After much of this week has been dominated by a cutoff low pressure system pinwheeling scattered showers through New England and keeping the region very fall-like, the system is now departing to the northeast. This low pressure will now be replaced with a ridge of high pressure building to New England' west. This will bring dry weather, sunnier skies and warmer conditions through the weekend.
This setup will prevent a deep southwest flow from developing initially, which will help keep humidity levels in check through Saturday. Dew points will be increasing over the next few days with Sunday noticeably more humid than Saturday, but truly oppressive humidity is not in the picture for this weekend.
Temperatures will be around seasonable levels, which means a return to more summery conditions over the fall-like weather for the past few days. As a matter of fact, Burlington experienced its first stretch of three consecutive days with highs in the 60s in August since 2014 Tuesday through Thursday. It was basically an anti-heatwave (which is officially three consecutive days of 90+°). The generally seasonable temperatures look to continue with no major warm ups or cool downs on the docket.
As far as the dry weather goes, it should last through at least Sunday night. Later in the day Sunday, a weak trough looks to bring a cold pool of air back to New England aloft. This will bring a round of unsettled weather from Sunday night through Monday. The most unsettled time will likely be Monday afternoon. A widespread washout is unlikely, but scattered to numerous showers and thunderstorms should be around the region.
500mb height anomaly on Monday afternoon showing (very) weak troughing over New England as a cutoff low pressure system moves over the region:
The overall large-scale pattern features less blocking than this past week, so this cutoff low should make a more timely exit than this past week, with a period of nice weather returning shortly after the low's exit. How long that lasts is up in the air as large timing differences remain for the next system next week.
Continuing to look at the large-scale setup, the main driver of the pattern will be a turn to an increasingly positive Eastern Pacific Oscillation (EPO) pattern. The basic setup for a positive EPO is troughing over Alaska with ridging building over Hawaii and often over Canada's Hudson Bay. This sort of setup typically allows mild Pacific air to flood across the United States, keeping much of the lower 48 under mild conditions.
The first photo shows a very general setup for a positive EPO. The second photo shows forecast 500mb heights (which basically depicts where troughing vs ridging will be) for Wednesday:
This setup supports zonal flow, when the jet stream lays generally flatter across the United States. Zonal flow typically sees weaker and lower impact weather systems move across the country rather than high impact systems. This kind of setup also keeps the weather generally moving along rather than getting blocked. For New England, this can mean high and low pressure trading places with neither one dominating for a long period of time.
This could be the case next week with a cutoff low to start things off followed by a period of high pressure. This could be followed by the next weather maker heading beyond the middle of next week. Time will tell how this plays out and what the timing of the highs vs lows will be.
The overall zonal flow for New England is supported by the North Atlantic Oscillation (NAO) forecast to hang around neutral conditions (or around 0). A positive NAO favors ridging and high pressure for New England while a negative phase favors troughing and low pressure. When this is neutral, there is no strong signal for neither ridging nor troughing. This often results in mostly tranquil and low impact weather.
NAO index showing mostly neutral conditions heading into the end of August and start of September (the forecast is the dotted line, the solid black line is the observed index for the past three months:
As always, it's important to remember that this is the large scale setup, and many small scale factors will come into play. These smaller scale factors can always throw a wrench in the overall pattern. While large-scale patterns can give a general look at what's coming, they do not rule above all else. If they did, long range forecasting would be much easier and more accurate.
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