Premium: Cooler than average trend to persist through June
While many long range forecasts have been banging the “heat drum” for several weeks now, that heat has yet to materialize in any fashion throughout the Northeast United States during the first few weeks of meteorological summer. The next few weeks, additionally, are unlikely to offer any chances for prolonged heat or above average temperature departures.
In fact, there are growing signals, from both the hemispheric pattern evolution and climate phenomena, that below normal temperatures and unsettled conditions may return to the area to close out the month of June.
This weekend, temperatures are expected to continue to remain slightly warmer than average. To make things even more pleasant, the humidity is expected to be quite low, as a high pressure area nudges into the area between two disturbances on our opposite sides.
A ridge axis is expected to extend from the Central Plains into the Eastern United States during this time frame — from essentially this evening through Tuesday. A cut-off low (which has been discussed to premium members for several days now) is expected to form off the Southeast Coast late this weekend into early next week, which actually will enhance the southwesterly flow into our area. In terms of sensible weather, this means warmer temperatures with highs in the upper 80’s to lower 90’s.
The next chance for meaningful precipitation will come on Tuesday, as the ridge of high pressure which had been keeping our area dry begins to collapse. This will allow a cold front to surge toward our area from Southeast Canada. There is the potential for some strong or severe thunderstorms if shear and atmospheric forcing can juxtapose with instability and moisture — but forecast models remain inconsistent with this timing currently (an article will be published regarding this potential during this current weekend).
Afterwards, model and ensemble guidance begins to show the ridge over the East Coast breaking down, while the center of the ridge axis retrogrades and becomes established over the Southwestern US. This is an important development of our area. It suggests that troughs and disturbances will have plenty of room to amplify and meander in our area — with cooler than normal temperatures likely.
What’s more? Forecast models indicate that this pattern may continue through Days 11-15, which takes us into early July.
CFS weeklies showing a ridge over Western US and trough over the Eastern during end of June and beginning of July
Going further into last week of June and and into first half of July, the CFS and ECMWF weeklies continues the trend of a stronger ridge over the Southwest US and lower heights throughout the Eastern US. Temperatures could average near or slightly below normal during this period with a +PNA ridge supporting a large trough over Eastern US (the +PNA pattern drives a large ridge along the Western USA Coast). This pattern shift coincides with the MJO coupled with Kelvin wave propagating from the Indian Ocean/Western Pacific then into Western Hemisphere. This MJO propagating into Western Hemisphere occurred already this season — and supported a cooler than normal/active pattern during the first week of June.
CFS 200mb Velocity Potential anomalies indicating tropical forcing returning into the Western Hemisphere during final week of June and first couple weeks of July
It remains to be seen if will this pattern will continue into the second half of July. So far, ENSO conditions remain neutral while weekly departures have actually risen over the past two weeks. The 30-day moving SOI values also currently show neutral ENSO conditions with values remaining in the -7.00 to +7.00 range this month. Compared to the hot summers around this time during 1998 and 2010, weekly sea-surface temperatures departures are running warmer than those previous years. The last models have also trended towards a weak or moderate La Nina later in the year.
Most models showing departures in the Nino 3.4 region at warmer than -0.8C in July. This indicates a very weak La Nina at best for mid-summer
From the summer forecast, our +PDO with decaying moderate to very strong El Nino analogs showed July to still be warmer than normal. So we don’t feel a weaker La Nina will have much impact on our warm forecast for July. We still anticipate the month will average above normal temperatures with more surges of heat and humidity, ahead of troughs — especially later in the month. August still remains the month with the greatest uncertainty in regards to temperatures and precipitation.
As for severe weather, if the aforementioned ridge remains over western or central parts of the United States this will likely shift the baroclinic zone and dynamics farther south and west with a larger trough moving into the Eastern US. EML advection with steep mid-level lapse rates will more likely be occurring over the Ohio/Tennessee Valley and Southeast US regions. The Northeast and northern Mid-Atlantic regions, then, would likely be underneath more stable and cooler airmasses with possibly more stratiform rainfall events.
Stay tuned for more updates on the long-range pattern. We updated our long-range forecast on Friday for the next three weeks. Stay tuned for a new video on the long-range forecast by Sunday.
This article was written by Miguel Pierre and edited by John Homenuk.