Rain showers should get your attention

Too many pilots see the word “showers” and think it’s just some harmless light rain event.  When, in fact, a forecast for showery precipitation should get your attention since rain showers are a result of a convective process.  In many cases, forecasters often use showery precipitation (SHRA or VCSH) when they have a fair amount of uncertainly about a future convective event.  Forecasters won’t include TSRA or VCTS in a TAF unless they feel certain thunderstorms will impact the airport’s terminal area.

Here’s an example.  Consider you are making a short flight from Cedar Rapids (KCID) in Iowa to Chicago Executive (KPWK) around 12Z on the morning of May 11th.  The previous evening you fire up WeatherSpork and go to the Imagery view and take a look at the new HREF Model graphics.  You notice an area of convection with echo top heights greater than 30,000 feet is forecast to be poised just to the west and northwest of your route by 08Z as shown below. With such a high probability of tops above 30,000 feet MSL, this area of weather is likely to contain dangerous convective turbulence at 08Z.

However, as you forward the loop to 12Z (below) you notice that the high probability of 30,000 ft echo top heights has diminished, indicating this area of convective weather is expected to weaken with time.  Therefore, forecasters also will likely treat this weather system as a weakening area of weather.

So the next morning you take a look at the radar depiction and notice that at 11Z, the pattern to the weather looks remarkably like the echo tops height forecast from the HREF Model you saw the previous evening.

You go to the WeatherSpork Airports view and find the latest TAF issued for Cedar Rapids.  The TAF doesn’t mention any possibility thunderstorms during the morning.  However, you do notice that there’s a forecast for light rain showers (-SHRA) beginning at 15Z.  So what’s that all about?

KCID 111120Z 1112/1212 08015G21KT P6SM OVC050
FM111500 08017G25KT P6SM -SHRA OVC035
FM111800 09017G25KT P6SM OVC020
FM120000 09014G21KT 6SM BR VCSH OVC015
FM120500 05013KT 5SM -SHRA BR OVC012

You then take a peek at the Area Forecast Discussion (AFD) out of the Quad Cities NWS weather forecast office. The aviation section explains the forecaster’s thinking.  The forecaster also believes this was a weakening area of showers and isolated storms. Then, the last sentence of this section goes on to say that thunderstorms were not included in the TAF for KCID because of the limited predictability due to the weakening system.

.AVIATION…(For the 12Z TAFS through 12Z Saturday Morning)
ISSUED AT 525 AM CDT Fri May 11 2018

A weakening area of showers and isolated storms will move through
the area late this morning into early this afternoon and have
timed a 2-3 hour window of rain at each site. Later this afternoon
and tonight, scattered showers and isolated storms will be most
favored at KDBQ, with lesser coverage expected to the south.
KDBQ/KCID/KMLI can expect periods of MVFR ceilings and/or
visibilities beginning as early as late morning. With limited
predictability, have not include mention of thunder in TAFs.

Nevertheless, the showery precipitation forecast is a placeholder for an uncertain thunderstorm event.  If the weather doesn’t dissipate as expected, the forecaster will amend the TAF once the event becomes more certain.  Even if this area of convection doesn’t meet the thunderstorm criteria to make it into the TAF, showery precipitation implies vertically-developed clouds from convective debris of this weakening system and may still contain dangerous convective turbulence.

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

New weather imagery for you jet jockeys

If you are flying in the flight levels this summer and especially the upper flight levels, we have some new static weather Imagery for you!  We recently added to WeatherSpork version 4.3.4 new guidance that is going to be very popular with you jet jockeys out there.   It’s from the High Resolution Ensemble Forecast (HREF) which is updated four times a day and provides two probabilistic forecasts for echo top heights at hourly intervals with a lead time of 36 hours.

Two probabilistic forecasts are available in WeatherSpork.  You will find this in the Imagery view under the HREF Model.  Choices include echo top heights greater than 30,000 feet MSL and echo top heights greater than 35,000 feet MSL.  The height of the echo top is a forecast for the highest simulated radar echo top (top of the precipitation core) with an expected reflectivity not less than 18 dBZ.  Remember, these are forecasts, not actual NEXRAD echo tops.  And note, the cloud tops are often higher.

Notice the scale at the bottom of the chart.  The echo top height forecast depicted on the chart is a calibrated probability.  The highest probabilities are on the right side of the scale (red and purple) with lowest probabilities on the left (green and blue).  So a forecast that’s depicted in red means there’s a 90 to 100% chance at the forecast valid time, there will be tops greater than 30,000 feet (or 35,000 feet depending on the forecast you are using).   Therefore, flying through or near areas of red and purple will likely require flying at an altitude higher than 30,000 feet (or 35,000 ft).  They may be areas you want to avoid.

A flight at 30,000 feet (or 35,000 feet) through areas without colors depicted will very likely be above the highest echo tops or in an area without precipitation forecast.

The valid time is posted at the top of the chart as shown above (underlined in red).  It uses a YYYYMMDD HHMMZ format. For example, the chart abvove is a 33 hour forecast valid at May 9, 2018 at 03Z (20180509 0300Z).  The time on the left side is the initialization time of the model or what is referred to as the “run time” or “cycle time” of the HREF forecast model.

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

Newest enhancements to WeatherSpork – version 4.3.0

WeatherSpork just got even better! WeatherSpork version 4.3.0 is now available in the App Store.  In addition to a few bug fixes, we are excited to tell you that we raised the roof of WeatherSpork to FL450.  You’ll now enjoy in the Route Profile view a depiction of clouds, wind and en route advisories from the surface to 45,000 feet MSL.  In this release we also added cross track distance to the Route Profile view allowing you to see the distance from fly-by airports to the proposed route.  Lastly, we added the High Resolution Ensemble Forecast (HREF) guidance for ceiling and flight category to the WeatherSpork Imagery view.

Watch the video below to see an overview of these enhancements to WeatherSpork. Enjoy!

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

 

New static Imagery from the HREF model

With WeatherSpork version 4.3.0, we’ve extended the static Imagery to include aviation forecasts from the High Resolution Ensemble Forecast (HREF) model.  The HREF is updated four times a day and provides a forecast lead time of 36 hours at one hour time increments.  In WeatherSpork you can find these in the Imagery view right below Graphical MOS under HREF Model as shown below.

These six choices provide the probability of a flight category of Marginal VFR (MVFR), IFR and Low IFR (LIFR) as well as the probability of a ceiling below 3,000 ft, 2,000 ft and 1,000 ft AGL.  In the sample below, the colors represent probabilities shown on the scale at the bottom of the chart.  Colors such as red and purple are indicative of ceilings that are forecast to be less than 3,000 feet as they are in northern Ohio; of course, they could be much less than 3,000 feet in this area.  Keep in mind that non-filled areas do not mean clear skies per se.  This simply means that there is a very low chance the ceiling will be less than 3,000 feet in these areas. This 21 hr forecast is valid at 1500Z on April 28, 2018.

The probabilistic ceiling forecasts above are additive in nature.  In other words if there’s a high probability of a ceiling less than 1,000 feet, there’s also a high probability of a ceiling less than 2,000 feet and 1,000 feet respectively.

Flight category combines both the ceiling and visibility as shown in the table below. These flight categories are sometimes improperly referred to as flight rules in many flight planning apps.

In addition to the probabilistic forecasts for ceiling, the HREF also provides a probabilistic forecast for specific flight categories that include LIFR, IFR and MVFR.  The example below has a forecast lead time of 22 hours and is valid at 1600Z on April 28, 2018.  Similar to the probabilistic ceiling forecasts above, higher probabilities of a MVFR flight category are shown in red and purple colors like you see in Indiana and Ohio in the example below.  Non-filled areas simply mean that the chances of a MVFR flight category are very low.  Unlike the probabilistic ceiling forecasts above, these forecasts are not additive.  That is, in an area where there’s a high probability of a MVFR ceiling, there may be a very low probability of a LIFR ceiling.

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

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