The North American 500 mb Maps for January
The month began with a ridge dominating the eastern third of the country and an upper-level trough over the west. This trough quickly moved to the northeast only to be replaced by another. Around 10 January, there was a pattern shift and the western third of the country came under the influence of an upper-level ridge while the east was dominated by an upper-level trough with the mean axis just off the Atlantic east coast.
Dominant high pressure over the southwestern part of the country provided much warmer than normal temperatures for most of the country from the central plains westward. Near normal temperatures were observed from the Ohio Valley eastward. A prevalent onshore flow provided ample precipitation for Oregon, northern California and portions of the central and northern Rockies while the southwest, central plains and the south were drier than normal. The eastern third of the country averaged near normal for precipitation.
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 - January
Preliminary data for January 2000 indicate that the monthly mean temperature averaged across the contiguous United States was warmer than normal and ranked as the 14th warmest January since 1895. Nearly 30% of the country was much warmer than normal while less than one percent of the country was much cooler than normal.
The preliminary national standardized temperature index ranked January 2000 as the sixth warmest January since records began.
The map to the right, based on approximately 250 airport stations, shows departures from the 1961-1990 normal of average temperatures for January 2000. Much of the western and central U.S. had much above normal temperatures for the month. The Hawaiian stations averaged colder than normal and the western half of Alaska averaged much colder than normal.
The map animation provided to the left shows the geographical pattern of temperature anomalies for the last 12 months, compared to a base period of 1931-1990.
The January temperature variability map over the contiguous U.S. is shown to the left. January 2000 started out generally warm across much of the contiguous U.S. Major winter storms developed during the last half of the month in the eastern third of the country, bringing very cold temperatures and widespread snow. In spite of the cold outbreaks, however, the nation (as noted above) had the tenth warmest January on record.
A large area from the northern and central Rockies, across parts of the Plains then into the Southeast, experienced lower than normal day-to-day temperature variability. Below-normal variability also occurred over parts of the Hawaiian Islands. Temperature variability was higher than normal across parts of California, the Gulf Coast, the mid-Atlantic to Northeast states, and Alaska.
National Precipitation - January
Based upon preliminary precipitation data, January 2000 ranked near the long-term mean. About ten percent of the country was much wetter than normal while about seven percent of the country was much drier than normal.
The preliminary national standardized precipitation index also ranked January 2000 near the long-term mean.
Dominant high pressure over the southwest kept Pacific moisture from influencing California while the trough that dominated the eastern-third of the country seldom tapped available moisture from the Gulf of Mexico. This provided for much drier than average surface conditions along the central and southern Mississippi Valley, a large area of the Midwest, parts of the central Plains, most of Texas, and the interior valleys of California.
The map to the right, based on approximately 250 airport stations, shows January 2000 total precipitation as a percent of the 1961-1990 station normals. Most of the Alaskan stations were wetter than normal, while most of the Hawaiian stations reported a drier than normal month. On the mainland U.S., stations were predominantly drier than normal from the Great Lakes to the northern and central Plains, and from Florida to southern California. Above normal precipitation occurred at stations in the Carolinas, Ohio Valley, parts of Texas, the central Rockies, the Great Basin and from Oregon to northern California.
Long-term drought areal coverage (as measured by the Palmer Drought Index) increased when compared to December, with about 18% of the country in severe to extreme drought during January. The percent area of the country experiencing severe to extreme wetness dropped steadily during the last four months of 1999, remaining nearly steady during the last two months at about five percent. The drought area expanded this month to its greatest extent since mid-1996 (see graph below left). However, several droughts in the past hundred years covered a much larger area (see graph below right). The January 2000 drought is only about half as extensive as the 1988 drought was at its peak.
The snow cover anomaly map to the left reflects the general storm track across the U.S during January. The dominant ridge in the west prevented the normal accumulation of snowfall for the central and southern Rockies as well as the northern Plains while aiding the Pacific onshore flow in providing positive snowfall anomalies in the Oregon Cascades and the Bitterroots of Washington and Idaho.
The prevalent trough along the east coast provided positive snowfall anomalies as far south as central South Carolina and the mountains of northeast Georgia.
An animation of daily snowcover for North America is available.
Tornadoes - January
During January 2000, 16 tornadoes were documented across the contiguous United States. The 46-year average is 20. The most tornadoes observed in the January record was 216 in January 1999, while no tornadoes were observed in January 1986.
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.