Atmospheric pattern may support periods of heat by mid July

Warmer temperatures have returned this week, with highs reaching back into the middle and upper 80s. But the pattern still has yet to change to support any type of heat waves — and it doesn’t look like it will over the next week or so. Another cooling trend during the latter half of next week will bring an end to the month of June. For those who are looking ahead to the warm, hot days of summer, or trying get a heads up on any excessive heat for crops — this may escalate the amount of questions in regards to how this summer will turn out. Are there any changes expected to the atmospheric pattern over the next few weeks?

A ridge amplifying over the West Coast early this week will lead to a large trough over much of the Central and Eastern US for the later half of this week and into the July 4th holiday weekend. This pattern evolution can be largely attributed current +PDO influence and MJO progression during neutral ENSO conditions thus far this meteorological summer. This pattern should continue for another couple of weeks, before any significant changes take place to support more heat over Northeast US..

Ahead of this large trough this week will be a slow-moving cold front that could cause some showers and thunderstorms with possibly some heavy rainfall from late tonight through Wednesday. After this cold front moves passed the region, high pressure builds in briefly for Thursday with temperatures near average in lower or 80s. Then more embedded disturbances moving around this trough could provide more chances for showers and thunderstorms on Friday and then possibly over the July 4th holiday weekend.

The ECMWF, as of Monday morning, is more bullish than the other reliable models with a wave of low pressure developing over the Delmarva region on Saturday night or Sunday. However that model solution currently has little support from it’s own ensembles. So we don’t favor this wetter solution at this time. Much of the weekend could still turn pleasant for outdoor plans and fireworks festivities. But again no major heat appears to be in store at this time for the holiday weekend. High temperatures will likely be near or slightly below average, in the lower to middle 80s,

GFS ensembles showing temperatures near average over the Northeast for July 4th

GFS ensembles showing temperatures close to average  over the Northeast for July 4th. Image credit: PSU Ewall

There are some signs on the guidance we might get into gradually a more hot and humid pattern, beyond this period. First, the GEFS and ECMWF ensembles still suggest a large trough with near normal height anomalies in the means (or averageS) over the east during the during the July 1st to 10th period. This will support warm temperatures that aren’t very far from average for July, assuming we aren’t seeing any days with cloud cover, rainfall and/or an onshore flow dominating.

The GEFS showing lower 500mb heights near normal persisting the 6-10day period over the Northeast US.

The GEFS showing lower 500mb heights near normal persisting the 6-10day period over the Northeast US.

Then both the CFS and ECMWF weeklies start building a ridge with higher height anomalies over the much of the CONUS including into Northeast going into second or third week of July with  +AO/NAO patterns. If correct, this will support more surges of heat and humidity into the Northeast US. This pattern on the CFSv2 do indicate an active, convective pattern with perhaps northwest flow/EML events over this ridge rather than more sustained, or extreme heat.

The CFS weeklies showing an expanding 500mb mean height anomalies into Northeast US for late July period

The CFS weeklies showing an expanding 500mb mean height anomalies into Northeast US for late July period

Next, the MJO is current very amplified over Western Pacific, and will have an influence the atmospheric pattern. First over Pacific, and then into North America and the Atlantic — aiding in keeping a ridge over portions of the West Coast of North America and more troughiness over the East Coast. Model and ensemble forecasts show this MJO heading into the circle of death of next few weeks. Looking at the 200mb Velocity Potential anomalies we also see the strong tropical forcing over Western Hemisphere during first 15 days of July.  With the MJO also going strongly into Western Hemisphere we might also need to carefully monitor for tropical cyclone development over the Atlantic or Eastern Pacific Basins during this period.

CFS 200mb Velocity Potential anomalies showing tropical forcing with the MJO and Kelvin wave over the Western Hemisphere

CFS 200mb Velocity Potential anomalies showing tropical forcing with the MJO and Kelvin wave over the Western Hemisphere

The MJO begins to weaken and progress into Atlantic Ocean and then Africa as we go into the later half of July. This could also be an indication that walker circulation with the La Nina is beginning to take hold. However, we won’t know more about that until another a few weeks as the details in regards to the evolution become more clear. Some more cooling of sea-surface temperatures has occurred over the Pacific recently. But currently 30-day moving SOI values are still supporting little chance from neutral ENSO conditions.

When looking at the Bering Sea Rule (developed by Joseph S. Renken), the evolution appears to suggest more heat and humidity spreading into Central and Eastern US during mid-July. That follows the Robssy wave train over the Pacific, showing anomalous 500mb height anomalies over much the United States between the second and third weeks of July.

500mb positive anomalies indicate stronger ridge over much of the CONUS during the second to third week of July

500mb positive anomalies indicate stronger ridge over much of the CONUS during the second to third week of July

From our summer forecast, the +PDO and decaying moderate to very strong El Nino analogs also support July to trend above normal. These analogs were useful in prediction more heat with well above average temperatures in late May for our region. However, so far June is average to slightly above normal. These same forecasting tools showed June to be slightly below normal. So will have to see the if “cooler” temperatures late this month will be enough to bring anomalies down below normal on average.

Stay tuned for more updates on the long-range pattern. We updated our long-range outlook on Friday for the next three weeks. At this point, we are still maintaining hot July from our summer forecast, with more warmer than average temperatures likely occurring during the second or third week of the month. The next long-range update will be by Tuesday. We also more have video discussions on the medium on long range as well.