Updated: May 1, 2019
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 the Imagery view and take a look at the 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 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™
Weather Systems Engineer
CFI & former NWS research meteorologist