Storms flying under the radar: when radar gaps and down time turn dangerous

From checking for rain to tracking tornadoes, doppler weather radar is an indispensable tool for meteorologists. When this staple of weather forecasting breaks down or is unavailable, however, the consequences can range from a simple inconvenience to lack of storm warnings and injury or death.

NCDC

NCDC

Albany’s radar out for weeks

The National Weather Service (NWS) doppler radar in Albany, NY experienced a “major hardware failure” on the evening of Tuesday, July 16, and will be offline through early August:

….NATIONAL WEATHER SERVICE DOPPLER RADAR WILL BE UNAVAILABLE THROUGH EARLY AUGUST. A MAJOR HARD WARE FAILURE WITH THE RADAR HAS BEEN IDENTIFIED. FIXING THIS WILL REQUIRE HEAVY DUTY EQUIPMENT AND A TEAM OF EMPLOYEES FROM THE RADAR OPERATIONS CENTER. WE APOLOGIZE FOR THE INCONVENIENCE.

The radar site – known by its designator “KENX” – is located about 16 miles west of downtown Albany and provides low-level (between 100 and 4,000 feet) coverage for a large part of eastern New York, western Massachusetts, and parts of Vermont and Connecticut.

NWS Doppler radars, often abbreviated as WSR-88D (for Weather Service Radar-1988 Doppler), go offline for maintenance with some regularity, and these outages often do not have any appreciable impact on a forecaster’s ability to issue warnings. But the lack of low-level radar sweeps due to outages or radar spacing can affect the detection of some severe weather, including events like small tornadoes.

Radar may miss shallow storms

The most serious threat posed by gaps in radar data are small, brief tornadoes that can spin-up undetected in squall lines or tropical systems. As opposed to the tornadoes that sometimes form in Oklahoma in springtime, where rotation may reach five to six miles up into the atmosphere, these smaller tornadoes  tend to be shallow (with rotation sometimes only a mile deep) and can form, touch down, and dissipate within a couple of minutes.

On an active weather day, it takes about five to seven minutes for a NWS Doppler radar to complete a full sweep of the atmosphere from top-to-bottom (usually between 12 and 15 different levels) and release the data over the internet. These small, spin-up tornadoes can occur in between these radar sweeps, meaning they form and die undetected by forecasters until damage is reported. Often times in severe squall lines, and especially derechos, forecasters will issue tornado warnings along little “kinks” in the line in anticipation of these weak tornadoes.

These tornadoes can, on rare occasion, go undetected and unwarned, as was the case on March 3, 2012 near Charlotte, NC.

The Charlotte radar gap

Charlotte is likely the largest city in the United States without detailed low-level WSR-88D coverage. The two closest sites are in Greenville, SC (KGSP) and Columbia, SC (KCRX), both about 80 miles away from the center of the Queen City. The next closest radar is in Raleigh (KRAX), a full 120 miles to the northeast. As a result, the lowest radar sweep over Charlotte and its immediate suburbs is around 8,000 feet above ground level.

charlotte

Locations of weather radars in the vicinity of Charlotte, NC. (Google Maps)

March 2, 2012 saw a particularly bad tornado outbreak across parts of Indiana, Kentucky, and Tennessee, killing 41 and producing two EF-4 tornadoes. As the storms crossed the mountains into North Carolina that evening and into the morning of March 3, the supercells merged into squall lines and the threat transitioned from tornadoes to wind damage.

One of these lines approached Charlotte around 2:30 a.m., the tail-end of which produced an EF-2 tornado that damaged almost 200 homes and injured four people. The NWS did not issue a tornado warning on this storm before the tornado hit – something that rarely ever happens anymore. A gap in Doppler radar coverage led to the lack of detection.

The tornado occurred just north of Hickory Grove, NC, or about seven miles northeast of Charlotte. At this location, the lowest sweep of the nearest radar (in Greenville, SC) was 8,300 feet above ground level. The rotation and tornado occurred well below this level, so the radar missed it. Here’s what it looked like:

(DATA: NCDC; RADAR: Gibson Ridge’s GRLevel2AE)

(DATA: NCDC; RADAR: Gibson Ridge’s GRLevel2AE)

An inspection of the radar image shows no hook on the base reflectivity (precipitation) image, and the base velocity (wind) image shows little discernible rotation in the area of the tornado. This is the main danger of these low-level gaps in radar coverage – tornadoes like this one can go undetected by radar, and do damage without warning.

The tornado was, however, detected by the Terminal Doppler Weather Radar (TDWR) located at the Charlotte-Douglas International Airport. TDWR sites are FAA-owned weather radars set up at most major airports in the country to aid air traffic controllers in protecting arriving and departing aircraft from dangerous wind shear. These TDWR sites have a higher resolution than most radar data available to the public, but cover a much smaller area. An area of weak rotation was detected by the Charlotte TDWR at 2:34 a.m., around the time of the tornado:

(DATA: NCDC; RADAR: NOAA Weather and Climate Toolkit)

(DATA: NCDC; RADAR: NOAA Weather and Climate Toolkit)

By the time the radar showed the rotation, it was too late – the tornado had already occurred and the rotation weakened, which is another probable reason the forecasters didn’t have time to issue a warning before the EF-2 touched down.

The Minnesota missed storm

A similar incident happened again earlier this week.  The town of Mahnomen, Minnesota experienced an EF-2 tornado early in the morning on Monday July 22. The tornado, which traveled 18 miles and reached 150 yards in width at one point, developed without a tornado warning being issued.

ValleyNewsLive, a news Web site based in Fargo, ND, interviewed NWS meteorologist Jim Kaiser about the incident. When asked why the NWS didn’t issue a tornado warning, Kaiser cited lack of radar coverage as one reason:

“The tornado and how small scale it was, and you try to figure that we’re looking at 7,000 feet above that circulation,” Kaiser said.  “You’re never gonna see it on radar.”

The D.C. area is covered

In the Washington D.C. area, we’re covered by the radar site over at the NWS office near Dulles Airport (KLWX). If that radar happens to go down, we’re lucky in that we have four TDWR sites covering the area. There’s one that covers Dulles (TIAD), National (TDCA), Joint Base Andrews (TADW), and BWI (TBWI).  Considering the local television stations that also have dedicated weather radars of their own, it would take a catastrophe of unprecedented levels for the D.C. metro to go without any sort of reliable radar coverage.

(Google Earth)

(Google Earth)

Coverage gaps are small, but consequences may be large

Overall, there aren’t many severe weather prone areas of the country that have to grapple with problems in low-level radar coverage unless a site goes down for maintenance. But in places like central North Carolina, Albany, and areas where a radar site goes down for an extended period of time, the results can be dangerous when severe weather – pardon the pun – moves in under the radar.

* Guest contributor Dennis Mersereau, who grew up in Woodbridge, Va., writes a weather column for Daily Kos.

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