(Premium) Will May Be Cooler and Wetter Than Normal?

High-latitude blocking returned this week. After a warm start early this week, confluence from an upper-level over Southeast Canada forced a frontal boundary south of the region. This produced cooler than normal temperatures for the rest of the week. As a result, April will likely finish near or slightly below normal on average. It appears that May will start not only cool, but also wetter. So will May also be a cooler and wetter month as a whole? Or will a warming, drier trend occur again later in the month?

For next week, high-latitude blocking over the AO and NAO regions will start to weaken and an upper-level low moves out of Southeast Canada.  But an active split flow pattern with Pacific energy undercutting the West Coast ridge remains into next week. This pattern will keep a succession of troughs moving over the Central and Eastern United States for the next week or two. These troughs will supports airmasses coming down from Canada with mostly below normal temperatures. There is still the potential for backdoor cold fronts and cut-off lows with this pattern as well.

The GEFS showing ridge over West Coast and trough amplifying over Eastern US next week

The GEFS showing ridge over West Coast and trough amplifying over Eastern US next week

For rainfall, there will be a better chance for more significant rainfall systems coming out the Southwest or Central Plains. Significant rainfall may fall from a couple waves of low pressure Sunday or Monday. Model and ensemble guidance also shows one system possible along the Mid-Atlantic coast during the middle part of next week. However, with high-latitude blocking over the AO and NAO regions weakening, the flow may become too progressive for a huge storm along the East Coast to produce prolific rainfall in the region. As previous mentioned in our public article earlier, rainfall deficits are between 4″ and 6″ in the local region since March 1st. This has enhanced the brush fire danger in our region. So any significant rainfall over next couple will help alleviate that threat.

So what is could be causing this split flow pattern? We look out into the Pacific Ocean for some possible answers. First: warmer than normal sea-surface temperatures still over the Gulf of Alaska and Northeast Pacific. The warmer anomalies have persisted for the last two or three years. The PDO value for March was +2.40. When the PDO is in a positive phase, this may be enhancing the ridge with the polar jet over the Northeast Pacific and West Coast. During the summer, these -EPO/+PNA patterns have prevented more prolonged heat waves from occurring.

Warm sea-surface temperature anomalies cover the Gulf of Alaska and Northeast Pacific.

Warm sea-surface temperature anomalies cover the Gulf of Alaska and Northeast Pacific.

Secondly, warmer sea-surface temperature anomalies over the tropical Pacific have diminished and cooler waters have been upwelling near Ecuador. This indicates El Nino is weakening and may transition to La Nina later this year. But double-digit negative 30-day and 90-day SOI values as of April 26 still indicate that El Nino conditions still persist and are having influence on the atmospheric pattern over the Pacific. So some El Nino enhancement of the subtropical jet over the Pacific may still be occurring. Also some guidance showing that tropical forcing may return over the Indian ocean and become active during late May and early June. This will need to be monitored for an influence on the atmospheric pattern during this period and beyond.

Some long-range ensemble guidance suggest a mean trough will position itself over the Southwest and thus builds back a ridge in the Northeast again during the second week of May with above normal temperatures returning. But this guidance had shown a similar pattern evolving for the end of this month, which turned out incorrect. So we have lower confidence in the guidance, especially at this range.

That being said, the Southwest trough building back a ridge in the Northeast is supported by our analogs — they were quickly calculated based on years that had a previously somewhat strong El Nino that was declining, while also having a +PDO:

Our May analogs support the long-range model guidance, with a Southwest trough and a ridge in the Northeast. That gives the idea of warmth returning later in the month more credibility. (ESRL)

Our May analogs support the long-range model guidance, with a Southwest trough and a ridge in the Northeast. That gives the idea of warmth returning later in the month more credibility. (ESRL). Precipitation in these years was near normal to slightly above normal.

This is a pretty strong signal; even when removing the very warm analog of 1942, the composite was still above average. This gives the idea of warmth returning later in the month more merit. Precipitation in these analogs was near normal to slightly above normal.

We made a long-range forecast update and along with video discussion for next three weeks on Tuesday. Stay tuned be another long-range forecast update coming tomorrow evening (Friday) with more data to analyze.

 

This article was written and edited by Miguel Pierre and Doug Simonian.