Late-Weekend System Update #2

  Good morning folks! As expected, things are a little more clear now regarding our setup for the end of the weekend. There are still uncertainties regarding small-scale details but the overall system is coming together. Freezing rain, "regular" rain, sleet, and snow are all in play across Central Plains into the Ohio Valley and I'm pretty confident we are going to see some problems starting in the Plains sometime late Saturday and locally late Sunday through Monday.

  What we we're going to do tonight is look over how the features I outlined in the previous post evolved over the past 24 hours and take a quick look at what the latest data is suggesting. Tomorrow is still the day that I really want to fine-tune everything and we'll be monitoring as things unfold on Sunday so we're right on track. 

  Quick analysis shows our pair of shortwaves upstream beginning to interact with each other, as expected. The circle labeled #1 is the wave that will bring winter weather to the Central Plains on Saturday and to us late Sunday. The #2 circle is the northern wave that will ultimately pass somewhere near the Great Lakes early next week. A shot of energy from the northern wave is actually wrapping up with energy from the southern wave, essentially seeding it, and I expect to see this energy eventually merge with the southern wave as they cross into the Plains which should result in the initial deepening of the trough. 

 

Current Mesoanalysis Data. Note the Warmer Air Beginning to Surge Northward Into the Southern Plains Creating a Thermal Contrast

 At the surface, high-pressure and an Arctic air mass are still in place across most of the central and eastern CONUS. In the Southern Plains however, southerly surface-flow is advecting warmer air sourced in the Gulf northward. This has set up a thermal gradient between the cold air and the warmer Gulf air. This stationary front is important because when our wave aloft dives southward into this region it will interact with this boundary and trigger cyclogenesis (the development of the surface low-pressure) sometime overnight Saturday. We will then see the Arctic air mass begin retreating with a warm front sliding northward and warm air advection becomes established across much of the eastern part of the Country. This is where some of the critical small-scale uncertainties come in. Now I think we will see some issues with ice and slick spots simply due to sub-freezing ground temps but the extent of what we see has a lot to do with how fast the Arctic air retreats. 

  We know we will have a classic "overrunning" setup (Isentropic Ascent) with warmer air riding up and over the warm front and producing wintry precip to its North. We will have ample moisture with trajectories from the Gulf source region directly into the warm sector. Unfortunately forecasting precip-type relies on the temperature throughout the atmosphere and that's where the greatest spread seems to be (naturally, right?). The GFS is quicker with the retreat and stronger warm air advection, though it has been trending cooler compared to its earlier runs. The NAM is slower and keeps temps right around freezing for this event. The NAM is typically better with air masses like this but the GFS has handled the overall evolution of the pattern better so far. Ensemble means suggest freezing temps for eastern KY during this event but then we quickly get above freezing on Monday. The warmest guidance seems a little too warm based on the way things look right now so I'm treating it as an outlier while cautiously keeping an eye on it.

Blend of Forecast Model Output For Temps For Overnight Sunday through Monday Morning

0z NAM Suggested Temperatures for Monday Morning During Event. I Personally Think The NAM Is Underestimating The Advance Of Warmer Air Northward And As A Result Is Too Slow Moving Our Cold Air Out. 

0z GFS Suggested Temperatures for Moday Morning During the Event. Notice It Has Stronger Warm Air Advection with the Arctic Air Mass Retreating Much More Quickly Than the 0z NAM (And All The Previous NAM Runs Too)

  So based on all of that here's what I think is the most reasonable solution for now. I can't ignore the strength of the warm air advection given decent low-level flow out of the Gulf of Mexico. Surface-winds should be in the vicinity of 20mph. I think we will see temps at or just above freezing which would support rain/freezing rain. I'm leaning toward locations in eastern KY, primarily along/East of I-75, to be the most likely to see freezing rain. In Western and Central KY I think locations along the Western Kentucky Parkway have the highest chance for freezing rain, although I expect a mix to be more likely.

SREF (Short-Range-Ensemble-Forecast) Model Guidance for the Probability of Precip Being Freezing Rain (Valid 9pmCST Sunday). I Actually Placed A Lot Of Weight On The SREF Output Because I Think It Has A Good Handle On The Development Of Our Surface-Low/Frontal-Zones 

SREF (Short-Range-Ensemble-Forecast) Model Guidance for the Probability of Precip Being Freezing Rain (Valid 12amCST Monday)

SREF (Short-Range-Ensemble-Forecast) Model Guidance for the Probability of Precip Being Freezing Rain (Valid 3amCST Monday)

 

0z NAM Model Guidance for Total Accumulation of Freezing Rain Through 6amCST Tuesday 

  Elsewhere in the region, we will see the heaviest snowfall across northern Indiana and Ohio as the northern wave we looked at earlier moves into the Great Lakes and lake-effect snow gets rolling. The heaviest freezing rain could occur North of the Ohio River (North of the warm front) and into the Midwest. 

WPC Forecast Probability of Freezing Rain More Than 0.01" from 6pmCST Sunday through 6pmCST Monday

That's going to do it for this morning's post. It's all about attention to detail. The good news is that we have reached the point where our wave will begin it's trek into the Central Plains within the next 12-18hrs and we can actually observe its development as it does. That's our next real decision point and that's when I'll make another update and fine-tune my precip forecast.

Later...

DKK