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

[Andrew Revering]

Well at 30 hours there is a brief period above freezing. Removing the time period above freezing from the equation I'm still calculating out 11-17" of snow from GFS over Minneapolis.

Here's a question for ya...

We've discussed the wet bulb or warmest wet bulb rules... I think Mike Umschied mentioned he prefers to use the environmental temperature... when looking at model data, the models produce precipitation and therefore shouldn't they already be lowering the environmental temperature appropriately, and therefore looking at the wet bulb temperature in model graphics is attempting to reduce the temperature twice?

Or do you still look at the wet bulb of the cooled model air because it's not capturing the actual cloud physics and just the air "around the cloud"

???

 

[Darren Addy]

OAX mentions the DDC/LBF moisture progs (that Jeff noted above) and says:

THERE WERE SOME DISCREPANCIES IN MOISTURE PROGS OVER
THE HIGH PLAINS...ESPECIALLY ABOVE 700MB...WHEN COMPARING 00Z NAM
AND DDC/LBF SOUNDINGS. HOWEVER ABUNDANT MOISTURE WAS DEPICTED ON
UPPER AIR CHARTS AND SATELLITE IMAGERY JUST TO THE WEST OF THESE
AREAS...AND THINK REALITY WILL CATCH UP WITH MODEL FORECASTS IN
THIS REGARD. UPPER TROUGH WAS STILL ROLLING INTO THE SOUTHWESTERN
U.S. WITH 120M HEIGHT FALLS NOTED IN SOUTHERN CALIFORNIA.
IMPRESSIVE TROPICAL MOISTURE PLUME WAS SURGING NORTH AND NORTHEAST
AHEAD OF THIS SYSTEM AS PER WATER VAPOR IMAGERY...AND IS ON TRACK
TO OVERSPREAD EASTERN NEBRASKA LATE TONIGHT. BROAD SCALE ASCENT
WILL BEGIN IN OUR AREA AFTER MIDNIGHT IN DIFFLUENT UPPER
FLOW...INDUCING STRONG CYCLOGENESIS IN THE CENTRAL ROCKIES.
INCREASING SOUTHERLY FLOW WILL PUMP LOW LEVEL MOISTURE NORTH AHEAD
OF THIS SYSTEM...AND STRONG ISENTROPIC UPGLIDE WILL ENSUE LATE
TONIGHT. 00Z NAM IS JUST A TAD SLOWER IN BREAKING OUT
PRECIPITATION OVER THE CWA...

GID has been silent since 5:55 PM CST (probably because they are quite busy puzzling this one out). All I can add is, "that's one heck of a trough coming this way!

 

[Mike Umscheid]

Here's a question for ya...

We've discussed the wet bulb or warmest wet bulb rules... I think Mike Umschied mentioned he prefers to use the environmental temperature... when looking at model data, the models produce precipitation and therefore shouldn't they already be lowering the environmental temperature appropriately, and therefore looking at the wet bulb temperature in model graphics is attempting to reduce the temperature twice?

Or do you still look at the wet bulb of the cooled model air because it's not capturing the actual cloud physics and just the air "around the cloud"

???

Yes, when the model generates precipitation, it is in effect "wet-bulbing itself", which is one reason why I use just the model temperature once precipitation starts in the model. Wet bulb temperature is most useful for me at the onset of precipitation, in any layer of the atmosphere, prior to saturation, of course -- but once the model produces precipitation and saturates the column, the model has "wet-bulbed" itself, which is why it's really no different, in my eyes as an operational forecaster, to use ordinary T versus Tw when forecasting precipitation type. Each model has its own precipitation parameterization schemes, but in my experience, the NAM does a better job of "wet-bulbing" model Temperature -- the RUC as well, or at least it is most noticeable in these models due to better resolution.

That said, I've noticed the NAM has a really dry bias on occasion, like near the center of an anticyclone in a very shallow near surface layer, such that when it generates precipitation through this layer, it wet-bulbs the temperature too much, b/c the model was too dry in some arbitrary shallow layer. This is the problem with models and trying to resolve the highly varied distribution of water vapor in the atmosphere, so sometimes when a high resolution model shows -20°C dewpoint, for instance, in reality, that particular level might actually be closer to -5°C, and thus the model would forecast too cool of a temperature at onset of precipitation and subsequent wet-bulb effects. I'm basing most of this on my own experience as a high plains winter wx forecaster

 

[Maggie Kahman]

according to the Omaha Wx Page has just extended the Ice Storm Warning from 12 AM (or now..lol) to 6PM Sat. night; to from 3AM Sat to 12AM Sun. Morning

 

[John Farley]

For what it is worth, this storm has been warmer than predicted in New Mexico. Snow levels thus far have stayed pretty much above 10,000 feet (except in the NE plains, where the air is much drier), versus earlier forecasts of more like 8,000. Hope it comes down tomorrow (as it is forecasted to do, but not sure how much this will occur before the precip lightens as the day goes on), as I am out here in hopes of getting in some decent skiing.

I'm not sure I can ever recall seeing such a widespread, persistent precipitation shield in Arizona - this storm has a ton of tropical moisture to work with. I would think that the warmth combined with the abundant tropical moisture would tend to drive the p-type gradients north, as it has raised the snow levels higher than predicted in NM, but time will tell.

 

[Ryan Pfannkuch]

watching sfc T trends for Nebraska, northern KS

Thought I would throw out a few tidbits as of 6z regarding guidance vs. reality:

Russell KS: Actual T=30 NWS fcst=28 0z MAV fcst=31 0z MET fcst=31
Hastings NE:Actual T=25 NWS fcst=21 0z MAV fcst=27 0z MET fcst=21
Omaha NE: Actual T=25 NWS fcst=24 0z MAV fcst=24 0z MET fcst=21

Not to make too much out of this...but at least at 6z it would seem that in general...reality seems to be trending at least a few degrees warmer than the NWS forecasts...and 4 degrees warmer than the MET fcsts for HSI and OMA. Up until now, I had thought all along that the GFS was way too warm with the sfc T evolution vs. the colder NAM...but now I am starting to wonder if WAA may be kicking in a little better than I expected.

Taking it one step further...I noticed that the 5z GFS LAMP output had a very good handle on the 6z obs. So looking 9 hours ahead...here is a forecast comparison for 15z:

Russell KS: NWS fcst= 38 LAMP fcst= 41 0z MET fcst=38
Hastings: NWS fcst= 22 LAMP fcst= 36 0z MET fcst=29
Omaha: NWS fcst= 28 LAMP fcst= 33 0z MET fcst=31

Hmm...in this case the GFS LAMP fcsts are anywhere from 3 to 14 degrees warmer than NWS...and even the "cold" MET numbers are 3 to 7 degrees warmer at OMA and HSI. So, bottom line here is that at a quick glance...it appears maybe the WAA may surge north quicker this morning than currently reflected in the NWS grids...which could mean a shorter duration ice/sleet event...especially across northern KS and southern NE. Of course, this is a very simplistic analysis...and does not necessarily account for factors such as substantial cooling from wet-bulbing etc. That being said, its gonna be interesting to see just how close (or far off) these warmer guidance numbers play out and any impact this would have on ice accumulations.

Update at 8z: Sometime around 145am CST very light freezing rain kicked in here in Hastings...with a surprisingly decent little glaze already covering my vehicle. Sidewalks are just starting to show that "feel" of getting slick. At any rate...didn't expect this for at least a few more hours so things may be getting underway a little early. I figured this might start off with some sleet first...but so far just light ZR.

 

[Craig Maire II]

From this mornings Desmoines NWS forecast discussion:

COBB OUTPUT SUGGEST ONE HALF TO ONE INCH
AMOUNTS (of ice) ON TOP OF ANY SNOW WHICH COULD BE QUITE CRIPPLING. ON TOP
OF ALL THE PCPN...EXPECT TIGHT GRADIENT TO PRODUCE AS STRONG
SOUTHEAST WIND WITH GUSTS OF UP TO 35 MPH OR SO WHICH WILL AGGRAVATE
THE SITUATION BY DOWNING POWER LINES AND BREAKING TREE BRANCHES.
WILL CONTINUE TO HIT IT HARD WITH QUARTER TO HALF INCH ICING AMOUNTS
IN THE SOUTH AND HALF TO ONE INCH ACRS THE NORTH. THIS MAY RIVAL
THE FEBRUARY ICE STORM AND CERTAINLY MAY BE MORE WIDESPREAD.

The February ice storm was the worst ice storm that I have ever seen so if this statement pans out we could truly be facing a VERY serious situation in my area. My next door neighbor has a generator so we will go there when our power gets knocked out to stay warm etc. I have warned all of my friends and family about the potential seriousness of this weather dituation so they too are as prepared as they can be.

 

[Rhyuan Janssen]

Well it's only 22 degrees here and we have mostly freezing rain with a tad bit of sleet. DVN says this could rival the ice storm of last year and certainly could be more widespread with more power outages and the like...
YAAY for ice!!

 

[Joel Wright]

Yeah, the 12z models are pretty much unanimously agreeing on a short lived, yet fairly impressive icestorm all across IA into IL. The band from just north of SUX, to Ames/Waterloo IA, to about Clinton IA, on towards Sterling IL look to get the most ice accums. Probably about 1/2", with some local higher amounts.

The latest GFS has a bullseye of 1"+ QPF in a six hour time period this afternoon over northeast IA, into southwest WI, and extreme NW IL. That's very intense for just stratiform precip.

The million dollar question is just how fast will the melting temperatures in MO and SW IA make it north to put an end to the icing. The NAM and RUC look way too cool, keeping the freezing line south of I-80 still at late this afternoon. The GFS is probably closer to reality, yet may be just a bit to bullish in bringing the warm air northward, taking the freeze line well into northern IA by late afternoon. Temps there now are still in the upper teens. I think a compromise of those models is the way to go. The GEM looks way too warm as well, yet it is usually very reliable.

In either case, still a substantial icing event to be sure, yet somewhat short lived...

 

[Robert Dewey]

I'd like to point interested people to the NSSL WRF. For the most part, it's usually pretty damn accurate in forecasting radar reflectivity... In this case, it pretty much nailed the 16Z-17Z radar output at the 16-17 hour time step; not too shabby if you ask me.

Quite a widespread area appears to be looking at 0.50 inches of QPF, with many areas approaching one inch or more.

http://www.nssl.noaa.gov/wrf/