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A Sign of the Times: Some readers may be wondering when more persistent autumn temperatures will arrive in the Northeast beyond the chilly mornings we have already experienced during the past week. Saturday afternoon's Weather Prediction Center surface weather chart offered a reminder that the seasonal transition is well underway farther north, with widespread daytime temperatures in the 40s and 50s across parts of western Canada and considerably colder air already present in the Canadian Arctic.

As summer wanes and autumn approaches, the Sun follows an increasingly lower path across the Northern Hemisphere sky and the days become shorter. With less solar energy reaching the surface each day, northern Canada cools increasingly rapidly and colder air masses become more common.

That does not mean the cold air simply advances steadily southward. Its movement depends on the evolving atmospheric circulation. As the jet stream becomes more active and stronger troughs, fronts, and pressure systems develop, some of that increasingly cold Canadian air will periodically be drawn southward into the United States. Eventually, one of those patterns will deliver the first more widespread and persistent taste of autumn to the Northeast and Maritimes.

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Sunday, September 13 – Tuesday, September 15

Overview: A progressive weather pattern will bring one more period of unsettled conditions before quieter weather returns. High pressure that provided Saturday’s dry and warm weather has shifted into the Maritimes, while an upper-level trough and cold front approach northern Maine from Quebec. A separate low-pressure system developing farther south will keep its deeper moisture and heavier precipitation largely south and east of the Saint John Valley.

Clouds will increase today, with scattered showers developing this afternoon and becoming more numerous tonight as the cold front crosses the region. An isolated thunderstorm is possible, but limited instability should prevent organized severe weather. Rainfall amounts are expected to remain lighter than earlier forecasts suggested.

The upper-level trough will keep showers around Monday morning before cooler, drier northwest flow develops. High pressure builds in Monday night, bringing clearing skies, colder temperatures, and the possibility of localized valley fog or frost before dry, seasonably cool weather Tuesday.

Sunday: High pressure that brought Saturday’s sunny and warm weather has shifted east into the Maritimes as a cold front approaches northern Maine from Quebec. Clouds will increase through the day, with scattered showers developing mainly during the afternoon. An isolated thunderstorm is possible, but significant severe weather is not expected. Highs will reach the upper 60s, with south winds around 10–15 mph. Showers become more widespread tonight as the front crosses the Saint John Valley, with a slight chance of evening thunder. Rainfall should generally remain below 0.30 inch. Lows will fall into the upper 40s to lower 50s as winds turn west behind the front.

Monday: The cold front will be east of the Valley, but an upper-level trough crossing northern Maine will maintain showers during the morning. Precipitation should gradually diminish toward midday, followed by increasing dry periods and some afternoon sunshine. Cooler and drier air will arrive on northwest winds, with highs in the lower 60s. Northwest winds will become breezy during the afternoon. High pressure builds in Monday night, bringing clearing skies and diminishing winds. Temperatures will fall into the upper 30s, with patchy valley fog and localized frost possible toward Tuesday morning.

Tuesday: Any early patchy valley fog will dissipate after sunrise as high pressure settles across northern New England. The remainder of the day should be dry with mostly sunny to partly sunny skies and considerably lighter winds than Monday. Temperatures will recover into the mid-60s, close to seasonal normals. Clouds will begin increasing Tuesday night as the high gradually shifts east, with lows moderating into the upper 40s.

Wednesday, September 16 – Saturday, September 19

Overview:  As the upper-level trough and cold front responsible for Sunday night’s showers move east, cool Canadian high pressure will build into northern New England Monday night and Tuesday, providing a brief period of quieter weather. The high will shift east Wednesday, allowing south to southwest flow to develop ahead of the next Canadian low and frontal system. Temperatures will rebound as moisture and cloud cover gradually increase.

The late-week pattern remains somewhat uncertain. Current guidance supports another frontal passage between Thursday and Saturday, bringing a renewed chance for showers to the Saint John Valley. The system currently appears more likely to produce ordinary frontal showers than heavy rain or thunderstorms, with little significant instability indicated. The main forecast question is timing: a faster-moving front would bring showers Thursday followed by earlier clearing, while a slower system could postpone the wetter period until Friday or Saturday.

Wednesday: After any early valley fog dissipates, Wednesday should be mainly dry with a mix of sun and increasing clouds as high pressure moves east and southwest flow develops. Temperatures should rebound into the upper 60s to lower 70s, several degrees above normal for mid-September. Winds will generally be light from the southwest to west. Clouds increase Wednesday night as the next system approaches, with lows remaining relatively mild in the upper 40s to lower 50s.

Thursday: The forecast becomes less certain as the Canadian low and associated front approach. Current NWS guidance introduces a chance of showers while maintaining at least some dry periods. Skies should range from partly sunny to mostly cloudy, with highs generally in the mid to upper 60s. Shower chances should increase as the frontal boundary draws closer, although whether the more organized precipitation arrives during Thursday or holds until Thursday night remains uncertain.

Friday: Friday currently appears likely to remain at least somewhat unsettled as the frontal system moves through or begins pulling east. Periods of showers are possible, but considerable timing uncertainty remains. A faster frontal passage would allow clearing and drier northwest flow to develop during the day, while a slower solution would maintain clouds and showers longer. Temperatures should generally reach the lower to mid-60s, near seasonal normals.

Saturday: Confidence is lowest by Saturday and will depend heavily on the speed of the late-week system. The slower guidance could keep some clouds and lingering showers across northern Maine, while a faster solution would allow high pressure and drier air to become established. At present, a cooler, increasingly dry trend appears more likely as the system eventually moves east, with highs generally around the lower 60s.

Long Range: Sunday, September 20 – Saturday, September 26

Normal temperatures across the Saint John Valley during this period are generally in the lower 60s for daytime highs and lower 40s for overnight lows, with temperatures continuing their gradual decline through September. CPC outlooks favor near-normal temperatures and below-median precipitation across Maine. A developing trough over the Northeast early in the period should gradually weaken as ridging shifts east, while stronger surface high pressure favors drier conditions across northern Maine. Confidence is below average because of the transitional pattern.

Outlooks updated 7-9 AM weekdays & 8-10 AM weekends and holidays. Long range outlooks indicate broader temperature and precipitation trends rather than precise day-to-day forecasts.

Updated every 3-6 hours from NWS Caribou

Weather Notice

Apex Wx forecasts are based on available observations, computer model guidance, and other meteorological data. Conditions and forecast guidance can change, and information displayed here may not reflect the latest official forecast or warning information. Always visit NWS Caribou for the latest official forecasts, watches, warnings, and advisories for northern Maine.

Apex Wx is a private weather information service unaffiliated with NOAA or the National Weather Service and operated by Joseph Becker, PhD. Dr. Becker earned a Certificate in Weather Forecasting from Penn State University and is a trained NWS SKYWARN Storm Spotter.

Credits

• Sun and Moon data from the U.S. Naval Observatory, with additional astronomical information from timeanddate.com.
• NOAA warning banner provided by WillyWeather.com.
• Through December 2026, additional archived Apex Wx weather data are available through the University of Utah's MesoWest.
• Current and archived Apex Wx weather data are available through Synoptic Data, the successor to MesoWest.
• Apex Wx logo by Joanna Becker © 2016. Visit Bushi and Brush Arts Dojo.

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