You are based in Palmerston North and intend to complete a local flight in the Manawatu area this morning. With limited budget, you only want to fly on good weather days so you and your passengers can enjoy it. You’ve been monitoring weather forecasts in the days leading up to the flight and have an idea that there should be a front going through and then conditions should improve. The latest charts are shown here.
These synoptic charts confirm that a front should clear the area by 1 pm. A south to southwest (onshore) flow affects the east of the North Island, but Manawatu should see an improvement due to being sheltered from this direction.
Scenario 1 One A
Reports and Forecasts
There are no SIGMETs in effect. Have a look at the Sanson ARFOR on the right, as well as the TAFs and METARs below.
Your flight is booked for 10 am (NZDT). The front is forecast to move out of the region during the morning. Behind the front, broken stratiform cloud layers are expected to become scattered. Although a base of 500 ft is forecast in rain, this is expected to clear early morning behind the front. By 10 am NZDT, the cloud base is expected to be broken at 2000 ft and should be gradually breaking up (becoming scattered).
Next, have a look at the latest TAFs and METARs.
Tip – Look at the last 6 (or 12) METARs to assess whether conditions are improving or deteriorating.
The latest observation from Ohakea is a SPECI, issued at 2005Z. This was issued because conditions improved from BKN008 to SCT009. There are a set of rules that determine significant changes and when SPECIs should be issued. These rules also apply to when a change can be included in a TAF and when a TAF must be amended.
The Ohakea METARs and SPECIs also have a TREND forecast appended to them. Be aware that a TREND is valid for 2 hours from the issue time of the METAR or SPECI and supersedes the TAF for that period. The TREND forecast appended to the 2005Z SPECI is “NOSIG”. This doesn’t mean NO SIGNIFICANT WEATHER, it means NO SIGNIFICANT change is forecast to the conditions reported in the SPECI.
Cloud bases are currently lower in all three locations in the METARs than what the TAFs indicate.
There is very little wind on the ground at Palmerston North. Between 11am and midday (NZDT) the wind is forecast to increase to 12kt, from 250°T. By the time you land, the wind could be quite different from when you take off. Notice that it is forecast to increase at Ohakea earlier. This is something you can monitor during your flight.
Scenario One B
Satellite and Radar
Have a look at the satellite and radar imagery. The latest images only are shown here, but in MetFlight you would want to look at a loop of a sequence of images. Just remember that MetFlight will display the latest 6 images, so during the morning, some images are from yesterday evening.
Notice the thick slab of cloud over the lower North Island. This is consistent with what METARs are currently indicating.
Ohakea is reporting showers in the vicinity of the aerodrome. These not appearing on either the New Plymouth or Wellington radars because they are below the beam of the radar.
Conditions appear to be slower to improve than forecast. The cloud base is currently 1600ft at Palmerston North, but it is likely that there are areas of stratus with a lower base, given that both Wanganui and Ohakea are reporting 900ft. The TREND on the Ohakea forecast indicates that forecasters are still anticipating an improvement. Once the wind increases, the base should lift due to mixing. However, beware that the developing westerly may advect this low cloud over Palmerston North and bases could temporarily decrease there before improving. Also, because it’s summer, warming of the surface should also cause the base to lift as the morning progresses.
What are you going to do?
This exercise focusses on the lower North Island. Without looking at anything other than the chart shown here, what do you think conditions might be like in New Plymouth at the current time (1 pm NZDT on 9 December)? What about Wellington?
Scenario Two A
It’s likely that you’re thinking both New Plymouth and Wellington should be not too bad in this situation – perhaps a bit windy in Wellington with the isobars squeezing through Cook Strait. You should also be able to anticipate from this chart that conditions in the west of the South Island are likely to be bad with an approaching warm front (warm advection bringing deteriorating cloud base and visibility), while the east of the South Island should have higher bases and be generally not too bad, although turbulence could be a problem.
Gisborne and Hawke's Bay are obvious cloud-free regions, as they appear darker. Most of the North Island has cloud over it, as does most of the South Island. Remember, this image tells you nothing about the base of the cloud. You may have noticed the clear slot just east of the Southern Alps – this is a signature of the northwest arch, so cloud in the east of the South Island is probably wave cloud. From the isobaric chart, you can guess that the bases will be lower in the west than in the east.
Scenario Two B
The infrared shows that cloud tops over the North Island are relatively warm, so the cloud is in the lower levels of the troposphere. We still don’t have any information about the base of the cloud though.
In the east of the South Island, the cloud tops are very cold – less than -50°C in places. This is consistent with the thinking that there’s wave cloud in the east of the South Island.
Cloud tops in the west of the South Island are relatively cold. There could be a deep layer of cloud that extends from the low levels all the way up to where the temperature is around -40°C, or there could be several layers of cloud. We can’t really tell from satellite imagery.
Scenario Two C
There are some very small radar echoes on the New Plymouth radar, but nothing that leaps out at you.
There are a few more echoes on the Wellington radar, mostly around the hills. However, there doesn’t appear to be a lot of precipitation.
So far, we haven’t seen a lot to be concerned about around New Plymouth or Wellington. Finally, take a look at the METARs, TAFs and ARFORs.
Scenario Two D
Reports and Forecasts
It should be obvious reasonably quickly that we won’t be flying in either New Plymouth or Wellington today!
Notice the deterioration in cloud base and visibility at Paraparaumu. Although there isn’t much precipitation on the Wellington radar, visibility has been down to 1400 m. Low cloud is also affecting Wellington airport, although the base isn’t as low due to the surrounding hills and 25-30 kt wind. Meanwhile, at New Plymouth, where the air is coming directly off the sea, the base is down to 300 ft. There is also precipitation at New Plymouth. It must be very light as it is hardly detected by the radar, but visibility is still significantly reduced.
Also important to note is the TAFs for New Plymouth, Paraparaumu and Wellington have all been amended and both the Straits and Te Kuiti ARFORs have also been amended.
What the information given here doesn’t show is that 36-48 hours before this time, a weak warm front moved across the lower North Island and up as far as about Auckland. This dragged a lot of low-level moisture with it. As the next frontal system approached New Zealand (west of the South Island at the time of this example), the developing northwest flow dragged all this low-level moisture southwards again and into New Plymouth, Paraparaumu and Wellington. While the old warm front was too weak to be marked on synoptic charts by this time, (and it had certainly lost its frontal characteristics), the associated low-level moisture remained, and will persist until there is a change in airmass.