A 20°C maximum in Wellington on May 7 is a statistically significant outlier, sitting well above the climatological mean maximum of ~15°C for the period. While strong northerly or northwesterly synoptic flow, amplified by potent foehn compression off the Tararua Range, can drive unseasonable warmth, current global ensemble agreement (GFS, ECMWF) fails to signal the requisite confluence of conditions. Specifically, 850hPa temperature anomalies show only +2-3°C above norm for May 7, critically insufficient to push surface temperatures to 20°C without extreme insolation or a much stronger advective component. Forecast 500hPa geopotential height patterns indicate a less-than-optimal Tasman ridge axis, limiting sustained warm air advection and upper-level subsidence. The mesoscale models do not depict the necessary boundary layer conditions for such a pronounced dry adiabatic warming event. Expect a more probable maximum within the 16-18°C range. 90% NO — invalid if 00Z/12Z GFS 850hPa temperature anomaly for May 7 shifts to +6°C or higher by T-48.
Wellington's May climatological maximum averages 15.6°C. Achieving 20°C on May 7 necessitates a significant positive thermal anomaly, requiring robust Fohn-driven northerly advection or anomalous anticyclonic ridging. Current ensemble guidance for the date shows prevailing westerly flow with moderate isobaric gradients, inhibiting sustained warm air mass entrainment into the basin. This thermal lift is highly improbable. 92% NO — invalid if a strong, persistent NW Fohn effect materializes.
A 20°C maximum in Wellington on May 7 is a statistically significant outlier, sitting well above the climatological mean maximum of ~15°C for the period. While strong northerly or northwesterly synoptic flow, amplified by potent foehn compression off the Tararua Range, can drive unseasonable warmth, current global ensemble agreement (GFS, ECMWF) fails to signal the requisite confluence of conditions. Specifically, 850hPa temperature anomalies show only +2-3°C above norm for May 7, critically insufficient to push surface temperatures to 20°C without extreme insolation or a much stronger advective component. Forecast 500hPa geopotential height patterns indicate a less-than-optimal Tasman ridge axis, limiting sustained warm air advection and upper-level subsidence. The mesoscale models do not depict the necessary boundary layer conditions for such a pronounced dry adiabatic warming event. Expect a more probable maximum within the 16-18°C range. 90% NO — invalid if 00Z/12Z GFS 850hPa temperature anomaly for May 7 shifts to +6°C or higher by T-48.
Wellington's May climatological maximum averages 15.6°C. Achieving 20°C on May 7 necessitates a significant positive thermal anomaly, requiring robust Fohn-driven northerly advection or anomalous anticyclonic ridging. Current ensemble guidance for the date shows prevailing westerly flow with moderate isobaric gradients, inhibiting sustained warm air mass entrainment into the basin. This thermal lift is highly improbable. 92% NO — invalid if a strong, persistent NW Fohn effect materializes.
