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April 20-21 late season snow potential


Hoosier
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At the end of the NAM you can see what appears to me to be some lake enhanced/effect snow around Chicago and northwest IN (maybe going on before then but harder to pick out?).  One would expect some of that in this setup with delta T getting into at least the mid teens and sufficient inversion heights.  The issue is that it's countered by the warmer temps near the shore courtesy of the onshore flow.  So while it may not actually result in higher snow amounts, it's possible that it could help smooth things out a bit so that total snow amounts don't drop as much closer to shore.

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The ECMWF ensemble suite shifted a bit southeast vs 12z, but still a good hit of QPF across much of the Chicago metro. Agree on treating the UKMET as the outlier that it is for now. The Euro, GEM, and NAM all have initial f-gen driven banding starting Monday night over northern IL, that would probably be a narrower area of appreciable accums.

Big question mark is when and where does the synoptic system really take off as mid-level wave goes neutral to near negative tilt, and allow a strong deformation axis to develop. The 12z operational ECMWF more or less is the middle ground between the farthest northwest NAM and farther southeast 12z GFS and GEM.

At this point, seems more likely than not 1-3" amounts from the initial banding into the metro, with Tuesday PM the wild card. If the intense defo banding sets up just southeast, subsidence northwest of it would probably quickly shut off meaningful snow. Weighing probabilities at this still pretty early juncture, within LOT CWA, southeast of I-55 and particularly northwest Indiana probably have the best chance of a higher impact headline worthy event.

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I'm about as bullish as I can be on a 3-6" type hit in the middle of April.  I think the max band will be heavier than that but it would likely require quicker deepening to bring it in here.

This should easily be the most interesting wx event around here in the past several weeks, and the overall rarity of doing this with the trees leafing out as they are adds an element of intrigue. 

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00z HRRR is slower with the cold front than 18z.  This would probably lead to our system tracking farther north.  How much stock you want to put in the extended range of the HRRR is up to you though.
NAM fits my general idea for this system so I'll ride it lol. 850 mb front much quicker this run than 18z, so baroclinic zone sets up farther southeast and the top down and bottom up intensification of the synoptic system takes place just too late for most of the IL metro, on Tuesday evening. Looks like non efficient accums, mostly on colder surfaces, during the day on Tuesday over northern IL.

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8 minutes ago, RCNYILWX said:

NAM fits my general idea for this system so I'll ride it lol. 850 mb front much quicker this run than 18z, so baroclinic zone sets up farther southeast and the top down and bottom up intensification of the synoptic system takes place just too late for most of the IL metro, on Tuesday evening. Looks like non efficient accums, mostly on colder surfaces, during the day on Tuesday over northern IL.

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One thing I like for mby is that good rates look to linger into early evening.  So even if accumulations struggle more than anticipated in early-mid afternoon, there should be a few hour window (I'll say around 6 pm and later is when we'll see the diminishing effects of the sun angle) where it accumulates more efficiently.

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One thing I like for mby is that good rates look to linger into early evening.  So even if accumulations struggle more than anticipated in early-mid afternoon, there should be a few hour window (I'll say around 6 pm and later is when we'll see the diminishing effects of the sun angle) where it accumulates more efficiently.
Completely agree, I think that it's gonna rip in that rapidly intensifying defo zone Tuesday evening, 1"+/hour type stuff. I currently like south burbs and south in LOT CWA into NW/N IN and lower MI for this event.

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