We’re well aware of the concerns about dust being blown around during a period of stronger winds expected in the Christchurch area from later Tuesday to later Wednesday.

As explained in yesterday’s blog about this, Christchurch city is not expected to be subjected to the severe gales forecast for the Canterbury foothills and over higher parts of Banks Peninsula.

Here’s the most recent issue (11:43am) of the forecasts for Tuesday and Wednesday that we’re providing to those involved in the emergency response. It has as much detail as we can sensibly provide in a worded forecast about the winds and potential for dust.

Tuesday – 01 March 2011
Extensive high cloud. Possibly a few spots of rain late afternoon or evening but probably not enough to wet the ground.
Northeast rising 30 km/h gusts 45 km/h this afternoon, rising to 40 km/h gusts 55 km/h this evening and 45 km/h gusts 65 km/h tonight. Northeast winds could start raising dust from this afternoon.
Rainfall amount: Nil
Maximum temperature 23C
Minimum temperature overnight 18C

Wednesday – 02 March 2011
High cloud. Brief rain around dawn, then skies clearing and becoming fine late morning.
Northerly 45 km/h gusts 65 km/h turning northwest about dawn. Westerly may rise to 55 km/h gusts 80 km/h at times from late morning to early afternoon. Winds easing to westerly 30 km/h late afternoon and winds easing to 10 km/h in the evening. These strong winds during the day may raise clouds of swirling dust.
Note that winds will be stronger about the higher parts of the Port Hills with northwest gales gusting 110 km/h during the morning.
Rainfall amount: 2-5mm
Maximum temperature 27C
Minimum temperature overnight 9C

As of the time of publishing this blog (3:00pm Monday 28 February 2011), there’s a Severe Weather Warning in effect for severe gales in inland areas of Canterbury and the higher parts of Banks Peninsula, but not for Christchurch. The Warning has been issued well in advance of the severe gales: they are expected on Tuesday night and Wednesday morning.

In particular, the Warning states “Severe gales are not expected in Christchurch city but west to northwest wind gusts could reach about 90km/hr from early Wednesday morning until early Wednesday afternoon.” We’re doing our best to nail down the timing: make sure you keep up-to-date with the latest Severe Weather Warning.

Right now, there’s not much wind in Christchurch, and MetService isn’t expecting it to pick up significantly until later on Tuesday.

The threshhold for blowing dust is around 30km/h, though a lot of other factors influence how much dust gets into the air. So, it is reasonable to expect dust clouds around from later on Tuesday (when the wind picks up) to later on Wednesday (when it eases off again).

Wind gusts are the other thing to be mindful of, particularly since there’s quite a few tents up around the Christchurch area. Now’s the time to make sure your guy ropes are well secured.

If you’re interested in why MetService expects Christchurch to be less windy during this event than some other parts of Canterbury, here’s the explanation.

The Christchurch area is expected to experience steady northerlies, around the 40km/h mark, from about sunset on Tuesday 01 March to about sunrise on the morning of Wednesday 02 March. Near-surface effects (in particular, the likely strengthening of an inversion during the night hours) are likely to keep the stronger winds further up in the atmosphere from getting down to the land surface. During Wednesday morning, there’s a general shift in the Canterbury area to winds which are from just north of west, so that they travel across the plains (instead of over the sea) before reaching Christchurch. This, together with the weakening of the inversion through daytime heating (the sun warms the ground, which in turn warms the air above it), is likely to allow the stronger winds from higher up to “mix down” to the surface more easily. This “mixing down” is a turbulent process: that is, it happens from time to time and from place to place, not constantly. In this particular event, the gusts experienced in Christchurch will be little bursts of stronger wind mixed down to the surface.

This is the last post in this series of forecasts for Christchurch. The forecast team are paying extra special attention to the Christchurch forecast and today we’ve added high resolution radar imagery to metservice.com. We’re also putting considerable effort into providing forecast support to those involved in the emergency response.

Mercifully, there has been no severe weather in the Christchurch area since Tuesday’s earthquake. The Severe Weather Outlook and the Severe Thunderstorm Outlook show that none is currently on the horizon. Both of these outlooks are issued at least once a day by members of MetService’s Severe Weather Team; text versions are available by email.

Summary: Friday to Tuesday
Dry weather is expected until the end of Monday; rain is likely on Tuesday. No strong winds expected. Cool some nights.

Friday
Cloudy; a little light rain clearing this evening. Light or moderate southerlies.

Friday night, temperature falling to around 11 C.

Saturday
Fine, apart from morning and evening cloud. Southerlies dying out and moderate northeasterlies developing in the afternoon.

Early morning minimum temperature around 8 C. Afternoon maximum temperature around 18 C.

Sunday
Soime morning cloud, then fine. Northeasterlies, freshening in the afternoon, especially about coastal suburbs.

Early morning minimum temperature around 11 C. Afternoon maximum temperature around 19 C.

Monday
Fine with high cloud. Northerlies.

Early morning minimum temperature around 14 C. Afternoon maximum temperature around 23 C.

Tuesday
Thickening high cloud, with northerlies. Late southerlies and showers.

Early morning minimum temperature around 13 C. Afternoon maximum temperature around 24 C.

Summary: Friday to Tuesday
Rain easing to showers late this afternoon, then clearing. After that, dry weather is expected until the end of Monday; rain is possible on Tuesday. No strong winds expected. Cool most nights.

Friday
Rain, with briefly moderate falls around the middle of the day, easing to showers late afternoon. Southwesterlies, fresh for a time around the middle of the day. This evening, showers clearing (but remaining cloudy) and southwerlies dying away.

Afternoon maximum temperature around 16 C. Friday night, temperature falling to around 11 C.

Saturday
Mostly fine, after early morning low cloud and fog patches clear (expected by 9am). Light winds in the early morning hours; northeasterlies from afternoon, possibly fresh for a time.

Early morning minimum temperature around 8 C. Afternoon maximum temperature around 18 C.

Sunday
Mostly fine at first; cloud increasing during the afternoon. Northeasterlies, freshening in the afternoon, especially about coastal suburbs.

Early morning minimum temperature around 11 C. Afternoon maximum temperature around 22 C.

Monday
Fine and dry with passing cloud. Northeasterlies, fresh during the afternoon.

Early morning minimum temperature around 13 C. Afternoon maximum temperature around 23 C.

Tuesday
Thickening high cloud, with a few spots of rain possible. More persistent rain possible after dark. Becoming warm. Northeasterlies may tend northwest later.

Early morning minimum temperature around 14 C. Afternoon maximum temperature around 27 C.

Summary: Thursday to Monday
Apart from a period of rain on Friday, dry weather through to the end of Monday. No strong winds expected. Cool most nights.

Thursday
Fine weather, with high cloud. Moderate northeasterlies turning to moderate northwesterlies after dark.

Thursday night, temperature falling to around 15 C.

Friday
A change just before dawn to fresh colder southwesterlies and rain. Brief moderate falls around mid/late-morning. Showers clearing toward evening and winds easing, then dying away at night.

Early morning minimum temperature around 14 C. Afternoon maximum temperature around 16 C. Friday night, temperature falling to around 8 C.

Saturday
Mostly fine, though a risk of early morning fog and low cloud. Light winds in the early morning hours; northeasterlies from afternoon, possibly fresh for a time.

Early morning minimum temperature around 8 C. Afternoon maximum temperature around 19 C.

Sunday
Mostly fine at first; cloud increasing during the afternoon. Northeasterlies, freshening in the afternoon.

Early morning minimum temperature around 9 C. Afternoon maximum temperature around 22 C.

Monday
Fine weather, though some high cloud. Northerlies.

Early morning minimum temperature around 13 C. Afternoon maximum temperature around 23 C.

Summary: Thursday to Monday
Apart from a period of rain during Friday, dry weather through to the end of Sunday. No strong winds expected. Cool nights.

Thursday
Fine weather; high cloud increasing from late afternoon. Northeasterlies, freshening for a time this afternoon, especially about coastal suburbs.

Afternoon maximum temperature around 22 C. Thursday night, temperature falling to around 16 C.

Friday
A change just before dawn to fresh colder southwesterlies and rain. Brief moderate falls around mid/late-morning. Showers clearing toward evening and winds easing, then dying away at night.

Early morning minimum temperature around 14 C. Afternoon maximum temperature around 16 C. Friday night, temperature falling to around 8 C.

Saturday
Mostly fine, though a risk of early morning fog and low cloud. Light winds in the early morning hours; northeasterlies freshening in the afternoon.

Early morning minimum temperature around 8 C. Afternoon maximum temperature around 19 C.

Sunday
Mostly fine at first; cloud increasing during the afternoon. Northeasterlies, freshening in the afternoon.

Early morning minimum temperature around 8 C. Afternoon maximum temperature around 22 C.

Monday
Fine weather, though some high cloud. Northerlies.

Early morning minimum temperature around 13 C. Afternoon maximum temperature around 23 C.