Premium: Tropical forcing likely to support warmer late May pattern

Cooler temperatures are on their way tomorrow and early next week with a large anomalous upper-level low moving over the Northeast. Some waves along a stalled frontal boundary may also affect the weather during the middle of next week. But we are still anticipating warmer temperatures in late May.  We have already discussed a number of reasons why we believe more warmth is on the way. But a major influence in this pattern shift will likely be tropical forcing from an active MJO event occurring over the Indian Ocean now.

First, the ensembles guidance still show the AO/NAO turning positive with more troughiness over Greenland/Davis Strait. This will not support as much troughiness, nor an upper-level low over the Northeast or Southeast Canada. Also a -PNA pattern will be developing with troughiness digging over the Southwest US. This will causes heights to build over much of the Central and Eastern United States.

GEFS showing positive height anomalies over the Northeast

The GEFS showing positive height anomalies over the Northeast

The GFS and ECMWF ensembles have been consistent with higher heights and warmer temperatures in the East. May analog years with a +PDO and decaying strong El Nino event, along with the MJO/tropical forcing shifting Martime Continent or Indian Ocean (phases 3/4 ) which are typically warmer phases for the East Coast, leads to high confidence in above average temperatures overall during late May — particularly the week leading up to Memorial Day Weekend.

200mb Velocitiy Potential anomalies show the MJO with tropical forcing over Indian Ocean and Indonesia over the next two weeks

200mb Velocitiy Potential anomalies show the MJO with tropical forcing over Indian Ocean and Indonesia over the next two weeks (Image credit. Michael J. Ventrice)

 

At this time we aren’t anticipating any heat waves. There are some signs of a +PDO/+ENSO influence with a split flow or -EPO pattern still out West. Thus, troughs could still return over Southeast Canada downstream of a ridge building into Western Canada, at times. An active southern branch of the jet could lead to a few days with onshore flow, clouds, or rain. But with the AO and NAO still positive, these features have a better chance of being more progressive.

gfs-ens_T2maMean_us_11

Ensemble guidance still shows warmer then average during this period. But a split flow with more heights build out suggest more troughiness moving over the Northeast regions. As we move into the early June period, the MJO/tropical forcing will shown to be weakening at this point. So there is not as much confidence of this warm pattern continuing into early June.

So if you looking forward to warmth, it’s still on the way. We updated the long-range forecast for the next three weeks on Friday evening. We break down what to expect for each week. Stay tuned for a new long-range video and another update on Tuesday. We will also be releasing our Summer forecast to premium members on Friday, May 27th.