RCNYILWX

This is a very tough ptype forecast, especially between the I-80 and I-88 corridors and about 10-15 miles either side. Despite what I posted earlier, having looked more at the soundings, I'm not buying sleet being much of an issue other than mixing in at times.

 

It should be primarily either snow or freezing rain. The big question is the northern extent of the warm nose and if it's enough to get full melting or do heavier rates keep it more as snow. The NAM and RGEM are by far more aggressive with the magnitude and northern extent of the warm nose around 850 mb, with the other guidance much more muted and farther south.

 

Given lack of stout low level cold wedge around 925 mb, it doesn't appear there will be enough refreezing energy for widespread full freezing of melted hydrometeors into sleet, except maybe far north for a time if warm nose does get that far north.

 

The pivotal weather soundings show this better with the temperature and wet bulb profile positive and negative energies. A modified version of the wet bulb is what we're using for ptype derivation in our grids, rule of thumb is negative energy of 100 j/kg or more gives 100% sleet probability and less than 50 gives 0% sleet, while for positive energy, I believe 20 j/kg gives full melting. So the big thing to watch on mesoanalysis this evening is the 850 mb zero line. Within the vicinity of that will be transition zone likely driven by rates to go over to all snow and far enough south will be solidly freezing rain.

 

 

 

 

Unless the short range guidance 2m and sfc wet bulb temps are too cold this evening into tonight, would think we have to extend headlines south at least to I-80 corridor counties. With brisk east-northeast surface winds coming from the cool dry high pressure to the east, doesn't look good for temps warming rapidly above freezing.

 

NAM12 verbatim has warning criteria ice all the way to 2 tiers south of the current advisory. Even if that's too far south and overdone with the ice accums, latest HRRR has headline worthy ice accums in the I-80 counties not currently in the advisory.

 

The other challenging aspect is exact p-type. Most recent RAP/HRRR are colder and have mainly sleet for I-88 counties and snow for WI border counties. From what I recall, RAP/HRRR did a good job picking up on the more stout low level cold wedge for our pinger fest on 12/28/2015.

 

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Pivotal and COD 24 hour kuchera snow algorithm output from 12z GFS.

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So much time to go but the 12z GFS is save worthy for northern IL and southern WI. What a run.

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GFS bumped south again. Stays mostly frozen I-80 and north in IL, mid to high end advisory type snow and a bit of freezing rain near to a bit north of I-80.

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Then GEM comes in stronger than previous run. Obvious that models are still struggling with handling the synoptic setup for this system, particularly at h5.

The GFS and NAM continue to baby step south, but GFS had less backside snow on this run than previous, so end result wasn't great for northern IL. As Hoosier alluded to, would be surprised if there wasn't a freezing rain/mix zone in between predominantly snow area and rain area to south.

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The huge run to run swings the past few days on the operational models show that we won't know much on this system for a few days.

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To illustrate previous post, MJO RMM plots from CPC and 500 mb height and anomaly forecast valid 12z Saturday 2/16 from the GEFS and EPS. I suspect that the nearly off the charts MJO is leading to the much more pronounced southeast ridging look on the GEFS and this MJO projection is unlikely to be correct. The BCGEFS MJO and CFS MJO is still relatively high amplitude into p8 but more realistic looking and overall EPS looks most realistic.

 

You can see that the -EPO is similarly strong on GEFS and EPS, which is something that the GEFS has also caved to EPS on from previous runs a few/several days ago. The position of that EPO ridge and western trough is not so far west that there should be such a pronounced southeast ridge. If I'm right that this is MJO related, then expect the southeast ridge/WAR signal to mute on the GEFS and for the OP runs to back off the cutterish look in the longer ranges.

 

EPS as mentioned in previous post looks like a favorable -EPO/-PNA combo for an active storm track biased colder, with possible further help coming from -NAO/-AO if that signal comes to fruition.

 

 

 

 

 

IMO too many assumptions being made based off operational runs 5+ days out. Next Tuesday-Wednesday system very well could be a cutter and congrats MSP but recall that the operational ECMWF and GFS both had much colder solutions with it as recently as last night. Still plenty of spread in EPS members, so too early to call how things go, especially considering how much medium and even short range guidance has struggled this season.

 

Beyond that system, I continue to like the look of the pattern, especially central and western subforum. I don't trust the crazy MJO amplitude of the GEFS on the RMM plots, also a reason to doubt the 500 mb pattern on it as well as op GFS. EPS has and continues to look more reasonable with MJO forecast. It has a very strong and stable -EPO along with lower amplitude MJO moving into p8 and then p1, which are cold phases.

 

Maybe somewhat of a gradient pattern with modest positive height anomalies in the southeast and east but looks nothing like a massive torchy southeast ridge. In addition, remains to be seen if it's a phantom signal but today both EPS and GEFS went toward a negative AO and NAO beyond 2/13, which would be another reason to doubt a predominantly warm rainer/cutter pattern shown on clown range on some op runs. Should this pattern work out, beyond whatever happens next Tue-Wed, I see it being continued active and biased cold for a good chunk of the sub, so I'm pretty optimistic for it turning snowy again.

 

 

 

 

I'll tell you what, that system on Thursday would be a major severe wx/tornado outbreak in late March/April for the region should that type of setup come back around.

Too early to start a spring discussion about how the most recent Euro weeklies are showing western troughing and eastern ridging in mid to late March?

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Do you recall that even in Jan 2014 after the Jan 5-7 cold shot, a few days later we had 5 consecutive 40s? One of those things easy to forget about that epic winter. Hopefully the upcoming pattern delivers and we rebuild what we're losing through Monday. It's a good look on the European ensemble mean in the medium and long range.

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ORD got back to zero at 1014pm on the 1 minute data stream we have in the office, so we're going with 52 hours in the data we're putting out.

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Records recap for Chicago and Rockford
 

 

 

I was working on some more numbers from this stretch, plan to add them to the site tomorrow. Was going to put out the stat about this being only the 5th occurrence of consecutive -20 or colder lows.

In discussing with a coworker how ORD only got to -21 this morning, we speculated how cold it may have gotten before all the new runways and development in the area and also a change in the siting of the ASOS. Considering that DPA got to -29, it's possible ORD may have been very close to the record everything else being equal.

Also, hard to say how much closer it would've been if winds had been calm all night. I know beavis mentioned that it may just be the site is not a good radiational cooling site anymore and we'll need the advection part of the cold shot to deliver in future extreme cold air masses.

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The departure at ORD yesterday was -40, which may be 2nd largest departure on record to 12/24/1983 at about -44. The largest departure in March 2012 was +35 on 3/20. If anyone can think of others, let me know.

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-17 at ORD
-30 there is out of the question.  Getting to the record is iffy.  Probably a photo finish if it happens.

If it can get to -19 or even better -20 by 06z, it *may* be doable. I'll stick with what I forecast what I did the grids yesterday, -26, though wouldnt be surprised at anywhere from -23 to -25.

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We dialed into the ASOS and ORD officially got down to -23 this AM. The -24 was due to a rounding issue on the 5 minute obs site.

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ILX may threaten it's lowest observed 850 mb temp at 12z. Looks to be pretty close based off consensus h8 progs.

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RC, is there a good reason why the Euro is so cold during the daytime?  Should have plenty of sun, right?  You'd expect more of a recovery than what it shows, especially in urbanized Chicago metro.  

Was just looking at that how it's even colder with PM highs than 12z run was. Really not sure why it would show that signature from Chicago on west, with some spots west not getting above low -20s. Trying to think of a good physical explanation.

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The Euro came in with -31 for Thursday morning low at ORD, 12z run was -30. Will be fascinating to see how it goes on a radiational cooling night. Need daytime temps as cold as possible tomorrow.

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Down to -11 at ORD, was 10-11 minutes too late LOL.

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ORD -10 as of midnight. All time record low max temp of -11 will stand.

Secondary Arctic surge probably literally 5 mins too late for the record tie.

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ORD needs to pick it up soon.  A -11 midnight temp would be disappointing, even though it would go down as a record since ties count.

Looking fairly unlikely to set standalone record low max, hopefully can salvage the tie.

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Amount of recovery tomorrow will be important.  Got to have as cold of a starting point as possible tomorrow night.  A radiational cooling setup is not the way you'd draw up breaking the all-time record low for Chicago, but normal rules sort of go out the window if the high can't get much above -15.

That's one of the reasons I went as close as I did to the record there Thursday morning, only have a PM high of -15 forecast. Thinking it should fall short in the advection cold tomorrow morning but it could be close there too, I went with -24.

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I think there's a very good possibility of -40 in northern IL Thursday morning at one of the favored spots as long as high clouds don't move in quicker than expected. Though I did forecast -26 at ORD Thursday morning, I'm somewhat doubting that it will actually break the all time record coldest. Hopefully I'm wrong. I think RFD has a better chance, could certainly do it tomorrow morning, but if not Thursday AM looks primed.

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Trust me, there has been an internal debate on this else where. I'll let [mention=3997]RCNYILWX[/mention] explain more if he wants to.
But for now it stays WWA, and an upgrade may or may not happen.
I fully agree a WSW should be n effect though. Amounts will be near or exceed criteria, combined with winds, commute on a Monday morning...

I'm not going to be critical of decisions made today at my office, was there for the internal discussion, certainly not an easy call. However, personally I would've done the upgrade from Lee and Ogle and eastward to Cook for many of the same reasons mentioned (impacts to rush hour, heavy rates, wind, etc) plus pretty good likelihood of getting warning criteria amounts in well under the typical 6"/12 hour standard. We'll see how it all plays out.

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