Monthly Review for July

With the end of another month comes another time to examine what drove the weather during the month and see if any climate extremes were set for the month of July.

Temperature departures from normal via PRISM research.
Temperature departures from normal via PRISM research.

 

July’s average temperatures were above normal from the Northeast into the Southeast and westward into southern portions of the West. On the contrary, below normal temperatures occurred in the North and into portions of the Middle Mississippi Valley. One of the main features that helped to create temperature extremes was a strong high pressure system that slowly moved across the US from the west to the east from around the 22nd of the month to around the 27th of the month. Above normal temperatures were observed across a large portion of the US during that time frame and multiple areas received heat wave(s) as a result. Also record max minimum temperatures were at least forecast for many places along the eastern seaboard from the 25th to the 28th indicating the intensity of the humidity working into the area at the time (as mentioned in a previous post). Furthermore, record July average maximum temperatures were observed in much of the Southeast and even Reading, Massachusetts and Bangor, Maine saw record average maximum temperatures for the month of July.

Temperature rankings for July. Notice the top amounts in the Southeast and Northest.
Temperature rankings for July. Notice the top amounts in the Southeast and Northeast.

On the contrary, below normal temperatures were observed especially in the Northwest. This appears to be a result of an upper level low that originated as energy from a low pressure system in Central Canada, but then slowly meandered west into the Pacific. Upper Level low pressure systems often have cooler weather associated with them as a result of cloud cover and, occasionally, precipitation that has a marine origin. Perhaps Canadian cold air outbreaks contributed to the below normal temperatures, as well, as cold fronts passed by.

Meridional flow contribues to warming in the Southern Plains and cooling the Northeast and Northwest.
Meridional flow contributes to warming in the Southern Plains and cooling the Northeast and Northwest.

 

Also, average monthly meridional flow, or only the north-south component of the wind, shows above normal strength or southerly flow into the Gulf Coast at the same time that below normal strength or northerly flow occurred into Northeast and just west of the Pacific Northwest. This would help to explain most of the temperature anomalies. However, the northerly flow into New England paired with above normal temperatures doesn’t add up. We did end up with above normal temperatures in the Northeast so northerly flow paired with above normal surface temperatures seems to indicate a setup conducive to severe weather. There were numerous days in July that had severe weather in July so that seems like a likely explanation.

Precipitation departures from normal via PRISM research.
Precipitation departures from normal via PRISM research.

 

Above normal precipitation was observed from the Pacific Northwest into the Middle Mississippi Valley. Severe weather, including a squall line and other severe weather in the Middle Mississippi Valley during the week between the 16th and the 23rd seemed to contribute to above normal precipitation there. The Northwest was once again affected by the nearby upper level low with additional precipitation. At the same time, drier than normal weather was observed in southern portions of the United States as well as in the Northeast during the month of July. As a result the drought activity picked up in the Northeast and the chances for severe weather did not chip away at the increasing drought. Also, the increasing drought continued to make it harder to chip away at the drought as lower soil moisture content makes it harder to rain higher amounts.

Analysis for the EPO (PNA) on the top (bottom) during July shows a positive (negative) trend.
Analysis for the EPO (PNA) on the top (bottom) during July shows a positive (negative) trend.
General affects of a negative PNA on temperature in the United States. Via NCSU.
General affects of a negative PNA on temperature in the United States. Via NCSU.

 

Average placement of pressure systems seems to indicate the EPO/PNA pattern controlled the month as a low pressure system in the Northwest and a high pressure system in the Southeast indicates a negative PNA/positive EPO pattern. That’s what the analysis shows for the end of the month as the EPO and PNA trended towards their positive  and negative phases respectively. The average temperature for the month alludes to the fact that the negative PNA was controlling the month by having the placement of above normal temperatures matching what the signal for a negative PNA contributes to.

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