Saturday, January 26, 2013

UPDATED: Sunday Breakdown: Many things to look at

Noon update:  A mixture of sleet, freezing rain and even snow has moved into northern Illinois and southern Wisconsin.  What looks like heavier rain or even possibly thunderstorms on the radar (yellow and orange colors) is what we call 'bright banding'.  This is usually an indication of sleet and or freezing rain as the radar beam reflects differently from the precipitation.  Looking at current surface temperatures and we're rising close to 30° as the precip continues to move in. 

Sunday 7:30am Update:  A Freezing Rain Advisory is now in place across all northern Illinois counties.  This replaces the Winter Storm Watch from Saturday evening. 
Freezing rain, mixed with sleet at times, has been falling in eastern Iowa from Burlington to Cedar Rapids.  Current radar trends suggests the leading edge of the precipitation crossing the Mississippi River after 9am and spreading across northern Illinois and southern Wisconsin after 10am.  Temperatures now are in the upper 20s but where the sleet and freezing rain are occuring temperatures are in the 30°-32° range.  It still looks like a 0.10" up to 0.20" of ice accumulation is still possible.  Travel is not advised this afternoon. 

Saturday Evening Update:

9:45 pm Update:  From what has come in of the evening model runs, the going forecast looks pretty much the same.  Temperatures tonight will hold steady in the low to mid 20s.  Of interesting note the NAM model has come in with slightly higher QPF (total precipitation amount) totals by Sunday night.  If temperatures do in fact remain below freezing throughout Sunday afternoon ice accumulations closer to a quarter of an inch, or more, will be likely causing significant icing problems.  Winds will increase as well so it's possible power outages may occur if ice builds up on power lines and tree branches.

Previous forecast discussion:

A Winter Storm Watch (counties in blue) has been issued for northern Illinois beginning Sunday morning and a Winter Weather Advisory (counties in brown) has been issued for Green, Rock and Walworth counties beginning Sunday morning.

An icy Sunday is possible as low pressure currently in the southwest moves northeast late tonight.  As it does, southerly winds will increase by morning warming temperatures just a few thousand feet above the surface close to 40°.  Meanwhile, temperatures at the surface will start in the 20's and rise to roughly 30° by early afternoon.  This setup within the atmosphere poses the risk for sleet but also freezing rain; especially if temperatures at the surface remain below freezing throughout the day - and that's the million dollar question!  Just how long will surface temperatures remain below 32°???  If temperatures warm sooner than anticipated then the threat for ice accumulation will be significantly reduced.  If temperatures are slow to warm then ice accumulations near a quarter of an inch will be likely.

Here's the way it looks right now:
The atmosphere Saturday night and early Sunday morning will likely be too dry to allow for anything falling from the clouds to reach the surface.  However, after about 10 am is when the precipitation will begin to move in from the west.  Initially, it should begin as a mixture of snow and sleet with maybe a little freezing rain.  It looks like the warm layer aloft in the morning may not be deep enough to allow for complete melting to take place.  Southeast winds will increase due to high pressure over the Ohio River Valley.  It's important to note that temperatures in that region this afternoon were in the upper 20s and low 30s and this is the source of where our air mass will be coming from Sunday.  After about Noon it appears as if the invasion of warm air aloft will become fairly significant while temperatures at the surface sit between 30° and 32°.  This is when we could see the wintry mix transition to more of a freezing rain scenario.  Temperatures at the surface look to stay either right at or slightly below freezing until after about 6pm/7pm Sunday evening.  As the temperature warms Sunday night any freezing rain would then switch to just rain.  Again, the big question Saturday night is just how fast, or slow, surface temperatures will warm above that magic 32° mark.  Right now, total ice accumulations look to be anywhere from 0.10" to 0.20".  If temperatures warm sooner then those ice totals will be reduced.  (Let's hope that's the case).  Subtle changes in the atmosphere will be major players for Sunday's weather.  Keep checking back for the all the latest updates, but to be on the safe side try to have any last minute errands finished by tonight. 

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