface pressure gradient (valid 06Z) |
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At 12Z January 6, the surface pressure gradient was still the predominant driving force. The ETA analysis had smoothed the gradient giving the appearance that the surface pressure gradient had weakened (6 mb) (Fig 2B). The 12 hr forecast from 00Z (valid 12Z, not shown) had a slightly better solution with 7-7.5 mb difference. However, according to the surface observations, the surface pressure gradient had strengthened to 8.5 mb between CDC and GCN. Badger Spring winds peaked at 72 mph at 14Z in response to this tighter surface pressure gradient.
Support from aloft had not changed much as 700 mb winds were 25 kts and the thermal advection decreased to 0.
| FIGURE 2A - 12Z Jan 6, 1997, analysis of 700 mb thermal advection (valid 12Z) | FIGURE 2B - 12Z Jan 6, 1997, analysis of surface pressure gradient (valid 12Z) |
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As this system furthered deepened over south central Arizona, between 12Z Jan 06 and 00Z Jan 07, the surface pressure gradient remained strong over Washington County and northern Arizona (8 mb CDC-GCN) (Fig 3B) and the 700 mb winds increased significantly to 45 kts (Fig 3A). Winds increased again at Badger Spring, reporting a max gust of 74 mph at 18Z, but more importantly winds remained strong throughout the afternoon, gusting between 65 and 70 mph through 23Z. With surface temperatures in the afternoon at St George rising above 5C and 700 mb temperature remaining about -12C, the atmosphere became unstable. This likely resulted in the transfer of momentum from aloft down to the surface. After 23Z, surface wind observations showed a steady decline, likely a result of both surface pressure gradient relaxing and stabilization as surface heating was abating. Once again thermal advection was weak (-2.0 to -2.5)(Fig 3A) through this period and was not a factor.
At White Reef, winds were sustained about 35 mph with gusts in the range of 55-60 mph, barely reaching HWW criteria. A spotter report from Virgin indicated winds gusting to 66 mph at 23Z.
| FIGURE 3A - 12Z Jan 6, 1997, 06 hr forecast of 700 mb thermal advection (valid 18Z) | FIGURE 3B - 12Z Jan 6, 1997, 06 hr forecast of surface pressure gradient (valid 18Z) |
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By 00Z Jan 07, the surface pressure gradient between CGN and CDC had weakened to 6 mb (Fig 4B). However, winds aloft were reaching their peak (45-50 kts)(Fig 4A) with the 700 mb low at its deepest point. With a moderate surface pressure gradient still present the higher momentum winds were still able to mix down to the surface during the evening hours keeping a WA valid through midnight.
| FIGURE 4A - 00Z Jan 7, 1997, analysis of 700 mb thermal advection (valid 00Z) | FIGURE 4B - 00Z Jan 7, 1997, analysis of surface pressure gradient (valid 00Z) |
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Finally, by 06Z Jan 07, the surface pressure gradient was down to 5 mb (Fig 5b), which is marginal for WA criteria based on gradient alone. The surface pressure gradient relaxed even more after midnight and winds at the surface fell below WA criteria despite the 700 mb winds still strong at 45-50 kts (Fig 5a). Without either a moderate surface pressure gradient or a moderate thermal advection mixing of the boundary layer ended and the atmosphere became decoupled.
| FIGURE 5A - 00Z Jan 7, 1997, 6 hr forecast of 700 mb thermal advection (valid 06Z) | FIGURE 5B - 00Z Jan 7, 1997, 6 hr forecast surface pressure gradient (valid 06Z) |
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CASE SUMMARY - The predominant feature for driving strong winds in three-of-four other HWW cases, has been the surface pressure gradient alone or in combination with strong thermal advection. In this case, HWW winds were driven by a moderate-to-strong surface pressure gradient which was enhanced during the afternoon by transfer of momentum downward through turbulence mixing upward created by surface heating. Despite 700 mb winds increasing to 45-50kts and surface pressure gradient still a respectable 5 mb at 00Z, surface winds started decreasing after 23Z with the lost of surface heating. The moderate surface pressure gradient maintained WA type winds through the evening.
Hand drawn analyses and keeping track of hourly surface observations showed their importance in this case study. Surface pressure gradients smoothed by the ETA model made a difference of at least 2 mb across Washington County. This difference between observed and computer model derived surface pressure gradient made the difference between a HWW and WA, respectively.
The December 17-18, 1996, case was associated with a deep full-latitude trough over the central U.S. and a strong ridge along the West Coast. The combination provided for strong northerly 700 mb winds (40-50 kts) over Washington County for an extended period of time (over 26 hrs). High wind warning type winds surfaced during two distinct periods of this episode. The first peak occurred several hours behind the passage of the surface cold front as the thermal advection intensified to 8.0 and the surface pressure gradient increased to 5.0 mb over this area and the second occurred about 15 hours later in the late afternoon of the 17th when the thermal advection had increased to above 10.0 and the 700 mb winds were about 45 kts. A combination of downward transfer of momentum with an increased tightening of the surface pressure gradient due to the passage of a secondary trough were the primary causes of this second peak. All-in-all, winds at White Reef were at or above WA levels for 26 hours with 9 hours worth of HWW criteria winds.
Winds increased to WA levels by 05Z on the 17th with the passage of a cold front. While Figure 1b shows at best only a 3 mb difference between CDC-GCN, the actual 06Z observations (not shown) indicate a nearly 5 mb gradient with the tightest portion of the surface pressure gradient (4 mb) over Washington County just behind the cold front which was near St. George. Although the 6 hour ETA forecast of the strength and orientation of the surface pressure gradient was not handled very well compared to 06Z surface observations, the net result of WA to possibly HWW level winds could still be inferred from the forecast.
| FIGURE 1A - 00Z Dec 17, 1996, 6 hr forecast of 700 mb thermal advection (valid 06Z) | FIGURE 1B - 00Z Dec 17, 1996, 6 hr forecast of surface pressure gradient (valid 06Z) |
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Between 06 and 15Z the surface pressure gradient remained at about 5-6 mb with the thermal advection increasing to 8.0. By 09Z this combination resulted in surface winds increasing to HWW criteria (58-60 mph) at White Reef which continued through 13Z before dropping down to WA levels. The 12Z ETA analysis indicated a surface pressure gradient of 5 mb between CDC-LAS (Fig 2b) and 700 mb winds of 45 kts with a thermal advection of 8.0 (Fig 2a), both of which support HWW type winds.
| FIGURE 2a - 12Z Dec 17, 1996, Analysis of 700 mb thermal advection (valid 12Z) | FIGURE 2b - 12Z Dec 17, 1996, Analysis of surface pressure gradient (valid 12Z) |
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Strictly using the ETA 6 hr forecast from 12Z Dec 17, valid 18Z, would support HWW conditions to continue. The thermal advection and winds at 700 mb were about 8.0 and 020 degrees at 45 kts (Fig 3a), respectively, both strong attributes. The surface pressure gradient indicated at least moderate support with about 5 mb difference between CDC-GCN (Fig 3b). Having two strong and one moderate attribute should, by definition developed in this study, support HWW type winds. However, 18Z surface observations showed that the surface pressure gradient over southern Utah was weak with a strong surface pressure gradient over Arizona. (Caution should be used when evaluating ETA's strength of the surface pressure gradient as it has been found in many of these cases to smooth the isobars evenly.) In reality, the effective surface pressure gradient over Washington County was only about 3 mb. This then translates to WA type winds which were observed at White Reef (43-48 mph) between 15-20Z.
| FIGURE 3a - 12Z Dec 17, 1996, 6 hr forecast of 700 mb thermal advection (valid 18Z) | FIGURE 3b - 12Z Dec 17, 1996, 6 hr forecast of surface pressure gradient (valid 18Z) |
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Winds at White Reef increased to above 58 mph for 3 hours centered on 23Z. Reasons for this increase are likely related to a combination of a slightly stronger thermal gradient of 9.0-11.0 at 700 mb (Fig 4a) and a slightly stronger surface pressure gradient of 5 mb analyzed by the ETA (Fig 4b), which was close to the 6.5 mb observed between CDC-GCN at 00Z. This increase of surface pressure gradient was due to a secondary surge of colder air moving across southwest Utah which was not forecast by the 12Z ETA model. However, this secondary baroclinic zone was analyzed across northeast Nevada into central Utah at 12Z by our hand drawn analyses and then again from south central Nevada to just south of GCN Arizona at 00Z. The 00Z ETA model run revealed this feature in the 700 mb thermal advection field which had increased over southwest Utah and northern Arizona at the same time the 700 mb winds shifted to more northeast, likely indicating the passage of a trough (Fig 4a).
HWW type winds decreased to WA levels between 00 and 01Z. This rapid decrease was a result of a few factors; 1) the relaxation of the surface pressure gradient after the passage of the secondary trough, 2) the lost of downward momentum due to surface heating causing turbulent mixing upward and, 3) strong thermal advection at 700 mb mixing downward.
| FIGURE 4a - 00Z Dec 18, 1996, 0 hr forecast of 700 mb thermal advection (valid 00Z) | FIGURE 4b - 00Z Dec 18, 1996, 0 hr forecast of surface pressure gradient (valid 00Z) |
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Despite the 700 mb winds forecast to increase to 50 kts, the thermal gradient to remain above 9.0 (Fig 5a), and the surface pressure gradient forecast to be between 4-5 mb (Fig 5b) at 06Z, winds at White Reef already had decreased to under 50 mph by 02Z. This supports the idea that even minimum surface heating in the middle of winter can still turn over the boundary layer. Surface observations indicate that the surface pressure gradient was weaker than forecast across Washington County with the tightest gradient located over Arizona, resulting in the decreased winds. Without the benefit of a strong or even a moderate surface pressure gradient to enhance the mixing, the strongest winds that can be realized at the surface would be a direct one-to-one relationship to that of the strength of the 700 mb winds themselves. In this case, WA level winds continued for only a few hours, but then decreased even more as the surface pressure gradient went to < 2 mb by 09Z.
| FIGURE 5a - 06Z Dec 18, 1996, 06 hr forecast of 700 mb thermal advection (valid 06Z) | FIGURE 5b - 06Z Dec 18, 1996, 06 hr forecast of surface pressure gradient (valid 06Z) |
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CASE SUMMARY - This case was different from the closed low case as it had strong thermal advection throughout the event with on-average moderate surface gradients. The times at which HWW conditions were met were in the beginning of the event behind the cold front and then again late in the afternoon when a combination of parameters came into play. Downward transfer of momentum due to surface heating mixing air upward and strong 700 mb thermal advection both appeared to play an integral part in the development of the second peak in winds. This is similar to the prior case where high winds surfaced late in the day but diminished rapidly with the onset of nightfall.
From observing winds in this case it is apparent that the strength of thermal advection is important, but only if and when the energy can be released. This occurs when some other force acts on it or works in concert with it, such as a moderate or strong pressure gradient creating mechanical mixing near the surface that interacts with the turbulence aloft and brings the high winds to the surface. Another means of tapping the thermal advection energy, is by surface heating, which causes thermal mixing from ground up creating turbulence and net transport of momentum downward. Both of these processes were present in this case. When either occurred, HWW type winds infact surfaced.
| FIGURE 1A - 00Z Sep 27, 1996, 6 hr forecast of 700 mb thermal advection (valid 06Z) | FIGURE 1B - 00Z Sep 27, 1996, 6 hr forecast surface pressure gradient (valid 06Z) |
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At 12Z, the strongest thermal gradient along with 25 kt northerly winds were over Washington County (Fig 2A), but both of these values are weak to marginally moderate, which typically is not enough for an event. The main driving force remains to be the surface pressure gradient which is moderate at 4-5 mb (Fig 2B).
| FIGURE 2A - 00Z Sep 27, 1996, 12 hr forecast of 700 mb thermal advection (valid 06Z) | FIGURE 2B - 00Z Sep 27, 1996, 12 hr forecast surface pressure gradient (valid 06Z) |
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By 12Z, the 700 mb support weakens with the winds down to 20 kts and thermal gradient between 2.8-3.0 (Fig 3A). The surface pressure gradient is now 4 mb (Fig 3B), but surface winds remain sustained at 20-30 mph with gusts 30-35 mph.
| FIGURE 3A - 12Z Sep 27, 1997, analysis of 700 mb thermal advection (valid 12Z) | FIGURE 3B - 12Z Sep 27, 1997, analysis of surface pressure gradient (valid 12Z) |
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By 18Z, this non-event was forecast to fall apart even more with the most influential parameter in this case, the surface pressure gradient, decreasing down to about 2 mb (Fig 4B). The consistent 20 kt winds at 700 mb were never a factor.
| FIGURE 4A - 12Z Sep 27, 1997, 6 hr forecast of 700 mb thermal advection (valid 18Z) | FIGURE 4B - 12Z Sep 27, 1997, 6 hr forecast surface pressure gradient (valid 18Z) |
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CASE SUMMARY - This case was mainly driven by a surface pressure gradient which reached moderate levels, but never had sufficient support from aloft. A totally driven surface pressure gradient type event requires the gradient to reach at least 6 mb in most cases before WA type winds will occur.