El Nino ramps up - what it means to Mid-South weather

Climatologists and environmental scientists expected El Nino to appear over the winter of 2014-'15 and, though it never officially did, it appears it was only tardy, not absent. As we head into summer, NOAA has now indicated that a weak but strengthening El Nino is present and could become one of the strongest in quite some time later this year. In fact, NOAA is forecasting a higher than 80% chance that El Nino will continue through the remainder of the calendar year.

Sea surface temperature anomalies (departure from normal - orange to red above normal) increased over the past 3 months in the El Nino region from the west coast of South America into the central Pacific indicating a strengthening El Nino.

The various long-range computer models almost all support a strengthening El Nino this year with some indicating it could become very strong. 

El Nino's effects on winter weather are fairly well documented and understood (though local variations certainly exist and no two El Nino patterns are the same) but the effects in summer tend to be more muted and less predictable. There have also been many fewer instances of a strong summertime El Nino in recent decades, thus there are fewer cases to look back to and draw conclusions from.

So how will this summer's El Nino affect the Mid-South?

Dr. Jeff Masters, who writes on Weather Underground's Wunderblog, recently discussed the same topic and recounted the effects of two similar cases in particular, the summers of 1982 and 1997. Both of these summers were generally cool and wet across a large part of the continental U.S. Both of these summers were slightly below normal temperature-wise in the Mid-South as well.

El Nino also tends to bring wetter than normal conditions to the southern U.S. owing to a persistent subtropical jet stream. Masters indicates that "signals remain positive for widespread summer moisture." In fact, NOAA's June-August climate outlook shows the Mid-South on the eastern periphery of a large area of cool and wet conditions that they believe will dominate the central U.S. So, despite short-term trends that indicate a warmer-than-normal start to June, the summer average could end up a bit cooler (not to the exclusion of some periods of hot weather that we know are in store!).

The June-August temperature trends (left) as forecast by NOAA indicate a better than even chance of cooler than normal temperatures across the central U.S. with the western part of the Mid-South possibly cooler than normal. Precipitation-wise (right image), a large part of the Rockies, central, and southern U.S. could be wetter than normal, including the Mid-South.

It's also worth noting that there tends to be a marked decrease in Atlantic tropical activity during El Nino summers as well, owing to increased storm-destroying wind shear in the favored tropical formation regions. However, that could be partially offset by elevated sea surface temperatures (SSTs) in the Gulf of Mexico and Caribbean. So while the overall pattern could be quiet this year, it only takes one storm to feed on higher than normal SSTs near the U.S. to cause big problems for coastal residents. Tropical outlooks for 2015 that have been issued so far have indicated a below normal season. NOAA's Atlantic basin outlook, released  May 27, calls for a 70% chance of below normal activity due to El Nino. More specifically, NOAA predicts 6-11 named storms, 3-6 hurricanes, and up to 2 major hurricanes.

We'll see how the summer plays out - it will definitely be interesting to monitor!

Erik Proseus
MWN Meteorologist

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Rare "roll cloud" appears in the Memphis sky

So I was driving into work this morning and saw this...

A roll cloud! It extended north/south as far as I could see and was moving east across the city. I kept an eye on it before it disappeared over the eastern horizon. These pics have gotten a fair bit of attention on our Facebook, Twitter, and Instagram feeds and several people asked for more details, specifically how does it form? So here's the scoop on the rare "roll cloud" from the Glossary of the American Meteorological Society:

"A low-level, horizontal, tube-shaped arcus cloud associated with a gust front of a convective storm (or occasionally a cold front).  Roll clouds are relatively rare; they are completely detached from the convective storm's cloud base, thus differentiating them from the more familiar shelf clouds. Roll clouds appear to be 'rolling' about a horizontal axis [similar to a tornado on its side] because of the shearing effects and horizontal vorticity [rotation about a horizontal axis] provided by the differing air masses."

Roll clouds and shelf clouds (which are much more common and we have seen regularly lately as storms move into the area, pushing an outflow of cold air from the parent storm out ahead of the storm) are both of the arcus variety. The difference, as noted above, is that shelf clouds are attached to the parent storm while roll clouds are completely detached from other clouds.

In this case, the roll cloud likely originated as a shelf cloud from the squall line of storms that moved into central AR last night before falling apart. The outflow from the storms continued, and with no "parent storms" to attach to, the shelf cloud likely became a roll cloud. The fact that it made it this far east is fairly impressive!

A comparison of the two types of arcus clouds, from Ecography.com.

Roll clouds are most common in the U.S. along sea breezes and coastal locations, but are most likely (and more easily predicted) in Queensland, Australia, where they are referred to as the Morning Glory. (Check out this link for more on the Morning Glory and impressive pics!) We happened to luck out with one overhead this morning! Don't ever miss an opportunity to look up - you never know what you'll see!

Erik Proseus
MWN Meteorologist

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