Earth-Sun Distance at its Closest Tonight

Happy Perihelion!

Tonight at roughly 7pm local time, the earth was at its closest point to the sun in its annual orbit. This point in the earth's orbit is called its "perihelion." That's right, January 4th (give or take a day or two, depending on the year) is when the earth is closest to the sun! One may not think so, given that this is the winter season. But the distance between perihelion and aphelion (when the sun is furthest away in early July) is only about 1.5 million miles. That's just 1.5 percent of the earth's average distance from the sun -- roughly 93 million miles.

The earth's 23.5 degree tilt on its axis is far more influential in determining seasons than its distance from the sun. At this time of year, the northern hemisphere is tilted away from the sun at about 23 degrees and we get colder temperatures as a result. In the summer, the northern hemisphere is tilted 23 degrees toward the sun and our temperatures are warmer.

The image above shows the inner planets' elliptical orbits with perihelion and aphelion points.

Lack of snow cover really helps our temperatures

The weird and mild winter weather will continue for the next week or so as temperatures are expected to warm right back up into the 40s and even possibly the 50s to our south and west later this week.

We've talked a little about the jet stream and how the positive NAO has had an affect on it so far this year.  Now let's talk about albedo.  Albedo is the amount of solar energy that is reflected from earth back out to space.  Snow and ice have a higher albedo while water, forest and land have a lower albedo.  When the ground is covered with snow roughly 85% of the sun's rays (shortwave energy) is reflected back to space keeping the surrounding atmosphere cool.  When there is very little or no snow on the ground most of the shortwave energy is absorbed by the earth, therefore heating the surrounding air mass.

The fact that we're lacking any sort of snow on the ground combined with our wind source originating from places that have either very little or no snow is what's helping keep temperatures well above average.  Highs the next couple of days will warm into the middle and possibly upper 40s by the end of this week.  This is the time of year when we start to typically see, climatologically speaking, our coldest temperatures.   

Tuesday morning chills followed by end of the week warmth

Temperatures early this morning fell just on either side of 10° with Rockford hitting 9° for the low.  There were even a few spots that had wind chills below zero.  The last time we were that cold was back on the 10th of December.  While this afternoon will be a chilly one, the cold won't last.  The deep trough centered across the U.S. will begin to weaken and push into the Atlantic as mild Pacific air moves back in.  The jet stream pattern by the end of this week will turn a little more zonal pushing the cold air back north and allowing temperatures to modify back above average.  The fact that we don't have any snow cover is also aiding in the warmer temperatures.  Snow has a high albedo, meaning the majority of the sun's rays are reflected back out to space.  When this occurs the surrounding atmosphere doesn't warm.  Since we have no snow on the ground the sun's rays are absorbed into the ground, thus heating it.  Temperatures by the end of the week will reach the low to mid 40s on the heels of a brisk southwest wind.  Unfortunately, with this type of pattern we're in there are no major winter storms expected in our future.  

Stretch of Mild Weather Ends

After experiencing the 8th warmest December on record in Rockford, a reality check is well underway. Tonight's low will be near 8 degrees, some 6 degrees below average. Although this doesn't sound too bad, this will be the coldest we've been all winter.

In fact, today will likely go down as the first below average day we've seen since we were 9 degrees below average on December 10th. The cold will not last, however, as highs will be back into the 30s and lows in the 20s and 10s by Wednesday.

Cold blast felt across much of the U.S.

Not only are we going to experience some bone chilling cold temperatures come Tuesday morning, but much of the central and eastern U.S. will as well.  Temperatures will tumble all the way into the upper 20s and low 30s as far south as the gulf coast!  Freeze and Hard Freeze Warnings have been issued for the state of Florida.  The cold won't last long as highs are forecast to rise above average towards the end of the week.

Wind and chills continue today

Scattered light snow showers will continue this morning tapering off to a few flurries later this afternoon.  Temperatures will go from the low 20s this morning to the teens by late this afternoon.  Combine that with wind gusts as high as 30 mph and wind chills will be lucky to make it past 10°!  Bottom line - it's very cold outside! 

High pressure will move in from the west late today helping erode away some of the cloud cover and relax the winds some.  Not only is this high pulling down an arctic air mass, it also has some pretty dry air associated with it.  The clearing skies, somewhat calming of the winds and dry air will likely cause temperatures to fall into the upper single digits by Tuesday morning.  Winds won't go entirely calm and we'll still have a little breeze from the northwest as high as 10 mph so this could create wind chills as cold as -10° to -5° by Tuesday morning.  There will be some kids who will be heading back to school after the holiday break so it's important to make sure they have on the layers!  Especially for those who have to wait at a bus stop.  Winds Tuesday won't be nearly as gusty as what we experienced Sunday or today but it will still be chilly despite the sunshine.  Highs will remain in the middle 20s.

The Wind Advisory has been cancelled. No more wind gusts up to 45 mph are expected.  It will continue windy overnight with northwesterly winds 18 to 28 mph with gusts to 35 mph. The winds will remain very brisk through midnight on Monday night before quieting down. There could be a few flurries tonight.  The coldest weather so far this season is settling into the Stateline from Monday through Tuesday night.  Low temperatures tonight will drop to near 15 degrees.  Temperatures will not go up much on Monday... rising to only around 20, and wind chills will be in the 0 to -10 range Monday through Monday night into early Tuesday morning. The cold snap will be short lived with a nice warm up starting on Wednesday. After a cold high of only 23 degrees on Tuesday,   The arctic high will shift to the east, and temperatures will not drop much much on Tuesday night. The low will be in the upper teens starting another string of above normal weather.  The high on Wednesday will be warmer than average... in the middle 30's. It will be mostly sunny and warmer on Thursday with a high in the upper 30's, and back into the middle 40's on Friday. No precipitation is expected during this first week of January.