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Dec. 10-11 Severe Weather


Indystorm
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...Lower/Mid Mississippi Valley to the lower Ohio/Tennessee Valley region... Strong upper trough will progress across the Rockies into the Plains by 11/00z then into the mid/upper MS Valley by the end of the period as an intense 500mb speed max (100-110kt) translates into IL late. In response to this feature a surface low will deepen as it tracks from southern KS to north of CHI by 11/06z then into northern lower MI by daybreak Saturday. Severe thunderstorms are expected to develop ahead of this strongly dynamic trough. Serious consideration was given to higher severe probabilities along the MS delta region of eastern AR into the lower OH River valley. Despite the nocturnal initiation, strong tornadoes are possible with supercells Friday night.

Southwesterly LLJ is expected to increase across eastern TX/lower MS early in the period. This southerly branch of the LLJ will encourage higher-quality boundary-layer air mass to advance north into the mid MS Valley ahead of the surging cold front. Early this morning, lower 70s surface dew points are noted as far north as PSN to IER over eastern TX/LA with lower 60s dew points into central AR. There is little reason to doubt moisture will advance into MO/IL ahead of the front as capping will prevent appreciable convection until mostly after sunset. Latest model guidance suggests moisture advection/cooling profiles aloft will result in destabilization ahead of the surface low such that surface-based parcels should become buoyant by 11/02-03z across MO. There is great disparity among the latest HREF regarding timing/spatial distribution of convection during the initiation phase after sunset. Based on forecast soundings have opted to increase severe probabilities farther west across the warm sector ahead of the surface front in MO. Latest thinking is scattered supercells could develop by 03z then track northeast within a very strongly sheared environment. With thunderstorms not expected to develop until after dark diurnal heating will not contribute appreciably to buoyancy across this region. Even so, parameter space appears very favorable for supercells, and with dew points expected to rise into the lower 60s, a few strong, longer-lived tornadoes seem plausible.

They considered a moderate risk, wouldn't be surprised to see one later given the size of that 10 hatched in the newest D1.

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I don't know this for sure but I would imagine it is pretty rare to see these kind of probabilities (10% hatched tornado, 30% wind) north of the Ohio River for a nocturnal severe weather threat in the month of December.  It looks like a dangerous environment though for anything that is rooted at or at least very near the surface.  Deepening surface low should help keep the low levels fairly well mixed.

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3 hours ago, METALSTORM said:

Would someone smarter than me (which is most) have an analog to compare this possible event to?

Although CIPS's analogs only go back to 1979, could the 12/18/1957 outbreak be a comparable analog to today's threat?  That included an F4 in Murphysboro, IL and an F5 in Perry County.  However, the bulk of that outbreak was late afternoon and early evening; as opposed to the entirely nocturnal threat tonight (ILX is timing spotter activation for 7PM-3AM tonight and early tomorrow morning.

https://en.wikipedia.org/wiki/Tornado_outbreak_sequence_of_December_18–20,_1957

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I-70 corridor looks primed for the potential of significant tornadoes tonight. HRRR has been relatively consistent bringing a long tracked discrete supercell through a fairly volatile environment. Additionally, with the rate of moisture advection being observed on stations to the south, I wouldn't be surprised if the true environment along I-70 ends up being in the neighborhood of 67/65 or so. Given that thermos are the biggest thing that the HRRR sees as limiting that environment, better than progged moisture would certainly make things interesting. I feel as if the moderate should've been expanded northward to account for this possibility.

hrrr_2021121015_011_39.png

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1 hour ago, andyhb said:

Have some 2/28/2017 vibes from that 12z HRRR run. We are dealing with some pretty incredible moisture for December (or any cold season month, really) and it should get pretty far north given the lack of morning convection.

Nail on the head, dews are approaching 70° in Memphis & Jackson already. Once that cap breaks tonight, holy smokes. So many ingredients in place for long track twisters tonight

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^To that point, moisture return is coming along nicely with 60 degree dews currently into southern IL.  

Figuring out the threat level with northward extent is a bit tricky, but I generally agree with how and where SPC laid that out with the tight gradient of 2% and 5% tornado probs on the northern end around the I-80 region.

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57 minutes ago, CheeselandSkies said:

18Z HRRR big :yikes: for southern Chicago metro. @Malacka11

 

:twister::twister::twister:

 

SPC maintains smallish moderate @ 20Z.

Concerning run.  Really the main question for the LOT cwa later on is whether storms will be able to be rooted at the surface or close enough.  As a previous LOT afd said, going to need frequent analysis of the environment later tonight.  

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