Surface Air Pressure Patterns and Winds Applications

Investigation 1B

In the late-summer of 2020, weather across much of the contiguous U.S. had been quite warm. In some western locations, however, it was stifling, with fires raging and Death Valley, CA temperatures cresting at a scorching 130°F. Central and western states were dominated by large areas of high pressure with numerous Heat Warnings and Heat Advisories issued by the National Weather Service. Wildfires prompted hasty evacuations, while mitigation efforts were immensely challenging for firefighters. Weather conditions on the East Coast weren’t nearly as warm, and with lower pressure, more moisture abounded, bringing precipitation throughout New England and Florida.

The Southeast experienced classic, late summer weather with bouts of small, slow-moving thunder- storms. Many of these storms were found proximate to fronts that didn’t surge far enough to cool off temperatures. The weather data for this investigation show a weak wind field, bereft of strong low-pressure systems. This is typical of late-summer weather patterns across the U.S.

Figure 1B-1 (Pressures), acquired from Weather Studies Maps & Links, reports surface air pressure (corrected to sea level) rounded to the nearest whole millibar (mb) on 18 August 2020 at 17Z. UTC, or Z time, is four hours ahead of Eastern Daylight Time (EDT), so the 17Z map of August 18 depicts conditions at local times of 1 p.m. EDT (12 p.m. CDT, 11 a.m. MDT, 10 a.m. PDT, etc.).

Figure 1B-1. Map of surface atmospheric pressure at selected stations at 17Z 18 AUG 2020. We recom- mend clicking on the map to view and print a full size image to analyze.

Most weather map products from the Weather Studies Maps & Links are created by the National Weather Service’s (NWS) National Centers for Environmental Prediction (NCEP) at the National Oceanic and Atmospheric Administration (NOAA), as noted in the lower image margin: Unidata/NWS/NOAA.

As the course proceeds, we will discuss the geographic regions across the United States. For your refer- ence, common NWS terminology for these are found at Link 1B-1 and Link 1B-2.

1. The highest plotted air pressure observed on the map of 1022 mb is located in ______.

a. Texas b. California c. Florida d. Nebraska

2. The lowest reported pressure was ______mb, in the Canadian province of Quebec (north of New England).

a. 1007 b. 1010 c. 1013 d. 1016

3. The isobars in the conventional series to complete the pressure analysis between the lowest and highest values on this map are ______mb.

a. 998, 1002, 1006, 1010 b. 999, 1003, 1007, 1011 c. 1008, 1012, 1016, 1020 d. 1013, 1018, 1023, 1028

To complete the isobars in Figure 1B-1, refer to the Tips on Drawing Isobars in Investigation 1A. More than one isobar of the same value should be drawn if pressure values located in separate sections of the map require it. Consider each pressure value at the center of the number. Isobars with values of 1008, 1012 and 1016 mb have already been started but look for other areas that require the additional 1016 and/or 1020 intervals as well. The labels for the isobars are added at the end of the land mass or at the map’s boundary. For closed isobar references in a circle, the number is placed in the line itself.

Draw and label isobars of the series where they existed over the remaining portions of the U.S. and Canada. (Some isobars were intentionally left incomplete.) After completing all the isobars, label the positions with the lowest value in the U.S. mid-section with bold Ls. Label the highest pressure in the southeastern U.S. with an H.

4. In Pennsylvania, the 1012-isobar bisects the state near Williamsport with a line running north-south. Given the 4-mb interval convention, this line will separate values where the lower values on the map remain to one side while higher values are on the other side. The pressure values of locations west of Williamsport are ______1012 mb.

a. less than b. equal to c. greater than

Figure 1B-2 is the analyzed surface pressure map from Weather Studies Maps & Links produced from NOAA’s National Centers for Environmental Prediction (NCEP) for 17Z 18 August 2020. It shows the locations of isobars, air pressure system centers, and fronts at 15Z, 2 hrs before those on the Figure 1B- 1. Compare your analyzed map to Figure 1B-2. Note, the uniform dark green in Kansas, Oklahoma, and Texas are mostly “false returns” from the NOAA weather radar network.

Figure 1B-2. Analyzed NCEP surface weather map for 17Z 18 AUG 2020 with isobars (blue lines), pressure systems, and precipitation.

5. The overall isobar patterns on the two maps over the coterminous U.S., particularly for the Great Lakes Low and the central High, are generally ______. Also included are shadings for precipitation around the country based on radar reports.

a. very different b. similar

The isobars in Figure 1B-2 are computer generated and based on a more complete set of pressure values than those in Figure 1B-1. This degree of detail can be seen, for example, on a near real-time surface map available from Link 1B-3. (This resolution may account for some of the variations between your analysis and Figure 1B-2. The computer-based analysis is the source of some additional plotted Hs de- noting more local marginally higher-pressure centers and Ls for lower pressure centers.)

By analyzing the pressure values reported on weather maps to find pressure patterns, you can locate the centers of local highest and lowest pressures. We will see that these pressure centers often mark the mid- points of major weather systems, either regions of fair weather or stormy conditions, respectively.

Figure 1B-3 is the “U.S. - Data” map from the Weather Studies Maps & Links, Surface Maps, for 17Z 18 August 2020. The data depict weather conditions at individual locations across the contiguous U.S. plotted using a coded format called the “surface station model.” Station models will be examined in more detail in Investigation 2A.

Figure 1B-3. Unanalyzed NCEP surface weather map for 17Z 18 AUG 2020 with station model plots con- veying surface weather observations of temperature, moisture, pressure, cloud cover, wind speed and direction. The centers of major high- and low-pressure systems are depicted with letters H and L respectively.

Selected weather-reporting stations are shown on the map as circles and the wind directions shown by a line, which can be thought of as an arrow shaft, depicting the air flowinto each circle. In meteorology, wind at a station is identified by the direction the air is flowingfrom . (Air arriving at the station from the north is called a north wind.)

6. Therefore, the wind direction at Grand Junction, in western Colorado was generally from the ______. (Because of the map projection used, the north may not be precisely at the top of the map. In the Colorado area, a north-south line would be nearly parallel to the Colorado-Utah straight-line border.)

a. north b. south c. west d. east

7. Knowing the direction from which the wind at Grand Junction was blowing, it would be reported as a(n) ______wind.

a. north b. south c. west d. east

Optional: One knot (1 kt = 1 nautical mile per hour = 1 NM/hr) is about 1.2 land (statute) miles per hour. Wind speed is reported by a combination of long (10 kts) and short (5 kts) “feathers” attached to the direction shaft. Grand Junction had a 5-kt wind (one short feather). A double circle without a direction shaft, such as seen in Brownsville, TX, Denver, CO, and Reno, NV, signifies calm conditions. A shaft plotted without feathers would denote 1–2 kts.

One tool for wind speed conversions between miles per hour and knots (as well as other quantities) and their formulas can be found at Link 1B-4.

8. Three bold blue Hs are marked on the map for the centers of high pressure. Compare the hand-twist model (Link 1B-5) of a High to the wind directions at stations in the several-state area flanking the high-pressure centers. (The central H is a slightly better example due to a lesser topographic influ- ence.) Wind direction at these stations suggest that, as seen from above in Figure 1B-3, the air was circulating ______around the Northern Hemisphere high-pressure center.

a. clockwise b. counterclockwise

9. The winds at stations in the states around the high-pressure center indicated the air also spiraled ______the high-pressure center.

a. outward from b. inward toward

10. This wind flow pattern, with respect to the high center, is therefore ______thehand-twist model of a High. (Refer to Investigation 1A in the Manual for the hand-twist models of Lows and Highs.)

a. contrary to b. consistent with

In addition to the forcings of large-scale pressure patterns in the western portion of the country, air flows are influenced by the mountainous, high-elevation terrain that stretches from western Montana and Idaho southeastward to New Mexico and western Texas. As such, wind flow patterns around the high-pressure system may not fully display the anticipated rotation.

Link 1B-6 dynamically displays the forecast wind flow in both direction and speed across the U.S. Mov- ing your cursor across the map will give the wind speeds at specific locations. Compare the wind flow patterns seen at this site to those of the latest surface weather map’s positioning of Highs and Lows. Two other interactive global wind maps are at Link 1B-7 and Link 1B-8. Summary

When the current weather map available on Weather Studies Maps & Links shows centers of Lows or Highs near your location, you might consider your local wind direction (from weathercasts or even seen in a flag flapping in the wind) with map circulations and thehand-twist model of weather systems. The typical designation of the L and H as centers of stormy and fair-weather systems, respectively, can be compared to satellite views showing clouds across the U.S. Check to see if the region around a Low is generally cloudy or the broad area centered on a High is mostly clear.

Further details for deciphering station data can be found in the “User’s Guide” (linked from the Extras section of Weather Studies Maps & Links). The reporting surface weather stations plotted on course maps can be identified from the “Available Surface Stations” link on theWeather Studies Maps & Links, Surface as well as in the “User’s Guide.” Also, a map of National Weather Service (NWS) offices can be found at Link 1B-9.