Climate Summary for September 2010

"Not Much to Remember"

William Hurley, EIU WeatherCenter Analyst
October 13, 2010

This summary is a requirment for Mr. Hurley's Independent Study with Cameron Craig.

September in Illinois is a month usually characterized by a fresh change of tempo from the parched summer months. Cool, comfortable air begins to settle into the region, leaves give the opening acts of their annual dance of colors, and people begin to look forward to the coming holiday season. Usually on the drier side, September marks the opening of the autumn season with the Autumnal Equinox, which this year occurred on Sept. 22. Most Septembers are usually counted upon to bring fair weather as well, and this year has more or less been squarely in the middle of the record. Examining the record, the month can be fairly evenly divided into three distinct sections, each lasting for ten days, which will be the primary reference and timeline here.

At the opening of the month, a weak cold front disintegrated across Eastern Illinois, bringing a slight drop in temperature from the last day of August, with high temperatures dropping from 85̊ F on the 31st to 76̊ F on the 1st of September. No rainfall was experienced from this event, however, with the front ceasing to exist shortly after passing through this region. The rain that did fall (to the northwest of the area) though, was quickly returned to the atmosphere however, as a low pressure system quickly spun up over southern Manitoba/northwest Minnesota, pulling warm air to the northern parts of the Great Lakes. In Charleston however, the result was cloud cover for most of the day. In the evening hours, rain began to fall and accumulated to about a third of an inch by the end of the day. All day Thursday, September 2, scattered rain mixed with a few intervals of sun endured as Charleston lay under the influence of a hastily built and slightly disorganized warm front. Resulting precipitation was a little more than a trace. Thunderstorms threatened the area overnight on the 2nd as the cold front approached, however they died down just to the west of Charleston, as they quickly lost energy after the setting of the sun and thus brought only a few passing rumbles of thunder.

By the morning of Friday, September 3rd, the cold front had passed through, dropping low temperatures to around 60̊ F that morning. On the back side of the cold front, a strong high pressure cell pushed into the area and remained entrenched for the next several days, bringing beautiful weather for the Labor Day weekend. High temperatures were in the mid to upper 70’s for the weekend, with partly cloudy skies during the first half of the weekend from upper-level moisture that was left over and wrapping around from the low pressure system. By the end of the long weekend, skies had cleared, and temperatures peaked in the low 80’s on Labor Day.

But while Charleston enjoyed fair weather that Monday, a low pressure system spun up over the Great Plains in a north-south elongated surface trough and classic 500mb area of divergence. Pushing into the area Tuesday morning was a cold front flowing from an already-occluding low pressure system now centered over northern Wisconsin and Michigan, which brought a decent amount of cloud cover and rainfall totaling less than a tenth of an inch. High temperatures for the day were in the upper 70’s.

On the heels of this system though was a ridge of high pressure that brought the air pressure up to around 1016 mb for the remainder of the week with clear to partly skies prevailing Tuesday afternoon through the early AM hours of Friday. Winds were mainly out of the northwest, slowly shifting over to the north and northeast during the middle part of the week, which helped to keep high temperatures in the mid 70’s for the 7th, 8th, and 9th of September. Thus, the opening days of September experienced two low pressure systems with associated cold fronts, neither of which left Charleston with much more than damp conditions and only a brief drop in temperatures.

Moving into the middle of the month, the most noteworthy weather system passed over the area on the 10th and 11th of September. On the morning of Friday, September 10th, overcast skies blanketed Charleston with winds out of the southeast at 10-12 mph. These winds were the product of a low pressure system spinning up over Kansas, whose warm front had yet to reach Charleston (Map 1). The low temperature that morning only dropped down to 55̊ F, matching the dew point, and scattered drizzle ensued. Morning rainfall accumulations totaled .06 inches as the warm front pushed north into the area. Cloud cover continued throughout the day, limiting the daily high to 67̊ F ahead of the warm front. The warm front itself passed over Charleston at the surface level in the early AM hours of Saturday the 11th, where cloudy skies and a southerly wind flow of 8 mph allowed the low temperature to drop only to 63̊ F. This nice, warm setting was smashed in the early afternoon hours of the 11th however, as a cold front pushed into the area, not from the system that brought the warm front, but from a stronger system that had developed and quickly occluded over south-central Canada (Maps 1 and 2). The warm front that had finally reached Charleston overnight stalled just to the north of the area as its’ low pressure system was dissolved and swept aside in favor of the stronger system to the north (Map 2). However, the moisture from this system was already in place over the area, and the encroaching cold front harnessed this energy to drop .45 inches of rain by the afternoon. Dew points for the afternoon quickly dropped from 66̊ to 58̊ F, and air pressure started to rise from a low of 1010 mb. Being on the southern extremity of the cold front though, air temperatures were not profoundly impacted by it. High temperatures for the 11th still managed to reach 79̊ F that afternoon under quickly clearing skies, despite winds out of the northwest as high as 15 mph.

Map 1. Surface Chart of September 10, 2010. NOAA
Map 2. Surface Chart of September 11, 2010. NOAA.

On the backside of the low, a high pressure system brought the air pressure up to around 1019 mb by the afternoon of September 12th, with mostly clear skies that witnessed only scattered clouds and allowed temperature to climb to 83̊F. This was also aided by the movement of the center of the high pressure to the southwest of Charleston, giving the town a west-southwest wind between 5 and 7 mph. This high pressure was the primary weather-driving system for the next several days, lasting until the early morning hours of Thursday, September 16. Air pressure stayed between 1016-1018 mb Monday, Tuesday, and Wednesday, before dropping to 1015 mb and further late Wednesday night/early Thursday morning. High temperatures were in the upper 80’s, with 87̊ F on both Monday and Tuesday, and 88̊F on Wednesday. Skies were predominantly clear for these days as well, with winds mainly from southerly directions between 3-7 mph for the duration; beautiful high-pressure influence weather. The only scattered clouds that did appear were the result of a fast-fading cold front that stalled out over northern Illinois and dissipated as it moved south into the region on Tuesday.

Thursday the 16th though, brought a moderately strong cold front with maximum winds around 16 mph and gusts up to 22 mph. AM low temperatures only reached down to 68̊F from the system’s warm front. Though passing over Charleston in the open stage of cyclogenesis (Map 3), the system lacked any amounts of moisture, with dew points ahead of it around 58̊F in Charleston, and only briefly peaking at 64̊F on the leading edges of the cold front, with no rain falling at any point. Air pressure did not fall below 1011 mb, and the high temperature for Thursday the 16th was still able to reach 80̊F. So clearly, this system was lacking in intensity in the areas that crossed Charleston, as evidenced on Map 4, where a gaping hole in the map shows a lack of precipitation across Illinois, Iowa, and Northeastern Missouri, despite these areas being relatively near to the center of the system, and presumptuously experiencing significant weather events.

Map 3. Surface Chart of September 10, 2010. NOAA
Map 4. Precipitation totals for September 11, 2010. NOAA.

Afterward, high pressure again took hold over the region with partly to mostly clear skies for the 17th, and 18th of September. High temperatures were allowed to rise under the clear skies, from 80̊F on the 17th to 86̊F on the 18th. Low temperatures for both days were around 60̊F. Then overnight Saturday the 18th to Sunday the 19th, a trailing and dissipating cold front settled over the area once more, however this time it passed over Charleston before fully stalling out and dissipating. This limited temperatures to 78̊F for a high on the 19th under mostly cloudy skies, but again, no precipitation was experienced from the system. Winds also only briefly turned to the northwest at 7 mph after comfortable southerly flow had prevailed for the past few days, before returning to southwest winds at 6mph by the morning of Monday the 20th as high pressure sat over the Southern United States. So in brief summation, these middle ten days of September saw three weak cold fronts pass through, only one of which left the area with any measurable rainfall.

The next few days of September witnessed the warmest temperatures of the month, with high temperatures climbing to 90̊F on Monday the 20th and 94̊F on the 21st. These temperatures were the product of southerly flow from a deep high pressure system that extended from the surface all the up into the 500 mb layer for both days (Maps 5, 6, 7, 8). The position of this high pressure to the south and southeast of Charleston during these days allowed clockwise high pressure circulation to pump warm temperatures into Illinois, along with and represented by a warm front (Maps 5 and 7), ahead of a growing low pressure system over the northern Great Plains. Cloud cover for these two days was clear to partly cloudy as well, which allowed plenty of solar radiation to reach the surface and heat air temperatures. Putting all these factors together, it is of little wonder why temperatures were enabled to rise into the 90’s. Overnight temperatures were quite warm too- 68̊F on the morning of the 20th and 22nd, and 69̊F on the morning of the 21st.

Map 5. Surface Chart of September 20, 2010. NOAA.
Map 6. 500mb Chart of September 20, 2010. NOAA.
Map 7. Surface Chart of September 21, 2010. NOAA.
Map 8. 500mb Chart of September 21, 2010. NOAA.
Map 9. Surface Chart of September 22, 2010. NOAA.
Map 10. Surface Chart of September 22, 2010. NOAA.

Conditions briefly cooled off on Wednesday the 22nd as a trailing cold front approached from the system linked with the warm front of the previous two days, which rapidly moved to the northeast across Canada (Maps 5, 7, 9). The passage of this cold front was also marked by the high temperature reaching 85̊F in the morning hours, with rapid cooling having then ensued by 2 pm, when temperatures had already dropped to 77̊F. The southwest winds that had been experienced over the last few days picked up to 10-14 mph that morning, then switched over to a northeast direction at 4 mph that afternoon. Cloudy skies also covered the region with the passage of the cold front, which produced .22 inches of rainfall in Charleston on the afternoon of the 22nd, marking only the second time for the month that any noteworthy precipitation occurred. However, by Thursday the 23rd, hot temperatures had returned, when the high temperature soared to 93̊F that afternoon to set a new record high for the day. As seen only a few days prior, high pressure again dominated to the southeast of Charleston at the surface and the 500 mb, and again the effects were amplified by a warm front that surged northward from a low pressure spinning up over the Great Plains (Map 10). The might of this northward surge of warm temperatures was also able to be observed from the strong southwest winds that accompanied this push, with speeds in excess of 20 mph. On the morning of the 24th, cloudy conditions and southwest winds around 16 mph prevailed as the cold front that spun up over the Great Plains approached the area (Map 10). As the front pushed into the area during the late morning hours, only a trace of precipitation was observed. The strong southwest winds that had been experienced were exchanged for strong northwest winds of 12-13 mph and the cloud cover quickly pushed out of the area. The humidity at the surface also fell somewhat quickly, dropping from a 60̊F dew point on the morning of the 24th to 51̊F by the morning of the 25th, and then down to 47̊F that afternoon. The 24th and 25th were both quite tolerable days, seeing only scattered clouds and high temperatures of 69̊F and 75̊F respectively, as high pressure pushed the barometer up to 1016-1017 mb for the duration of both days. Low temperatures were also quite comfortable, dropping to 51̊F for both nights.

Early in the morning on the 26th of September, a series of light showers passed through Charleston, bringing 0.14 inches of rain. While seemingly oddly timed, these showers came about from an upper-level low pressure system that was being squeezed out of the Jet over the Midwest at that time (Map 11). This brought northerly winds at higher altitudes, which combined with northeasterly winds from a ridge of surface high pressure over the same area, pushed cool air southward over Charleston, causing the modest amounts of moisture in the air to condense and precipitate (Map 12). This marked the third and final time for the month that any even moderately noticeable rain fell over Charleston. This also helped to keep temperatures cool for the 26th, with a high of only 61̊F.
The remainder of the month saw high pressure of 1014-1018 mb dominate the Charleston area, with partly cloudy to clear skies, and winds from the north and west at 3-9 mph. The only two brief variations were when one, when air pressure dropped to 1006 mb on the 28th as a low pressure worked its way up the East Coast just close enough to drop local air pressure on the backside of the low. And two, on the last day of the month, when an extremely weak cold front brought winds of around 15 mph as it pushed through. This cold front was so weak that it did not produce any change in the mostly clear conditions that were prevalent at the time, and only lowered the high temperature of the 30th three degrees lower than that of the 29th. The high pressure that sat over Illinois during the closing days of September was likely welcomed by many people, as the clear skies and modest winds allowed day time temperatures to slowly climb over the course of the 27th-29th, from 69̊F to 75̊F to 81̊F for highs, before that extremely weak cold front dropped temperatures threes degrees to 79̊F to finish the month out.

Map 11. 500mb Chart of September 26, 2010. NOAA.
Map 12. Surface Chart of September 26, 2010. NOAA.

Looking back over the course of the month, few things stand out that would be noticeable to an average person. Though there were a few brief spikes of warm temperatures, mainly around the 20-24th, temperatures were in the upper 70’s for both the opening and closing of the month, and partly to mostly sunny conditions prevailed for the majority of days. Daily high temperatures never dropped below 60̊F either, helping the air temperature to feel rather comfortable throughout the month, and thus not leave any serious lasting impressions. No major storms passed over either, with only three instances of even noticeable rainfall, and hardly a rumble of thunder for the entire month. The only thing that stands out when looking over the records is the high number of weak cold fronts that passed through without producing any precipitation, and which only moderately affected temperatures. But being one of the drier months of the year, this is within acceptable limits, making this month’s weather not much to remember.

References and Further Information

National Oceanic and Atmospheric Administration
Charleston, Illinois Climate Data (EIUWC)
Cameron Craig, cdcraig@eiu.edu