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Feb 24-25th Severe Weather


Anti tornado

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15-20F dewpoint depressions along the CF and an occluded low doesn't excite me for severe potential. Euro basically agrees and shuts out most of the warm sector until it's further east and the flow is fully veered. Good if you like a snaky line or elevated hailers north of the warm front.

Better moisture return or a later occlusion/deeper low would help.

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SPC discounts severe chances in the Plains/Miss. Valley due to that pesky Gulf low
that's been discussed here, but believes that moisture return will be sufficient 
for those of us farther east on Friday. 
.....Models and their ensembles indicate that an
elevated mixed layer originating from the Great Plains will be
advected into the highlighted area along with a strong consensus for
marginal low-level moisture (50s degrees F boundary-layer
dewpoints).  Given the expected setup, sufficient confidence is
attained for some severe risk to be highlighted graphically, with
the expected severe hazard to be mainly damaging winds.

 

 

 

day6prob.gif

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5 hours ago, HillsdaleMIWeather said:

New SPC outlook has a large area highlighted

Yes, and the geographical placement is a bit farther north than I would have initially expected.  It seems the threat at present is for a strong squall line.  But after what happened last night in northern San Antonio you never know.

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The northern extent looks plausible to me at this point.  Whatever magnitude of threat this ends up becoming, one thing about it is that there's no snow on the ground to have to melt off, so the northern extent of the real warm sector won't be contaminated in that way.  Shouldn't be a situation where you have to go 200 miles south of the surface low to get into a threat. If you're anywhere along/south of the surface low track, you're pretty much fair game, at least until after it occludes.

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11 hours ago, Hoosier said:

The northern extent looks plausible to me at this point.  Whatever magnitude of threat this ends up becoming, one thing about it is that there's no snow on the ground to have to melt off, so the northern extent of the real warm sector won't be contaminated in that way.  Shouldn't be a situation where you have to go 200 miles south of the surface low to get into a threat. If you're anywhere along/south of the surface low track, you're pretty much fair game, at least until after it occludes.

Correct.  I might be in Highland In on Friday visiting relatives and coming back to the Indpls area on Sat. so we'll see what happens in our state.

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This thing being progged as being in a steady state more or less suggests that sfc winds won't have much trouble veering. Tornado threat isn't zero but it's hard to imagine something appreciable barring significant changes.  There's a decent slug of CAPE for this time of year, so hail and areas of damaging winds could occur.

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16 minutes ago, Hoosier said:

Here's a forecast sounding near the IN/MI border at 18z Friday.  There's some modest low level turning but not much.  

2017022100_GFS_090_41.8,-85.67_severe_ml.png

I would wait until this gets into the NAM range before looking at hodographs, GFS is notorious for overly veering surface winds. There should be some veering but not to the magnitude it is showing.

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3 minutes ago, Stebo said:

I would wait until this gets into the NAM range before looking at hodographs, GFS is notorious for overly veering surface winds. There should be some veering but not to the magnitude it is showing.

Fair enough, but what's going to keep the winds more backed in the warm sector (say 180 or less)?  

I guess an area to watch for perhaps locally enhanced tornado prospects would be right on the warm front, assuming the thermodynamic environment is good enough.

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1 hour ago, Hoosier said:

Fair enough, but what's going to keep the winds more backed in the warm sector (say 180 or less)?  

I guess an area to watch for perhaps locally enhanced tornado prospects would be right on the warm front, assuming the thermodynamic environment is good enough.

If the system is still strengthening that could help, though I don't know if we will see 180 or less. Anything 200 or less would be fine though or at least give a tornado potential.

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There's not much that supports backing of low-level wind fields in the warm sector on Friday. The warm front has been fairly consistently progged to lift north through IN Friday morning and into MI by peak heating. While there may be a marginal tornado threat here, dew-points in the mid-50s won't cut it. 

Farther south, the guidance is unanimous in veering winds in the lowest km, yielding a largely unidirectional wind field. (00z EC included) Sure, given the magnitude of speed shear, it won't take much subtle backing to enlarge hodographs. The most likely convective mode will be linear with the potential for a few QLCS spin-ups, assuming dew-points can rise into at least the upper 50s.

If moisture return is greater than currently progged with northward extent, watch the warm front, but that seems fairly unlikely given this setup and the time of year. While many analogs show some substantial severe events, most of these occurred in March and featured 60+ dews. 

A quick assessment of model progs also suggests the surface low occludes several hours before peak heating, further suggesting a veering wind profile with southward extent. This includes the 84hr NAM, FWIW. Terrain influences are not helpful and if anything, support a more of a SWrly low-level wind field than a Srly or SErly immediately west of the Appalachians.

Thursday looks increasingly interesting SW of this sub-forum, but marginal low-level moisture and capping will most likely keep any threat limited across MO and surrounding areas. 

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Just an observation from today in regards to Friday.  Dewpoints have had no trouble rising into the mid/upper 50s in a corridor as far north as Chicago, with even some 60 degree readings near St. Louis. The moisture really won't get shunted south to a major degree in the next couple days, so all this adds confidence in the model progs that are getting mid/upper 50s dewpoints even well into Michigan on Friday.  

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On ‎2‎/‎20‎/‎2017 at 9:14 AM, Indystorm said:

Yes, and the geographical placement is a bit farther north than I would have initially expected.  It seems the threat at present is for a strong squall line.  But after what happened last night in northern San Antonio you never know.

True. There was only a 2% tornado risk down here on that day. All it takes is a little more low level shear and you get a situation like what happened here on Sunday.

(For reference; we're up to 6 tornadoes. 2 EF2, 2 EF1, 2 EF0 and 1 major damaging wind event with estimated wind speeds between 120-140 mph. Crazy)

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