Monday, January 14, 2013

Round 2 of "Winter Weather Week 2013" set for tomorrow

What likely was just round 1 of "Winter Weather Week 2013" brought a whopping 0.04" of freezing rain and sleet this afternoon (with some snow flurries at times), but delivered a TKO to many in the metro as bridges and overpasses iced up and exposed surfaces glazed over.  Even as I write this, some 5 hours since the last sleet pellet hit the ground, bridges over the Mississippi River are backed up for miles.

So why call it round 1?  Because round 2 is set to arrive tomorrow and the potential icing associated with this round is higher than it was for today's weather.  There are a couple of key differences from today's system:

1.  Precipitation amounts are expected to be higher in the metro.
2. Temperatures appear to be slightly warmer.
3. Precipitation duration will be longer.

Nearly all numerical guidance (model data) from this afternoon indicated that the amount of precipitation expected tomorrow will be just under 0.25", while the human-generated forecasts indicate about half that will  fall.  The duration of the precip event, while today was just 4 hours, could be as long as 8 hours.  Daytime temperatures today peaked at 30-31.  Tomorrow, a pessimistic forecast has a high of 32 but computer models indicate it could get to 36.  (Computer models were also warm with today's event by a few degrees...)

18Z GFS model precipitation total forecast from noon-6pm. Much of this precipitation would be in the form of ice.

The National Weather Service has extended the Winter Weather Advisory for most of the metro to 6pm Tuesday, while Marshall and Tate Counties, and points east and south, are now under a Winter Storm Watch.  The delineation between the two lies at the 1/4" accumulation mark - a watch was issued for areas expected to get more than 1/4" of ice, the advisory for less than 1/4".

What we expect:

Period of chief concern: 10am-8pm Tuesday
Main precipitation types: freezing rain, possibly becoming rain for a couple hours in the afternoon and possibly mixed at times with sleet
Intensity of the precipitation: light to occasionally moderate (0.10"-0.20" ice)
Chance of precipitation: 80%.
Temperature during potential precip: 30-32 degrees
Expected impacts: Moderate - glazing of ice on trees and power lines that could cause spotty power outages and hazardous conditions on elevated road surfaces and possibly secondary roadways if precip is moderate for short periods of time.  The most hazardous conditions will be from mid-afternoon through rush hour if temperatures stay at or below 32.

In sum, tomorrow's winter weather event has the potential to be more hazardous than today's, but not quite to "ice storm" level, if temps do not reach 33+ for a couple of hours in the afternoon.  We already know what 0.04" of ice looks like.  Plan ahead for the likelihood of hazardous travel conditions in the afternoon and evening.  Have an ice scraper in your vehicle when you leave home in the morning and plenty of gas in case your afternoon commute takes longer than usual.  Know your alternate routes in case bridges are iced up.  And above all, be patient, drive slowly and carefully, and watch out for the other guy that doesn't read this blog!

Finally, stay with for the latest on this developing winter weather event, including our nowcasting on Twitter and Facebook. The MWN mobile app has StormView Radar with precipitation typed by color so you know what to expect, as well as latest conditions from multiple metro locations, and StormWatch+ severe and winter weather alerts for the latest warnings and advisories. Links to our services are below.

(Last note: for those that like to look at the model data and have looked ahead to Thursday - we have zero confidence in the snowfall forecast from a certain model run earlier today.  It's an outlier and the situation is very fluid. Rest assured, if a round 3 appears later in the week, we'll be sure to provide our "no hype" forecast and opinion on this blog as it approaches.)

Follow MWN on Facebook, Twitter, and Google+
Visit on the web or on your mobile phone.
Download our iPhone or Android apps, featuring a fresh new interface and StormWatch+ severe weather alerts!

No comments: