Wednesday, November 4, 2015

First storm event of secondary severe weather season

For those who have lived in the Mid-South for at least few years know, November into early December is considered secondary severe weather season. While most severe weather in our region occurs in the spring, high wind, large hail, and even tornadoes have been known to occur year-round. National Weather Service statistics show a second peak in the occurrence of these phenomena during the fall transition season as more potent weather systems - with stronger wind throughout the atmosphere and sharper temperature gradients - move through the region.
Frequency of Mid-South tornadoes by month since 1950. Graphic courtesy NWS-Memphis. Note the secondary peak of tornadoes in November as summer transitions to winter. Notice also that tornadoes can occur any time of year in the Mid-South.

Storm Prediction Center severe weather outlook

The first of these types of systems to move through the Mid-South this fall arrives Thursday night as a strong cold front moves into the area. The Storm Prediction Center has placed parts of the Mid-South, specifically northeast into central AR, under a Slight Risk of severe weather (see graphic below). This risk level, which is the 2nd of 5 severe weather outlook categories, indicates scattered severe storms are possible. The Memphis metro is in the lowest of the 5 severe weather categories, a Marginal Risk, which can produce isolated severe thunderstorms. The main severe weather threat Thursday night in the Memphis metro will be straight line wind damage.

Severe weather outlook categories, as defined by the Storm Prediction Center, define the threat of severe weather in a given area. Click for larger image.

Severe weather outlook for Thursday into Thursday night for the Mid-South. A Marginal Risk (category 1 of 5) encompasses the Memphis metro. Graphic courtesy Storm Prediction Center.

Atmospheric setup conducive to a few severe thunderstorms

You've probably already noticed that, despite cloud cover the past few days, temperatures have been warm, especially in the mornings. In fact, this morning's low of 67° was nearly 20° above normal and set a record for the warmest low temperature on this date. Low temperatures Thursday and Friday mornings, also projected in the mid to upper 60s, could also set daily warm minimum records, while high temperatures reach well into the 70s to near 80° (or about 5-10° above average) despite abundant daytime cloud cover.

A warm airmass covers much of the U.S. east of the Rocky Mountains, including temperatures still above 70° at 7pm Wednesday evening. Click for larger image. Graphic courtesy
The above-average warmth is due to two factors: cloud cover overnight, which acts like a blanket trapping heat near the Earth's surface rather than letting it radiate out into space, and humid air from the Gulf of Mexico flowing into the region on southerly wind. The humidity of the air in an area is best represented by dewpoint temperatures. When the air temperature drops to the dewpoint, the air is said to be saturated and relative humidity is 100%.

Dewpoint is an absolute measure of humidity, describing the amount of moisture in the air irrespective of the temperature. With dewpoint temperatures running above 60° ahead of the approaching front, there is plenty of moisture in the air to provide one ingredient for thunderstorms. In fact, as shown in the graphic below, dewpoints will likely rise into the mid to upper 60s as the front moves through on Thursday night, as a southerly flow of air from the Gulf pushes even more moisture into the region.

The GFS (Global Forecast System, American computer model) depicts southerly wind (wind barbs pointing from south to north) and dewpoints well into the 60s (orange) as the cold front moves through the metro about 3am Friday. The "valley" in the solid black pressure lines indicates the position of the front. Graphic courtesy
In addition to abundant moisture in the atmosphere, strong thunderstorms also require a fair amount of wind energy. In this case, we examine the forecast wind at 5,000 feet (or 850mb by pressure level) shown in the graphic below. At 3am Friday, as the cold front is approaching, wind is forecast to be from the southwest at up to 50 knots, or 55-60 mph. The magnitude of the wind at 5,000 feet in this scenario is plenty sufficient for the development and maintenance of strong storms. It also is an indicator that the storms could be moving fairly quickly as they sweep through the region.

The GFS/American model forecasts strong southwest wind in the Mid-South, on the order of  55-60 mph, as the front approaches early Friday. Graphic courtesy
Even higher up, at the upper levels (about 35,000 feet / 250 mb, shown in the graphic below), a very strong jetstream is over the Midwest U.S. and Great Lakes region, on the order of 100-140 knots, or 115-160 mph. The Mid-South is positioned on the southeast side of the highest wind. This is referred to as the "right rear quadrant" or "right entrance region" of the jet, which is a favored area for severe thunderstorms due to upward or rising motion of the air in this area. So, while the best wind energy overall is a bit north of the region where wind is stronger, there is more than enough wind to support strong to possibly severe thunderstorms in the Mid-South.

The GFS/American model shows a very strong jet stream over the Midwest Friday morning with the Mid-South positioned in the right entrance region of the jet, a favorable location for severe thunderstorms as described above. Wind approaching 100 mph will exist along the western periphery of the metro according to the GFS model. Graphic courtesy
Finally, the last ingredient we consider relative to the severe weather threat is instability. Instability is similar to the heat from a stove burner under a pot of water. It serves as the fuel for thunderstorm development. An unstable airmass is one in which there is sufficient heating to cause air parcels to rise once they are lifted, for instance by a front. The rising motion of the air results in updrafts in the thunderstorms. The faster the rising air, the stronger the updrafts, and the more likely the storm is to be severe.

Thursday night, instability is the one negative factor that is present. Because the storms are arriving during the coolest part of the day - the early morning hours - daytime heating is at a minimum. Certainly the combination of temperatures in the upper 60s and dewpoints in the mid 60s will be enough to provide a modest amount of instability as described above, but it is not strong enough to result in a high threat of severe storms. The graphic below shows that the most unstable air will reside over the Gulf of Mexico early Friday morning with small amounts of instability over the Mid-South, as measured by an index called CAPE, or Convective Available Potential Energy.

Once again, using the GFS model forecast early Friday, instability is limited in the Mid-South with the most unstable air well to our south over the Gulf of Mexico. Graphic courtesy
The combination of an approaching front to provide lift, strong wind aloft, but weak instability, results in a likelihood of thunderstorms but a muted chance of severe weather with the most likely threat being the strong wind at 5,000 feet being pushed to the surface by downdrafts in thunderstorms. Thus, the Storm Prediction Center's Marginal risk of severe weather for the Memphis metro is a good call.

What to expect in the metro

So here is the bottom line: rain and thunderstorms are expected to move into the Mid-South Thursday overnight, following a day that will be mainly dry, but warm (upper 70s), humid (dewpoints in the 60s), and breezy (south wind at 10-15 mph). The most likely time for storms will be between 1am-5am Friday. A few storms will be capable of producing strong wind gusts to 60 mph, but widespread severe weather is NOT anticipated. Rainfall totals will likely be on the order of 1/2" or so due to the fairly progressive, or fast-moving, nature of this system. Prolonged heavy rainfall is not anticipated, though downpours are likely as storms move overhead.

The GFS/American computer model forecasts total overnight precipitation Thursday night to be near or above 1/2" in the metro with heavier amounts further west where storms are expected to be stronger. Graphic courtesy
Behind the system, look for a continued chance of showers Friday into Friday evening with sharply cooler air behind the cold front. High will remain in the 60s Friday as wind switches to the north, clouds remain in place, and showers dot the region. By this weekend, conditions will be dry with cloud cover moving south of the area and cooler, much less humid air infiltrating the metro as high pressure originating in Canada moves by to our north. For the latest details, check out the MWN Forecast on desktop, mobile, or in our apps.

As always, we encourage you to have multiple ways of receiving severe weather information prior to a potential event. At nighttime, it is particularly important that you have methods that will wake you up, including a NOAA Weather Radio programmed for your county. We also highly recommend that you download the mobile app if you have a smartphone. In the app, you can activate StormWatch+, our precision severe weather notification service that only alerts you for the specific location(s) you input and for the types of watches and warnings you wish to receive. Learn more and/or download the app by clicking here.

Erik Proseus
MWN Meteorologist

Follow MWN on Facebook, Twitter, and Google+
Visit on the web or on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

No comments: