Understanding Weather – not predicting – 25 January 2013
What happened? What do we believe: our eyes or our charts? For long years, centuries in fact, mankind’s navigators only had eyes, then came instruments to offer values, now most recently we have technology. With only eyes, sailors set out and, most importantly, usually returned. The eyes had it! Today, our reliance on our most up to date presentations has become overbearing: rightly or wrongly? A week ago eyes and charts focussed on a worthy weather pattern so close to our skies. The probability of some rain provided a combination of desire and need. The response was confusing. By the new week, the well-developed equatorial low pressure core had progressed, taking away its rainy potential, leaving the entire country beneath an upper air anticyclonic core extending far to the west and identifiable at all levels throughout the Troposphere. To burden matters further the high pressure core was, throughout, identifiable along the 30oS latitude (the climatological norm) while ensuring a zonal flow (west to east) more in keeping with an ENSO synoptic pattern. The one respite was that an east wind flow persisted above our northern skies. But more optimistic patterns were at best remote and this in mid-January! Both eyes and charts remained unrewarded. Then came Wednesday. Both charts and eyes saw little to enthuse: Search, though, revealed-a satellite image featuring lower cloud levels and surprisingly a distinct vortex circulation off-shore our central coast. This is a cold water area, so this had to be an upper air circulation, despite the limiting chart patterns. By mid-morning cumulus cloud development was evident. Outbreaks of rain occurred on a wide range! What’s coming? Improved prospects for the next few days are based upon the outline of an anticyclonic lower level core breaking away from the major, almost stationary, centre, so that it thrusts around the Cape. The centre of this still-developing core making its track along the 40oS latitude, so that by and during the weekend, easterly flows are persisting, flowing into the sub-continent but also flowing around an obstinate vortex in the Mozambique Channel, but not being absorbed by this system. Hence some moist advection is to be expected, rainy prospects improve accordingly. Yet these outlooks do little to see a collapse or retreat of the considerable upper air high pressure domination based on 30oS, extending westward. Are we alone in this? While the Atlantic outlook only slowly weakens, the positive La Nina-style Pacific Ocean pattern appears to infiltrate eastward, so bringing our synoptics closer to a mid-summer orientation, coupled with equatorial moist air in abundance so close to our northern borders. The rain-bearing potential is still limited in both extent and intensity for these next few days.