The Outlook

Throughout the month, normal highs drop eight degrees, falling to the lower 70s. Average logs move from the upper 50s to the upper 40s.

September's average precipitation usually drops below three inches (2.70 is the Dayton area average) for the first time since February. The days most likely to be dry are the 6th, 10th, 11th, 13th, 15th, 19th, 25th, 26th, and 28th, each having a 20 percent chance or less for rain (the 6th and the 28th have less than a ten percent chance).

The wettest September days, those with a 40 percent or better chance for precipitation: the 9th, 18th, 21st, 22nd. September's cloudiest days, those with better than a 40 percent chance for completely overcast conditions throughout the day are: the 12th, 18th, 21st, and 22nd.

The sunniest days, those with at least a 75 percent chance for sun, are the 5th, 6th, 7th, 8th, 9th, 10th, 11th, 19th, 20th, 24th, 25th, 26th, 27th , 28th and 29th, with the 6th, 7th, 8th, 10th, and 28th having a 90 percent chance for clear skies.

The warmest September days, those with a 50 percent chance of highs above 80 degrees, usually occur during the first 11 days of the month, the warmest days of all being September 1st, 2nd, 6th, 7th, 9th, 10th, 11th, and 30th.

The chilliest days of the month, those with better than a 30percent chance for highs only in the 60s, are September 19th, 22nd, 23rd, 24th, 25th, and 26th.

Lows in the 30s occur as early as September 6th, but there is usually little danger of light frost until the end of the month's third week. The September mornings most likely to bring a light freeze are 24th and the 27th (20 percent chance). Each day after the 19th carries about a ten percent chance for lows in the low 30s.
September takes 75 minutes from the day's length. On the first of the month, Yellow Springs has 13 hours and five minutes of daylight each 24 hours. After 30 days, there are only 11 hours and 50 minutes between dawn and sunset.

Frostwatch
The following chart shows the chances that frost will have occurred by the date indicated. Calculations are based on average frequency of freezing temperatures in central Ohio during the month of September. The data can be adjusted by adding five percent for each 100 miles north or south of Dayton(or the 40th Parallel).

Date Chance for Light Frost Chance for Killing Frost
September 1: 5% 0%
September 10: 10% 1%
September 15: 15% 2%
September 20: 30% 3%
September 25: 55% 5%
September 30: 80% 8%

WEEKLY WEATHER
WEEK 1
The likelihood for rain remains at 35 percent though September 3rd, then diminishes to 25 percent on the 4th and 5th, finally dropping to ten percent on the 6th - the lowest of the entire month.
The effects of the first September cold wave usually appear by the 2nd, which is the first day since June 4th that 90s become unlikely. Then on the 3rd: a 55 percent chance for highs only in the 70s, and the chances for frost suddenly become one in a hundred as far south as the Border States.
September 4th is often cool, but warmer conditions typically return on the 5th and 6th. Nevertheless, the second high-pressure system of the month, which arrives between the 5th and 11th, has brought lows into the 30s one year in 20.

WEEK 2
Early fall arrives this week. Temperatures, which began to cool at the pivot time of August 10th, decline more noticeably. Chances for highs in the 90s hold at less than ten percent each day this week, the first time that has happened since the last days of May. Cold 60s occur another ten percent of the time (with the slight possibility of 50s for the first time since June 4th), and 70s and 80s share the remaining 80 percent.
The rainiest days this week are historically the 9th and the 12th, each having a 40 percent chance of showers. The other days of the period carry about half those odds.
Frost is rare at this stage of September, but chances for a light freeze increase on the 13th and 14th as the third high-pressure system of the month comes across the country.

WEEK 3
This third week of September brings one of the most radical autumnal shifts so far in the season. The likelihood for warm 90s or 80s falls sharply throughout the period, with September 18th bringing only a 20 percent chance for highs above the 70s, the first time that has happened since May 6th.
The mornings are chilly, and the possibility of a light freeze grows steadily. Two weeks ago, the odds were high against frost. Now the chance that freezing temperatures will occur in a seven-day period rises to 40 percent. Next week it will be 50 percent; in two more weeks it will be 80 percent, in three weeks almost 100 percent.
Each day this week brings at least a 30 percent chance for showers, with the 18th having the highest chance: 45 percent.

WEEK 4
The likelihood of days in the 90s disappears after the 22nd of September all across the northern half of the nation. Even 80s will be gone in only three weeks. The odds for an afternoon in the 50s or 60s this week doubles over those odds last week - to 40 percent.
And the season of light frosts deepens in Ohio, Indiana and southern Michigan; the 24th and the 27th carry a 20 percent chance of a mild freeze—the greatest chance since May 10th. On the 23rd and the 26th, chances for a high below 70 degrees are better than 50 percent, the first time that has happened since May 4th. Precipitation is lightest on the 28th (just a 15 percent chance for showers on that date).

THE DAILY WEATHER HISTORY
September 1
Temperatures today are usually in the 80s (a 60 percent chance) or 70s (a 30 percent chance), with only a ten percent chance for a hot day in the 90s. There is a 35 percent chance for a thundershower, 25 percent chance for completely overcast conditions, 50 percent chance for a pleasantly cool night below 60 degrees.
September 2
Today is another pivot day in the approach of autumn: for the first time since June 4th, the chances for an afternoon in the 90s fall to five percent. Highs are in the 80s seventy-five percent of the time, in the 70s the remaining 20 percent. Morning lows fall below 60 a fourth of the years. Skies are partly cloudy ten years out of a dozen; but a storm develops one third of September 2nds.
September 3
Between today and the 6th there is an increased chance for an afternoon in the 90s (about 15 percent chance). On the other hand, 30 percent of the afternoons are in the 80s, and 55 percent are in the 70s. Skies are clear to partly cloudy 65 percent of the time. Chances for rain: 30 percent. Lows in the 50s come a third of the early mornings, with 40s coming once in a decade.
September 4
Today carries a 25 percent chance for highs in the 90s, thirty percent for 80s, forty percent for 70s. And from today through the 13th, for the first time since the middle of June, there is between a five and ten percent chance for highs only in the 60s. Clear to partly cloudy occur nine days in a dozen; rain falls one year in four. Cool nights below 60 degrees occur four nights in ten.
September 5
The second cool front of the month has often passed through by now, and sun ordinarily dominates the days through the 11th.
Today, 90-degree temperatures come only 20 percent of the time, 80 degrees 40 percent, 70 degrees 30 percent, 60 degrees ten percent. Lows are in the 50s forty percent of the mornings, in the 40s ten percent, and a dip to the 30s is possible (a five percent chance of that). A shower passes through just one day in four.
September 6
Statistically, this is one of the three driest days of September, with only a five to ten percent chance for rain (the 10th and the 28th are the other two days). Highs are in the 90s fifteen percent of the time, in the 80s on 45 percent, 70s twenty-five percent, 60s fifteen percent. Today is the first day of the season on which there is five percent chance of light frost. And, for the first time since June 11th, the likelihood of early morning temperatures below 60 degrees rises above 55 percent.
September 7
Today is usually sunny or partly cloudy with little chance of frost. Totally overcast conditions occur only once or twice every two decades, and rain comes just 25 percent of all the years. Highs are in the 70s on 40 percent of the afternoons, in the 80s on 50 percent; there's just a five percent chance for an afternoon above 90 or below 70. Lows remain in the 60s four nights out of ten.
September 8
September 8th is typically a sunny day with only a five to ten percent chance for completely cloudy conditions. Thundershowers, however, occur 30 percent of the time. Today's highs: five percent chance for 90s, forty percent for 80s, fifty percent for 70s, and five percent for 60s. Evening are pleasant, with lows in the 50s half the time, in the 60s the other half.
September 9
Odds for rain are higher today (40 percent chance) than at any time in the first third of September. Still, the sun comes out 90 percent of the days; and highs reach 90 ten percent of the afternoons, reach 80 on 35 percent, and 70 on 55 percent. Frost is quite unlikely through the 12th, but nighttime lows drop below 60 one night in two.
September 10
This is the second of the three driest days in September. The sun almost always shines, and temperatures are typically slightly warmer than yesterday's: 55 percent chance for 80s, and 35 for 70s, ten percent again for 90s. Half the nights fall below 60, but frost almost never occurs.
September 11
Chances for completely overcast conditions rise from yesterday’s five percent up to 30 percent, and showers occur one day in three. Highs seldom reach the 90s but make it into the 80s sixty percent of the afternoons, are in the 70s thirty-five percent, and in the 60s five percent. Evening temperatures dip below 60 sixty percent of the time, but frost almost always stays away.
September 12
This is typically one of the two cloudiest days in September; a full 40 percent of September 12ths are completely overcast, and rain falls four days in a decade. Today also marks the beginning of a decline in percentage of daily sunshine, a decline that continues through December (the year's darkest month). There is a 40- percent chance for highs in the 80s, 45 percent for highs in the 70s, fifteen percent for 60s. Nighttime temperatures are typically mild for the next two days, with 60s occurring more than half the time.
September 13
The 13th and the 14th are the last two days in the year on which there is a 15 percent chance for a high above 90 degrees. Eighties occur 40 percent of the time, 70s another 40 percent, with a cool high in the 60s coming the remaining five percent. Lows are usually close to 60; the third cool wave of the month brings frost, however, almost ten percent of the mornings.
September 14
As the sun moves to within a few degrees of equinox, late summer's grip grows measurably weaker. From now on, it is more likely that highs will occur in the 70s than in the 80s, and an afternoon in the 60s is four times as likely to be recorded as during the first week of the month. Although highs in the 90s occur 15 percent of September 14ths, the same percentage applies to 80s, odds not seen since May 24th. Rain falls 40 percent of the time, and the sun does not appear four years in ten. Odds for frost: one in ten.
September 15
Today is one of the more decisive times in the movement toward autumn, a dramatic shift in low temperatures occurring on this date. Overnight between September 14th and 15th, the chances for a cold dawn in the 40s or 50s leaps from the early September average of 40 percent up to 80 percent. Although frost rarely strikes on this date, the stage is set. The 15th is one of the dryer September days, carrying only a 20 percent chance for rain. There is a 35 percent chance for completely overcast conditions; highs in the 70s come 60 percent of the time; temperatures rise only into the 60s twenty-five percent of the time. One afternoon in four warms above 80 degrees.
September 16
Today is typically one of the coolest and wettest days in early fall. Chances for clouds and rain: 40 percent. There is a 15 percent chance for temperatures in the 80s or 90s, thirty-five percent for 70s, and just a 30 percent chance for highs in the 60s - the greatest chance of that since spring. And for the first time since June 12th there is the possibility of afternoons warming only into the 50s. Four mornings in ten drop below 50 degrees.
September 17
Highs in the 80s come one year in four. Seventies occur 40 percent of the time, 60s twenty-five percent; and from now through the end of the month, the chance of highs just in the 50s remains steady at ten percent. This morning, for the first time in the fall, low temperatures fall into the 40s fifty-five percent of the time. Chances for rain: 35 percent.
September 18
There is a five percent chance for highs in the 90s today, and a ten percent chance for 80s; most of the days, however (65 percent), are in the 70s. The remaining days: fifteen percent chance for 60s, five percent for 50s. Skies are totally cloudy 45 percent of all September 18ths, with rain occurring 40 percent of the time. Nights are typically mild, with lows below 60 occurring more than half the time.
September 19
On average, the 19th is one of the warmer days in middle September; highs in the 90s occur one year in ten, with 80s three in ten, 70s and 60s also three in ten. The possibility of rain declines to 25 percent, and skies are clear to partly cloudy 80 percent of the time.
September 20
Thirty-five percent of today’s afternoons rise into the 80s; fifty percent warm to the 70s, fifteen percent to the 60s. Rainfall occurs one year in three on this date, and the sky is clear to partly eight years in ten. Morning lows dip below 60 degrees 80 percent of the time, the first time this fall that such cold is so likely.
September 21
After today, the chance of a light freeze is a minimum of ten percent per night until the first week of October – when odds quickly increase. Highs on the 21st are in the 80s three days in ten, with the same likelihood for 70s or 60s; one cold afternoon in the 50s fills out the spectrum. Rain occurs 40 percent of the time, as do totally overcast skies. “South wind today means a warm autumn,” says traditional lore.
September 22
This afternoon, 80-degree temperatures occur about 25 percent of the time, and highs reach into the 70s another 25 percent, into the 60s thirty-five percent, and remain in the cold 50s on 15 percent of the afternoons. Morning lows are typically in the 40s or 50s, with mild 60s and chilly 30s the exceptions. Chances for rain and overcast conditions today are 40 percent. Pollen season usually ends with the precipitation near this day.
September 23
Partly cloudy to clear 60 percent of the time with showers coming four years in ten. Lows are in the 30s on 15 percent of the mornings. Afternoon highs are in the 50s five percent of the years, in the 60s fifty-five percent, 70s twenty-five percent, 80s fifteen percent. After today, warm 90s usually do occur until next summer.
September 24
The chance for a high in the 80s is 15 percent today. Seventies come 40 percent of the years, 60s forty percent, 50s five percent. Lows fall below 60 seventy-five percent of the time, and there is a greater likelihood of light frost this morning (20 percent) than on any other morning of September. Rain occurs 30 percent of the days. Sky conditions are clear to partly cloudy eight years in ten.
September 25
There is only a 20 percent chance for rain today, with skies clear to partly cloudy nearly 90 percent of the time. Highs reach 80 two years in a decade, the 70s four years, the 60s four years. Frost comes only five to ten percent of the mornings, but lows are in the 50s one fourth of the time, and in the 40s more than half the time, leaving only a little room for milder 60s. Average temperatures, which varied only one or two degrees at the height of middle summer, now start to fall at the rate of four degrees per week.
September 26
Eighty degree afternoons come 30 percent of September 26ths; 70s are observed 15 percent of the time, 60s forty-five percent, 50s ten percent. Nine days in ten are clear to mostly sunny, and rain falls just once or twice in a decade. Lows in the 30s are rare today.
September 27
Today marks another pivot point for autumn: this is the first time since May 21st that there is a 25 percent chance for highs in the 50s. Temperatures warm to the 60s another 25 percent of the days, to the 70s thirty percent, and to the 80s twenty percent. Chances for rain increase over those for the 26th: three days in ten are wet. Skies are totally overcast 20 percent of the time. There is a 20 percent chance for light frost this morning.
September 28
This is one of the warmest, sunniest, and driest days of late September. Rain falls only ten percent of the years, and the sun fails to shine just five percent of the years. Highs in the 80s come 25 percent of the time, 70s fifty percent, 60s twenty percent, 50s five percent. Frost occurs 15 percent of the mornings. Twenty percent of the nights remain in the mild 60s.
September 29
Only five percent chance for an 80 degree high today, but a 60 percent chance for 70s. Cold days in the 50s come 20 percent of the time, and 60s occur 15 percent of the years. Chances for rain are 30 percent; skies are overcast 25 percent of the days. Nights in the 40s and 50s are the rule, with frost on the lawn one dawn in twenty.
September 30
A surge of warm air overtakes the nation’s midsection half the time, and highs rise into the 80s half of the afternoons on this date - just like they did back during the first two weeks of the month. Chances for highs in the 70s are 25 percent, for 60s fifteen percent, for 50s five percent. Clouds and rain occur four years in a decade. For the first this season, lows drop below 60 a full 90 percent of the nights, but frost occurs only one morning in fifteen.