02/24/07 FCST: NE/SD/MN/IA/WI/IL (WINTER PRECIP)

[Bill Schintler]

Winter WX to affect Eastern IA this weekend...

10:50 PM CST, 02/22/07

A well advertised winter storm will affect much of the central portion of the country Saturday and Sunday. Here are forecasts for specific locations in eastern IA:

Cedar Rapids:
Freezing rain and sleet will start at 1 AM Saturday morning and continue throughout much of the day before changing over to snow at 5 PM. Ice and sleet accumulations to 0.75 inch will be likely. Snowfall will accumulate to 4 inches on top of the ice before the snow ends Sunday evening. The other story will be the wind. Saturday will be very windy, with easterly winds gusting to 35 mph.

Iowa City:
Rain, freezing rain, and sleet will start at 1 AM Saturday morning and continue for much of the day before changing over to snow at 7 PM Saturday. Ice and sleet accumulations to 0.25 inch will be likely. Snowfall will accumulate to 3.5 inches on top of the ice. Saturday will also be very windy, with easterly winds gusting to 35 mph.

Marengo:
Rain, freezing rain, and sleet will start at 12:30 AM Saturday morning and continue for much of the day before changing over to snow at 7 PM. Ice and slush accumulations to 0.35 inch will be likely. Snowfall will accumulate to 3.5 inches on top of the ice. Saturday will also be very windy, with easterly winds gusting to 35 mph.

Discussion:
Not much time for discussion other then to say that models have now come to reasonable agreement on storm track, QPF, and temperature structure of the atmosphere. Very strong easterly flow near the surface should advect significant amounts of cold air, therefore the temperatures suggested by guidance seem reasonable. Precipitation onset has also been delayed by the latest model runs, probably because the SFC flow is now being forecasted better. Everything points towards a significant icing event, especially north of US-30. NAM is more bullish with the freezing rain threat, and probably has the best handle on the shallow cold air mass.

- bill

 

[Robert Dewey]

Latest 00Z GGEM still pretty heavy on the QPF even this far east (25-35mm)... Until I see some consistency in the QPF fields over here in the GL, I'm going to go relatively dry. If the 06Z NAM and GFS bump up QPF, then I might say we're seeing a trend toward wetter... But I won't hold my breath.

 

[Joel Wright]

Wow, a very tough forecast here in northern IL. The combination of changing temp profiles, low pressure tracks, and possible dry slotting are making things very complicated.

It looks as though freezing rain and sleet will develop late tonight/early saturday morning, and continue much of the daylight hours. The cold air at the surface will hold steadfast, largely maintained by east winds gusting over 40mph by afternoon!

Colder air aloft will begin to seep southward with time as evening approaches saturday. The tricky part here is how fast will it cool. Due to the very strong dynamics of this storm, the models may in fact be underdoing the amount of cooling. Time will tell if this is true or not.

A dry slot will move northward fairly quickly early saturday evening, and that may shut down the precip for awhile, or at least significantly weaken it, until the low centers pass with additional bands of precip.

A very tough forecast here. For my immediate area I'm expecting about 1/4-1/3" of ice, followed by 4-6" of very heavy/wet snow. Probably on the order of 8:1 snow to water ratio.

Just north the snow will be MUCH heavier. Those areas won't have to deal with dry slotting and mixed precip cutting down on amounts as much. If the low centers track just a bit south, heavier snow above 6" would come down closer to I-80.

 

[Robert Dewey]

NAM has been trending wetter over the GL... The western half of the state is at 1.00 to 1.25 inches of QPF, with 0.75 to 1.00 across the rest of the area. That trend is occuring on the NAM only, as the GFS has been getting drier and drier.

The GGEM was pretty wet on the 00Z run, it'll be interesting to see what the 12Z GFS/GGEM think.

 

[Jeff Miller]

Ominous depictions for Nebraska, Iowa, Minnesota and South Dakota. This is a very powerful storm, and may be one of the more memorable and intense winter storms to impact this region in some time. Copious moisture combined with incredible dynamics.

HIGHLIGHTS:

1. Model features gaining strength from previous runs.
A. Moisture is currently advecting north.
B. Extremely strong H5 Frontogenesis inow develops in Eastern SD and slowly moves east.
C. NAM/GFS show powerful jet stream curling back into SD, placing SD in left exit region.
D. Very strong lift Saturday night with massive upper -div q.
E. Tight gradient will cause strong winds to 20 to 30 MPH and gusty.
F. Large isentropic lift into Iowa with 60 knot LLJ causing widespread outbreak of precipitation.
G. Frontogenesis in Central Minnesota will enhance snowfall in this region.
H. Mid level warming will cause a serious ice situation in eastern winter storm areas. Temperatures will be crucial in borderline areas of ice and snow.

POSITIONING:

1. Low pressure center to be over IA/MO border on Saturday Night, moving east SLOWLY past 72.

WARNINGS:

Winter Storm warnings issued for SD, IA, MN, NE NE, W WI.
Blizzard Watches issued for C NE, NW KS
Winter Storm Watches issued for N IL, N IN, N/C IA, S SD, N MN
EXPECTATIONS:

NW KANSAS:

Blizzard conditions with likely whiteouts. Winds gusting to over 50 MPH and 3 to 6 inches of snow.

NEBRASKA:

Blizzard conditions with 6 to 10" of snow and winds frequently over 30 MPH and gusting to over 40 MPH. Snow will be heavy. Expect blizzard warnings and winter storm warnings to dominate. Whiteout conditions will be likely.

SOUTH DAKOTA:

Blizzard conditions with 7 to 12" of snow and winds frequently over 30 MPH and gusting to over 45 MPH. Snow will be heavy. Expect blizzard conditions and prolonged heavy snow. Whiteout conditions will be likely.

IOWA:

Up to 4 tenths of an inch of ice likely in central Iowa north of Des Moines, and up to 2 tenths of an inch in Northern Iowa before changing over to heavy snow. 5 to 8 inches of snow is likely, and this may be conservative.

MINNESOTA:

Very heavy snow with 12 to 17 inches of snow likely in much of southern Minnesota. Blowing snow likely as well. This snowstorm could reach record criteria in Minneapolis/ST Paul as there has never been a snowstorm with more than 12" of new snow in February.

NORTHERN ILLINOIS:

Mixture of rain sleet and snow with potential for significant icing to 3 tenths of an inch.

NORTHERN INDIANA

Ice accumulations may exceed .25" causing terrible driving conditions.

 

[Robert Dewey]

12Z GGEM is nearly the same as last nights run, with an inch of QPF across southern MI between 12Z SUN and 12Z MON. 12Z GFS is a little more moist than the 06Z run, but still much drier than the previous runs.

All in all, the GGEM seems to be pretty damn consistent in showing a solid band setting up from the ND/SD eastward into lower MI. Models also show pretty windy conditions setting up with sustained SFC winds of 20-30knts.

 

[rdale]

GRR is likely issuing a blizzard watch for the western parts of MI, I'm not ready to go with winds that extreme but I am going to raise my 2-4" for our area up a little more and reduce the ice risk.

MOD: Add MI to the title please...

 

[Robert Dewey]

GRR is likely issuing a blizzard watch for the western parts of MI, I'm not ready to go with winds that extreme but I am going to raise my 2-4" for our area up a little more and reduce the ice risk.

MOD: Add MI to the title please...

I'm a little worried about a bunch of things with this system. While "blizzard watch" excites me, snow amounts don't. GFS has been backing off of QPF for the past several runs, and with northeasterly flow I could see this being a reality.

For what it's worth, your snow ratio algorithm is in the 10-20:1 range which surprised me (I was thinking 8:1).

 

[rdale]

Use the cumulative snow-ratio more than the hourly, for the sites I've checked around MI they are in the 8:1 - 12:1 range which I can go with. The other concern that I've been talking with GRR about, is that I can't recall a weakening system cause that much havoc around here. A steady-state sure, strengthening of couse, but not one that is winding down...

 

[Taylor Pechacek]

WI/IL Blizzard?

Wisconsin

NWS considering a blizzard watch for srn wi. Fairly rare occurance. The last time we had a blizzard warning was December, but before that was in 2 Jan 99.

here is link to mkx text:
http://www.crh.noaa.gov/product.php?site=MKX&product=WSW&issuedby=MKX

http://www.crh.noaa.gov/crnews/display_story.php?wfo=mkx&storyid=6085&source=0

- taylor

 

[Jeff Miller]

BLIZZARD WARNINGS now issued for NW KS. they are mentioning drifts over 3 feet and the possibility I 70 will be shut down. If any chaser is using I 70 they need to seek alternate routes. A similiar scenario will be likely elsewhere.

 

[Robert Dewey]

Use the cumulative snow-ratio more than the hourly, for the sites I've checked around MI they are in the 8:1 - 12:1 range which I can go with. The other concern that I've been talking with GRR about, is that I can't recall a weakening system cause that much havoc around here. A steady-state sure, strengthening of couse, but not one that is winding down...

Ah, okay.

Yeah, the rapid weakening is definitely a concern for me as well. I'm searching through Google for some case studies of weakening systems, but the ones that pop up are the "models failed in forecasting the weakening trend..." -- systems in which the model actually forecasted weakening, but the weakening did not occur or occured more slowly than anticipated. While that's a possibility, I think the chances of that are pretty low (< 15%).

While the SFC-850MB system weakens, the upper level low holds it's strength for quite a bit longer, leading to a relatively long period of lift over the MI area. The dryslot seems to get washed out and pushed east as the comma head setups up...

Synoptically speaking, I'm seeing the three things I don't want to see during a winter storm;

1) Rapid weakening
2) Strong dry northeasterly flow
3) Dry slotting

I can just imagine watching the precipitation taking forever to move into the area, finally making it in just as the dryslot punches up.

It'll be interesting to watch.

 

[Ross Lazear]

A nice surprise here in the Madison area... After MKX NWS had our zone forecast getting "up to an inch of accumulation" overnight tonight, models trended this initial frontogenetic precip farther southeast... Since about 6 PM, we've been seeing on-and-off heavy snow here with very strong easterly winds. Right now, the visibility appears to be down to about 1/12 mile....very heavy snow and lots of blowing snow. A good 6" out of this portion of the system, coupled with tomorrow night's event, and we could be looking at up to 16" in Madison. Excellent.

 

[Robert Dewey]

NAM still pretty consistent with its previous runs... GFS is *slightly* wetter, depending on where you're at. The current stuff over WI might actually help moisten up the airmass here a bit in lower MI, at least according to a non-operational WRF run, so I'm not as worried about that.

What I do see on the NAM and the RGEM is a tremendous amount of vertical lift at 700MB pushing ENE through the GL area as the upper level low begins to stall... This leaves most of MI in a TROWAL with wrap around moisture, so I envision a long duration event with some heavy stuff in the beginning, followed by a long term light to moderate snowfall.

Cross sections are pretty interesting (intersecting the 1000-500MB thickness across PTK), at least on the NAM. I seen signs of convective instability (CI) between 00Z SUN and 00Z MON; a pretty lengthy period of time. However, low levels don't really moisten up until 06Z SUN... At that time, there is a significant amount of lift present in the -15C to -20C region, with RH AOA 90%, and within the CI region. That persists until 18Z, when the dry slot shears off moisture above 700MB. The column still shows decent lift and CI, but with no moisture, I don't think much will be happening (a lull in precipitation?).

Between 21Z SUN/00Z MON, the column moistens back up, with moderate lift through the -15C to -20C region (minus the CI or CSI). If the stuff over WI is able to "seed" the airmass in the lower levels a little bit over southern MI, we might be able to start precipitating a couple hours earlier.

So, we're looking at a solid 8-12 hours of MDT to HVY snowfall, followed by a brief lull, then LGT to MDT snow for another 6-8 hours. The first punch might be accompanied by convective banding if the cross sections are right... While the second punch won't be nearly as impressive.

NAM outputs 0.75 to 1.00 inch of QPF, while the RGEM is around 1.00 inch to slightly above. Depending on your location in MI, I am going to say your snow ratio could be anything between 8:1 all the way up to 15:1 (further northwest). Right now, I'll go with 5-10 inches of snow north of a DTW to AZO... With 8-12 inches northwest of GRR to MBS. Any convective banding could really screw with the totals, but those are my numbers until things setup.

 

[Jim Zandonai]

With Blizzard warnings now up here in N.IL it will be interesting to see where the real dividing line will be between heavy snow and the mix. Up in the Madison area you'll no doubt be all snow. In Rockford we are going to be near the heavy snow mix area where near the I-88 area will be less with a mix. I figured the ICE Storm would be a little south and as it is in North IN.
Now we just wait n see what happens with the temps in low 30s expected its a best guess on precip type.