Inflow To Heat Low

[J07]

Saw a good example today (which has happened the last 3 evenings, and will happen tomorrow evening also).

30km/h gusts are not very high, but considering this is a fairly sheltered inland land-site and the pressure gradient is apparently minimal....it is a fair bit.

It's probably more relevant for boaties on the Lakes. A whole day of light winds would go by, and then quite suddenly they would get hit by a 15 knot (possibly 20 knot) southeast wind in the evening. That can be quite significant.

Anyway, the place in question is in the inland south of the South Island, at about 45S. As you can see from the analysis, there is not much in the way of synoptic scale pressure gradient.

However, due to heating throughout the day, a small scale heat low forms inland. Coastal locations are pegged to around 20C by some low cloud and northeast sea breezes. Inland, this is not a problem and by late morning temperatures have reached 27C-29C, and continue to rise during the afternoon with a heat low forming.

The inflow to this heat low reaches inland locations from about 4PM-6PM, and is fairly sudden, and quite a change from earlier in the day. It can drop temperatures by about 3C, and rise dewpoints by about 7C.

Compared to how quickly things change in these places with actual airmass changes (eg 10C-15C in an hour during summertime southerly changes) this is quite small - but it's interesting because there are no synoptic features causing it.

eg at 7PM, the Wanaka site is at 31C with humidity up to 25%, a southeast at 25km/h, gusting to 35km/h.

[J07]

Progression of southeasterly gradually bringing in moisture.

5:30PM: 32/06 - Southeast 19km/h gusting 30km/h

7PM: 31/10 - Southeast 25km/h gusting 35km/h

8PM: 29/11 - Southeast 33km/h gusting 50km/h