Hoosier

Down to 20 here as well, colder than forecast and colder than any model had. Given the proximity of the high, it makes sense why. But again, these little things will start to matter at some point, at least in terms of ground impacts, because now we'll have to rise all the way back through the 20s instead of starting out at 25 or something. Interestingly, the 00z HRRR is colder here and now has temps AOB freezing until like 21z.

Temp has dipped to 23 here. Not that this means that temps won't rise above freezing tomorrow, but the longer it takes to climb back through the 20s does have some ramifications on the paved surface response.

Yeah, 12/28/15 ended up with more sleet here than what it had been looking like. It reminded me a bit of what I experienced with GHD in LAF, just not as intense. Been looking back at previous setups and comparing them to this one. As far as 12/28/15, that had a stronger surface high and also had a stronger surface low than what we have this time.

Shame that we can't get a wide snow band given these surface conditions. But oh, that's not how it works. Probably gonna stay in as much as possible tomorrow. Even as temps rise to/just above freezing, the paved surfaces will likely lag and be a lingering disaster (especially untreated ones).

Final CFS

It looks like the HRRR started drinking early on this New Year's Eve. Not sure what it is doing in northern Indiana. At first I thought maybe it is using heavier precip as a reason to warm temps above 32, but that doesn't explain why the area south of there is still freezing rain. In reality, it should be a more uniform look without plain rain embedded in the freezing rain shield.

Regarding what LOT said about the max temps in the warm layer aloft... If you look at the model progs of where the heavier band of ice is depicted, I wonder if the northern part of it would have a chance to accrete a little better/not waste as much precip. The warm layer in that area maxes out more like 4-6C instead of 7-9C. So in the LOT cwa, that would be a line from Pontiac-Kankakee-Valparaiso and maybe 10-20 miles north of there.

We got a FRAM reference in the LOT afd. Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 254 PM CST Thu Dec 31 2020 .SHORT TERM... 254 PM CST Through Friday night... Near term through tonight... Quiet conditions are expected through tonight we close out 2020 and welcome in 2021. For details on Friday`s winter weather event, see the following paragraph. Large scale subsidence over the area this afternoon appears to be suppressing what was a fairly shallow stratus deck under a 2kft inversion. Some stratocumulus has filled in where snow cover is less common south of I-80, but should erode with the loss of daytime heating. Confidence is low on whether stratus will redevelop under the inversion tonight or remain confined to pockets. A narrow window of a few hours exists this evening where temps may quickly drop over the snow cover north of I-80 before thickening high clouds moderate conditions by later in the evening. WAA will ensure overnight with increasing cloud cover, allowing temps to likely rise a couple degrees. Friday... From a "big picture" view, our thinking with the upcoming storm system is unchanged. As a Gulf of Mexico low pressure system tracks northeastward and along a line from just south of Springfield, IL, to Indianapolis, IN, a broad area of precipitation will spread across northern Illinois. With the low-level thermal profile becoming progressively cooler and deeper with northward extent, precipitation types will range from all snow on the northern fringe of the shield to freezing rain on the southern edge, with a sloppy wintry mix in-between (and right over the Chicago metropolitan area). As the freezing line inches northward Friday afternoon, freezing rain and the wintry mix should convert to "plain" rain for areas generally along and south of I-88 (though the exact location may not be known until, well, it happens). All areas should convert back to snow as the low pulls away Friday evening. Total snow accumulations should range from 1 to 4 inches generally along and northwest of a line from Mendota to Bolingbrook to Evanston (highest as one goes northwest). It`s worth noting the city of Chicago and immediate shoreline may remain above freezing for much of this event thanks to wind off the relatively warm lake, and hence only see liquid rain. Explicit output from the Freezing Rain Accumulation Model (FRAM) with the outgoing forecast package topped 1/2 to even 2/3 an inch of ice across central Illinois, including in Livingston and Ford counties. At face value, those values seem too high, considering: * A deep warm nose with max wet-bulb temperatures pushing +7 to +9 C (that`s rather extreme and should heat raindrops enough to prevent freezing upon falling into the near-surface sub-freezing layer) * Precipitation rates pushing 0.1 to 0.2"/hr when freezing rain would be occurring, which would encourage run-off rather than freezing * Relatively short period of time when trees, power lines, and elevated surfaces would be weighted down by ice given rising temperatures and switch-over to liquid rain. As such, we manually lowered ice totals to max out in the range from a quarter to third of an inch. While those values along would not normally lead to widespread downed tree limbs and power outages, breezy northeast winds of 25-35 mph could be the straw that breaks the camel`s back. As such, we felt there was enough justification to issue an Ice Storm Warning for Livingston and Ford counties (ending at 6 PM CST, though actual freezing rain should end after the noon hour). North and east of the Ice Storm Warning, total ice up to a quarter inch is expected which may yet lead to pockets of power outages and downed tree limbs when combined with the wind. Borchardt/Kluber

Trends overnight are definitely more important than what is happening right now. I agree with that. NWS has a low of 26 for Springfield tonight, which will probably end up being relatively close considering it is 27 there now.

Congrats Alek

The backside changeover to snow prospects are getting my attention more. Depending on how much ice there is and how much melts before the changeover to snow, that could be an added thing to stress the trees as whatever falls on the backside looks to be a wetter snow variety around here.

Good point. Hopefully that is factored in to the NWS advisories/warnings in some way. In some areas that have leftover ice, getting a quarter inch of ice may be more like getting a half inch, or getting a half inch may be more like getting three quarters, etc.

Don't have the better maps but it looks pretty icy.

Interesting how the NAM and GFS are still less bullish on ice overall. The differences are laughable in some places. But the preponderance of evidence points toward a band of 0.25-0.50" of ice (possibly a fairly wide one), and I'd certainly lean toward the idea of a band of 0.25-0.50" We'll see what the Euro shows shortly.

More important to see where you're at overnight tonight.

So as an example of how minor details can have an impact, take a look around Lafayette, IN at 16z Friday. The HRRR has a temp around 29 as heavier precip swings through. If that is right, that makes a difference. But for all we know, it could end up being 31 or 32 when the heavier precip comes through.

I am definitely getting more bullish on a nice burst of snow around here at the end late Fri/early Sat.

Will be curious to see if the offices hit the ice threat harder in the overnight discussions. Most guidance (except the NAM and GFS) is suggestive of a band of around 0.25-0.50" ice accretion in MO/IL and into IN/MI. Devil is in the details of course. Do precip rates get too high and hurt accretion more than what the FRAM output suggests?

Tricky forecast around here for various reasons. It looks feasible to start off as snow/sleet, but that should be brief and my expectation is an inch or less of snow/sleet before a transition to freezing rain. Exactly how long it takes to transition to freezing rain will play a role in ice amounts. The other big question is what happens with surface temps as precip lightens and the flow starts to bend more northeast, and thus off of Lake Michigan. Indications are that temps will nudge above freezing, if only by a little. However, if that manages to not happen due to the inland penetration of the marine influence being overdone, the little bit of snow/sleet at onset being enough to feedback, or whatever other reason, temp profiles aloft would support freezing rain continuing in a lighter/more showery regime until enough cooling of the column occurs Friday evening.

03z RAP... oh boy

Hey, so am I!

Digging these FRAM maps.

Looks like a clown solution around your area in more ways than one (what it does with the precip shield, how it handles sfc temps, etc). Wouldn't put much stock in that depiction for northern IN area.

Prelim guess on the icing is for a band of 0.25"+ accretion embedded in a larger area of 0.1-0.25" accretion. I'm not sure how wide the 0.25"+ will be though.

Definitely a factor to consider. A setup with a 10C warm layer aloft and 31F at the surface will not accrete as well as a setup with a 4C degree warm layer aloft and 31 at the surface... all else being equal.