Aggressive upper-level ridging is establishing over the Pacific Northwest for May 5th, driving robust positive 500mb geopotential height anomalies. Deterministic GFS 00z and 12z runs consistently project 850mb temperatures soaring to +17°C to +18°C over Western Washington, a significant departure from climate normals. This potent thermal advection, coupled with deep boundary layer mixing and extended daylight hours, ensures efficient downward transport of warmth. While ECMWF deterministic is slightly cooler, the ensemble mean from GEFS and EPS shows a clear probability spike, with over 68% of members indicating surface highs above 78°F for KSEA, with the mean forecast around 79-80°F. The PNA index is firmly positive, reinforcing this high-pressure dominance. The synoptic setup is prime for unseasonable heat.
Aggressive upper-level ridging is establishing over the Pacific Northwest for May 5th, driving robust positive 500mb geopotential height anomalies. Deterministic GFS 00z and 12z runs consistently project 850mb temperatures soaring to +17°C to +18°C over Western Washington, a significant departure from climate normals. This potent thermal advection, coupled with deep boundary layer mixing and extended daylight hours, ensures efficient downward transport of warmth. While ECMWF deterministic is slightly cooler, the ensemble mean from GEFS and EPS shows a clear probability spike, with over 68% of members indicating surface highs above 78°F for KSEA, with the mean forecast around 79-80°F. The PNA index is firmly positive, reinforcing this high-pressure dominance. The synoptic setup is prime for unseasonable heat.
