Summary:
- The return of high-pressure in the last week of April brought a nice warm-up, but it was short-lived as the circulation has fallen back to troughing with low-pressure areas dipping south along the west coast. The combined cool SSTs offshore and meridional circulation over the west kept temperatures below average* in April for the entire western US, especially in the Great Basin and Rockies.
- April turned dry across the southern portion of the west while much of the PNW made up some of the short-term deficit experienced since the start of the year.
- Continued north-south flow over the west coast is allowing moderately cold core low-pressure areas to spin to our west. The result is the short-term forecast calling for unsettled conditions with temperatures running near seasonal to below average over the west with precipitation spinning onshore bringing spring thunderstorms to light rain over most regions. This should give way to more seasonal conditions in the second half of May.
- A warm-up at the end of April started snow melt, but cooler conditions have slowed the onslaught. Snow Water Equivalents range from 90-110% of normal across the northern states in the west to 200 to 800% above average in California, the Basin, and the rest of the Rockies. Note that I have not mentioned the D word here!
- The 30-day forecast has the odds tilted to California seeing average to below average temperatures and above average precipitation in May while the PNW will likely be warmer than average and see close to average precipitation. The 90-day forecast continues to show a slightly warmer scenario for the PNW, while most everywhere else is likely closer to average. El Niño is forecast to come into play over the next couple of months while the PDO remains in a strong negative phase with cold near-shore SSTs along the western coast of North America. My take is that El Niño will produce a warm summer, but the cold PDO will make it slow to come.
