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

[Dustin Wilcox]

In any case, pay close attention to where you think that snow cutoff line is going to lay... because that's the key.

Key and almost next to impossible to pin down this far out, and might be hard to pin down all the way right up to crunch time. The 12Z NAM is much more agreeing with previous GFS runs and brings the Warm Air Advection much further North then previous runs. The temperature Gradient is sharp. http://grib2.com/animate/nam218nojava.php3?fcsthour=60&type=SFC_SLPTMPDPT&region=C-PLAINS This mornings NAM favors the Sioux city NE East Into IA for a sig Ice Event

 

[Mike Hollingshead]

Anyone looked at just how cold the northern plains are right now?

http://www.rap.ucar.edu/weather/surface/sfc_pir.gif

Low single digits early afternoon in northern ND! That is heading this way(yeah modifying, but jeesh). I'm having a pretty hard time imagining that kind of air sinking southeast, them being quickly slammed back northeast on Saturday with 60s prog'd in se NE! Someone's getting nailed on this one I'm affraid. Very strong warm air advection off the ground returning north above what looks like a very very cold air mass. Sure it will modify some and want to warm from the top down a bit.....but. Should prove very interesting to see what that boundary is like Saturday. RUC is already indicating a fair amount colder than the GFS by 6z Friday.

 

[Chad Cowan]

LOT has just issued a Winter Storm Watch:

.DISCUSSION...
256 PM CST

MAIN CONCERN THIS AFTERNOON IS WINTER STORM EXPECTED ACROSS THE
REGION SATURDAY AFTERNOON AND SATURDAY EVENING. HAVE DECIDED TO
ISSUE A WINTER STORM WATCH WITH THIS FORECAST...MAINLY DUE TO THE
THREAT OF SEVERAL HOURS OF FREEZING RAIN.

...AS THE WARM AIR LIFTS NORTH OVER THE COLDER AIR AT THE SURFACE...
SOUNDINGS SHOW THE POTENTIAL FOR A 3-5 HR PERIOD...PERHAPS LONGER
OF FREEZING RAIN. BEST TIMING ON THIS PERIOD WOULD BE FROM ROUGHLY
20Z SATURDAY TO 03Z SUNDAY. QPF AMOUNTS RANGE WIDELY FROM A TENTH
OR SO ACROSS THE SOUTH TO A HALF INCH OR MORE TOWARD THE IL/WI
BORDER. WHETHER ALL THIS FALLS AS FREEZING RAIN OR AS A MIXED BAG
IS TOO DIFFICULT TO PIN DOWN THIS FAR OUT. BUT THE LESS MIXED
PRECIP...THEN THE MORE SNOW OR AT LEAST POTENTIAL FOR SNOW...AGAIN
WITH THE MOST QPF EXPECTED TOWARD THE WI BORDER. AS WITH ANY
WINTER STORM SITUATION...VARIATIONS IN THE TRACK OF THE STORM AND
EXTENT OF COLD AIR AT THE SURFACE CAN HAVE LARGE IMPACTS ON THE
FORECAST. BUT GIVEN THE THREAT OF A PROLONGED PERIOD OF FREEZING
RAIN...A WINTER STORM WATCH IS REASONABLE.

Henry Margusity at Accuweather is dead set on the system taking a more southerly track than has been shown by recent model runs. His reasoning is that model initialization is poor with not being able to sample the UA where the main storm is (currently off Baja). This, he says, is resulting in initialization of the system being stronger than it actually is which, when it comes ashore and is absorbed by the trough, a weaker storm won't buckle the jet as much as progged resulting in a more southerly track.

 

[Jason Bolt]

DDC also seems confident on a more southerly track as a Watch has been issued for SW KS... Funny to me DDC has been predicting storm to hit their CWA from 5 days out, LOT has been predicting it to hit their area, and if you read the posts from forecasters on here, everyone seems to be wishcasting too, predicting it to be in their backyard. I say SD/NE can have it!

 

[Dick McGowan]

EAX just issued a winter storm watch for tomorrow night through Saturday afternoon for just a couple of counties north of here. They think Highway 36 (in Northern Missouri) and further north are going to get the ice/sleet accumulation the worst. I'm hoping it's further south, I have a class on Saturday and don't feel like going!

SIGNIFICANT ICE AND SLEET ACCUMULATIONS APPROACHING ONE QUARTER OF AN
INCH WILL BE POSSIBLE ACROSS THE WATCH AREA BY LATE SATURDAY MORNING. AS
WARM AIR SURGES NORTHWARD...PRECIPITATION WILL GRADUALLY TRANSITION TO
RAIN BY THE EARLY AFTERNOON HOURS ENDING THE THREAT OF WINTRY WEATHER.
HOWEVER...WITH A RAPID PRECIPITATION RATE EXPECTED ACROSS NORTHERN
MISSOURI DURING THE HEIGHT OF THE EVENT...SIGNIFICANT ICING COULD OCCUR
IN A SHORT DURATION OF TIME.

And from their AFD from this morning:

WHILE THE NAM-WRF IS THE OUTLIER
VERSUS THE GLOBAL WAVE MODELS...FEEL HOLDING THE COLD AIR FURTHER
WEST LONGER IS IN PART DUE TO BETTER BOUNDARY LAYER RESOLUTION...AND
PAST EXPERIENCE SUGGESTS NAM HANDLES THESE TYPE OF SHALLOW COLD
AIRMASSES BETTER THAN THE GFS

 

[Maggie Kahman]

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.

 

[Mike Hollingshead]

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.

 

[Patrick Marsh]

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

 

[Craig Maire II]

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... lol With this forecast can I possibly be wrong? 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 Umscheid]

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.

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.

 

[Jeff Miller]

NEW NAM

http://rap.ucar.edu/weather/model/eta42hr_sfc_prcp.gif
http://rap.ucar.edu/weather/model/eta42hr_sfc_ptyp.gif

New NAM really kicking out the QPF on the wintery mix! Quite a bit more QPF then the last run.

 

[Jeff Miller]

http://rap.ucar.edu/weather/model/eta48hr_sfc_prcp.gif

Ice Storm anyone??? That right there is some pretty incredible ice storm QPF.

 

[Mike Hollingshead]

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...

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).

 

[Mikey Gribble]

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

 

[Mike Hollingshead]

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

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

That is pretty much what I was wondering on. I'm actually sort of surprised liquid will refreeze that quickly.