An approach to forecasting potentially strong winds reaching either wind advisory or high wind warning levels in southwest Utah (Washington County) has been developed using surface pressure gradient and 700 mb thermal advection. Strong north-south surface pressure gradients are known to produce strong winds in central Washington County due to topographic channeling of the flow (a.k.a. gap winds) through the narrow nearly north-south oriented canyons. Examining nearly two dozen cases from two years of data revealed another meteorological process that directly influences the strength of these gap winds; magnitude of 700 mb cold advection.
From a quantitative approach of evaluating the strength of thermal advection and and surface pressure gradient a decision-tree table was developed. This table enables forecasters to easily determine wind speed potential based on ETA forecast's strengths of surface pressure gradient and 700 mb thermal advection. Two wind advisories, two high wind warnings, and one non-event cases are presented to illustrate the strengths and limitations of this procedure as it might be applied operationally. By recognizing which parameter is most important during an event, forecasters are able to predict its duration.
Limitations of this procedure are directly related to the accuracy of the ETA model forecasts. However, as revealed in this paper, identifying ETA model biases, can reduce these limitations.
The purpose of this paper is to provide guidance (based on 13 case studies from Jan 96-Mar 97) to NWSFO SLC forecasters on what parameters are necessary for the occurrence of these events and how to determine the magnitude and duration.
Data were collected from 13 case studies from January 96 through March 97. Various parameters collected for each case include analyses and forecasts from the AVN/NGM/ETA of surface pressure, lowest level winds, 850 and 700 mb winds and temperatures, three hour hand drawn analyses, hourly surface observations from AWOS, METAR, and RAWS sites across portions of both southern Nevada and Utah, and spotter reports. Due to a lack of full documentation of each parameter for each case and to keep evaluations of each case simplified, guidance developed from these events are based on surface and 700 mb fields. Of particular importance are the surface pressure (gradient and orientation), 700 mb temperature advection, and 700 mb wind (strength and direction) over southwest Utah. Radiosonde data are used for a quick reference and rough evaluation of the conditions encompassing an event, but due to sparse radiosonde network in this region leading to wide interpretation of isotherm analyses by NWSFO SLC forecasters, 700 mb model analyses with gridded data winds are used exclusively for case-to-case comparisons and evaluations. As for surface pressure, model forecasts are used prior to the commencement of an event, but model analyses are used in conjunction with three-hourly hand drawn analyses whenever possible during an event. Model analyses inherently smooth isobars, so that subtle areas of tight gradients are not analyzed properly. In the Case Study portion of this paper, mention is made as to when hand drawn analyses were used in place of model analyses, due to this smoothing. Hourly, pressure differences between both Cedar City (CDC) and Las Vegas (LAS) and CDC and Grand Canyon (GCN) (whichever is greater) are also used to qualify the strength of the surface pressure gradient. However, caution is needed in using strick pressure differences between stations to quantify the surface pressure gradient because frequently this gradient is not homogeneous, but contains varying strengths. If the strongest surface pressure gradient is south of the area of interests, then much weaker winds can be expected.
Locations of data sources used in verifying an event will be discussed under the Topography section.
The main canyons of interest in Washington County are the three north northeast oriented canyons nearly parallel to each other in the east central part of the county. These canyons slope from north to south dropping nearly 2,000 feet in elevation. The I-15 corridor lies in the western and most well defined canyon between the Hurricane Cliffs to the east and Pine Valley Mountains to the west. Within this canyon, observations are sometimes received from a spotter located at Anderson Junction (which is the intersection between I-15 and SR-17, where the word White is written, Fig 1). However, the most consistent observations and best representation wind strength are received from the White Reef automated sensor. This sensor is located next to I-15 about 15 miles northeast of St. George and 6 miles south southwest of the canyon mouth (Fig 1) at an elevation of 3500 feet. Due to its prime location and hourly observations, most of the case study verifications are based on White Reef reports.
Two lesser sized canyons farther to the east; La Verkin Creek and North Creek canyons have the towns of La Verkin and Virgin, respectively, located near their mouths. Spotter reports are frequently obtained from these towns.
Another RAWS site is located at Badger Spring at an elevation of 3990 feet on the
western slopes of the more north-south oriented Beaver Dam mountains. Due to its
open exposure away from any enhanced channeling effects of terrain, the observed
winds are likely the direct result of the pressure gradient and downward transfer
of momentum, more representative of the area in general. However, when
700 mb winds are oriented more from the northeast and east, winds tend to increase
at Badger Spring, possibly signifying mountain wave effects from the Beaver Dam
mountains. During these episodes winds at Badger Spring are typically 10-25 mph
stronger than at White Reef. Therefore, events used in this case study are based
on observations from White Reef and from spotters in and around the mouths of the
canyons where true channeling effects of the winds occur.
Introduction
Figure 1. Washington County Terrain Enhanced Image
| Type | Sustained Winds | Gusts |
|---|---|---|
| Wind Advisory | >= 31 mph/ 27 kts for 3 hrs |
>= 45 mph/ 39 kts for 3 hrs |
| High Wind Warning | >= 40 mph/ 35 kts for 1 hr |
>= 58 mph/ 50 kts any time |
| Surface Pressure Gradient (mb) CDC-LAS or CDC-GCN |
||||||
|---|---|---|---|---|---|---|
| Temp Advection Degrees(C) |
700mb Wind(kts) |
Weak | Moderate | Strong | ||
| 0-1 | 2-3 | 4-5 | 6-7 | >= 8 | ||
| 0-6 | <25 | none | none | none(3)/WA(1) | WA(1) | WA(1)/HWW(1) |
| 25-40 | none | none | none(1) | WA(1) | HWW(1) | |
| >40 | none | none(1) | none(1)/WA(1) | WA(1) | HWW(1) | |
| 7-15 | <25 | none | none(1) | none(2)/WA(2) | WA(2) | HWW(1) |
| 25-40 | none | none(2) | none(2)/WA(1) | none(1)/HWW(2) | HWW | |
| >40 | none | WA(1) | HWW(1) | HWW | ||
| 16-24 | <25 | none | none(1) | WA(1)/HWW(1) | HWW | |
| 25-40 | none(1) | none(1)/WA(2) | HWW(1) | HWW | ||
| >40 | WA(1)/HWW(1) | HWW(1) | HWW | |||
| 25-32 | <25 | none(1)/WA(1) | WA(1)/HWW(1) | HWW | ||
| 25-40 | none(1)/WA(1) | none(1)/WA(1) | HWW(1) | HWW | ||
| >40 | WA(1) | WA(1)/HWW(1) | HWW | HWW | ||
| 33-41 | <25 | none(1)/WA(1) | WA(1)/HWW(1) | HWW | ||
| 25-40 | WA(1) | WA(1)/HWW(1) | HWW | |||
| >40 | WA(1) | WA(1)/HWW(1) | HWW | HWW | ||
| >= 42 | <25 | WA(1)/HWW(1) | WA(1)/HWW(1) | HWW | ||
| 25-40 | WA(1)/HWW(1) | HWW | HWW | |||
| >40 | WA(2)/HWW(1) | HWW | HWW | |||
In open trough systems, a strong pressure gradient alone with little if any thermal advection may produce HWW, but will usually last only 2-6 hours. However, combine the following "strong" attributes;
While the orientation of the surface pressure gradient could vary greatly between roughly 290-070 degrees (essentially any northerly component) the winds aloft at 700 mb had the greatest influence when they nearly paralleled the canyon orientation between 360 and 060 degrees. When the flow aloft was oriented more northwest (320-350 degrees), causing winds to flow over much higher terrain of the Pine Valley mountains before descending into central Washington County, strong winds were not observed at White Reef. Also, in the same manner, when the 700 mb winds became oriented more easterly (>=70 degrees), winds at White Reef tended to subside. This suggests, at least in the area of concern, that mountain wave effects are minimal if any. However, at Badger Spring winds tended to increase under a more easterly 700 mb flow, possibly a result of mountain wave effects of the Beaver Dam mountains which run nearly north-south just east of Badger Spring.
Individual Cases | Summary |