Synoptic Discussion - April 2014


Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.


NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.


Summary

April is in the middle of climatological spring, which is the season of transition from the atmospheric forcings of cold, dark winter to a new set of forcings behind the hot radiance of summer. This change was reflected in the long-wave circulation pattern for April 2014 over North America, which reflected a weakening of a mode of atmospheric variability (East Pacific-North Pacific) which has influenced the weather of the last several months. The circulation pattern consisted of a retreating upper-level trough over central Canada which extended into the north central U.S., with short-wave troughs traversing the contiguous U.S. in a fast westerly flow. This long-wave pattern still reflected the influence of an atmospheric mode of variability (West Pacific) associated with the North Pacific driver, whose teleconnections include colder-than-normal temperatures in the north central U.S. and warmer-than-normal temperatures in the Southwest, although signs of the influence of other modes of circulation were evident in the temperature, precipitation, and upper-level circulation anomaly patterns. The short-wave troughs generated large areas of precipitation east of the Rockies, and some produced severe weather when slowed by blocking patterns in the atmospheric circulation. Strong winds kicked up by frequent cold frontal passages contributed to large wildfires and, in the Southern Plains, numerous dust storms. The circulation pattern inhibited beneficial precipitation across much of the West and Plains, where drought expanded or intensified. See below for details.


Synoptic Discussion

Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.

In the Northern Hemisphere, April is in the middle of climatological spring which is the time of year when solar heating forces the jet stream and circumpolar vortex to contract poleward. Cold polar air masses can still influence the weather, but the warm, dry subtropical high pressure belt begins to exert more influence as it shifts northward, with warm southerly air masses moving into the contiguous United States (CONUS) more frequently. In April 2014, a strong westerly flow in the upper-level circulation sent several weather systems rippling across the CONUS, which played havoc with a cold long-wave trough that tried to maintain its grasp over eastern and central North America.

Animation of daily surface fronts and pressure systems for the month
Animation of daily surface fronts and pressure systems for the month.

These upper-level weather systems generated low pressure systems with their associated warm and cold fronts at the surface — warm air flowed ahead of the lows, with cold air surging southward behind them. There were more than one and a half times as many record cold daily highs and lows (2799) as record warm daily highs and lows (1583), but the parade of competing cold and warm air masses resulted in a monthly temperature pattern where monthly extremes were washed out. Snow cover generally decreased as the month wore on and temperatures in general warmed, but the air was still below freezing in the Plains and northern states behind cold fronts at mid-month and near the end of the month to result in temporary expansions of the snow cover area.

Severe weather developed in the Central to Southern Plains and eastward in association with these weather systems, with major outbreaks occurring at the beginning, middle, and end of the month. A large, slow-moving upper-level trough near the end of the month triggered severe outbreaks of tornadoes along and ahead of its surface low and fronts, resulting in significant destruction and dozens of fatalities. The end-of-month outbreak brought the preliminary count of 217 tornadoes well above the average count of 155 tornadoes for April.

The short-wave weather systems moving in the upper-level flow brought precipitation to many parts of the country, but the long-wave trough over central and eastern North America imparted a slight northerly component to the overall circulation which inhibited precipitation in the western and central CONUS. The short-wave troughs intensified as they moved into the long-wave trough and tapped Gulf of Mexico moisture to lay down bands of heavy precipitation in the Southeast, along the Ohio Valley, and in the Upper Mississippi Valley to Great Lakes, giving five states their tenth wettest, or wetter, April. The rain from these weather systems shrank the drought and abnormal dryness areas in the Midwest, Southeast, and Pacific Northwest, but drought expanded in the West and Plains where the month was drier than normal, resulting in the national drought area remaining relatively unchanged compared to a month ago. The frequent frontal passages in the Southern Plains kicked up numerous dust storms. The wind, low humidities, and prolonged dryness contributed to the development of large wildfires in the Plains and Southern Appalachian regions throughout the month.

Monthly upper-level circulation pattern and anomalies
Monthly upper-level circulation pattern and anomalies.

When integrated across the month, the circulation produced a pattern of below-normal 500-mb heights (stronger-than-normal long-wave trough) over central Canada extending into the north central CONUS. Above-normal 500-mb heights extended over the southwestern CONUS and along the east coast of North America, reflecting an upstream and downstream response to the long-wave trough in the westerlies as well as hints of intensifying and expanding North Atlantic and North Pacific subtropical highs.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Above-normal precipitation fell in the Upper Mississippi Valley to Great Lakes, in the Ohio Valley, Southeast, and Mid-Atlantic Coast where upper-level weather systems and surface lows were able to tap into Gulf of Mexico moisture. Pacific weather systems moving in the upper-level flow brought a few areas of above-normal precipitation to the West and Central Plains, but the overall pattern of western ridging and a component of northwesterly flow brought widespread below-normal precipitation to these areas. In between the bands of heavy precipitation in the east, April was drier than normal across the southern Great Lakes and along the Appalachian chain to New England. The strong westerly flow over the Pacific kept weather systems away from Alaska, which was mostly drier than normal.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

With the roller coaster pattern of warm fronts and cold fronts traversing the CONUS, monthly temperatures averaged near normal in many areas. A colder-than-normal pool was located in the north central states, in association with the upper-level long-wave trough, and in New England and the Lower Mississippi Valley. Warmer-than-normal temperatures were found in the West, Ohio Valley, and parts of the coastal Southeast. Temperatures in Alaska averaged warmer than normal in the west and north, and cooler than normal in the southeast.

The upper-level circulation over North America in April is an example of an interconnected and amplified meridional pattern. The atmospheric circulation is interconnected around the world, and this interconnectedness was manifested this month by a pattern of paired above-normal and below-normal 500-mb height anomalies over North America, the North Atlantic, and Eurasia. Similar to last month, a dipole of below-normal heights can be traced from the high latitudes over north central Asia, extending across the Arctic into lower latitudes over central Canada. Above-normal heights dominated Europe and East Asia. The above-normal heights were reflected at the surface over Eurasia as widespread above-normal temperatures and below-normal snow cover, with areas of below-normal precipitation.


Atmospheric Drivers

Subtropical highs, and cold fronts and low pressure systems moving in the storm track flow, are influenced by the broadscale atmospheric circulation. The circulation of the atmosphere can be analyzed and categorized into specific patterns. The tropics, especially the equatorial Pacific Ocean, provides abundant heat energy which drives the world's atmospheric and oceanic circulation. The following describes several of these modes or patterns of the atmospheric circulation, their drivers, the temperature and precipitation patterns (or teleconnections) associated with them, and their index values this month:


Upper-level circulation pattern and anomalies averaged for the last three months
Upper-level circulation pattern and anomalies averaged for the last three months.

Examination of these circulation indices and their teleconnection patterns, and comparison to observed April 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in April was most closely related to the jet stream and ocean-atmosphere interactions over the North Pacific Ocean. ENSO was neutral, and thus not a player. Equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state, which was affecting the MJO, but the MJO still may have exerted some influence on temperature and precipitation. The teleconnections for the NAO match April's temperature and circulation patterns in the Southwest and East, and precipitation patterns in parts of the Central Plains and West, but not elsewhere, so the match could have been coincidence. Likewise, the AO teleconnections match for precipitation in parts of the country, but don't match for temperature, so the match could have been coincidence, but the upper-level circulation pattern suggests the AO did exert some influence. The PNA index shows essentially no significant correlation to the April weather patterns. It is the North Pacific indices which have the best correlations — EP-NP and WP with the April temperature and circulation anomaly patterns. But the decreasing Northeast Pacific SST anomalies, and limited agreement with April temperature, precipitation, and upper-level circulation anomaly patterns, suggest that the EP-NP's influence is weakening. The undercutting of the western North America ridge by moist short-wave troughs in recent months may also be contributing to the weakened EP-NP influence. The WP shows some agreement with the precipitation anomaly pattern where teleconnections exist, but so do the MJO, AO, and NAO. It should be noted that precipitation relationships with these indices are generally weak. This month illustrates how the anomaly patterns can be strongly represented by an index (WP) measuring one atmospheric driver (the North Pacific Ocean) for circulation and temperature but also have elements of several other drivers (or modes of atmospheric variability), and how precipitation can be a reflection of several of the drivers.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for April 2014, published online May 2014, retrieved on November 27, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2014/4.