Looking at the first 15 days of May, temperatures have been above average across most of the East with well above average temperatures from the lower mid-Atlantic states into the Carolinas. The central Plains have been slightly above average with well below average temperatures across the upper Plains. The long-term drought has gotten worse across the southwestern Plains and points west.
The April 15th outlook for May called for generally above average temperatures for much of the Plains and into the Ohio Valley. A shift in the pattern moving into late May is expected to bring heat well to the north across the Plains, helping to offset some of the below average temperatures back closer to average. At the same time, more troughiness across the Northeast will likely result in the final third of the month ending up near to slightly below average in terms of temperatures there.
Click here to review the previous outlook for May
Looking ahead to June, there is a fairly strong model consensus for above average temperatures across much of the country, especially for the first half of the month. The latest projections show a ridge developing and positioning itself further north than might typically be expected in early June. While this may keep most areas warm, eastern New England may stay under the influence of troughing more often than not. In addition, the upper Great Lakes may avoid significant positive temperature anomalies due to still well below average lake surface temperatures. Continued drought conditions may effectively extend some positive temperature anomalies back into the southwestern Plains, although the warmest temperatures with respect to average are forecast to be displaced closer to the center of the expected ridging, from Kansas up into Iowa. It is possible that the immediate Gulf Coast could verify with somewhat below average temperatures as the ridging is displaced north. Some model guidance hints at a quasi-rex block setting up with some weak low pressure underneath this trough. If that were to verify, that would increase the likelihood of cooler than average temperatures in the vicinity of the Gulf states
Starting with the 17/12 European ensemble, the model shows a nearly zonal pattern setting up to start June with nearly across the board positive 500mb height anomalies. The Euro weeklies (15/00) showed that same general pattern persisting into the first half of June with slightly above average temperatures and a developing ridge in the Plains. Also, the weeklies showed some residual troughing across eastern Canada.
The CFS mean forecast for June indicates slightly above average temperatures from the northern Plains into the Great Lakes and Northeast. The CFS also indicates well above average temperatures for the mid-Atlantic and Northeast states. This northeast extension of such temperature anomalies is probably overdone. While three of the four CFS members show a warm start to June, the members diverge significantly by mid-month and have poor consistency moving into the second half of the month.
MJO signs are mixed and show little skill as both the GFS and Euro show variability near the center of the MJO phase diagram. With no clear phase expectation(s), this forecast is not focusing on the MJO.
The extended GFS ensembles are virtually warm for the eastern half of the country leading into early June, with the exception of Texas, Florida and portions of the Carolinas.
Severe thunderstorm considerations:
Overall, 2014 continues to follow near-record low severe thunderstorm levels. The combination of early spring troughing and cooler temperatures over the Great Lakes/Northeast and drought conditions in the southwestern Plains has not favored much in the way of significant severe activity. For June 2014, an uptick in activity is forecast in two areas, although overall activity across the country is forecast to remain below average.
The first focus area would be across the upper/High Plains. With ridging into the mid and upper Mississippi Valley, this would favor a tight gradient pattern evolving on the northwestern fringe. States such as the Dakotas could expect more severe thunderstorm activity in the coming weeks, although the threat of any major outbreaks is relatively low.
The second focus area is slightly less certain, but lies from Michigan southeastward into the upper mid-Atlantic states. With temperatures rebounding and a (mean) northwest flow setting up, this could favor at least a few severe thunderstorm events. Troughing to the east could mitigate this activity, but at the same time may favor a tighter thermal gradient and enhance thunderstorm activity. Climatologically, some of the more significant events in this region have been under a west or northwest flow.
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