Forecast: Unsettled, cooler pattern not far away

Despite the fact that pleasant weather has essentially dominated late summer and Autumn thus far, the pattern of late has trended a bit more unsettled. This is fairly typical for this time of year, as the first signs of wavelength changes are beginning to be seen in the atmospheric pattern and jet stream. However, a more dramatic pattern change is being forecast by most models in the medium range — with noticeable wave amplification throughout the Northern Hemisphere. In the short term, unsettled weather will begin to impact the area as early as Wednesday with a weak system meandering offshore beginning to back towards a coast slightly. Another frontal system back to our west will eventually approach and cross the area around mid-week, bringing in the first of several shots of cooler air.

NAM model showing multiple disturbances in the mid levels (500mb) of the atmosphere later this week.

NAM model showing multiple disturbances in the mid levels (500mb) of the atmosphere later this week.

After a pleasant Wednesday, low clouds are forecast to shift westward (inland) from the Atlantic in response to a weak low pressure system which is meandering offshore. The increasing clouds and moisture, owing to southeast winds in the low levels, will bring the potential for some drizzle and showers along with the clouds — especially along the coast. This will be the beginning of an unsettled pattern, which is expected to last through the end of the week. In addition to the weak system offshore, multiple disturbances over the Mississippi and Ohio Valleys will eventually shift northeastward into the Great Lakes and Canada, dragging cold fronts through our area.

Clearing is expected by the upcoming weekend (we have been on a great stretch of pleasant weekends, in case you haven’t noticed) with temperatures averaging around normal. Highs in the 60’s are likely, with cooler nights and crisp mornings. Should be the peak weekend for fall foliage in our area, especially after the wet and damp conditions during this week. By the middle of next week, however, forecast models are hinting at several reinforcing shots of cooler than normal air. Driven by a large scale pattern change and amplification, a large ridge is forecast to perk up on the West Coast into British Columbia. Across the Eastern 1/3 of the United States, below normal height anomalies at 500mb are forecast by most guidance.

GFS Ensemble mean and spaghetti plot, showing height anomalies or departure form normal at 500mb on the left, and individual ensemble height lines on the right.

GFS Ensemble mean and spaghetti plot, showing 228 hr height anomalies or departure form normal at 500mb on the left, and individual ensemble height lines on the right.

It remains to be seen exactly how direct the cool shot will be — and how it will affect our sensible weather. At this range, forecast models (especially ensembles) are best used as guidance and not taken literally. Notice, on the right side of the above image, that there is still a great amount of variability amongst ensemble members. That being said, the indications are fairly strong that the pattern will flip to feature colder air by late October — with the -EPO and potential – NAO forecast in the medium range. For more on the EPO and average -EPO temperature departures per month in the US, click here. Our article from yesterday also detailed the pattern change.

 

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