The North American 500 mb Maps for July
The dominant upper-level flow pattern during July showed two separate and distinct flow patterns. The first half of the month saw progressive mid-latitudinal waves moving west to east across the country while the second half was dominated by a trough in the west and a stagnant ridge in the east. The trough in the west allowed for cooler than normal temperatures from the central Rockies westward, while the dominant ridge in the east provided for a much warmer than normal temperature scenerio from the central Plains eastward.
Due to the upper-level trough over the Pacific Coast and the return flow from high pressure centered over the southern Mississippi valley, the West and Southwest Regions of the country were wetter than normal. The western Great Lake states were also wetter than normal due to the proximity of frontal systems which rode eastward on the 500-mb storm track along the Canadian border.
| The rest of the country was out of the predominant storm track due to the influence of the upper-level ridge or, as in the case of the Pacific Northwest, the storm track was much further south. This promoted drier than normal conditions for most of the southeastern half of the country as well as portions of the Northwest.
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Additional information on drought conditions in the United States can be found on the NCDC Drought in the U.S. page. Additional information on hydrometeorological analysis and forecasting can be found at the Hydrometeorological Prediction Center's Web Page. The principles behind the 500 mb flow are briefly explained Here.
National Temperature - July
| Preliminary data for July 1999 indicated that the monthly mean temperature averaged across the contiguous United States was warmer than the long-term mean and ranked as the 22nd warmest July since 1895. About 12% of the country was much warmer than normal while about four percent of the country was much cooler than normal.
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The national temperature index expresses temperature departure from the 60-year mean in terms of standard deviations. Each year's value is computed by standardizing the temperature for each of 344 climate divisions in the U.S. by using their 1931-90 mean and standard deviation, then weighting these divisional values by area.
| These area-weighted values are then normalized over the period of record. Positive values indicate warmer than the mean and negative values indicate cooler than the mean. The preliminary July 1999 national standardized temperature index ranked July 1999 as the 19th warmest July since 1895.
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The upper level flow across North America this month showed a fairly persistent upper level ridge of high pressure over the Great Lakes and a trough across the western portion of the continent. This pattern generally allowed for cooler than average temperatures from the central Rockies westward; the dominant ridge in the east provided much warmer than average temperatures from the central Plains eastward.
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During the first half of the month several mid-latitudinal troughs moved from west to east across the region. The cold bulls-eye in the Gulf of the St. Lawrence is erroneous, and efforts are under to correct the analysis. The temperature anomaly map above is based on a blend of surface station data and satellite data.
| The map to the right, based on approximately 250 airport stations, shows July 1999 average temperatures as a departure from 1961-1990 station normals. Temperatures were much warmer than normal over the northeastern third of the country in association with the upper-level ridge. Much cooler than normal temperatures dominated the western third of the country in response to the upper-level trough. The Hawaiian stations averaged slightly cooler than normal, while the stations in Alaska had mixed departures.
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The map animation to the left shows the geographical pattern of temperature anomalies for the last 12 months, compared to a base period of 1931-1990. On these standardized temperature anomaly maps, the color scheme ranges from blue (cold) to red (warm):
red = top 10 percentile
tan = 70-90 percentile
light blue = 10-30 percentile
dark blue = bottom 10 percentile.
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Temperature Variability
Monthly mean temperature maps show the average conditions during a month, but give no information about changes that occurred within the month. A measure of the day-to-day variability of temperature provides some insight into how temperatures changed during the month. Daily temperature variability is highly dependent on the weather systems and air masses that affect a region.
The daily difference in temperature may be lower in areas where a single air mass remains dominant. This can happen under a stable circulation pattern (at the jet stream level) that locks an air mass in place--for example, a strong zonal flow, or a stable ridge/trough pattern. The daily difference in temperature will be higher in areas that experience a greater frequency of frontal passages as cold arctic air moves southward and warmer, maritime air moves northward. This will happen under a variable circulation pattern, or along a stable storm track.
To quantify the variability in daily temperature, the average daily differences in temperature for the current month have been expressed as a ratio of the normal (1961-90) average daily difference. The magnitude of this ratio is expressed by the intensity of the shading on the map. Green shading indicates that daily variability in temperature was less than normal and may be a consequence of a dominant air mass. Red shading indicates that daily temperature variability was greater than normal reflecting a more frequent passage of differing air masses.
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Daily temperature variability was below normal in the south central states due to the dominance of a stagnant high pressure ridge during much of the month. The upper-level trough and associated frequent frontal passages in the West and parts of the northern Plains resulted in higher variability in these areas. Otherwise, daily temperature variability for July 1999 showed a less organized spatial pattern than in earlier seasons. This is typical for temperature variability in the summer months.
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National Precipitation - July
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Based upon preliminary data, July 1999 ranked as the 13th driest July on record. Nearly 13% of the country was much drier than normal while about 13% of the country was much wetter than normal.
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The national precipitation index expresses precipitation departure from the 60-year mean in terms of standard deviations. Each year's value is computed by standardizing the annual precipitation in each of 344 climate divisions across the U.S. using the gamma distribution over the 1931-90 period. The gamma statistical distribution takes into account heavy precipitation years and extremely dry years in the historical record (in mathematical parlance, "a zero-bounded skewed distribution"). These gamma-standardized divisional values are then weighted by area and averaged to determine a national standardized value for each year.
| These national values are normalized over the period of record. Negative values are drier and positive values are wetter than the mean. This index gives a more accurate indication of how precipitation across the country compares to the local normal (60-year average) climate.
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The preliminary national standardized precipitation index ranked July 1999 as the 21st driest July since 1895.
The map immediately below, based on approximately 250 First Order airport stations, shows July 1999 total precipitation as a departure from 1961-1990 station normals.
| The departure map, in general, shows the less coherent spatial pattern characteristic of summer precipitation, but some general characteristics can be seen. Dry conditions dominated much of the country during July, especially along the east coast and in much of the West. The western Great Lakes and upper Mississippi Valley were unusually wet. Localized summer thunderstorms brought heavy rain to a few stations in the southeast and southern Plains, and over a wider area in the desert Southwest. The Hawaiian stations were predominantly drier than normal, while stations in the middle third of Alaska were wetter than normal.
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Long-term drought coverage (as measured by the Palmer Drought Index) was up slightly over June, with July 1999 having about seven percent of the country in severe to extreme drought. The percent area of the country experiencing severe to extreme wetness decreased slightly to about 17%.
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National Temperature - January-July
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Preliminary temperature data indicate that the year-to-date, January-July 1999, was the eighth warmest January-July period since records began in 1895. About 18% of the country averaged much warmer than normal while about one percent of the country averaged much cooler than normal.
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| The preliminary national standardized temperature index ranked January-July 1999 as the 12th warmest January-July on record.
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National Precipitation - January-July
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Preliminary precipitation data indicate that the year-to-date, January-July 1999, ranked near the long-term mean for nationally averaged precipitation summed across the seven month period. About five percent of the country was much wetter than normal for this period while about four percent of the country averaged much drier than normal.
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| The prelimininary national standardized precipitation index for January-July 1999 also ranked near the long-term mean.
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Tornadoes - July and Year-to-date
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During July 1999, 82 tornadoes were documented across the contiguous United States. The 46-year average is 98. The most tornadoes observed in the July record was 240 in July 1993, while the least amount was 31 in July 1953.
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| For the year-to-date, January-July 1999, 1040 tornadoes have been documented. This ranks as the second most active year-to-date, behind the count of 1143 recorded in 1998. The least amount was 353 in 1953.
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It should be pointed out that the preliminary tornado count is traditionally higher than the final count and that the tornado observations have generally improved with time as better observing practices and instrumentation (especially weather radar and satellites) were utilized.
Current data are based on preliminary reports from River Forecast Center stations and First and Second Order airport stations obtained from the National Weather Service (NWS) Climate Prediction Center and real time Global Telecommunications System (GTS) monthly CLIMAT summaries. THE CURRENT DATA SHOULD BE USED WITH CAUTION. These preliminary data are useful for estimating how current anomalies compare to the historical record, however the actual values and rankings for the current year may change as the final data arrive at NCDC and are processed.
The following NCDC datasets are used for the historical U.S. data: the climate division drought database (TD-9640), and the hurricane datasets (TD-9636 and TD-9697). It should be noted that the climate division drought database consists of monthly data for 344 climate divisions in the contiguous United States. These divisional values are calculated from the 6000+ station Cooperative Observer network.
NOAA's National Centers for Environmental Information
