DAYS 1-5 (SEPTEMBER 18-22)

A saturated air mass has provided a foggy overnight and early morning for a fair portion of the WHW forecast area as the wind field has been very weak from the northeast with maritime modified polar air in place. Other than a few wind gusts around 20 MPH over Cape Cod, the region has weak wind between TS Odette well offshore and a weak approaching cold front from the northwest. The proximity of Odette will not allow the circulation to drag too much drier air into the region today, as well as being on the “humid” side of the approaching cold front. This combines to make for stubborn cloud cover and areas of fog. The fog will gradually dissipate where it is as we slowly warm the air mass enough to elevate the temperatures away from the dew points, but the clouds may be very slow to break, with the greatest chance for at least partial sun being away from the shoreline and especially southern NH through central MA to northeastern CT. If you see much if any sun in southeastern MA, for example, it will be a bonus. All in all though, not a terrible day if you can handle a little humidity without heat, and a lack of sunshine. It’s not a wash-out or a heavy rain event like we’ve seen a fair amount of times during the last 2 1/2 months of summer. The aforementioned cold front will be the introduction to drier air from Canada, and as the boundary crosses the region from northwest to southeast later this evening it can produce a few showers and even a brief thunderstorm in a few locations. Some areas will be missed by the showers, but nobody will be missed by the drier air moving in during the early morning hours of Sunday. This is the beginning of a stretch of very nice weather which is going to last for the remainder of astronomical summer, ending the season on a note much different than it will be remembered for. High pressure will still be centered to the northwest of New England Sunday, with dry air coming in on a modest northerly air flow under lots of sun. Pretty much 100% of the possible sun is expected both Monday and Tuesday as the center of the high slides across northern New England then into the waters east of New England, with enough dry air to keep ocean clouds from forming and moving ashore despite a regionwide northeast to southeast air flow Monday / Tuesday. By Wednesday, this high will have sunk a little further to the south, putting our region into a more southerly air flow. The forecast dilemma for that day when it was in the medium range was the timing of the next trough / frontal system from the west. Typical model differences have been present, with 2 out of 3 major global models holding the next front off until after Wednesday, keeping that day dry, warmer, and a little more humid. The other model, which has faster timing, has indicated shower activity and higher humidity for Wednesday, but the last 2 runs of this model have actually slowed the trough and front down a little, not quite to the timing of the other guidance, but slower nonetheless. Based on this, I’m leaning toward the rain-free scenario with warmer temperatures and a slight up-tick in humidity for the final hours of astronomical summer. The autumnal equinox occurs at 3:20 p.m. Wednesday.

TODAY: Cloudy with areas of fog and patchy light drizzle through mid morning. Mostly cloudy to partly sunny (best shot of sun inland areas) midday-afternoon. Highs 68-75, coolest coast. Wind NE to N up to 10 MPH.

TONIGHT: Variably cloudy. A passing shower possible mid to late evening from northwest to southeast. Slight chance of a thunderstorm. Lows 56-63. Wind N to NW up to 10 MPH but a few gusts around 15 MPH are possible.

SUNDAY: Mostly sunny. Highs 70-77. Wind NW 5-15 MPH.

SUNDAY NIGHT: Clear. Areas of ground fog in lower elevations. Lows 53-60. Wind calm.

MONDAY: Sunny. Highs 72-79, coolest coast. Wind variable to E up to 10 MPH.

MONDAY NIGHT: Clear. Areas of ground fog in lower elevations. Lows 53-60. Wind calm.

TUESDAY: Sunny. Highs 73-80, coolest coast. Wind SE up to 10 MPH.

TUESDAY NIGHT: Clear. Patchy ground fog interior lower elevations. Lows 56-63. Wind SE to S under 10 MPH.

WEDNESDAY: Mostly sunny. Highs 75-82. Wind S 5-15 MPH.

DAYS 6-10 (SEPTEMBER 23-27)

A trough and cold front arriving from the west is expected to bring lots of clouds and periods of showers to the region September 23. Current timing favors cooler/drier air arriving September 24 but a chance of a shower as an upper level disturbance crosses the region from west to east. High pressure brings fair weather to the region for the September 25-26 weekend. Next trough from the west brings an unsettled weather threat for the end of the period.

DAYS 11-15 (SEPTEMBER 28 – OCTOBER 2)

The overall weather pattern should feature a general west-to-east flow. Shower chances are present early in the period and again at the end of the period. We may see a brief but significant shot of cool air in the September 29 to October 1 window.