A Chilling Mid September

Frost and freezing temperatures have made an early appearance in the central and northeastern U.S. this year.  Last week's strong cold front that swept through the central U.S. brought the first freezing temperatures to the Midwest as far south as southern Wisconsin  Another sprawling cold air mass spilled south across much of the country east of the Rockies this weekend ending the growing season for many locations as far south as northern Missouri Sunday morning. Last night frost warnings were in effect for eastern Wisconsin, all of Illinois, Indiana, and Michigan (except areas close to the Great Lakes), and parts of Ohio, Pennsylvania, New York, Maine, West Virginia, Virginia, and North Carolina.

The areas in blue indicate the frost advisories in effect for Sunday night, September 23rd.

Low temperatures this morning reached the low to mid 30s south to the Ohio River, and frost was noted on the grass and rooftops in many areas.  On nights with strong radiational cooling, the vertical temperature gradient can be rather steep. The temperature at grass level can be degrees lower than the temperature at five feet, the approximate height of temperature sensors. If the temperature at sensor level is 33°F, the temperature right at the ground can be below freezing.  Even on such days, there may not be much damage to plants.  Damage to plants depends not only on whether or not it is below freezing, but how long the temperature is below freezing.

Minimum temperatures for the morning of September 24.

The colored dots on the map indicate locations where 32°F has occurred this fall.

The first occurrence of 32°F is about on schedule for the upper Midwest and Northern Plains, and anywhere from a week to two weeks early in the central U.S.  The temperatures in the low and mid 20s are a few weeks early.  The maps below show the extend of freezing weather so far this fall n the central U.S., and the median dates for the first occurrence of 32°F and 28°F.

This map shows the median date of the first occurrence of 32°F in the central U.S. Within a range, the earlier dates typically occur in the north and the later dates in the south. However, terrain and surrounding land use can have a significant influence on overnight low temperatures.

Same as the map above, except for 28°F.

September 11th

As I was on my way in to work this morning it struck me how much today's weather here in central Illinois was like that on the morning of September 11, 2001, one I distinctly remember as many of us probably do.  Today most of the country on the CoCoRaHS precipitation map is showing zeroes, and here in Illinois it was a cool, clear early fall morning without a cloud in the sky.  In 2001 our morning low was 54°F, this morning it was 51°F. The high temperature in 2001 was 82°F, and that's about our forecast high for today.

I thought I would take a look at the Daily Weather Maps for both days, and was amazed at how similar they are.  The map for this morning is about 12 hours "ahead" of the map for 9/11/01.

The surface weather map for 7:00 a.m. EST on September 11, 2001

The surface weather map for 7:00 a.m. EST on September 11, 2012

What started as a magnificent fall day in 2001 turned into a day of horror, shock, sadness, and grief.  As you enjoy the weather on September 11, 2012, pause to remember the victims and heroic efforts of those who risked and lost their lives on 9/11.

Thanks, Isaac!

Hurricane Isaac caused a lot of grief and damage in Louisiana and Mississippi, mostly from the heavy rain and flooding. However, the heavy rains with the remnants of Issac were most welcome in the central U.S. despite the fact they made for a soggy Labor Day weekend.

Most of the Midwest received from two to more than seven inches of rain from Isaac. However, the amount of rain depended greatly on whether or not your gauge happened to be underneath one of the heavier bands of showers and thunderstorms rotating around the center of Issac.

As you might expect, Isaac gave the drought a good kick in the teeth from Missouri eastward. If you compare the U.S. Drought Monitor of August 28 to that of September 4 the areas of improvement are clearly apparent. One thing to note, however, is that "islands" of Severe and Exceptional Drought (darker reds) persist within the larger areas of improvement.  These "islands" are a reflection of the banding of showers and thunderstorms in Isaac's circulation in which some areas received much less rain than areas not far away..

This is an animation created from maps of 24 hour precipitation amounts from
August 31 through September 3, 2012. The maps in this animation
are from the NWS Advanced Hydrologic Prediction Service.

Note that while Issac improved the drought situation by one to two categories in some areas, Severe to Exceptional drought is still firmly entrenched over much of the central U.S.