Hoosier

Temp has slid down into the upper 30s

4-7 am seems like a good timeframe to start mixing with snow here. Not sure if we'll be able to sustain a complete changeover then. Then forget about anything sticking after mid morning as precip lightens and now that we are dealing with a sun angle that is equivalent to the first few days of August.

Here's something you don't see a lot. A snow (or at least snow-ish) forecast sounding with a pwat not far under an inch (0.89")

The NAM looks too generous with precip amounts on the northern edge. Certainly the vast majority of evidence points against that.

Is the right way of interpreting that map being that if it were to strike in the US, it wouldn't be north of the yellow line?

No chance of verifying, but the 00z NAM is insane. Almost tempting to post the clown map to document the absurdity.

Can you tell we're deep into Spring with the pencil thin band of snow where cooling is maximized?

Thinking back on really fringe season anomalous snows that I have witnessed, the one that comes to mind on the other end of the spectrum is 10/7/2000. That was a couple inches of extremely wet snow that did a number on the branches since leaf drop really hadn't begun in earnest yet. If this just ends up being some snow mixed in, then there's obviously no comparison. But if we can somehow manage to pull out some accumulation, then it becomes an interesting thought as to which one stretches climo more.

Don't think any model had flakes that early.

Do you know how much of a heads up there will be for possible impact area after it enters the atmosphere? On the subject of this thread, I admit I am rooting for the snow at this point. I missed out on the recent May snows and haven't seen snow in May since I was a little kid. Would prefer a spring/summer like pattern for sure, but in this instance the alternative is a cold rain with temps in the 30s so why not cheer on the snow. If anything falls it will be gone soon enough.

They should've had Gino Izzi do the afd for this May snow, though that might have been cruel and unusual punishment for him. Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 247 PM CDT Sat May 8 2021 .SHORT TERM... 245 PM CDT Through Sunday... Mother`s Day will feature weather conditions more characteristic to St. Patrick`s Day, with cloudy, blustery, and very wet conditions for the morning. The main forecast messages are: * Rain onset spreads from west-to-east this evening with possibly brief ice pellets mixing in at first * Rain increases in intensity late this evening into overnight, with some embedded thunderstorms possible mainly south of I-80 * Potential for a mix with snow has increased for primarily a portion of north central Illinois and eastward toward the Fox River Valley part of northeast Illinois late tonight into early Sunday morning * Rainfall of 1 to 2 inches for most locations along/south of I-88, with isolated over 2 inches possible, though flooding threat should remain low * Gusty northeast winds keep temperatures down through Sunday, even as rain ends west to east late morning into afternoon A lot to unpack for the rest of this weekend. Looking upstream on GOES-16 water vapor imagery shows a well-defined, strong upper low over Montana moving southeast bringing with it 100 meter 500 mb height falls. In still a somewhat split flow pattern, a 100 kt subtropical jet moving into the Central Plains ahead of this wave will result in a deepening surface low below 1000 mb into Missouri overnight. This should spread into an area of some coupled jet divergence over our region overnight into Sunday morning, resulting in the mid-latitude system overall maturing as it heads over Illinois/Indiana. The synoptic pattern is such that a rain shield should gradually blossom through the evening followed by a steady moderate to more heavy rain with the stronger upper forcing overnight into early Sunday. Model guidance is in decent agreement with this synoptic evolution, though some spread exists and given where we are located within the baroclinic leaf, a subtle difference in low track will alter the heavier QPF axis, the area where a few hour wet snow mix is favored, and the magnitude of the winds. Given a hefty footprint of convection anticipated over the Central Plains into Missouri River Valley tonight, this could result in a more south track within the model solution envelope, such as the RAP has been advertising and the 12Z ECMWF depicts. The HRRR has also trended ever so slightly south from its earlier solutions today. This is the more favored route, but a more north solution such as the 12Z NAM cannot be fully discounted yet. All model guidance agree on the strong mid-level baroclinic zone to our west (18C difference from LBF to MPX on 12Z RAOBs) tightening over the area tonight and a sharpened frontogenetic vertical circulation. This is a longer duration zone of such forcing and this is the zone most favored to see the heaviest rain rates on strong moisture convergence and ascent. It`s also along this where enough wet bulb cooling may occur to support snow. Given marginal low-level melting profiles (<10 J/kg of positive energy using a local revised Bourgouin technique) and upright instability predicted on an area of steeper 650-500 mb lapse rates, there likely will be an area that does change over. As mentioned earlier, there certainly is some wobble room where this is, but a consensus/middle ground solution is toward I-88. Warm surface temperatures will result in quite a bit of immediate melting, but if a band of instability-driven heavier rates persists for 2-3 hours as mainly snow, then could certainly see accumulation on at least grassy and elevated surfaces. While a tail of the solution envelope, f-gen magnitude events such as this can provide a mesoscale-level (multi-county scale) quick several inches. There have been a few CAMs run that have shown something like this, though seem to be much too robust in their efficiency of turning 100% QPF to snow, and at too high of ratio. Further east with increasing easterly winds off the warmer waters of Lake Michigan, the potential for snow drops into the heart of the Chicago metro as well as far northwest Indiana. South of I-80, more melting energy exists thanks to the proximity of the 850 and 700 mb lows passing closely by to the south. It is always a true challenge with predicting snow during these tail end of season events, as thermal profiles are marginal and present one striking failure mode. Also there could be enough moisture transport robbing from the south by daybreak to start reducing the precipitation rates by early Sunday morning to the point where overcoming the marginal low-level temperatures becomes quite iffy.

Southward nudges continue. Obviously not focusing on amounts as much as the corridor in play.

It's interesting to look at the forecast soundings for this one. Take the 18z RAP for example. I was poking around the I-80 corridor and noticed that it's changing areas over to snow even with freezing level/wet bulb heights of like 3500 feet. It is very difficult to get snow to the surface with a wet bulb height like that (unless in higher elevations) but I think the reason that the model is changing it to snow may be because most of the layer under that is literally only a few tenths of a degree C above 0C. Regardless, to say this is a borderline setup is an understatement.

This is the last Euro snow map I plan to post for many months. Usual caveat about 10:1

I'll say this... there are probably worse lead-ins for getting snow to stick in May. Recent highs around here have been in the 50s and the last couple nights were in the 30s, including below freezing last night. Precip rates look fairly heavy so if it can actually change to snow around here, I think it would stick at least on colder surfaces. The time of day is pretty optimal as well.

The most recent HRRR runs are drifting south. Could be noise, or maybe not.

I'll add a couple more... ARR 26, DPA 30. Some of those interior places of northern IL really radiate well. I'm not exactly radiating capital USA here, but temps last night were colder aloft the farther east you go which probably explains why I was able to get within a few degrees of the prime radiating locations in northern IL.

Taking a blend of models, the southern fringe of the changeover would run somewhere around here. It's tricky to put odds on it. I think the warming influence of onshore flow will be largely deflected to the north of here at least through 12z Sun or so as the flow is E/ENE. After that time it does develop more of a northerly component.

The no taste thing is interesting. I know you guys had covid... did she lose her taste then?

Had a low of 30 here, with 29 a bit to my west at IGQ. Big gradient in low temps between here and GYY, the latter of which being so close to the lake and maintaining light onshore flow through the night.

Temp has dropped fairly quickly in the past hour or so. Down to 34.

Surface temps are of course a question mark, but one of the other challenges is the deep layer in the low levels that is straddling the 0C mark. Here's a NAM forecast sounding for west of Chicago. If this is even slightly too cold in the lowest several thousand feet, the precip will struggle to change to snow and would remain a rain/snow mix at best.

Fwiw, ORD hasn't officially had more than a trace of snow in May since 1989. The daily record for 5/9 is 0.1", set in 1923.

Although the thermal profiles in the lowest several thousand feet are marginal, I am pretty well sold on a changeover to snow at this point. The bigger questions imo are how long it lasts in any given area and how well it would be able to accumulate.

Words can't even say how anomalous that would be.