Wednesday, May 27, 2015

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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Tuesday, May 26, 2015

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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Monday, May 25, 2015

An active weather pattern continues into the holiday week

The first round of Memorial Day weekend storms moved through the region yesterday afternoon into the early overnight hours. A Tornado Watch was issued for the Mid-South until 10pm last night, but fortunately only scattered reports of wind damage were reported across the area.  

The bigger issue for some was heavy rainfall. Most afternoon and evening storms were scattered or fairly transient so no widespread flooding issues resulted. However, late in the evening, a north-south line of storms moving basically due north set up from Memphis south roughly along I-55. It took some time to slowly move east and weakened to general rainfall during the overnight hours. A Flash Flood Warning was issued for the eastern metro and Doppler estimated storm total rainfall amounts were in the 2.5-4" range within that warned area.

Scattered damaging wind reports were received across the region (yellow/green icons above) with most activity concentrated west of the metro closer to Little Rock. Yellow boxes are Severe Thunderstorm Warnings issued, green is Flash Flood Warnings, and red boxes are Tornado Warnings.
Doppler estimated rainfall totals for the Sunday/Sunday night event. The storm tracks with heaviest rain can be easily picked out as they moved north-northeast. Pink colors are >3" amounts.
We'll get a reprieve today for the most part, though a few scattered showers can't be ruled out. This should allow most picnics and ceremonies to be unaffected by precipitation, although it will be breeze and muggy with highs in the mid 80s this afternoon.

Flash Flood Watches cover much of the southern plains and Mid-South with Flash Flood Warnings across much of OK and along rivers and streams across OK, TX, AR, and LA.

By this evening, another round of storms is expected. Driven by another upper-level impulse, these storms will likely bring many reports of large hail, widespread damaging wind, and a few tornadoes to eastern TX and southeast OK today before moving east-northeast across AR and into the Mid-South overnight. Fortunately, the storm system will weaken considerably as it draws closer, but expect more potentially heavy rainfall and lightning after midnight tonight into Tuesday morning.

Monday severe weather outlook from the Storm Prediction Center and the general track of a developing storm system in west TX that will move into AR overnight and weaken.

High-res model simulated radar image valid at midnight showing the line of storms that will be moving across across AR. Line placement is approximate and one model's interpretation. This line will be in a weakening stage, likely reaching the MS River in the wee hours or near dawn Tuesday morning.
The remnants of the overnight system will affect us Tuesday morning, then there is a chance that storms could re-fire over the area Tuesday afternoon. This will be largely driven by how much rain is still around in the morning, as morning showers and clouds will help to stabilize the atmosphere and keep afternoon storms from forming.

The SPC convective outlook for Tuesday shows a Slight Risk of damaging wind or large hail mainly east of the MS River.
Heading into the latter half of the short work week, the Mid-South enters a more summerlike pattern, with fewer organized systems like early this week. Scattered showers and t'storms will be expected during peak heating in the afternoon and early evening hours as temps reach the mid to upper 80s with moderate humidity levels and southerly flow off the Gulf of Mexico.

To close, on this Memorial Day holiday, I simply say "thank you" to all of those past and present who have fought bravely to ensure the freedoms that we too often take for granted and God bless the families and friends of those who have lost their brave loved ones in the battle. You are not forgotten.


Erik Proseus
MWN Meteorologist

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Friday, May 22, 2015

Memorial Day weekend forecast - a little something for everyone!

There's a famous saying that is used in many places around the country but is also particularly applicable here in the Bluff  City: "If you don't like the weather, stick around - it'll change."

That is surely the case for the 72 or so hours spanning yesterday through Sunday. In the wake of a cold front, cool air under a Canadian high pressure system and abundant cloud cover served to keep temperatures in the 50s Thursday, prompting complaints from some about it being too chilly. As the clouds departed last evening and the high pressure system settled over, wind became light and temperatures fell into the 40s area-wide, including an official low of 48 at Memphis Int'l Airport and 43 at MWN in north Bartlett. (41 is the record low for the airport, so it was not in jeopardy.)

WxBell graphic showing temperatures at 6am CDT today with widespread cool weather across much of the U.S. except the southern periphery. Two-thirds of the nation was below 50 at that time. Click for larger image.

Today, temperatures are rebounding nicely with sunny skies, dry (low humidity) air, and easterly flow around the high bringing temps into the mid 70s. Today is a transition day as even warmer air arrives from the south as the high pressure moves east, resulting in low 80s on Saturday for the grand finale of the Memphis in May AutoZone Sunset Symphony, of which MemphisWeather.net is proud to be the Official Weather Partner! Fortunately, humidity will remain low, but sunscreen will be a good idea if you plan to be at Tom Lee Park, or outdoors for any length of time elsewhere.


By late Saturday night, a warm front will move north across the region and bring a small chance of rain or a t'storm. On Sunday, we'll be back in the "warm sector" with humidity back to late spring levels (dewpoints in the mid 60s), a chance of t'storms, and highs in the mid 80s.

The GFS model showing precipitation totals for Sunday afternoon indicates scattered t'storms will be a factor, but it won't be as wet as areas to our west. The system responsible for higher precip amounts to the west will move into our area Sunday night into Monday, promising higher rain chances.  Graphic courtesy WeatherBell Analytics.

Unfortunately for those who have seen enough rain of late, this pattern sticks around for much of next week it appears. Daily rain chances in the 40-60% range will occur beginning Memorial Day (have a plan B for your grill-outs and picnics) and lasting at least the first half of the week. Highs will be in the 80s, lows near 70, noticeable humidity, and southerly wind.  No severe weather is currently expected, though upper-air impulses that can't be timed just yet could serve to intensify precipitation somewhat during the first half of next week. Also of some concern will be rainfall amounts as daily storms could result in flash flooding  at some point next week - just in time for the kids to be out of school and summer fun to start!

The NOAA Weather Prediction Center's rainfall forecast through Wednesday. Another very wet several days across the rain-soaked Southern Plains into the Mid-South!

Stay tuned and we'll keep you posted on any threat you need to be aware of. Be sure to always get the latest via our social media feeds and MWN mobile app as well. All pertinent links can be found below.

Erik Proseus
MWN Meteorologist

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