June 28-30 Severe/Heavy Rain Threats

[Tim from Springfield (IL)]

Uncertainties might still exist on the severe side of the threat, but either way it looks like the upcoming storm system to close out June might be the biggest rain event since about mid-May for at least the west part of the subforum.

SPC Day 3 includes a Slight for most of IA, SW WI, NW IL, SE MN and N MO--and SPC is not ruling out an upgrade in parts of the threat area in later forecasts:

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

...Southern MN and WI into KS...
   Scattered showers and storms are likely to be ongoing across the
   upper MS Valley Wed morning, ahead of the low and in a zone of
   low-level warm air advection aided by a 40-50 kt low-level jet. This
   activity may not be severe due to weak instability initially.
   However, a reservoir of instability will grow to the southwest due
   to heating, extending from eastern NE and KS into IA, with an
   effective warm front near the MS river by afternoon. Rejuvenated
   convection is expected to form near the warm front/outflow boundary,
   possibly aided by outflows, with diurnally driven storms farther
   west along the front where heating will be strongest. The greatest
   severe threat will extend from the warm front southwestward into
   northern KS, with very large hail and damaging wind possible in all
   areas. A tornado or two appears most likely from IA into WI, where
   low-level shear will be strongest and with the lowest boundary layer
   Temperature/Dewpoint spreads. Storms are expected to increase in
   coverage into Kansas along the cold front during the evening with a
   more linear storm mode expected and damaging wind threat. Portions
   of the area may be upgraded in subsequent outlooks.
 

[Tim from Springfield (IL)]

SPC Day 4 has a 15% threat area from NE/KS into W IL for Thursday:

http://www.spc.noaa.gov/products/exper/day4-8/

Also 4.4-5+ inches of rain possible in SE IA before the system exits the region:

 

[Chinook]

Take a look at the 12km NAM soundings for Iowa/Wisconsin/Illinois on Wednesday. The 3km NAM has an MCS along I-80 somewhat early, then develops convection in the Davenport-Kansas City corridor, within a zone with pretty high severe parameters. However, it would appear that the higher tornado chances would never reach northeast Iowa or Wisconsin if the 3km NAM is right.

[AppsRunner]

Even with plenty of morning/early afternoon convection, the 0z 3k NAM run was much more aggressive with the afternoon convection, especially in southern/eastern Iowa. Won't be able to work out too many details until the morning stuff works its way through, but at least some of the mesoscale models show pretty robust destabilization during the early afternoon. 

DVN's scheduled move to their new office is supposed to start tomorrow, with their radar down for at least 2 days. That may become a concern with almost no alternative options for radar coverage in E/SE Iowa where the best supercell threat seems to be at this time. 

[Tim from Springfield (IL)]

SPC Day 2 keeps the slight, for now, across roughly the same area as yesterday's Day 3 but introduces hatched area from SW WI to N KS:

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

...Northern KS into southern WI...
   Scattered thunderstorms will be ongoing Wed morning from MN
   southward into northern MO, aided by strong low-level warm advection
   and in association with a shortwave trough. Some of this activity
   may contain severe hail, especially from the MO river into IA where
   instability will be strongest. This activity should generally weaken
   during the day but additional storms are expected from southeast
   MN/southwest WI into IA as destabilization occurs. The early storms
   may create an outflow boundary in the region, which would serve as a
   focus for supercell development with the possibility of a couple
   tornadoes as low-level shear will be favorable. Predictability is a
   bit low given variance in model output, and this area will
   experience rising heights in the wake of the shortwave trough.
   Therefore will defer a possible upgrade to later outlooks.

   To the southwest, additional activity will form late in the day
   along the stalled front across northern KS/southern NE, as areas to
   the south experience strong heating which will erode the cap.
   Midlevel flow on the order of 40 kt along with MUCAPE in excess of
   2000 J/kg will support large hail and damaging winds.
 

And SPC's Day 3 for Thursday introduces a Slight from KS/NE all the way to Cleveland:

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

...Central Plains into the mid MS Valley...
   While difficult to be precise due to likely effects of early storms
   and outflows, bouts of thunderstorm activity are likely throughout
   the day from eastern KS into western PA and NY, with the most
   concentrated area of severe weather expected to form after 18Z from
   eastern NE/KS across IA and northern MO. Here, instability will be
   greatest, with MUCAPE possibly reaching 2500-3000 J/kg. Indications
   are that an MCS could form along the stationary front across eastern
   NE and KS, with activity moving across IA and northern MO. Damaging
   winds are most likely, along with hail. Other storms are expected to
   form near the surface low and along the cold front into central KS
   near or after 00Z. Some of this activity could be in the form of
   supercells briefly before becoming linear, with hail and wind
   threat.

   ...Midwest into the Northeast...
   At least isolated strong to severe storms are expected by afternoon
   along the front from IL into OH in an elongating plume of
   instability supported by strong west-southwest flow. Winds aloft
   look to be parallel to the boundary, which should keep activity
   within a narrow west-east zone. Strong 850 mb flow, especially into
   NY and PA, suggest strong wind gusts will be the main threat as
   instability will be weakest there. Potentially, a supercell could
   occur as SRH will be maximized along the eastward-moving warm front
   into NY.

   ..Jewell.. 06/27/2017
 

 

[Tim from Springfield (IL)]

SPC not ruling out a Slight across Lower MI on Friday:

http://www.spc.noaa.gov/products/exper/day4-8/

...DISCUSSION...
   On Fri/D4, an upper trough will be centered over the northern
   Plains, with embedded shortwaves moving from the middle MS Valley
   into the upper Great Lakes, with a weak surface low over Lower MI by
   afternoon. A cold front will extend southwestward from this low,
   with a moist air mass to the east. Indications are that substantial
   rain and thunderstorms will be ongoing along this front, which lends
   uncertainty to forecast instability levels. For example, the ECMWF
   shows much less MUCAPE than the GFS, with minimal instability into
   Lower MI where the most favorable shear will be. While a Slight risk
   may eventually be issued, will defer to later outlooks given these
   uncertainties.
 

[Hoosier]

Specific details murky but looking active.

[CheeselandSkies]

WRF-ARW wants to slam southern Wisconsin while 3K NAM largely leaves us high and dry.

[cyclone77]

Yeah it's hard to get too excited about tomorrow's setup with all the questions regarding ongoing convection/cloud debris.  Active LLJ may keep anything from the early morning hours festering for much of the day over Iowa.  Instability axis could end up fairly muddled if that happens.  The overall synoptic setup is nice though, so hopefully thing's aren't too muddled up by the morning/early afternoon stuff.  

[AppsRunner]

HRRR has been fairly consistent in running the MCS all the way out of Iowa by 13z. In fact, it has MLCAPE values near 2000J/kg near DSM by 14/15z. 

Airmass recovery will be important, but convective initiation time will be critical, especially for tornado potential. Some CAMs delay afternoon convection until early evening, allowing the LLJ to kick and really helps low level shear increase. 

Lots of players to watch for here, but thinking an enhanced risk for most of South/East IA perhaps into southern Wisconsin as well

[Chinook]

This is a graphic from a paper by SPC forecasters John Hart and Ariel Cohen. It is used as an example of how the STP parameter may be somewhat useless in June through October severe storm forecasting. As you can see here, this is a situation that is not terribly different from tomorrow's possible setup, with high STP values in the Midwest.  The significant tornado of the day was a long way away from the 10 STP contour!  The authors show data that helps describe the limited usefulness of the STP in the summer, September and October.

Reference: Hart, J.A., and A.E. Cohen, 2016: The Challenge of Forecasting Significant Tornadoes from June to October Using Convective Parameters. Published in Weather and Forecasting.

I bring this up because I tend to get excited about high STP values, for example, forecast soundings for tomorrow.

 

[Quincy]

A great example about SCP and STP uselessness is evident this evening. 01z mesoanalysis shows values of >40 and >7 respectively in parts of the eastern half of Nebraska, yet all guidance and near term trends remain very consistent in showing a continued messy storm mode with any tornado threat being brief, very limited and constrained to QLCS processes. (Not only are tornadoes not particularly likely, but the threat of any significant tornado is close to zero)

There is an array of issues with the setup tonight, including, but not limited to considerable SB/ML CINH, weak upper level flow (~20kts at 500mb), storm mode, etc. 

For tomorrow, there are many issues as well. Just quickly looking at recent runs of the HRRR, SW near-surface flow across most of central/eastern Iowa at 18z is not very encouraging for supercell tornado potential. Yes, the strength of the low-level jet (unseasonably strong climatologically for midday) helps offset the lack of substantially backed flow somewhat, but despite modest 0-1km SRH, hodograph size is still relatively small in forecast soundings.

Outflow boundaries and an effective warm front will be important considerations, of which will be modulated by convective systems/elements tonight. 

An "enhanced" wind threat seems probable in almost any scenario, but unless short term trends start improving soon, I wouldn't get too excited about a tornado threat just yet. It seems likely to be another Iowa "letdown" given current expectations.

 

[cyclone77]

5 minutes ago, Quincy said:

A great example about SCP and STP uselessness is evident this evening. 01z mesoanalysis shows values of >40 and >7 respectively in parts of the eastern half of Nebraska, yet all guidance and near term trends remain very consistent in showing a continued messy storm mode with any tornado threat being brief, very limited and constrained to QLCS processes. (Not only are tornadoes not particularly likely, but the threat of any significant tornado is close to zero)

There is an array of issues with the setup tonight, including, but not limited to considerable SB/ML CINH, weak upper level flow (~20kts at 500mb), storm mode, etc. 

For tomorrow, there are many issues as well. Just quickly looking at recent runs of the HRRR, SW near-surface flow across most of central/eastern Iowa at 18z is not very encouraging for supercell tornado potential. Yes, the strength of the low-level jet (unseasonably strong climatologically for midday) helps offset the lack of substantially backed flow somewhat, but despite modest 0-1km SRH, hodograph size is still relatively small in forecast soundings.

Outflow boundaries and an effective warm front will be important considerations, of which will be modulated by convective systems/elements tonight. 

An "enhanced" wind threat seems probable in almost any scenario, but unless short term trends start improving soon, I wouldn't get too excited about a tornado threat just yet. It seems likely to be another Iowa "letdown" given current expectations.

 

That whole last paragraph would look great on a T-shirt lol.  

 

[Harry Perry]

3 hours ago, Chinook said:

This is a graphic from a paper by SPC forecasters John Hart and Ariel Cohen. It is used as an example of how the STP parameter may be somewhat useless in June through October severe storm forecasting. As you can see here, this is a situation that is not terribly different from tomorrow's possible setup, with high STP values in the Midwest.  The significant tornado of the day was a long way away from the 10 STP contour!  The authors show data that helps describe the limited usefulness of the STP in the summer, September and October.

Reference: Hart, J.A., and A.E. Cohen, 2016: The Challenge of Forecasting Significant Tornadoes from June to October Using Convective Parameters. Published in Weather and Forecasting.

I bring this up because I tend to get excited about high STP values, for example, forecast soundings for tomorrow.

 

Excellent read Chinook... this is where previous knowledge and critical thinking about the synoptic setup truly come into play. Seems the model could be re-written to somewhat accommodate time of year but then could be too muddled to be accurate. Regardless... Will definitely keep this in mind when I see high STP parameters this time of year.

[snowlover2]

Enhanced for E IA on new day 1.

Quote

 Day 1 Convective Outlook 
   NWS Storm Prediction Center Norman OK
   1246 AM CDT Wed Jun 28 2017

   Valid 281200Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   FAR EASTERN NEBRASKA...FAR NORTHERN MISSOURI...IOWA...FAR
   NORTHWESTERN ILLINOIS...SOUTHWESTERN WISCONSIN AND FAR SOUTHEASTERN
   MINNESOTA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
   ENHANCED RISK FROM PARTS OF THE CENTRAL PLAINS NORTHEASTWARD INTO
   THE UPPER MISSISSIPPI VALLEY AND WESTERN GREAT LAKES...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
   SLIGHT RISK AREA FROM PARTS OF THE NORTHERN ROCKIES AND CENTRAL HIGH
   PLAINS EAST-NORTHEASTWARD INTO THE GREAT LAKES...

   ...SUMMARY...
   Severe thunderstorms with large hail, wind damage and possibly a
   couple of tornadoes will be possible from the Mid Missouri Valley
   northeastward into the mid to Upper Mississippi Valley. Isolated
   severe storms will be possible in the central Plains with marginally
   severe storms possible in the central and northern High Plains.

   ...Mid to Upper Mississippi Valley/Mid Missouri Valley...
   A shortwave trough will move eastward into the mid to upper
   Mississippi Valley today. At the surface, a cold front will advance
   southeastward across the north-central states and should be located
   from central Nebraska northeastward into southeastern Minnesota by
   mid afternoon. Ahead of the front, a corridor of maximized low-level
   moisture will be located from eastern Kansas north-northeastward
   into Iowa where surface dewpoints are forecast to be in the upper
   60s and lower 70s F. A linear MCS will likely be ongoing at the
   beginning of the period across parts of the mid to upper Mississippi
   Valley. This may leave an outflow boundary across parts of the mid
   to upper Mississippi Valley on which storms can develop later in the
   day. The models show moderate to strong instability from southeast
   Nebraska east-northeastward across Iowa by mid to late afternoon
   with MLCAPE values reaching the 2000 to 3500 J/kg range. Scattered
   thunderstorms should initiate along this corridor by early afternoon
   and move eastward away from the instability axis. Additional
   thunderstorm activity should initiate further west along the front
   during the mid to late afternoon with several clusters of
   thunderstorms moving eastward across the enhanced risk area.

   Concerning the severe weather potential, the NAM is forecasting a 45
   to 55 kt mid-level jet to move eastward across Iowa during the
   afternoon. This feature will help to create moderate to strong
   deep-layer shear profiles along the instability axis. Forecast
   soundings from Des Moines, IA to Davenport, IA show 0-6 km shear in
   the 40 to 45 kt range with strong speed shear and some directional
   shear in the lower levels. This environment combined with steep
   mid-level lapse rates will be favorable for supercells with large
   hail. Hailstones of greater than 2 inches in diameter will be
   possible with the more dominant supercells. As the low-level jet
   strengthens during the late afternoon and early evening, a tornado
   threat should also develop. A transition to linear mode is expected
   to take place by the mid evening which would increase the
   wind-damage threat. An isolated severe threat will be possible in
   the late evening and overnight period mainly across southeastern
   Iowa, northern Missouri and northwestern Illinois as the convective
   system moves southeastward.

 

[AppsRunner]

Day 1 enhanced shifted just a touch to the south and east, but still 5/30hatch/30. Probably won't see much in the way of TOR potential, although there still could be decent LL helicity due to speed shear alone. Likely will see some very large hail reports in any supercells with MLCAPE >2500-3500J/kg. Wind potential a little iffier though.

Day 2 enhanced as well, but a large number of questions surrounding convective debris in the morning.

[Jim Martin]

Day 1 Enhanced Risk from South Iowa to the western border of Wisconsin/Illinois.

[hawkeye_wx]

The Nebraska/western Iowa MCS pooped out, but I was fortunate to just catch the bubbly warm advection band of storms that popped up overhead early this morning.  I picked up a nice 0.85".  Just west they got nothing.

I'm not sure what else we are going to get.  The HRRR has been gradually shifting today's afternoon/evening storms east and south.  Recent runs don't have much of anything nw of a Des Moines to Cedar Rapids line, although the model is currently missing the storms well north of Des Moines.  The Thursday into Friday morning convection is still up in the air.  The NAM and Canadian now have nothing else falling in most of Iowa after today as everything goes south.

[Harry Perry]

Day 2 has shifted both the Marginal and Slight risk areas north into northern Illinois, southern lower Michigan. 

 

From the SPC 

 

   ...Midwest into western PA and NY...
   Although a synoptic warm front should be located farther north into
   northern Lower MI into southern Ontario, a differential heating
   boundary extending from southern Lower MI through northern IL should
   be the primary focus for new thunderstorm development Thursday
   afternoon and evening.  The 15% (Slight) and 5% (Marginal)
   severe-risk areas have been expanded a little northward to include
   more of northern IL and southern Lower MI, with this change based on
   model consensus (12Z NAM/GFS and 00Z ECMWF) for the placement of
   showers and thunderstorms in vicinity of the aforementioned surface
   boundary.  Moderate instability and sufficient bulk shear oriented
   parallel to the convective initiating boundary suggest line segments
   with locally strong/damaging winds being the primary threat.

[iowahawkeyedave]

Got .15 of rain here around Iowa City from this morning's decaying MCS from Nebraska.  Not sure what we're going to get this afternoon, even though we're in the enhanced risk seems most of the short term models have been shifting the storms into south-central Iowa and moving ESE into southeast Iowa, missing east central Iowa for the most part.

[Quincy]

The environment over southern Iowa is actually quite favorable for tornadoes, if only shear vectors were more perpendicular to the front/mesoscale boundaries. Storms may tend to cluster fairly quickly.  Sharper backing of near-surface winds would also be more supportive. Already seeing >2000 J/kg MLCAPE with >20kts 0-1km shear, enlarged 0-3km hodographs, low LCLs (<1000m) and dew-points in the upper 60s to lower 70s around the area. 

 

The most likely scenario looks to feature initial supercells/supercell clusters, quickly transitioning to more of a linear convective system with embedded supercell structures. This could still yield a few tornadoes, but if discrete modes were to be realized for a longer time, then maybe you could argue greater tornado probabilities. 

 

We'll see. Any scenario from a convective mess to multiple tornadoes (even possibly a strong tornado, given 0-2km flow/shear) could unfold. I'd lean more pessimistic given climatology and the minor issues mentioned above.  

[farleydawg79]

Tornado Watch until 10:00 pm


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[Indystorm]

Nice string of tornadic supercells in IA at present before they may eventually line out as they move east.  Some tornadoes confirmed.

[andyhb]

Absolute beauty of a tornado near Stuart, IA with the cell closest to DSM.

 

[Chicago Storm]

Several sups producing in IA currently...

[Chicago Storm]

Absolute beauty of a tornado near Stuart, IA with the cell closest to DSM.

 

And with very little showing up on velocity.

[Stebo]

Wisconsin also has a supercell east of MSP producing.

[Indystorm]

Helicity in IA is a bit stronger than I expected it to be for this episode as I was thinking straight line winds and heavy rain.  We'll see what transpires. Confirmed tor near Van Meter heading for West Des Moines in current warning.

[andyhb]

 

[Chinook]

tornado near Macksburg, IA seems very possible