Some midlatitude weather forecasting rules-of-thumb
E. Linacre and B. Geerts
There are a few rules-of-thumb to guide weather forecasters in estimating future development of the synoptic situation, as shown by isobar patterns (1). These rules were important in traditional weather prediction, prior to reliance on NWP models, but they are still useful.
- The future position of a front can be extrapolated, assuming the same speed as the one observed during the last 12-24 hours. A refinement is to allow for a change of speed depending on the current rate of pressure change. Caution is needed near topography - fronts may decelerate, blocked, and/or deflected by mountains. A rule of thumb is that the front's speed is about 125% of the ground-level cross-frontal wind behind the front.
- Frontal cyclogenesis depends on
- the low-level baroclinicity (i.e. the cross-front temperature gradient),
- the amplitude of an upper-level short-wave disturbance travelling into the frontal trough,
- the regional pattern of pressure, e.g. cyclogenesis is less likely when the upper-level disturbance moves equatorward,
- the region, some being especially vulnerable, e.g. in the lee of higher terrain.
- Significant cold air advection immediately behind a trough at 700 hPa implies a strong and deep front and significant frontal uplift ahead of it, i.e. much precipitation.
- Isallobars (lines joining places with an equal recent change of pressure) point the direction of future movement of a low or high. Lows move parallel to a line between the isallobaric maximum and minimum, from the former to the latter.
- Lows tend to travel in the general direction of the 700 hPa wind, and at about 70% of its speed.
- Larger disturbances tend to move more slowly than small ones.
- Adjacent lows tend to merge.
- Lows advancing towards a large and intense high-pressure cell usually pass around it on the poleward side.
(1) Sturman, A. and N. Tapper 1996. The Weaher and Climate of Australia and New Zealand (Oxford Univ. Press) 476pp.