Don’t Let Satellite Weather Get You Down

The radar depictions you see from the SiriusXM broadcast are highly filtered to provide only real precipitation areas. Ground clutter, anomalous propagation, birds, insects and such are carefully removed to provide the cleanest and most representative image. But like any process, there will be times where non-precipitation returns do not get filtered out. More importantly, you may see real areas of precipitation filtered out as well.

While rare, the latter usually occurs in regions where WSI (the weather provider for SiriusXM) implements what is called a manual gross filter. This kind of filter is the most efficient way to eliminate any clutter in large areas that are not expected to see precipitation. But when that filter is left on too long, it’ll be just as efficient at removing real precipitation from the broadcast.

zoomed-in-gross-filter

Lightning and a single hail storm attribute marker with no radar depicted.

Here’s one such example depicted above. While connected to the SXAR1 I panned the map over Texas and I saw some lightning and a lone hail attribute marker showing echo tops at 45,000 feet in north-central Texas, but no radar returns. Hmmm?

I verified that I had the Radar Composite turned on (I did) and zoomed the display out as shown below to see that there are plenty of other precipitation areas shown to the northeast and southeast of this area. Given that the area wasn’t cross-hatched with “Radar not available” why wasn’t there any precipitation shown?

texas-gross-filter

Zoomed out to show the presence of other precipitation on the radar composite.

About 15 minutes later I came back to the map to see if there was any change. Notice below that plenty of lightning and storm attributes are being depicted here in north-central Texas; however, there are still no radar returns being rendered. Given this activity, you’d expect there to be some precipitation shown when both lightning and storm tracks are present. This is a classic indication that the real precipitation in this region was being erroneously filtered.

gross-filter

This is a classic signature for a gross filter being left on too long. With the radar composite on, no precipitation is being shown despite the presence of lightning and storm tracks.

Just five minutes later, the gross filter was removed by WSI and the returns suddenly popped into existence as you can see below.

gross-filter-pulled

Once the gross filter was removed, the NEXRAD returns associated with these thunderstorms were rendered.

I took a look at the NEXRAD archives and discovered that the first precipitation developed in this region around 12:05 p.m. CDT. The gross filter wasn’t removed until 12:50 p.m. CDT. That’s 45 minutes with no radar for this area of rapidly developing and potentially severe thunderstorms. Moral of the story is to always have lightning ON and be sure the SiriusXM Storm Markers are also set to ON in the Maps Settings menu (the gear button on the Maps view). Having both of these layers on will likely expose these kinds of uncommon events.

Tips On Using SiriusXM Satellite Weather In ForeFlight

With the release of ForeFlight Mobile 8.1 you now have the opportunity to use the best portable en route weather system available courtesy of our partnership with SiriusXM Satellite Radio. The SiriusXM Pilot for ForeFlight subscription tier has been uniquely designed to provide all of the essential weather data during every phase of flight. In fact, within about 15 minutes of turning on the SXAR1 and connecting to the ForeFlight Mobile app, you’ll have seamless access to a comprehensive set of weather products well before you close the door on the cockpit and depart. Here are some of my tips to safely use this unique collection of weather data.

Hurricane Hermine

SiriusXM radar depiction of Hurricane Hermine as it approached the Florida coast in early September.

The SiriusXM source label

Knowing the source of the data you are using is paramount since weather data ages quickly. When connected to the SXAR1, you’ll see a SiriusXM label under the tappable timestamp button in the upper left of the Map view. Moreover, every weather product provided through the SiriusXM broadcast includes a source label in parentheses along with its relative age like the one depicted in the image below. This is similar to the ADS-B label shown when connected to Stratus. While connected to the SXAR1 in flight, always be sure to check for the presence of the SiriusXM label. Seeing this label will confirm that you are using the most current weather available.

siriusxm-tag-taf

Products received from the SiriusXM broadcast and displayed in ForeFlight will be labeled with a SiriusXM tag along side the product’s age as shown here for a terminal aerodrome forecast (TAF) for the Cape Girardeau Regional Airport.

Lightning

During the warm season, lightning from ground-based sensors is perhaps one of the most critical weather elements to have available in the cockpit. Any area of weather that includes lightning means there’s a darn good chance you will encounter severe or extreme convective turbulence in and around that weather. While most of the serious thunderstorms will be included within the boundary of a convective SIGMET, not all thunderstorms will meet convective SIGMET criteria. Moreover, thunderstorms often occur outside of these areas, especially during a rapidly developing convective event.

Lightning is broadcast over SiriusXM every five minutes and provides pilots with a birds-eye view of where the truly nasty convective weather is located. Moreover, both cloud-to-ground (CG) and intracloud (IC) lightning are part of this broadcast. It’s quite important that both types are included since many severe storms are often dominated by IC lightning.

With SiriusXM not every lightning strike is broadcast. Instead, a single lightning symbol is shown anytime one or more strikes have occurred within a generous 0.5 nautical mile grid. So when you pinch-and-zoom way in on the ForeFlight map as shown below, you’ll notice the lightning bolt symbols are aligned in this 0.5 nautical mile gridded pattern. ForeFlight retains the most recent 10 minutes of lightning data which tends to align with the most recent radar depiction very well.

SiriusXM Lightning grid

A zoomed-in view of SiriusXM lightning reveals it’s gridded nature.

Lightning is detected even in regions where radar coverage is not present. This can be extremely useful when flying outside of the NEXRAD radar coverage area. You’ll see lightning depicted in regions over the Gulf of Mexico and Caribbean as well as the coastal waters of the U.S. in the western Atlantic and eastern Pacific Oceans. It will also include lightning in Canada, Mexico, Central America and the northern-most regions of South America. Although there is SiriusXM NEXRAD coverage provided around Puerto Rico and the U.S. Virgin Islands (using the base reflectivity from the lowest tilt), having lightning shown in other locations in the Caribbean will help pilots avoid the nasty tropical convection that occurs in these highly traveled areas where there isn’t NEXRAD coverage.

lightning-south-america

SiriusXM radar coverage is available using the base reflectivity layer from the lowest tilt around Puerto Rico and U.S. Virgin Islands. You will also see lightning depicted outside of the standard NEXRAD coverage area as far south as the northern portions of South America.

Storm attribute markers

Pilots have become accustomed to seeing echo top heights and storm track identification markers in ForeFlight. With SiriusXM you’ll get those same NEXRAD storm attributes. This includes a generic storm marker with an echo top height shown in 100s of feet in addition to cells that have signatures of hail, mesocyclone and tornadoes using the symbols shown below. Echo top heights are only shown for tops 20,000 feet and higher.

markers

Storm attribute markers include hail, mesocyclone and tornadic vortex signature. Under the settings, these SiriusXM Storm Markers can be switched on and off as desired.

In most cases these storm attribute markers will also contain a direction and speed of the cell being tracked. Similar to the other storm tracks you will see depicted on the radar mosaic in ForeFlight, SiriusXM tracks will contain an arrow showing the direction of movement as well as the speed. If the cell is moving at a speed of more than 10 knots, you’ll also see two black dots depicted on the arrow that loosely estimates the position of that storm cell in 20 and 40 minutes based on the cell’s current speed and direction provided. The arrowhead represents the estimated location of the cell in 60 minutes.

Confusing Storm Attribute Markers

During a rapidly developing convective event or when thunderstorms are dissipating, it’s quite common to see the storm tracks for adjacent cells point in opposite direction.

While these markers provide additional information about a storm cell, keep in mind that there will be times when the storm tracks for adjacent cells may provide conflicting information as you can see in the example shown above. It’s unlikely these cells are actually moving toward each other. This typically occurs during the initial stage of thunderstorm evolution especially when there’s an area of rapidly developing convection. Animating the radar is perhaps the best way to note the direction of movement of an area of weather.

tvs-hermine

Shown here are several storm attribute markers to include mesocyclone circulation and tornadic vortex signatures from Tropical Storm Hermine as it passed off the coast of South Carolina.

Radar layers

The SiriusXM composite reflectivity and base reflectivity from the lowest tilt have the same 2 km horizontal resolution as you may have experienced with the regional radar broadcast provided by ADS-B. On the left is the regional composite reflectivity mosaic broadcast by ADS-B using the Stratus 2 receiver. On the other hand, the right side is the SiriusXM mosaic just a minute earlier. While the mapping of dBZ levels to color may be a little different for the two composite reflectivity sources, the overall spatial resolution is the same.

ads-b-vs-siriusxm

Regional composite reflectivity from ADS-B shown on the left and composite reflectivity from SiriusXM shown on the right. Both have a similar resolution.

There’s no doubt that the overall qualitative glance value is practically the same between the two radar depictions above. You’ll find, however, that the latest SiriusXM broadcast will be about 5 minutes fresher on average than what you get through ADS-B.

Partial radar refresh

You may occasionally notice that both of the radar mosaics may take a short period of time to completely refresh the Map view for the entire radar coverage area when a new NEXRAD broadcast is being processed. During the refresh, it will be common to see “Radar not available” briefly depicted over regions where coverage is normally provided as shown below for the base reflectivity mosaic from the lowest tilt.

Partial refresh

Partial updates to both the composite reflectivity and base reflectivity from the lowest tilt should be expected when the newest radar broadcast is being processed.

This is because radar data received by the SXAR1 rarely comes as a continuous frame of data. Often this data is broadcast in blocks over a short period of time. This is especially true for the base reflectivity mosaic from the lowest tilt. To avoid holding back the entire radar mosaic until every single byte is received, we decided to provide the newest radar in pieces as it arrives. Whether or not this occurs and how long it takes to provide a complete picture, depends on the amount of radar echoes throughout the entire coverage area. During times of high convective activity or large-scale precipitation, expect the refresh to be a bit slower, typically 20 to 30 seconds.

If you believe in Murphy’s Law, this refresh delay will rear its ugly head at the most inopportune time. If the refresh takes uncomfortably too long while in flight, you can always switch to the other radar depiction in the short term.

Also includes Canada

Unlike ADS-B, the SiriusXM radar depiction from the lowest tilt does include Canadian Doppler radar information as well (Canadian radar is not included in the composite reflectivity mosaic). You won’t see any storm tracks or echo tops depicted by Canadian radar data, but this does extend the radar coverage to the southern most part of Canada for those pilots that fly to this area frequently. In addition to radar, you will see winds and temperatures aloft depicted in Canada as well as METARs, TAFs and PIREPs.

Winds and temperatures aloft

The winds aloft layer is populated by model-based winds (not observations) from the SiriusXM broadcast. These are an accurate representation of the current winds at 3,000 ft MSL up to FL480 at 3,000 ft intervals. This is a similar presentation to what you will find with the winds aloft layer when connected to the Internet. Tapping on any wind barb will provide the wind direction, wind speed and temperature at the altitude selected.

winds-aloft

While in flight, you will see updates to the current winds once each hour. At this time there are no forecasts of winds aloft provided through SiriusXM valid beyond the current time. Consequently, the SiriusXM winds are not used in performance calculations, so you should anticipate using the pack feature to have an estimation of winds aloft along your route while in flight.