Monday, September 1, 2014

Gravity waves from Missouri storms result in Labor Day showers in the metro

Pop-up showers around mid-day today were a bit of a surprise, as they were not indicated by computer models early this morning and, according to our forecasts over the weekend, Labor Day appeared to be a dry day. However, an event currently not easily "forecast-able" by the models was the cause for these showers - a set of gravity waves!

Sometimes these atmospheric waves can be detected in radar imagery, but if conditions are just right, they are more easily detected using visible satellite imagery, as was the case this morning. The image below, taken at 8:30am, shows an arc of clouds from OK through AR, into western TN and KY. This arc is the outflow boundary from the MO storms. However, the outflow was accompanied by a series of gravity waves as well, most clearly visible in OK in the circled region (the technical term for this phenomena is an undular bore). Click here for an animation of this satellite imagery clearly showing the waves propagating away from the storms between 7-10am CDT (animation download is large and recommended for high-speed internet users). Also, the waves can be faintly seen in this (large) radar loop from about 6am-10am this morning around Oklahoma City, Little Rock, and over east AR from the Memphis radar.

An outflow boundary from decaying thunderstorms in KS/MO moved across OK, AR, TN, and KY this morning as shown in the satellite image taken at 8:30am. Gravity waves were also present behind this outflow, best seen in the circled area in OK. Click here for a satellite loop clearly showing the waves propagating away from the storm complex (Warning: VERY large animation - best with high-speed connections).

So what conditions were responsible for the waves that occurred this morning and why did showers result in the Mid-South?

Just as when a pebble is thrown into a calm body of water, waves in the atmosphere spread out from a large disturbance such as a thunderstorm complex. In meteorology, these waves are called "gravity waves" and they form when the wave(s) are trapped in a stable layer of the atmosphere, rising like the crest of a wave on the water, then being pulled back towards earth by gravity. As long as the stable layer of air exists, the waves continue to propagate.

The waves are visible on satellite imagery when they move through an area of saturated air that exists in or just below the stable layer. Rising air from the wave causes clouds to form in the saturated air and sinking air promotes drying or clear skies. The alternating rising and sinking air result in cloud bands that form perpendicular to the direction the wind is blowing at that level (transverse bands). Satellite imagery is the best way to see these bands of clouds that are the result of the gravity waves!

The rising air from the waves can also be strong enough to promote precipitation from the resultant clouds, especially when they encounter less stable air, as was in place over the Mid-South at mid-day today. The showers were the result of the rising air in the gravity waves. Since we didn't know these waves would occur, nor that they would arrive as the atmosphere was destabilizing, we didn't have a good handle on the formation of showers either. As the waves dissipated, the showers ended by early afternoon.

Erik Proseus, MWN Meteorologist

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Thursday, August 28, 2014

Deep tropical moisture brings wet weather pattern this weekend

Surface Analysis Chart valid Thursday at 1 PM from the Weather Prediction Center
Above is today's surface analysis from the Weather Prediction Center. It's not a surprise that there's plenty of warm temperatures across the southern U.S., but the past couple of days we have been enjoying a bit lower humidity at the surface courtesy of easterly wind. This is beginning to change as our wind shifts toward the south and moisture rich air streams in from the Gulf of Mexico. The approaching low pressure system from the west will strengthen that southerly flow. By tomorrow afternoon, humidity will return to higher levels (along with a slight chance of t'storms).
Precipitable water values and low level wind early Saturday afternoon
The lowest levels of the atmosphere will not be the only portion seeing an increase in moisture. The map above shows precipitable water values, or basically how much precipitation would occur if all the mass of water vapor in the atmosphere was brought to the surface as rain. This indicates very deep moisture through western Tennessee with precipitable water values of over 2.0 inches. The wind barbs show southerly winds in the lower levels of the atmosphere bringing a plume of moisture northward from the Gulf of Mexico.

Quantitative Precipitation Forecast through Sunday evening from the Weather Prediction Center 
Above is the NWS Weather Prediction Center's quantitative precipitation forecast for this weekend. They're calling for nearly an inch and a half of rain, with higher amounts southwest of the metro. The Weather Prediction Center expects more of the rainfall to be focused on Sunday, while we're currently forecasting a good chance of showers and thunderstorms on both Saturday and Sunday. Unfortunately, that means decent chances of rain for both the season opener for the University of Memphis football team on Saturday evening and the opening weekend of the Delta Fair at the Agricenter.

William Churchill, MWN Social Media Intern

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Tuesday, August 26, 2014

Summer continues for now, but tropical moisture will affect holiday weekend plans

A massive upper-level high pressure ridge has receded a bit, but hot and dry weather continues until tropical moisture arrives, just in time for the holiday weekend!

Abnormally dry weather over the past month has resulted in very dry vegetation and soil. A strong upper-level ridge that was in place over the weekend combined with the dry conditions to create record heat, with the mercury reaching the century mark for the first time in over two years on Sunday and tying the record high for the date. (The last time Memphis hit 100 - actually 101 - was July 30, 2012). Another record was tied Monday morning as the low only dropped to 79. Thankfully the ridge receded a bit and temperatures have responded, only reaching the mid 90s Monday and lower 90s today. With wind out of the east to southeast during the middle of this week, dewpoints will also fall just a bit. Though highs will remain in the mid 90s through the end of the week, heat indices won't top 100 thanks to slightly lower afternoon humidity.

A large ridge of high pressure at the mid levels of the atmosphere (18,000' feet) dominates the eastern 2/3 of the nation Monday night. This is keeping hot and dry weather in place across the Mid-South. Hurricanes Cristobal and Marie are also noted. Neither pose a threat to land areas. Click for larger image.

However, to our south, tropical moisture is sitting over the northwestern Gulf of Mexico, off the Louisiana and Texas coastline. The National Hurricane Center is monitoring this area and a short-lived tropical depression is possible. However, even if it doesn't, as high pressure moves east, southerly flow will push that tropical moisture north, putting the lower Mississippi Valley and Mid-South in its path.

Precipitable water (PW) values as of mid-day Wednesday. PW shows areas with high moisture content in the atmosphere. In this case, tropical moisture is sitting over the northwestern Gulf off the coast of TX. This moisture will be pulled north this weekend on southerly flow. There is also obviously a great deal of moisture in Hurricane Cristobal.

We'll notice this pattern shift by Friday and especially into the holiday weekend as dewpoints (humidity) rise, cloud cover increases, and showers and thunderstorms enter the forecast. In fact, this weekend may bring the best chances of widespread rain to the area in over a month. For now, severe weather is not a big concern, though heavy rainfall will be possible in areas that receive multiple storms.

By Friday evening, the ridge over the Pacfic strengthens, the ridge in the middle of the country moves east to the southeastern U.S., and well-established southerly flow exists over the Mid-South. This setup pulls Gulf moisture north into the region, resulting in decent rain chances. Click for larger image.
Forecast rainfall amounts from Friday evening through Sunday by the National Weather Service. A welcome 0.5"-1" of rain is possible and more could be expected in areas that receive multiple storms. Click for larger image.
So, for the rest of the week, enjoy hot, but not overly humid, days and prepare for the possibility of occasional rain on outdoor plans this Labor Day weekend, including the University of Memphis football team's home opener against Austin Peay on Saturday evening and the opening weekend of the Delta Fair. Find the complete MWN Forecast on our mobile apps (links below) or click here for our mobile website forecast.

Erik Proseus
MWN Meteorologist

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Monday, August 25, 2014

What do they say about broken clocks?

From the Farmer's Almanac, via the NY Post, I find this screen capture incredibly ironic:


The article above, dated April 14th and quoting the Farmer's Almanac (which I pulled up this morning on the NYC Post website), indicates an "oppressively hot and humid summer" for the New York area. In the margin are suggested articles relating to weather (that are obviously more recent). There we find  a headline stating "Summer 2014 is the coldest in a decade" (again, relating to New York). Wait a minute...

Say it ain't so!? The Farmer's Almanac -which boasts an 80% accuracy rating - was wrong (again)??

I pulled this year's stats for Central Park for June 1-August 25 (today). The average temperature has been 74.2, which is almost exactly the median over the nearly 140-year record at Central Park. In other words, it was an "average" summer by that metric. However, as quoted in the "coldest in a decade" article above, I also discovered that the highest temperature so far this summer in Central Park has been 91 degrees, which is the 3rd coolest high temperature ever recorded during a Central Park summer since records began (1876).

Graph showing the highest temperature recorded between June 1-August 31 each year at Central Park. In 2014, the highest temp of 91 is the 3rd coolest high temp ever recorded.
So why do I bring this up?

Because people are already asking what this winter will bring. Winter outlooks are starting to pop up, including the latest from the Farmer's Almanac, which predicts "colder-than-normal and wetter-than-usual weather for three-quarters of the country east of the Rocky Mountains,” according to the AP.  (By the way, the Farmer's Almanac says "chilly and wet" for the southeast U.S., while the OLD Farmer's Almanac predicts "much colder than normal, with below-normal precipitation." Which Almanac should I trust??  Are OLD farmers better weather-guessers than your average farmer?

There are hints, signs, and long-range models that will point you one direction or another. I can show you "evidence" of everything from cold to warm and wet and snowy to dry for our neck of the woods. I can tell you what it "should" do based on the El Nino that is "imminent," and then I will tell you El Nino isn't behaving like the PhD's that are way smarter than me thought it would. The truth is, I honestly have no idea what winter will be like.

If you like reading the Farmer's Almanac (either one of them), by all means go right ahead. But don't bring me their forecast and ask me to refute or defend it. It won't happen. "But it was right back when _______" you'll say! And I'll remind you of the old adage: even a broken clock is right twice a day.

Erik Proseus
MWN Meteorologist

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