WINTER STORM FCST 11/30/07-12/02/07: IA, NE, SD, MN, MI, WI, IL

This was just issued at 3:22PM CST
From WeatherBug


The National Weather Service In Hastings Has Issued A Winter Storm Watch... Which Is In Effect From Friday Evening Through Saturday Evening.

A Widespread Wintery Mixture Of Precipitation In The Form Of Freezing Rain... Sleet Or Snow Is Expected To Develop Friday Night. The Mixture Of Precipitation Will Continue Into Saturday Morning... Then Warm Air Is Expected To Be Drawn North Into South Central Nebraska. The Precipitation May Transition To Rain From South To North... Or Potentiallly Remain As Freezing Rain If The Warm Air Does Not Arrive. Ice Accumulation Is Possible From The Freezing Precipitation Friday Night Into Saturday.

A Winter Storm Watch Means There Is A Potential For Significant Snow... Sleet... Or Ice Accumulations That May Impact Travel. Continue To Monitor The Latest Forecasts.
 
Is there any sort of maximum near ground freezing height to get freezing rain rather than sleet? Like is there a set 0c height where you become more likely to get sleet than freezing rain?

I guess that would depend on how thick any above freezing level may be too, as far as rain drop velocity...or maybe not. I should head on over to haby hints.
 
First of all, don't use the actual air temperature use the wet-bulb temperature. The air will cool toward the wet-bulb temperature as precip falls into any dry layer aloft and evaporates.

If the Warm Layer Wet-bulb temperature is greater than 3C then you will most likely get complete melting of the snowflake and thus freezing rain is likely. If the maximum warm-layer wet-bulb temperature is between 1-3C then you will only get partial snowflake melting and thus sleet is likely.

This only holds when you are in a non-convective environment.

You can also use the tau method to further refine sleet-freezing rain environments by comparing the mean temperature of the warm layer to the depth of the warm layer.

Edit: The following is taken from a PDF from the HPC on winter weather forecasting. http://www.forwarn.org/temp/tau.gif
 
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My forecast calls for blustery conditions in my area on Saturday and Saturday night with winds of 20 to 35 miles per hour along with rain, freezing rain, sleet and snow plus "chilly" temps. Precip chances stand at 100% for Saturday and 100% for Saturday night, precip accumulations may vary but we will get atleast a trace of something...:D lol With this forecast can I possibly be wrong? :p The Craig Prediction Center has now issued a Messed Up Weekend Weather Watch for the entire state of Iowa along with portions of Nebraska, Minnesota, and Wisconsin. A Messed Up Weekend Weather Watch is issued when the threat of nasty, potentially dangerous winter weather may screw up my plans for this or any other upcomming weekend, please stay tuned to this thread for updates and possible warnings regarding this potentially cra$py weather situation!
 
Mike Hollingshead said:
Is there any sort of maximum near ground freezing height to get freezing rain rather than sleet? Like is there a set 0c height where you become more likely to get sleet than freezing rain?

I guess that would depend on how thick any above freezing level may be too, as far as rain drop velocity...or maybe not. I should head on over to haby hints.
Click to expand...

Well there's two ways to get sleet. The more "traditional" way is when maximum warm layer aloft T (or Tw) is greater than about +0.5°C and less than about +3°C. When snow falls into this kind of a thermal profile, it will only partially melt. it takes less thermodynamic energy to re-freeze a partially melted snowflake as sleet than to re-freeze a completely liquid rain drop. The other more "non-traditional" way is to re-freeze a completely liquid rain drop (after it has fallen through a ~ +3°C or greater maximum warm layer aloft) in the shallow cold layer, if it is cold enough. There are some studies that have tried to correlate these kind of soundings to freezing rain vs. sleet, and from a purely simplified "minimum cold layer temperature" approach, if the minimum near-surface cold layer temperature approaches -6 to -6.5°C, then the probability of liquid freezing back to solid before hitting the ground becomes much greater. The latter is what happened during the central Oklahoma/southeast Kansas sleet storm last Mid-January.

To expand on what Patrick said, he is right in that the more appropriate way to diagnose precipitation type is to use wet bulb temperature vs. temperature. That said, an event that is underway, typically the sounding is pretty much saturated such that Tw and T are almost identical. Since water vapor quantity is a highly varied and hard-to-predict variable above the surface, resultant Tw is going to suffer from uncertainties due to water vapor uncertainties. This is when I develop tools at work, I just use T, b/c it is less sensitive than Tw.
 
Thanks for the advice Patrick and Mike. I'm rather clueless on it yet though, lol. I like the 1/3-5 thing on the haby page for wet bulbing. That may prove interesting/useful come Saturday morning around here.

I don't recall seeing such similar precip forecasts run after run. They almost always look the same each run. Compare that one to the gfs run from this morning Jeff...

gfs_p60_072m.jpg


The speed and the surface low track have been extremely consistent too. The biggest difference is the GFS sfc temps compared to the NAM. It will get nasty around here fast given the current NAM may still be warming things too much. Hell it's already 12 degrees as far south as Mitchell SD now. Back towards the center of the High headed for this region it's single digits below zero. NAM looks like it probably initialized 5 degrees too warm up there(given it shows 0-5 above zero at midnight where it's already -3 to -6......areas in southern SD already to the temps prog'd by midnight for there).
 
Here are some notes from Tim's Weather Forecasting Handbook that I had on my website Mike. They aren't as detailed as the other two explanations you got, but helpful none the less.

Rules of Thumb (Vasquez)
- solids that pass through a warm layer depth of at least 600ft. will melt partially
- solids that pass through a warm layer depth of 1,200ft. will melt completely
- liquid that passes through a cold layer depth of 800ft. will freeze
- liquid that refreezes will always form a frozen droplet. Snow is not formed in this manner

Snow Through a Warm Layer (Vasquez)
(this section is referring to layers of warm air near the surface that snow falls through)
- warm air layer > 1,200ft. = most likely rain
warm air layer of 900ft. = 50% chance snow 50% rain
warm air layer of 700ft. = 70% chance snow
warm air layer of 300ft. = 90% chance snow
- the freezing level often lowers 500-1,000ft. during the first 1-2 hours after the precipitation begins
falling and may rise again to its original level 3 hours after the layer becomes saturated
- be wary of evaporational cooling in the lower dry levels
- wet bulb temperature is usually about halfway between the temperature and the dewpoint
temerature
- it has been shown that 0.38 inches of liquid equivalent precipitation can contribute 8 degrees F of
cooling towards an air parcel's wet bulb temperature

 
Thanks Mike. Now those terms I can handle a little.

- liquid that passes through a cold layer depth of 800ft. will freeze
Click to expand...

That is pretty much what I was wondering on. I'm actually sort of surprised liquid will refreeze that quickly.
 
Wow, what an interesting storm. The warm air advection wing precip is going to be quite significant. Many areas are going to see all four forms of precip within a 12 hour period of time. Just look at that tremendous WAA at 850mb! Incredible.

The NAM shows quite a wide band of sleet from Nebraska through Iowa, and into Illinois for much of the day on Saturday. Judging by the QPF forecasts, some of these areas may receive quite a bit of sleet. For example, Des Moines and Davenport are forecasted to have sleet for over 6hrs. If that were indeed the case, it would accumulate quite significantly over that duration.
 
New GFS run in and pretty much everything totally on - with no significant changes then what we have here

Expect upgrades to Warnings in the morning package and suspect some of them will be of the Ice Storm variety.
 
Taking a look at the 0z NAM and GFS...

The NAM/WRF takes the low on a further south path, keeping freezing surface temps over the IL area the whole duration of the event. However, it still shows nice WAA advection northward, with 850mb hovering between 6-9 degrees Celsius. This would spell s nice ice storm setup, with surface temps forecast to be sub freezing by at least a couple degrees..

Taking a look at the GFS.. It continues to drag the low further north, this time taking the low into central and northern iowa up into central wisconsin!! The WAA did not cut back too much either, and this would mean a lot of rain for areas in Western IL into north central Illinois and eastern/southeastern IA.

Hopefully things will converge one way or the other here soon..
 
Remarkable how the GEM & EC had this track all week long - what do those foreigners know about long-range models that NCEP doesn't?!? Remember the GFS track from Sunday / Monday's runs that had everyone excited, as it brought the low across Tennessee :>
 
I do not buy into the GFS taking the low from SC Iowa and then up thru wisconsin. I think it will be further to the south; unless the low bombs out and is much stronger than anticipated

Some of our local stations where I live are saying an inch of ice on top of 3-6 inches of snow; then 35mph gusty winds. They are also mentioning thunder will occur with our freezing rain. I'd like to see that!! That would cause major issues for us, in fact, maybe worse than last years Feb/March Ice storm.

I would also agree with morning update that some of these WSW will become Ice Storm warnings (cent. IA and eastward); especially if thinking edges towards NAM QPF output.

In any solution someone is going to get a major ice storm IMO, and I hope it is me!
 
Still looks as though the Sioux City NE to Storm Lake to Spencer IA area looks most at risk for a sig icing event. Models have been pretty consistent warming areas south of a Columbus NE to Harlan IA line to above freezing, though some early morning frozen precip looks likely. I think being in Lincoln I will probably be missing out on this one, I would head North towards my hometown, and maybe even farther North to enjoy the storm, but have to stick around Lincoln just in case we get any snow, as its my weekend to move snow on campus.
 
Temps not verifying????

I am new to the winter weather discussion, usually only involved in the spring season....does anyone put any weight into the models not verifying the temps in the Upper Midwest during the latest 00Z run today....they are anywhere from 8 to 14 degrees warmer than the actual station plots....if I am correct, could this affect the precip type across southern IA and northern MO?
 
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What a mess.

Am Ice Storm Warning is hoisted for extreme NE KS and extreme NW MO.

Winter storm watches are now in effect for SE SD, Central and Eastern NE, Central and southern MN, all of Iowa, NW/NC KS, Northern IL, All of Wisconsin, Extreme N IN, All of MI, Extreme NW OH, West NY and Central PA.

Freezing Rain advisories are in effect for Central and portions of NE KS

Major winter storm developing and this one is giving headaches to forecasters all over this country this morning. There are many, many highly complicating issues at play and this one is going to be a storm that is going to be evolving even at and beyond crunch time, and I would expect frequent changes as the "now" becomes more real then the "forecast".

The biggest issue will be how strong the mid level warm air advection can overcome the shallow cold air at the surface for many locations. I've noticed some weather service offices this morning look at the strength of midlevel WAA and agree with models on a warmer scenario. However historically colder air is not as easy to shallow out as models like to indicate. It seems that in this case just a few degrees difference is going to spell out the difference between a paralyzing ice storm and a cold rainy day with little consequence.

At any note, this is going to be a nailbiter and a challenge to even the most experienced forecasters.

Again, what a mess.
 
The DVN NWS calls for up to 3/4" ice accumulation by Saturday evening. They note that internal gravity waves combined with VERY strong winds just off the surface may induce some wind gusts up to 50mph. That exact thing happened in the icestorm this past February and it was a real mess.

The real question for many of us though is how much of that ice will just be sleet. That would make a huge impact on glazing accumulations.
 
For me I am hearing very conflicting forecasts. Several NWS stations say it will NEVER get above freezing in NE Iowa and Far North NE and NW Iowa in which case, we are all in for it.
Then on the flip side of things some say rain by afternoon? IMO I still think the warmer air is being overdone and the shallow cold air will stick around longer than anticiapted. In fact this morning my local station said 1-2 inches of ice??!! I think they are overdoing that a bit.
Now DVN says possibly 50+mph winds and crippling ice storm and Des Moines is talkin about lightning and thunder with the freezing rain! If the wind and thunder truly occur I think someone in Iowa is going to have an Ice storm beyond what we had last march. Again, hoping is it as my house :):D
 
Glancing at the 12z WRF. It has come in somewhat slower and holds precip into sunday night. It has also shown warmer trends, likely due to the more northern track.

The 9z SREF shows probabilities of Freezing Rain being higher thatn Ice Pellets/Sleet.

The GFS in all honesty hasnt changed much... It is however slightly slower this run.. Not too much to add

Edit: Emergency Manager Conference Call has been scheduled by DVN for their CWA.
 
The NE forecasters discussion continues to be interesting:
BELIEVE THE 00Z MODEL RUNS DID NOT HAVE A GOOD HANDLE ON
THE STRENGTH OF THIS COLD AIR...WITH 1032MB SURFACE PRESSURES INTO
NORTHERN NEBRASKA. CLOUDS ALSO NOW SLIDING OVER THE CWFA. WINDS
PROGGED TO TURN NORTHEAST/EAST TODAY AS WELL. THOSE TWO FACTORS
DON/T SUPPORT MUCH WARMING...AND HAVE LOWERED TODAY/S HIGH TEMPS.
THAT ALSO MEANS THE COLD AIR WILL BE SLOWER TO DISLODGE TONIGHT.
Click to expand...

Naturally, they are discussing only their CWA, but they seem to think that the most significant ice will be north and east of Grand Island (at this time). It does sound like somebody is going to get a repeat of what we got in south-central Nebraska last year (and that ain't pretty):

BUT THE MOISTURE TRANSPORT
ASSOCIATED WITH MASSIVE WARM ADVECTION IS ANOTHER BIG POINT.
PRECIPITABLE WATER VALUES ARE UP TO 4+ STANDARD DEVIATION ABOVE
NORMAL FOR EARLY DECEMBER...AND ABOVE 1 INCH. SUCH ANOMALOUS
INGREDIENTS CAN RESULT IN ANOMALOUS RESULTS (SEE ICE STORM
2006).
Click to expand...

The good news is that this will be much more short-lived event with temps warming significantly rather quickly. (This is NOT altogether good news if you happen to be under a warming traffic light or business sign that decides to drop a significant block/sheet of ice off when you - or your car - is under it.) The most damage will occur where there is the most ice (weight) combined with wind which is often the straw that breaks the camel's (tree's) backs.

As someone who lived through multiple days of no power last year, I hope anyone who lives in the affected area is telling friends/relatives to stock up on food that does not require cooking, candles, etc. (Count your blessings if you have a gas stove/oven and gas water heater). If you live on a farm, you probably know the drill: The pump won't be pumping with no electricity, so fill the bathtub with cold water before going to beddie tonight. In particular, before the storm, check for branches over the lines that bring electrical power from the pole to your house. If that branch gives: you are unplugged from the grid, even if the grid is UP. That's the line that will be the lowest on the power company's priority list to fix.
 
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Really curious to see how the warm air works in tomorrow. Last GFS run puts me in the 8"-10". I'm still thinking may see a fair amount of sleet first really cutting down on the totals up here based on forecasted soundings for MPX. FWIW west metro suburbs started pre-treating the road surfaces this morning up here.

071130001.jpg
 
Well, crap.
The ice storm warning area shifted from this:
icestorm1.png

to this:
icestorm4.png

I can't believe we are going to get it again. This will be the third significant ice storm for this area in 12 months.
 
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