Aug. 20-23 Severe Threats

[Tim from Springfield (IL)]

The latest SPC Day 2 has a slight risk of severe for most of IA, SW WI and N IL/N MO--including part of the eclipse totality area in Northern MO.  Carbondale and southern IL not under any risk for tomorrow at this time:

http://www.spc.noaa.gov/products/outlook/day2otlk.html

...North central Plains into the Upper Midwest/Great Lakes...
   Scattered thunderstorm activity, largely driven by lower/mid
   tropospheric warm advection, may still be ongoing east of the mid
   Missouri Valley into portions of the Upper Mississippi Valley at 12Z
   Monday, before weakening while spreading toward the Great Lakes
   region.  In the wake of this activity, models suggest that
   seasonably high moisture content, beneath modestly steep mid-level
   lapse rates, will support sizable CAPE in excess of 2000 J/kg by
   late Monday afternoon, particularly near the nose of an axis of
   stronger pre-frontal surface heating extending northeast of the
   central Plains into areas near/east of the mid and lower Missouri
   Valley.  Guidance suggests that this will precede an upper impulse
   emerging from the southwestern monsoonal regime, before merging into
   increasingly cyclonic mid/upper flow to the south of the main upper
   trough, across the mid/lower Missouri Valley into the Upper Midwest.

   While some timing differences still appear to exist among the
   various model output, large-scale ascent associated with this
   feature seems to provide the most obvious focus for potential
   strong/severe storm development.  This includes the possibility of
   an evolving mesoscale convective system in the presence of
   strengthening westerly lower/mid tropospheric wind fields (to 30-40
   kt).  Given this ambient flow, and potential for heavy precipitation
   loading (aided by precipitable water increasing to 2+ inches),
   convection may be accompanied by a swath of potentially damaging
   wind gusts, possibly continuing into the Great Lakes region Monday
   night.

   ..Kerr.. 08/20/2017
 

[Tim from Springfield (IL)]

The Tuesday severe threat that SPC has had a 15% risk area since Friday still exists in the latest SPC Day 3:

http://www.spc.noaa.gov/products/outlook/day3otlk.html

...Northeast into the southern Plains...
   Spread evident within and among the output of the various models
   precludes greater certainty concerning the details of the potential
   convective evolution and more precise severe weather probabilities
   for this period.  In general, though, weak deep layer flow and shear
   may limit pre-frontal severe thunderstorm probabilities to the
   southwest of the Ohio Valley.  The degree of instability and timing
   of the arrival of stronger forcing for ascent are more of a concern
   northeast of the Great Lakes region, into northern New England.

   Strongest mid/upper flow may tend to lag to the west of the
   southeastward advancing cold front.  Still, weak to moderate
   pre-frontal boundary layer destabilization, coupled with
   strengthening of cyclonic lower/mid tropospheric wind fields to
   30-50 kts, probably will be conducive to severe storm development
   Tuesday into Tuesday evening.  Aided by mid/upper forcing for
   ascent, storms are expected to initiate to the lee of Lakes Ontario
   and Erie, probably evolving into one or more organized lines
   accompanied by potentially damaging wind gusts while spreading
   toward the Champlain/Hudson Valleys and northern Mid Atlantic Coast
   region.  Similar additional activity seems likely to initiate
   southwestward across the Ohio Valley, before spreading southeastward
   into the central Appalachians and Tennessee Valley Tuesday evening.

   ..Kerr.. 08/20/2017
 

[Tim from Springfield (IL)]

And FWIW, most of IA, as are parts of S MN, N MO and far W IL, are under a Marginal for today at this time:

...Central Plains/middle MO Valley to middle/upper MS Valley...
   A cluster of storms may be ongoing at 12Z this morning across
   southeast NE southern IA and northern MO per latest runs of the
   operational HRRR.  Extensive cloudiness attendant to these storms
   should be present across much of IA and portions of the surrounding
   states.  The presence of a rather moist air mass, steep mid-level
   lapse rates, and differential heating on the edges of this cloud
   shield and near the aforementioned cold front will result in
   moderate-very strong instability, with the greatest MLCAPE values
   (around 3000 J/kg) from western MO/eastern KS to western IA and
   eastern NE.  The southern extent of stronger westerly 500-mb winds
   in the upper MS Valley and some enhanced mid-level winds attendant
   to the IA/northern MO trough will result in sufficient effective
   bulk shear for organized storms. 

   The CAPE/shear parameter space will be more than sufficient for
   organized severe storms by afternoon into tonight.  However, given
   the potential extensiveness of the morning convective cloud shield,
   limiting surface heating across the cold front into southern MN and
   the placement of differential heating boundaries by afternoon, there
   remains too much uncertainty to introduce higher severe
   probabilities.  Certainty for storm development and greater storm
   coverage is further reduced by expected height rises and subtle
   mid-level forcing.  There is some model agreement suggesting storms
   will increase in coverage across much of IA and perhaps eastern NE
   later this evening into tonight/early Monday morning as low-level
   warm air advection increases along a strengthening southwesterly
   low-level jet from KS and eastern NE to IA.  Severe storms will be
   possible with this overnight cluster of storms, though coverage and
   overall severity remains too uncertain to increase severe
   probabilities during this time period.
 

[Chicago Storm]

Probably won't be anything significant, but a little something with MCS potential each day.

[Tim from Springfield (IL)]

Most of Iowa has been upgraded to a Slight for today in the 11:30 outlook.

Plus much of central/NE IL and N IN now in a Marginal for this afternoon--with an interesting feature on the 11:30 outlook: a short gap between the eastern extent of the marginal surrounding the Iowa slight risk and the western part of the IL/IN marginal (basically just the immediate Quad Cities area)

http://www.spc.noaa.gov/products/outlook/day1otlk.html

 

[CheeselandSkies]

Yech. Slight risk trimmed out of WI, tornado probabilities reduced in what remains, and it's STILL too cloudy to view the eclipse.