We're into the final month of meteorological summer. It will certainly feel that way at the very start with hot and humid weather. That may not last heading toward the middle of the month, however, as changes appear to be in store. Here's a look at some of the large-scale features for New England this month.
TEMPERATURES
The very beginning of August will feature peak summer heat and humidity continuing for New England. Through this weekend and into early next week (through August 5), west, southwesterly flows will generally dominate. This will keep the high temperatures and high humidity around. The jet stream will still be close enough to New England for unsettled weather to continue this weekend, which will be the only thing keeping temperatures a bit down during the opening days of the new month.
Heading into next week, things start to change. A trough looks to build over eastern Canada, with its influence reaching into New England. At the same time, a strong ridge will likely build over western Canada, keeping the west hotter. The more intense heat that will kick off the month. This change looks to take over in earnest heading into the middle of next week.
500mb height anomaly showing the aforementioned trough over eastern Canada and the strong ridge over western Canada:
As of now, temperatures look to slide to around seasonal averages or skew slightly below average. This should come as a change to a more persistent northerly flow out of Canada may replace the persistent southwest flow that has brought heat and high humidity up the east coast.
Heading toward the middle of the month, a renewed trough may move across the center of the country, bringing continued near average to below average temperatures for New England. The east may continue to be in near zonal flow (when the jet stream lays mostly flat across the region). This would allow the trough in the center of the country to swing eastward.
This zonal flow can be seen by looking at the North Atlantic Oscillation (NAO Index). When the NAO is positive, it can indicate ridging in the east, negative can indicate troughing. When the index is near zero, there is no strong signal for ridging or troughing in the jet stream, leading to zonal flow. As always, it’s important to remember that the NAO index is not the only, tell-all factor, it’s just one piece of the puzzle used to determine potential weather.
NAO Index showing generally zonal flow for the eastern United States to start off August. The solid black line shows the observed index for the past three months while the dotted line at the end shows the ensemble mean heading into August:
Moving into the second half of the month, there are signals that point toward the persistent ridge over the west coast expanding toward the east, to eventually cover much of the northern tier of the United States. This could indicate a renewed warm-up. This timing fits the overall, long-range pattern New England has been in for several weeks, with more sustained heat (that we’re in the middle of right now) on either side of a shorter cool-down.
Overall, we’re predicting a near-average August with a cooler middle flanked by a warmer start and potentially end. Depending on just how hot the warmer times end up being, August could end up running above average for the month as a whole. This would be especially true if the cool-down coming just brings us closer to average rather than well below average. As of now, that looks to be the case.
Only time will tell what the exact temperatures will be, but the level of heat during the warm-ups and how far temperatures go down during the cool-downs will determine how the month looks when averaged together.
PRECIPITATION
New England has entered into an unsettled pattern as of late, and with troughing influencing New England heading into next week, that will likely continue. With zonal flow in place and the jet stream close to New England, this would keep disturbance moving through New England. There is uncertainty next week in just how unsettled it will end up being, that will be determined by the placement of a front and low pressure systems. Overall, New England is skewing toward above average rainfall to start off August.
Overall, it looks like the east coast will be generally wetter than average this month, with areas farther south of New England being more wet relative to average. The unsettled conditions may also aid in keeping temperatures from reaching their full potential. With general ridging expanding east in the second half of the month, it could lead to a drier end.
The one big issue with August precipitation outlooks (and September into October) is the fact that hurricane season will be reaching its peak activity. Whether New England sees a direct impact from a tropical cyclone or remnants moving through, it could drastically alter how precipitation ends up being for the region, as seen last month with the remnants of Beryl bringing 4-8 inches of rain to northern New England.
LOOKING BACK
We predicted a generally above average July for temperatures with only brief cool downs. We also expected there to be very few days with high temperatures topping out at less than 80° for much of New England. Overall, we stated that July was shaping up to be a rather typical July for New England. This just about panned out as expected, with Boston seeing 23 days near or above average and 8 below average days. Burlington, Vermont saw pretty much every day close to or above seasonal averages.
Persistently warm overnight lows helped propel much of New England into a very warm month overall, with Hartford seeing their warmest July on record when factoring in daytime highs and lows.
Daily observed temperatures for Boston (1st image) and Burlington (2nd image). The bars show the observed temperature while the lines show seasonal averages:
There were no strong signals for above or below average precipitation in July for New England. A couple bouts of unsettled weather bringing large rainfall totals to portions of the region propelled those areas to above average rainfall. Overall, New England was very varied in rainfall, with some areas seeing a surplus in rain while others were abnormally dry.
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