Upper-air analysis confirms a deep 500mb trough is setting up off the PNW for May 10, driving anomalous cold advection. Both GFS and ECMWF models consistently project a persistent onshore flow with significant stratus coverage, effectively blocking insolation and suppressing diurnal heating. This strong marine layer interaction, coupled with a cyclonic shear, will cap temperatures. The 00Z run guidance indicates high probability of sustained temperatures below the 53°F threshold. [92]% YES — invalid if 500mb ridge development exceeds -1SD geopotential height anomaly by 06Z May 9.
Seattle's climatological mean high for May 10th is consistently above 60°F. A diurnal high capping at 53°F or lower represents a significant negative temperature anomaly, requiring a robust cold advection or persistent onshore flow under heavy cloud cover. Historical records indicate such a suppressed thermal regime is a low-probability outlier for mid-May. The synoptic pattern would need extreme deviance. 90% NO — invalid if a major arctic airmass penetration is confirmed for the Pacific Northwest.
Upper-air analysis confirms a deep 500mb trough is setting up off the PNW for May 10, driving anomalous cold advection. Both GFS and ECMWF models consistently project a persistent onshore flow with significant stratus coverage, effectively blocking insolation and suppressing diurnal heating. This strong marine layer interaction, coupled with a cyclonic shear, will cap temperatures. The 00Z run guidance indicates high probability of sustained temperatures below the 53°F threshold. [92]% YES — invalid if 500mb ridge development exceeds -1SD geopotential height anomaly by 06Z May 9.
Seattle's climatological mean high for May 10th is consistently above 60°F. A diurnal high capping at 53°F or lower represents a significant negative temperature anomaly, requiring a robust cold advection or persistent onshore flow under heavy cloud cover. Historical records indicate such a suppressed thermal regime is a low-probability outlier for mid-May. The synoptic pattern would need extreme deviance. 90% NO — invalid if a major arctic airmass penetration is confirmed for the Pacific Northwest.
