Let's dive right into it.
The cloud shield has already taken over the area as this large system takes shape in the Plains. It is expected to affect more than 30 million people with various weather elements in the coming days. For us, I think our impact will be relatively minor.
Timing of the moisture still looks to be tomorrow afternoon/night. The problem will continue to be nailing down the temperature at the surface...and dewpoint.
GFS/NAM still very much vary on both----not drastic---but enough that would result in 2 completely different impacts.
Here are the guidance numbers from both the models. I know there is a lot of data on this---but if you look at the top numbers of each outlined box--- those are the forecast temperatures. The GFS(red) drops to about 34 before rising overnight to the 40 degree range. The NAM (blue box) only drops to about 37 and then rises into the 40s.
The soundings from BUFKIT show the same results.
GFS--- you can see by 7pm...it indicates it may start off as some sleet/snow very briefly.
By 10pm, the red (temp) and green (moisture) lines cross over to the right of the freezing line (0°C) . This leads to rain to fall. But notice at the very bottom of the atmosphere---where we are---- it jogs back to the right to the freezing mark. Which leads to freezing rain.
By 1am--- the bottom freezing layer starts to warm as the lines start to head back to the right ...toward 0°.
By 4am--- the entire lower level of the atmosphere is now to the right of 0°, and it is nothing but pure rain.
It is worth noting that we can pull a sounding out of Madison, IN...and it keeps the lower level freezing layer until about 7am.
So that is the GFS. It indicates freezing rain after 8pm along and north of I 64. All rain by the morning commute Friday. Ice accumulationsof .10 to maybe .15" possible on cars/sidewalks/trees. Main roads should be okay with treatment as salt works really well with temps near that 32 degree mark...and I the the actual temp at the ground may not even reach 32...it may hover at 33/34, but areas elevated like trees/powerlines/porches may be just high enough to see some light icing. Still dangerous if walking around of course.
So now let's look at the NAM.
Its sounding is warmer. It never takes the temp line to the left of 0° for the entire event here in Louisville. All rain.
The Madison, IN sounding however--- does BRIEFLY reach that level near 9/10pm. Then all rain after that.
SO WHICH ONE IS MORE LIKELY?
These situations are always tough, but here the thing to keep in mind with this system. It is progressive. It is always a race between the moisture reaching the ground and the thermometer rising. These events are best handled when nowcasting as they take place as models are great tools....but when you are talking about trying to nail down an event based on a certain degree of temperature---models are not the best. The margin of error for model temps can be as much as 5 degrees in these events. As it stands now, the difference in the NAM/GFS is about 3-4 degrees. Still significant. I saw the NWS is not going to issue any advisories at this time because of that margin of error. I would agree with that. I think southern IN (based on the Madison data) is certainly more at risk for any ice accumulations. Here in Louisville and south---I don't see enough data to support a high concern for such. In fact, when you look at the road temp sensor at the Watterson/Taylor Blvd---the pavement temp is in the 40s!
So even if freezing rain fell in the Metro----only cars/trees would feel that effect---roads likely would stay wet.
So overall---this looks to be a temporary southern IN ice chance...and nothing but pure rain elsewhere. The warming early Friday morning should be substantial enough that we all see temps in the 40s and even 50s with the light rain easing by afternoon.
AFTER THIS SYSTEM....
The blocking pattern takes hold. A low pressure will move in toward the Great Lakes by Monday/Tuesday. This will set up a windy and chilly pattern next week. It won't have a crazy amount of cold weather. But flurries/sprinkles certainly look likely with the upper low locked in like that.
Now, because of this upper low----it will prevent a storm from forming over our area like the models showed yesterday. In fact, this upper low looks so strong that is suppresses the storm track way south. This means a cold and snowy pattern all the way through the first weekend of March. I don't see any big snows...but constant snow showers are possible on an almost daily basis. Not exactly the welcome to March I am sure many of you were hoping for. BRRRRR!!
I will definitely take this weather over the tornado outbreak we had at the beginning of March last year.
Posted by: Woof Ruby | February 20, 2013 at 06:19 PM