October 14 Severe Threat

[Hoosier]

This kind of crept up.  Haven't really looked much at the setup (and when I did it seemed like instability was really lacking, especially in the northern/eastern part of the risk area) but SPC went with 5% tornado and 30% wind on the 06z outlook.

 

Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1200 AM CDT Sat Oct 14 2017

   Valid 141200Z - 151200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS NORTHERN MISSOURI AND SOUTHEASTERN IOWA
   THROUGH NORTHERN ILLINOIS AND ADJACENT PORTIONS OF SOUTHEASTERN
   WISCONSIN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
   AREAS OF THE LOWER MISSOURI VALLEY...UPPER MIDWEST AND GREAT LAKES
   REGION...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK...INTO SOUTHERN PORTIONS OF THE CENTRAL PLAINS...

   ...SUMMARY...
   Severe thunderstorms are expected across parts of the lower Missouri
   Valley into southern portions of the Upper Midwest late this
   afternoon and evening, particularly in a corridor across northern
   Missouri and southeastern Iowa through northern Illinois, where
   damaging wind gusts and a couple of tornadoes are possible.

   ...Synopsis...
   Models indicate that mid/upper flow will continue to trend more
   zonal across North America through this period.  One significant
   short wave impulse has already accelerated out of larger-scale
   western Canadian/northwestern U.S. troughing, through northeastern
   Canada.  The most significant remnant upstream impulse (currently
   digging through the northern Great Basin) is forecast to accelerate
   east northeast of the Rockies, through much of the upper Mississippi
   Valley by 12Z Sunday.  The progression of this feature appears
   likely to contribute to the suppression of the persistent
   Southeastern subtropical ridge, while supporting significant
   cyclogenesis along a stalled frontal zone across the central Plains
   into the Great Lakes region.

   ...Plains into Upper Midwest/Great Lakes...
   Guidance continues to suggest the primary surface cyclone
   development/deepening (to sub-1000 mb) will occur late today through
   tonight, roughly in a corridor from southern Iowa into lower
   Michigan.  It appears that this will be accompanied by strengthening
   of lower/mid tropospheric wind fields to 50-60+ kt within the warm
   sector near the low, contributing to strong low-level and deep layer
   shear in the presence of seasonably high moisture content.  Moisture
   characterized by mid 60s+ surface dew points and precipitable water
   increasing in excess of 1.75 inches is forecast within at least a
   narrow corridor near the frontal wave.  While mid-level lapse rates
   may not be particularly steep, it appears probable that this will
   support a corridor of mixed layer CAPE on the order of 1000 J/kg
   across northern Missouri into northern Illinois by late this
   afternoon.

   In response to the boundary layer destabilization, increasing
   large-scale forcing for ascent, near and along the track of the
   developing surface cyclone, is expected to contribute to the
   initiation of storms.  Given the vertical shear, this may include a
   few discrete supercell storms, and probably the eventual evolution
   of a squall line or organized convective system, accompanied by the
   risk for a couple of tornadoes and a broader risk for potentially
   damaging wind gusts, particularly this evening as wind fields
   continue to strengthen with surface cyclone deepening.

   With further deepening of the cyclone overnight, forcing for ascent
   and momentum/shear may maintain a risk for damaging wind gusts and
   tornadoes across lower Michigan and adjacent areas of the southern
   Great Lakes region.  The extent of this threat, however, remains a
   bit more unclear due to the likelihood of more stable boundary layer
   conditions within the warm sector.

   Otherwise, additional thunderstorm development is possible ahead of
   the trailing cold front across parts of the central into southern
   Plains late this afternoon into tonight.  Initially weak to
   moderately unstable conditions may contribute to some risk for
   storms capable of producing severe wind and hail, but it appears
   that a tendency for storms to become quickly undercut by the
   southward surging cold front by this evening will be at least one
   factor limiting the overall severe weather potential.

   ..Kerr/Dean.. 10/14/2017

 

[Tim from Springfield (IL)]

ILX is having a Conference Call at 10 for those counties in their CWA that are in the slight or enhanced risk areas, including here in the SPI area:

https://forecast.weather.gov/product.php?site=ilx&product=HWO&issuedby=ilx

 

[Chicago Storm]

Severe threat looks meh for the eastern half of that ENH risk.

Bigger story will likely be the multiple rounds of storms, with heavy rain/flooding from E. IA into MI.

[hlcater]

AFD for DVN. Seems like they are hinging on destabilization and expect some sort of a TOR threat to evolve based on their AFD

 

Quote

As the strong upper trof crosses the central/northern plains by late
afternoon, deep synoptic lift, strong shear profiles and
unseasonable moisture levels all point to a autumn severe weather
event across portions of the area this afternoon and into the
evening. If the airmass can recover like projected in these southern
and eastern areas, bulk shear of 40-60+ KTS, 0-1km of 20-30KTs, and
0-3km SRH profiles of 300-400+ m2/s2 all suggest sctrd pre-frontal
supercells this afternoon and early evening especially south of I80.
These shear profiles, some sfc southeast wind component and low
LCL`s of 2000 FT AGL or less suggest a tornado threat. Then the
second punch of more linear segments or squall line type systems
with 60-70 MPH damaging wind threat to sweep in acrs at least the
southeastern half to third of the CWA this evening through 8 PM to
10 PM or so tonight. A spin up meso-tornado possible as well with
these evening line segments looking at 30 KT 0-3km shear vectors.
All in all, slight to enhanced severe weather outlook look on target
if we get the thermodynamics to go with the high shear. How far
north the main sfc low and triple point makes it still uncertain and
will be a player on how far north the main severe threat extends,
but most latest run models have trended north and west through this
evening.

 

[Quincy]

The environment will be supportive for tornadoes from northeastern MO into northern IL and possibly eastern IA, but the complication will be storm mode/convective evolution.

Some convection allowing models suggest that a sustained, discrete cell or two may develop in northern IL (south of I-80) this afternoon and ride an instability gradient eastward. If convection can both initiate and sustain itself, this area would be primed for a tornado or two.  

Farther west and north, storm mode may be messy/clustered, along with less impressive instability. However, remnant outflow and/or a surface boundary would be focal points for locally enhanced low level shear, boosting the tornado threat. 

Edit to clarify northern IL portion. Areas in far northern IL should remain fairly stable in the boundary layer. 

[Hoosier]

55 minutes ago, Chicago Storm said:

Severe threat looks meh for the eastern half of that ENH risk.

Bigger story will likely be the multiple rounds of storms, with heavy rain/flooding from E. IA into MI.

Yeah, I'm not sure about this one, especially in that area you mentioned.

I always like seeing a deepening surface low (like we have in this case) and the associated response with the wind fields as we head into the post-sunset hours -- to try to offset/slow down nocturnal cooling -- but I just don't know how much that area will be able to destabilize.

[Hoosier]

Enhanced risk was trimmed on the 1630z outlook

[SchaumburgStormer]

Going to be a solid rain producer regardless, well over 2” since midnight with a heavy rain falling at the moment. It is nice to hear thunder, been awhile. 

[CheeselandSkies]

Yeah, that makes more sense. Just a cool fall rain for us here.

[Hoosier]

Nice update just released from LOT.  They are skeptical about much of a severe threat from around I-80 northward.

[Indystorm]

Wagons south in my opinion on this event.  I do think west central/central portions of IL are more at risk in this event if for no other reason than current satellite showing increased insolation and increasing dews in that area moving northward along with some incipient cells developing as far south as Springfield, which is currently 81/66 at noon central time.

[Hoosier]

The HRRR is trying to blast the front northward and get decent CAPE into the I-80 corridor this afternoon, but it's kinda hard to envision that happening given radar/satellite trends.

[Indystorm]

Meso discussion out now for northern MO into west central IL.   But lapse rates are not all that great at present.   40% prob of issuance.

[SchaumburgStormer]

1 hour ago, Indystorm said:

Meso discussion out now for northern MO into west central IL.   But lapse rates are not all that great at present.   40% prob of issuance.

Issued a tor watch 

[Hoosier]

11 minutes ago, SchaumburgStormer said:

Issued a tor watch 

They have it extending north into an area that hasn't been able to destabilize much.  Will have to watch to see if the increasing low level wind fields allow that gradient to come north.

 

 

[Jim Martin]

[Indystorm]

Guess SPC expects the LLJ to increase this evening.  Helicity, both low and mid levels, looks good.  And sometimes I think helicity outranks lapse rates in being able to produce.

[Indystorm]

First tor warning  of event issued for se IA, ne Mo, and western IL.  (Keokuk area)

[cyclone77]

The cell west of Galesburg needs a tornado warning as well.

[Indystorm]

NWS went with svr warning for Galesburg storm.  I don't know.  Mid and low level helicity are both frighteningly high in that area.

[Indystorm]

Half dollar sized hail reported near the Monmouth airport at 5:50 p,m. central time.

[Jim Martin]

Possible tornado on radar near Bowen, Illinois currently.

[Hoosier]

Effective boundary still seems to be fairly far south.  That corridor from around PIA up to southern Lake Michigan has basically cleared out as far as precip so we'll see what kind of northward retreat of the boundary can occur in the coming hours.

[snowlover2]

Watch being considered up to the Chicago area.

 

Quote

Mesoscale Discussion 1714
   NWS Storm Prediction Center Norman OK
   0601 PM CDT Sat Oct 14 2017

   Areas affected...Northern Illinois and adjacent portions of the
   Upper Midwest

   Concerning...Tornado Watch 501...

   Valid 142301Z - 150030Z

   The severe weather threat for Tornado Watch 501 continues.

   SUMMARY...The risk for potentially damaging wind gusts and perhaps a
   couple of tornadoes is expected to increase across much of northern
   Illinois, including the Chicago metropolitan area through 7-10 PM
   CDT.  An additional watch or watch extension will probably be needed
   eastward into the Chicago metro area.

   DISCUSSION...Within moderate to strong southwesterly mid/upper flow,
   ahead of a significant short wave trough now turning east of the
   Rockies, forcing for ascent associated with a smaller scale
   perturbation is contributing to the evolution of at least one
   distinct mesoscale convective system.  The bulk of this activity has
   spread east of the Mississippi River and appears likely to continue
   progressing northeastward into the Great Lakes region this evening.
   As it does, inflow of weak to moderately unstable warm sector air
   likely will support further convective intensification through the
   01-03Z time frame. 

   Strongest storms seem likely to become focused along a remnant
   convectively generated or enhanced boundary near/north of Interstate
   74 into the Interstate 80 corridor of northern Illinois. This is
   where a zone of low-level warm advection will enhance large-scale
   ascent, and low-level shear will become maximized as southerly 850
   mb flow continues to strengthen to 50+ kt, in conjunction with
   further deepening of the surface cyclone migrating northeast of
   central Iowa.

   The risk for potentially damaging wind gusts seems likely to
   increase with the evolving convective system across northern
   Illinois during the next few hours, and could impact at least
   southern portions, if not much, of the Chicago metropolitan area by
   around 02Z.  Further, an isolated supercell or two within, or
   perhaps forming discretely ahead of the line, may be accompanied by
   potential for tornadoes as low-level hodographs enlarge.

   ..Kerr.. 10/14/2017

 

[Indystorm]

Augusta IL cell starting to get a hook on reflectivity....Velocity looks good as indicated above from Jim's post.

[SchaumburgStormer]

Broad rotation NE of Brinfield moving towards Dunlap

[Indystorm]

PIA 75/68    se wind at 12 mph...

[Jim Martin]

West of Chicago this hour. Starting to getting kinks within the line itself.

[SchaumburgStormer]

4 minutes ago, Jim Martin said:

West of Chicago this hour. Starting to getting kinks within the line itself.

Seems like flash flooding may be the bigger threat for the metro. Line isn’t in a particular hurry. I am over 4” since midnight. 

[Indystorm]

Nice little mini bow punching east of PIA.  I'd also watch the cell approaching Joliet.