Friday, November 27, 2015

A warm Turkey Day becomes a wet holiday weekend

Well, at least we can say it was a nice holiday! In fact, the high temperature in Memphis on Thanksgiving was 75°. That is not a record for the date, but I did go back 20 years into the archives and it was the warmest Thanksgiving over the past two decades.  In reviewing those years I found lots of highs in the 40s and 60s. I also found that rain has fallen on only 2 Thanksgivings in the past 20 years (2000 and 2003).
Despite a few sprinkles, Black Friday has also turned out pretty decent too, warm and cloudy but mainly dry so far. Temperatures have been in the mid 60s most of the day. If we don't drop below this morning's low of 63°, we'll tie the daily record for warmest low temperature. We can be thankful we're not dealing with the ice storm that some in the Plains are experiencing! That speaks to the strength of the cold front to our west, if not to its (lack of) forward motion.

Heading into the rest of the weekend, a persistent upper level pattern has caused the surface front to nearly stall out as it attempts to move east through the Mid-South. This upper pattern features a fairly strong ridge of high pressure over the lower Mississippi Valley and Gulf of Mexico and a deep upper level low over the western states. In between, a broad southwesterly flow extends from the Desert Southwest to the Great Lakes, or parallel to the surface front. When this occurs, there is very little to "push" the front through and our surface pattern also stagnates.
Jet stream level (39,000') setup as of Friday evening, showing a strong southwest to northeast flow across the U.S. with a ridge of high pressure over the Gulf of Mexico and a deep and large low pressure trough over the western U.S. A surface front lined up with the jet stream aloft typically doesn't move very fast.
Eventually, high pressure over the Plains will push the front through the Memphis area, likely around noon tomorrow. Though rain is expected off and on tonight into early tomorrow (with the heaviest to our northwest), the steadier rain will move into the metro behind the front later Saturday as moisture-laden air also resides behind the front, which stalls again over north MS. Upper level disturbances moving parallel to the front will keep rain chances high right through Sunday and into Monday. This is when low pressure forms along the front and helps to finally push it east as the upper level flow flattens out, becoming zonal (meaning the air moves nearly west to east). It could be early Tuesday before we finally shake the rain chances entirely however.
By Monday evening, the southern U.S. is dominated by a strong west-to-east jet stream (what we refer to as "zonal flow") which helps to keep frontal system more progressive at the surface. Also visible is the weakened trough that was in the western U.S. moving over the jet stream across the northern Plains.
In the meantime, rainfall totals across the metro will likely end up in the 3"+ range by Monday afternoon, most of that falling between Saturday evening and Monday morning, though light amounts are to be expected tonight and Saturday, as well as during the day Monday. Flash Flood Watches are posted just northwest of the metro in Arkansas where rain totals (which include that which has already fallen) could be upwards of 6".

NWS rainfall forecast from 6pm Friday through 6pm Monday. Though the rain has held off so far, a lot is on the way, particularly from Saturday PM through Monday AM. Graphic courtesy NWS.
So keep your galoshes, ponchos, umbrellas, and rain boots handy for the next few days! For those attending Senior Day at the Liberty Bowl Saturday morning as the Memphis Tigers football team plays their final regular season game of the year against SMU, periods of light rain are possible from early day tailgating right through the game, but we're not expecting major washouts. Temperatures will be mild in the morning (lower 60s) before starting to fall into the 50s by early afternoon as the front moves through around noon, so overall not bad temperature-wise.

Surface map valid 6pm Saturday evening showing the cold front just southeast of Memphis and widespread rainfall expected along and behind it as low pressure systems ride along the front from southwest to northeast. Graphic courtesy NWS.
Much of next week features cooler weather and mainly dry, though high clouds could be abundant. Lows will drop into the 30s with highs in the lower 50s at best by mid-week. Click here for the complete "human-powered" forecast from MWN and be sure to follow us throughout the weekend on social media as we bring you the latest information!

Also, for a few more hours (ending tonight at midnight), our mobile app for iPhone/iPad is FREE for Black Friday! Click the link below to grab it before it goes back up to $0.99! (Android users will still have to pay $0.99 since Google doesn't give us the flexibility pricing that Apple does. Sorry!) We also encourage you to upgrade within the apps to add StormWatch+, or precision severe weather alert technology. You'll find it to be a very advanced version of a NOAA Weather Radio that is mobile, works nationwide, and is about 1/4 the cost of a standard weather radio. Check it out and thanks for your support in helping to make the rest of what we offer free!

Erik Proseus
MWN Meteorologist

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Sunday, November 15, 2015

Heavy rain and strong wind on track for the Mid-South

A lot of rain to be sure, but Biblical in nature? Don't hunt for gopher wood or pair up the animals just yet...

Another strong transition-season cold front will move across the country this week. Similar to the past two events of last week and the week before, there will be plenty of wind at all levels of the atmosphere with this system, but little instability. The lack of unstable air will greatly reduce the threat of severe weather, but it won't make much difference in precipitation rates as the heart of the system moves through. Let's take the next few days a piece at a time.

Tonight / early Monday

Clouds thicken overnight tonight as upper level moisture increases on west-southwest wind aloft. Atmospheric energy originating from an upper-level low over the Desert Southwest will move over the area late tonight and early tomorrow. These "upper level disturbances" as they are sometimes called are areas where the air is lifted in the vertical. Where moisture is also present, the lifted air cools and condenses, resulting in precipitation. Thus scattered light showers that result will help to moisten the lower levels of the atmosphere and begin priming the pump for heavier rain to come. Rainfall will generally be 0.10" or less and should mainly affect areas north of I-40 with measurable precipitation.

North American Model (NAM) forecast depicting a few upper-level disturbances, including a couple strong ones in red/purple, moving through southwest flow aloft early Monday morning. They will trigger scattered showers, mainly for northern areas of the Mid-South. Graphic courtesy Pivotal Weather.

Monday afternoon / night

By Monday late afternoon and evening, steadier light rain is expected to begin, lasting through the overnight hours. This additional shower activity will be in response to increasing moisture content in the atmosphere, additional upper level disturbances moving through southwest wind aloft and providing lift necessary for precipitation, and a rapidly-strengthening low level jet stream Monday night. As low pressure over the Front Range strengthens and moves slowly east into the Central Plains, wind increases ahead (or downstream) of it. At the low levels (a couple thousand feet up), south wind will increase to nearly 50 knots (almost 60 mph) by early Tuesday morning! This strong wind above the surface but in the low levels of the atmosphere is termed a low-level jet (LLJ).

North American Model (NAM) forecast showing low-level wind increasing to over 50 mph straight off the Gulf of Mexico by Tuesday morning, bringing abundant moisture into the region. Graphic courtesy Pivotal Weather
With the low-level wind straight out of the south, a flood of atmospheric moisture from the Gulf of Mexico will inundate the lower Mississippi Valley and Mid-South. Despite this, overnight rainfall amounts Monday night should still be less than 0.50" as the best upper level dynamics remain well to the west of the region. Surface wind will also increase in response to the low level jet with steady south wind likely reaching or exceeding 20 mph.

The NWS Weather Prediction Center forecast of total rainfall from Sunday night through Tuesday at 6pm indicates only about 1/2" of rain is expected in the metro. The band of heaviest rainfall totals shows up to our west in OK, AR, and MO.

Tuesday daytime

A relative lull in precipitation will occur Tuesday, though the metro will continue to see the pump primed for heavy precipitation later as very strong southerly flow begins to increase moisture content in the mid levels as well. With no significant upper-level disturbances traversing the area, significant precipitation is not expected before dusk, though scattered showers certainly are possible given the amount of moisture in the air above us. Plan for plenty of leaves to fall as wind will remain strong and gusty at the surface Tuesday with south wind frequently gusting to 25-35 mph. A Wind Advisory may be needed on Tuesday. With a break from steady rainfall and south wind pushing warm air into the area, expect highs to eclipse 70°, even with overcast skies.

North American Model (NAM) forecast showing surface wind increasing to 20-25 mph with higher gusts Tuesday afternoon. Parallel lines from south to north indicate a flow of air from the Gulf of Mexico. Graphic courtesy Pivotal Weather.

Tuesday night

Tuesday night is the time period of most concern, with the main threat being flooding and flash flooding and, secondarily, wind. As the north-south aligned cold front moves through the area, it will provide the necessary lift promote heavy precipitation and will open the faucet wide. For 24 hours prior to the front arriving, strong southerly wind carrying Gulf moisture-laden air will provide abundant moisture for the front to tap into. Upper level dynamics increase as wind increases to around 30 mph at the surface and 60-75 knots (70-85 mph) from 2,000 feet to almost 15,000 feet. In addition, jetstream wind about 35,000 feet up will increase to over 100 knots (115 mph). Wind Advisories will likely continue into the night.

Atmospheric profile from the NAM model at 9pm Tuesday showing very strong southerly to southwesterly wind in the low levels of the atmosphere and a strong southwesterly upper level jetstream above. In addition, the red line (temperature) and green line (dewpoint) are very close together from the surface to about 25,000', indicating high relative humidity through most of the column of air above Memphis.
With all of this wind coming from the south, it is originating from the Mid-South's biggest moisture source - the Gulf of Mexico. Precipitable water (PW) values, which are a measure of total atmospheric water content, will approach 2.00", which would be record territory for this time of year according to climatology provided by the Storm Prediction Center. With a slow-moving cold front and record atmospheric moisture levels, the stage is set for possible flooding and flash flooding Tuesday night. Various computer model solutions and Weather Prediction Center forecasts paint a picture of rainfall totals between 2-3" in a roughly 6-hour window overnight.

North American Model (NAM) forecast showing precipitable water values reaching near-record levels by Tuesday evening with abundant moisture in all levels of the atmosphere. Notice also behind the cold front how quickly moisture falls off. Graphic courtesy Pivotal Weather.

There are 3 big concerns related to the flooding threat on Tuesday night:

1) Precipitation rates. Widespread 2-3" amounts in a roughly 6-hour window, on top of precipitation that has already fallen from Monday morning through Tuesday, will lead to high water in the usual places. Drainage ditches, streams, and creeks will fill rapidly.
2) Leaves. It's fall, y'all! I drove around some today and saw piles of leaves sitting on curbs and in gutters, not to mention all over yards. We have a lot of trees in the Mid-South and the leaves are falling (and if they aren't yet, wait until the wind has been blowing 20-30 mph for 24 hours!). The leaves will block natural areas that water flows, namely along the sides of streets in the gutters. With drainage grates potentially blocked by leaves, street flooding becomes a bigger hazard as the water can't run off as quickly.
3) Darkness. All of the heavy rain will occur when it is dark out. Pair darkness with torrential rain and leaves blocking gutters and we have a recipe for big problems for motorists. Streets that haven't flooded in a long time will. We have one suggestion: if you don't HAVE to get out Tuesday night, DON'T! Get your errands done on the way home from work then stay put overnight.

Moving on, the threat of severe weather will be low Tuesday night as the lack of instability (as with the previous systems the past couple of weeks) and overall saturation of the atmospheric column will limit the formation of strong storms. Convective precipitation (that which occurs in an area of rising air) and thunder is certainly possible and would serve to increase rainfall rates, but the bulk of the severe weather should remain to our southwest. If there is a severe weather threat locally, it will be strong(er) wind from a possible squall line of storms. Again ,we expect that threat to remain to our south, but it's worth mentioning.

The severe weather threat on Tuesday will be southwest of the Mid-South where instability is greater and a squall line could develop, according to the Storm Prediction Center convective outlook.

As the cold front passes sometime in the wee hours Wednesday, the faucet will shut off as wind shifts westerly, cutting off the feed from the Gulf and ushering in drier air from the west (as seen in the precipitable water graphic above). By dawn, the rain will probably be over (and your rain gauge could be overflowing). Your school's car line should be dry.

Wednesday - Friday

The second half of the week will be a time to dry out as high pressure of Pacific origin moves overhead. Temperatures will remain seasonal with highs in the mid 60s and lows in the 40s. Clouds will depart early Wednesday, leaving the rest of the week mostly sunny.

Follow us on Facebook and Twitter and on our mobile apps (all links below) for the latest information on this week's wind and heavy rain threats, as well as any changes to the forecast.

Stay safe and (relatively) dry!

Erik Proseus
MWN Meteorologist

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El Niño, part II: Global & local effects / MWN Winter Outlook

In part 1 of this blog series on El Niño, we examined the basics of ENSO (El Niño Southern Oscillation), including what exactly it is and how ocean temperatures affect global weather patterns, a little history as to how it was discovered, and how it is measured. In this final installment, we examine the typical effects of El Niño globally, as well as what we have experienced locally in prior El Niño years, and finally what we might expect this winter. This blog will also serve as the "2015-'16 MWN Winter Outlook" post that is commonly requested this time of year.

Global effects of El Niño

Though effects on the atmosphere over North America, and the rest of the world, vary with each El Niño, there are some typical atmospheric responses. The effects are most visible during the winter months, since that is when the waters of the Pacific tend to be warmest during El Niño years, and include:
  • Increased precipitation across the southern tier of the U.S, including drought-stricken California, due to a more active Pacific jet stream
  • Decreased precipitation across the Pacific Northwest and Ohio Valley
  • Warmer than average conditions across the northern half of the U.S. and southern Canada, as the polar jet stream is displaced further north
  • More coastal storms affecting the eastern U.S., and
  • In summer, suppressed activity in the tropical Atlantic and increased activity in the eastern Pacific.
Below are the typical El Niño patterns during the winter (top) and summer (bottom) months globally. These "typical" effects are often be offset by other, shorter-term, regional or global patterns that are much less predictable beyond a couple of weeks. For instance, just because a region is usually dry or cool doesn't mean the entire winter will fall into that pattern.

Regional effects of El Niño

Mid-South weather can be fairly fickle during El Niño winters, as we are typically positioned between the persistent Pacific jet stream that brings wet and cool conditions to the southern U.S. and a pronounced dry area over the Ohio River Valley, which can extend as far south of the Tennessee River Valley. If either of those areas shifts slightly due to other variables, Mid-South weather could end up either wetter or drier than average, particularly for short durations over the course of the winter.

Mid-South temperatures over the course of an El Niño winter are generally driven by shorter-term regional variations that occur. El Niño itself generally places us in a "near average" temperature regime, between cooler than average weather to the south (nearer the Pacific jet stream) and warmer than normal weather to the north.
Typical climate pattern over North America during El Niño winters. Graphic courtesy NOAA/CPC.

Other factors: Teleconnections

While El Niño will be the primary factor on our winter weather pattern, there are other climate influences (or "teleconnections") that may last only for a week or two at a time that frequently determine local weather conditions. These are much less predictable more than a couple of weeks in advance. These include the North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Pacific Decadal Oscillation (PDO), Pacific-North American Pattern (PNA), and Madden-Julian Oscillation (MJO), among others. Here is a brief description of a few that can affect our weather:

North Atlantic Oscillation (NAO) - Outside of El Niño, one of the primary influencers of winter weather in eastern North America and Europe is the NAO. A positive NAO occurs when atmospheric pressure over the high latitudes of the North Atlantic (i.e., Greenland) is below average and areas in the central North Atlantic have above average pressure. A strong jet stream across the eastern U.S. into the north Atlantic keeps the coldest winter air bottled up to the north, resulting in above average temperatures for the eastern U.S. A negative phase features above average pressure over the high latitudes and usually results in below normal temperatures and a snowy pattern for the eastern U.S. as cold air is allowed to dip into the region due to a weaker jet stream. The NAO can shift from positive to negative multiple times within a season or may vary in strength but remain in the same phase for several months at a time. At least through the early stages of the winter season, a positive NAO is expected.

The North Atlantic Oscillation (NAO) measures pressure anomalies over the North Atlantic Ocean. Higher than normal pressure over the northern latitudes of the north Atlantic typically results in a cold and snowy pattern for the eastern U.S. in winter. Graphic courtesy

Arctic Oscillation (AO) - A positive AO occurs when the ring of winds circulating around the polar region (commonly referred to as the "Polar Vortex") is strong, keeping cold weather confined to the highest latitudes around the North Pole. Higher pressure results in the mid-latitudes, along with less frequent intrusions of Arctic air. In a negative AO phase, this wind circulation weakens, allowing the cold polar air to penetrate south into the middle latitudes and increasing storminess in these areas as the polar jet stream dips south. This is what has been referred to in the media last January as the "Polar Vortex" invading the U.S., when in actuality, it is always there, just not always as far south. The AO is difficult to forecast more than a couple of weeks into the future.

The Arctic Oscillation (AO) measures anomalies in pressure in the Arctic region. A negative phase occurs with higher pressure at the north pole, resulting in intrusions of cold air into the mid latitudes. A positive phase occurs when the cold air stays bottled up at the pole, resulting in higher pressure in the mid latitudes. Graphic courtesy
Pacific Decadal Oscillation (PDO) - As its name implies, this teleconnection generally lasts much longer (years at a time), varying in strength but generally staying in the same phase for long periods. The PDO is defined by ocean temperature anomalies in the northeast and tropical Pacific Ocean, with a positive phase occurring when warmer than average sea surface temperatures are positioned along the Pacific coast and cooler than average temperatures are located in the interior northern Pacific. After about 16 years in a negative phase, the PDO went positive about two years ago and remains that way now. It is a fairly recently-described concept and thus it effects are not yet well understood.

Pacific-North American Oscillation (PNA) - The PNA describes a pattern of mid-level pressure readings in distinct areas across the Pacific and North America. Usually, these pressure readings (or "heights") are similarly anomalous in the Aleutian Islands of Alaska and the southeast U.S. In the positive phase, heights are above average around Hawaii and in western North America and lower in the North Pacific and southeastern U.S. The positive phase tends to result in cooler and drier weather for the eastern portion of the country in the winter and also tends to occur during El Niño conditions, but not always.

Average temperatures during a strongly positive PNA regime in January 1981, courtesy of the State Climate Office of North Carolina. The country was virtually split in half with very cold air in the east and warmer than normal air in the west.
Madden-Julian Oscillation (MJO) - The MJO is an "eastward moving disturbance of clouds, rainfall, winds, and pressure that traverses the planet in the tropics and returns to its initial starting point in 30 to 60 days, on average" ( Because it is an intraseasonal tropical climate variable, it can change over the course of weeks. Its phase (of which there are eight) can have dramatic impacts on mid-latitude weather, including cold air outbreaks over the eastern U.S. in winter, flooding rain, and jet stream changes.

As you can see, the state of each of these teleconnections can alter or even reverse the impacts of a "typical" El Niño season for periods during the winter, which then affects the overall averages. It's important to remember that "climate is what we expect, weather is what we get." Thus there can be a difference between long-term, or seasonal, averages and day-to-day weather that makes up that average.

Winter outlooks from other sources

As I state every year, I am not an expert climatologist. At MWN, the focus is, nearly exclusively, on the short to mid-term forecasts - those out to a week to 10 days. So, as usual, in putting together our Mid-South winter outlook, I consulted multiple sources and researched data from winters past that I felt were comparable to what we might experience this year from a general pattern perspective (called analogs, listed at the end of the post).

Significant weight was given to previous winters that had strong El Niño conditions but consideration was also given to winters with weaker El Niños which had similar sea surface temperatures outside the Pacific ENSO region, namely the eastern north Pacific and western north Atlantic, since they would be most likely to have an effect on U.S. weather. In particular, I feel the warm waters near the Pacific coast and those in the north Atlantic will play more than a passing role in weather patterns over the U.S. and influencing the Mid-South.

The winter outlooks that I gave the most credence to were those from NOAA and WeatherBell Analytics, as their reasoning is sound and I generally agree with their premises of their respective outlooks, even though they differ slightly. You'll find those outlooks shown below.

NOAA Winter Outlook

NOAA's temperature outlook for this winter leans heavily on climatological expectations associated with El Niño. Percentages represent the likelihood of  above/below normal temperatures. In other words, there is a >40% chance that much of Texas will see cooler than normal weather this winter.

NOAA's precipitation outlook for this winter also leans towards El Niño climatology, though it depicts southern U.S wetness extending north into the southern Plains and Front Range, as well as up the east coast. Percentages are read similarly to the temperature map above. "Equal chances" means there is no signal to indicate a greater chance of above or below normal precipitation.

WeatherBell Winter Outlook

WeatherBell temperature forecast for the winter season. Colors represent departures from average.  In many respects, this outlook agrees with NOAA depicting a warm winter for the north and cool winter for the south.  The eastern 1/3 of the U.S. is where they differ the most.

The snowfall forecast for this winter from WeatherBell, expressed as percent of normal. Recall that for the south, where values are forecast well above average, snowfall amounts are typically not high, so a 150-200% of normal forecast could be only a few inches difference. NOAA does not expressly predict snowfall in their winter forecast. 

The MWN Winter Outlook

1. Temperature
All of that said, there is fairly strong consensus that Mid-South winter will start off warmer than it will end. In other words, the temperature anomalies will be positive (above normal) through December, then descend into negative (below normal) territory by February and March. In fact, the average temperature in December for the ten El Niño winters examined (analogs listed below) was 0.9° above normal while the average February temperature was 3.4° below normal. Thus I expect temperatures will be above normal, on average, through the end of the year before beginning to trend downward. Similar to last year, February and early March have the potential to be a fair amount below normal in the temperature arena.

2. Precipitation
Precipitation-wise, a warmer early season tends to support more atmospheric moisture than a cold season and our analog winters support that as well. By the time the latter half of the winter arrives, precipitation in the analogs falls below normal. In a classic El Niño setup, precipitation is usually near average in the Mid-South, but drier anomalies in the Ohio Valley can sometimes creep south into west and middle TN as well. Therefore, we are of the opinion that the winter will start off with above normal precipitation and the latter half of the season will be slightly below normal.  We also believe that severe weather activity will continue to be below average, which is fairly common in El Niño years.

3. Snowfall
The most anticipated part of the forecast, snowfall tends to be near the long-term average for the season, though it can be fairly variable. This is because one snow storm (or the lack thereof) can result in a season that is well above (or below) normal, since not much typically falls. (Memphis International Airport averages 3.4" of snow each year.) Last year, the December-February snow total was 2.3", or about an inch below normal. However, the first light accumulation actually occurred very early (0.1" in mid-November) and the biggest ice/snow event of the season actually occurred in the first week of  March, both outside the typical "winter" season. So for 2015-'16, we are predicting slightly above normal snowfall for the season (4-5") with the majority of that likely to occur later in the winter, not dissimilar to last year. With a forecast of cooler air in place in February to early March, we believe there is an above average probability of a late season snowfall in the Mid-South once again.

So there you have it - the predictions for 2015-'16 winter! If you're interested in how I did last year, take a look at this blog post from last March. In a nutshell, the winter of 2014-'15 was cold, dry and snowy. I had forecast temps below to slightly below average (correct), precipitation near average (incorrect), snowfall near to above average (correct), and large temperature swings with periods of severe weather (partially correct).

The analog years we examined for this year's outlook were 1919-20, 1957-58, 1972-73, 1982-83, 1991-92, 1994-95, 1997-98, 2002-03, 2009-10, and 2014-15. These included the 3 strongest El Niños on record (to this point): 1997-98, 1972-73, and 1982-83. Click here to view the raw data for each of these winters.

Erik Proseus
MWN Meteorologist

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Friday, November 13, 2015

El Niño, part I: The basics

With words like "Godzilla" and "strongest on record" being thrown around in relation to this winter's pending El Niño, you might be wondering just what El Niño is and what effects it will have on Mid-South weather. This is the first of a two-part blog series answering those exact questions.

In this installment, we'll answer the question "What is El Niño?" by putting the semi-technical terminology and atmospheric explanations on a shelf where you can reach it. In part 2, we'll look more closely at what the effects of El Niño conditions usually are globally and in our little corner of the world.

History of El Niño

El Niño traces its roots to a warming of the Pacific Ocean along the Ecuador/Peru coast in December, when the fishing season typically ends. El Niño literally means "The Little Boy" or "Christ child" in Spanish because the warming typically coincided with the Christmas season. Despite some annual warming, it was discovered that every few years the warming was more pronounced and coincided with heavy rainfall in typically dry coastal regions of Ecuador and Peru. Mid-20th century scientists discovered that the warming wasn't confined to the far eastern Atlantic, it actually spread across the eastern into the central Pacific near the Equator.

Link to the atmosphere

Further study in the 20th Century resulted in a discovery that the warming of the ocean water was also coupled with an atmospheric pressure oscillation called the Southern Oscillation. The oscillation of pressure results in changes in trade wind patterns (a prevailing pattern of surface winds blowing from the east found in the tropics) and rainfall distribution.

In "normal" conditions which are not El Niño or La Niña (commonly called "La Nada"), trade winds across the Pacific blow towards the west, causing warmer water near the surface of the ocean to be pushed westward. The temperature difference between cooler water to the east and warmer water in the west results in rising air over the western Pacific. This air evacuating the low levels of the atmosphere lowers the air pressure near the surface. In combination with warmer water that produces more evaporation in the same area, more clouds form and abundant rainfall occurs in the western Pacific. At the same time, in the east Pacific (off the coast of South America), atmospheric pressure is higher due to sinking air and rainfall is minimized.

Normal ("La Nada") conditions are shown above in a graphic from the Australian Bureau of Meteorology. Easterly trade wind pushes warmer water westward, resulting in lower pressure and increased precipitation over the west Pacific and cooler, drier conditions in the eastern Pacific.

In El Niño conditions, the trade wind weakens (or even reverses course in the strongest El Niños) and the warmer water shifts to the central and eastern Pacific, rather than the west Pacific. Rainfall patterns also shift, as rain tends to fall over warmer water for the reasons described above. Thus the normally-dry western coast of South America becomes wet and rainfall slacks off in the western Pacific. The shift in atmospheric heat, as a result of the displacement of warmer Pacific waters, disrupts atmospheric circulations globally, causing changes in weather and climate in places far removed from the Pacific Ocean, including across the United States.

El Niño conditions are shown above in another graphic from the Australian BOM. Easterly trade wind weakens, or even reverses, resulting in warmer water in the central and eastern Pacific and increased precipitation in the same regions. The western Pacific becomes drier with higher average pressure readings.

The ENSO Cycle

The complete name for El Niño is "El Niño/Southern Oscillation," or ENSO. The complete cycle from La Niña (the reverse condition of El Niño) to El Niño make up the "ENSO Cycle." El Niño episodes tend to occur every 2-7 years and typically last about 9-12 months. Much like a human being with a fever, there are varying degrees of El Niño and La Niña conditions, some relatively weak and others quite strong. It would follow that influences of ENSO can also vary widely, including the timing, duration, and intensity of the conditions.

How is ENSO measured? (this is a little more technical...)

The typical measurement that the National Oceanic and Atmospheric Administration (NOAA) uses to officially declare that El Niño conditions are occurring is based on sea surface temperatures (SST) in key areas of the equatorial Pacific, more specifically sea surface temperature anomalies (departure from normal). NOAA's Climate Prediction Center defines El Niño conditions as existing when three conditions are met:

  • a one-month positive SST anomaly of 0.5°C or greater is observed in the Niño-3.4 region of the Pacific Ocean (see map below),
  • an expectation that the 3-month Oceanic Niño Index (ONI, defined below) threshold will be met, and
  • an atmospheric response typically associated with El Niño is observed over the equatorial Pacific

The Niño regions along the equatorial Pacific are shown above. Niño 3.4, which is the key region for declaration of an El Niño pattern, encompasses the central portions of the Niño 3 (in red) and Niño 4 (in yellow) regions.

The ONI is a running three-month average of SST anomalies for the key Niño 3.4 region. "Events" (El Niño or La Niña) are defined as five consecutive overlapping 3-month periods at or above the +0.5°C anomaly for warm (El Niño) events and at or below the -0.5°C anomaly for cold (La Niña) events. In other words, El Niño is not officially declared until five consecutive three-month averages are at least 0.5°C above normal, which happens seven months into an El Niño event, since running averages are used.

For example, ONI values over the past year (shown below) featured three consecutive three-month periods that met the 0.5°C threshold (from OND '14 through DJF '15), but fell short of official El Niño status when the JFM '15 anomaly was just below the 0.5°C threshold. However, since FMA '15, the Pacific has produced five consecutive ONI values at or above the 0.5°C threshold (in red below), including the most recent 1.7°C value. Thus, El Niño officially began in February 2015, but was not declared until the June-Aug. '15 average was calculated in early September.

Oceanic Niño Index (ONI) values since winter 2014. The past 7 three-month periods have equaled or exceeded 0.5°C. When 5 of these periods reach 0.5°C consecutively, an El Niño is declared by NOAA.
According to NOAA, El Niño conditions are expected to peak this winter and then decline heading into next spring and beyond, thus El Niño will be a major driver of weather and climate throughout the upcoming winter.

For more information on ENSO, see NOAA's Climate Prediction Center website, NOAA's El Niño Portal, or this page from the Pacific Marine Environment Laboratory.

In the second part of this blog series, we will look more closely at 1) the effects of El Niño both globally and locally, 2) Mid-South weather in historical El Niños, and 3) what we expect from the weather this coming winter! Will it mean a colder or wetter winter? Find out here!

Erik Proseus
MWN Meteorologist

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