National Overview - February 1999
NCDC added Alaska climate divisions to its nClimDiv dataset on Friday, March 6, 2015, coincident with the release of the February 2015 monthly monitoring report. For more information on this data, please visit the Alaska Climate Divisions FAQ.
National Temperature - February
Preliminary data for February 1999 indicate that temperature averaged across the contiguous United States was much above the long-term mean. February 1999 tied with 1976, 1991, and 1992 as the third warmest February since 1895. About 48% of the country was much warmer than normal while about one percent of the country was much cooler than normal.
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 are warmer and negative values are cooler than the mean. The preliminary national standardized temperature index ranked February 1999 as the third warmest such month on record.
National Precipitation - February
larger image February 1999 was the 30th driest such month since 1895. Over 24% of the country experienced much drier than normal conditions while about nine percent of the country was much wetter than normal.
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.
The prelimininary national standardized precipitation index ranked February 1999 as the 30th driest such month on record. This standardized Z-score is estimated to be accurate to within 0.122 index units.
larger image Long-term drought coverage (as measured by the Palmer Drought Index) showed a slight rise over January, with February 1999 having about four percent of the country in severe to extreme drought. The percent area of the country experiencing severe to extreme wetness dropped to about 17%. The core dry areas included portions of the mid-Atlantic region and central Florida. The core wet areas included the Pacific Northwest, northern Rockies, and northern Plains.
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.