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March 26-28 Severe Threat


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15% tornado area too.

Quote

DAY 2 CONVECTIVE OUTLOOK  
NWS STORM PREDICTION CENTER NORMAN OK  
0100 AM CDT FRI MAR 27 2020  
 
VALID 281200Z - 291200Z  

 
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF  
NORTH-CENTRAL AND NORTHWEST ILLINOIS...  
   
..SUMMARY
 
 
A SIGNIFICANT SEVERE WEATHER OUTBREAK IS POSSIBLE FOR PORTIONS OF  
THE MIDDLE MISSISSIPPI VALLEY ON SATURDAY. THE THREATS WITH THE  
STRONGER STORMS INCLUDE TORNADOES, A FEW WHICH MAY BE SIGNIFICANT,  
LARGE TO VERY LARGE HAIL, AND SEVERE GUSTS.  
   
..SYNOPSIS
 
 
AN INTENSE 150-KT UPPER SPEED MAX OVER THE SOUTHERN GREAT PLAINS  
SATURDAY MORNING WILL QUICKLY MOVE INTO THE MIDDLE MS VALLEY BY MID  
AFTERNOON, AS A NORTHEAST-MIGRATING MID-LEVEL LOW MOVES FROM THE  
CENTRAL HIGH PLAINS INTO THE UPPER MIDWEST. AN INTENSE BELT OF  
SOUTHWESTERLY 500MB FLOW (100+ KT) WILL CONCURRENTLY MOVE FROM OK  
INTO IL BY SATURDAY EVENING AND INTO THE SOUTHERN GREAT LAKES BY  
EARLY SUNDAY MORNING. A 990MB SURFACE LOW WILL DEVELOP NORTHEAST  
AND DEEPEN FURTHER INTO NORTH-CENTRAL IA BY EARLY EVENING BEFORE  
BECOMING OCCLUDED OVER WI BY DAYBREAK SUNDAY.  
   
..MID MS VALLEY
 
 
---CONFIDENCE IS INCREASING FOR A POTENTIALLY POTENT SEVERE WEATHER  
SETUP AS INGREDIENTS NEEDED FOR THIS ARE APPEARING TO FAVORABLY  
ALIGN ON SATURDAY.---  
A CLUSTER OF ELEVATED THUNDERSTORMS WILL LIKELY BE ONGOING SATURDAY  
MORNING ACROSS THE MID MS VALLEY ASSOCIATED WITH WARM AIR ADVECTION  
NEAR A NORTHWARD-ADVANCING WARM FRONT. STRONG SOUTHERLY FLOW WILL  
TRANSPORT LOW-LEVEL MOISTURE NORTHWARD DURING THE DAY WITH SURFACE  
DEWPOINTS EXPECTED TO REACH THE MID 60S SOUTH OF THE WARM FRONT. AN  
ELEVATED MIXED LAYER WITH STEEP 700-500 MB LAPSE RATES (7.5-8 DEG  
C/KM) WILL ADVECT NORTHEAST INTO THE REGION WITH AN ASSOCIATED  
CAPPING INVERSION INHIBITING SURFACE-BASED THUNDERSTORM DEVELOPMENT  
THROUGH THE MORNING. BY MIDDAY, STRONG HEATING WILL RESULT IN  
MODERATE BUOYANCY DEVELOPING TO THE EAST OF A DRYLINE/PACIFIC FRONT.  
SCATTERED DISCRETE THUNDERSTORMS ARE FORECAST TO DEVELOP FROM NEAR  
THE SURFACE LOW SOUTHWARD ALONG THE BOUNDARY AND MOVE TO THE  
NORTHEAST WITHIN A DESTABILIZING AIRMASS AND STRENGTHENING LOW-LEVEL  
SHEAR. FORECAST HODOGRAPHS INDICATE LONG-TRACK, DISCRETE SUPERCELLS  
ARE PROBABLE GIVEN THE INTENSE SHEAR COUPLED WITH MODERATE  
INSTABILITY. THE SEVERE THREAT WILL LIKELY PEAK DURING THE  
AFTERNOON AND PERSIST INTO THE EARLY EVENING. IT APPEARS THE  
TORNADO RISK WILL FOCUS IN THE WARM SECTOR TO THE SOUTHEAST OF THE  
SURFACE LOW, ESPECIALLY WITH SUPERCELLS INTERACTING WITH VORTICITY  
RICH LOW LEVELS NEAR THE WARM FRONT. LARGE TO GIANT HAIL IS  
POSSIBLE WITH EITHER LEFT OR RIGHT-SPLITTING SUPERCELLS. ELEVATED  
STRONG TO SEVERE STORMS ARE POSSIBLE NORTH OF THE WARM FRONT ACROSS  
THE GREAT LAKES WITH MAINLY A HAIL RISK. STORMS MAY CONTINUE WELL  
INTO THE EVENING ACROSS PARTS OF THE OH VALLEY/SOUTHERN GREAT LAKES  
POSING A SEVERE RISK.  
   
..LOWER MS VALLEY INTO THE TN VALLEY
 
 
MODELS SHOW THE DEVELOPMENT OF MODERATE INSTABILITY DURING THE DAY  
ACROSS THE LOWER MS VALLEY WITH ISOLATED TO SCATTERED DIURNAL STORM  
ACTIVITY. RECENT MODEL TRENDS IMPLY STORM DEVELOPMENT PERHAPS  
INITIATING IN THE VICINITY OF LOW-LEVEL CONFLUENCE BANDS DURING PEAK  
HEATING. STRONG DEEP-LAYER SHEAR WILL FAVOR ORGANIZED STORMS WITH  
THE MORE ROBUST UPDRAFTS. HAIL/WIND APPEAR TO BE THE PRIMARY  
HAZARDS BUT A TORNADO RISK COULD MATERIALIZE WITH ADEQUATE LOW-LEVEL  
SHEAR FORECAST. SOME OF THIS ACTIVITY MAY LINGER INTO THE OVERNIGHT  
HOURS AND SPREAD EAST/NORTHEAST INTO PARTS OF THE TN VALLEY LATE.  
 
..SMITH.. 03/27/2020

 

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4 minutes ago, BrandonC_TX said:

With that drastic risk increase, I am getting more concerned that a scenario comparable to the November 17, 2013 tornado outbreak could be within the realm of possibilities, at least for Illinois.  (keep in mind that 11/17/13 had a MDT as far east as Buffalo, NY; I believe this was due to the risk for a serial derecho)

It is a pretty good setup, but 11/17/13 had a larger area of better tornado potential than this does.  I am not sure I agree with the comparison even if we limit it to Illinois, but you can make a better case with that vs a larger regional comparison.  

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1 minute ago, Hoosier said:

It is a pretty good setup, but 11/17/13 had a larger area of better tornado potential than this does.  I am not sure I agree with the comparison even if we limit it to Illinois, but you can make a better case with that vs a larger regional comparison.  

I agree with that.  April 9, 2015 may be the better comparison here, especially with the similarities in expected areal coverage for significant tornadoes.  11/17/13 was generally further east with the favorable area for tornadoes extending across Indiana and even into Ohio (though few tornadoes occurred in OH, several touched down in northern IN).

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Just now, LansingWeather said:

Is it possible to chase this with the lockdown in Illinois?

Technically yes. Just as long as you’re smart & avoid gathering in groups while waiting, being smart while getting gas (gloves, hand sanitizer, etc) & just being smart it should be fine. Planning on heading out but am gonna bring all my own food and water 

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1 minute ago, CoalCityWxMan said:

Technically yes. Just as long as you’re smart & avoid gathering in groups while waiting, being smart while getting gas (gloves, hand sanitizer, etc) & just being smart it should be fine. Planning on heading out but am gonna bring all my own food and water 

I would be coming from Michigan, not sure if crossing into states is permitted or not. I guess I will need to do some reading.

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06z HRRR has a major storm complex moving through Illinois, Indiana, and Ohio (and even into SW PA/N WV) during the morning hours on Saturday (overnight/early morning in IL), before the main action would get started in Illinois.  While generally elevated, some of this complex is modeled to get fairly close to some SBCAPE.  If this complex verifies we might see outflow boundaries come into play later in the day, but my experience with HRRR models is that they are quite bullish.

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Best setup I've seen in years for you guys. I would absolutely be driving from OH to IL if I still lived there but I digress.  

I'm REALLLLLYYYYYY impressed with lapse rates for tomorrow. While chasing would be cool, you have to take into account that there will be some giant hail tomorrow.  This kind of explosive growth should aid supercell development tremendously, and bring strong downdrafts down leading to quite a few tornadoes.  A sub-990 mb low and dry line is some serious stuff for this area. Promising but kind of terrifying considering the possibilities it may bring. Would not shock me to see multiple long track tornadoes tomorrow in IL leading into a fierce MCV-Bow Echo that blows through IN/OH

 

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3 minutes ago, nwohweather said:

Best setup I've seen in years for you guys. I would absolutely be driving from OH to IL if I still lived there but I digress.  

I'm REALLLLLYYYYYY impressed with lapse rates for tomorrow. While chasing would be cool, you have to take into account that there will be some giant hail tomorrow.  This kind of explosive growth should aid supercell development tremendously, and bring strong downdrafts down leading to quite a few tornadoes.  A sub-990 mb low and dry line is some serious stuff for this area. Promising but kind of terrifying considering the possibilities it may bring. Would not shock me to see multiple long track tornadoes tomorrow in IL leading into a fierce MCV-Bow Echo that blows through IN/OH

 

Wouldn't shock me if SPC goes high and/or expands that MDT area. All going to be dependent on morning convection.

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26 minutes ago, crossbowftw3 said:

The HRRR greatly oversold 5/20 so YMMV

This one is a bit different where basically every CAM is in agreement on a string of pearls ahead of that front. We have a very clear focus for initiation this time in the warm sector. I do believe the HRRR parameters may be a bit overblown in terms of instability, but its wind profiles are extremely impressive.

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8 minutes ago, andyhb said:

This one is a bit different where basically every CAM is in agreement on a string of pearls ahead of that front. We have a very clear focus for initiation this time in the warm sector. I do believe the HRRR parameters may be a bit overblown in terms of instability, but its wind profiles are extremely impressive.

That they are. This one seems a bit less obvious than 5/20 at first but seems more probable to verify. Do they go high? It still boils down to what kind of boundaries morning convection leaves/how fast atmosphere recovers.

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Turning to Saturday`s potentially higher end severe risk, there
are some lingering uncertainties to consider. Continued strong
isentropic ascent across the warm frontal zone could keep elevated
showers and some embedded thunder threat going into the morning,
though likely less t-storm coverage than Friday night-early
Saturday. The 00z guidance continued to offer a rather classic
synoptic pattern for upper echelon severe weather potential, with
a sub 990 mb low occluding into the evening, warm front with rich
boundary layer moisture lifting northward, and an arcing Pacific
front/dryline. These surface features will be topped by extremely
impressive wind fields, including south-southwesterly 40-50+ kt
at 850 mb, southwesterly 90-100+ kt at 500 mb, and 150+ kt 250-300
mb upper level jet core, along with still steep mid- level lapse
rates.

The main uncertainties with this set-up are the northward progress
of the warm front, owing to the possibly slowing progression
east-northeastward of the surface low, threat for continued
convection (non-zero severe hail risk) north of the warm front
that could temporarily reinforce the front, and likelihood of
extensive cloud cover that hindering insolation and perhaps
destabilization. While there is overall agreement in the big
picture in the guidance, any further slowing of the warm front
and then Pac front/dry line advancing from the psuedo-triple
point could shift severe risk a bit farther west and threat into
the CWA a bit later, likely into the evening. The ECMWF and UKMET
indicated this sort of possibility. That all being said, with the
inherent uncertainties aside, the set-up has the hallmarks of a
classic midwest severe weather producer. Advection and kinematics
will play a large role in compensating for likely lacking
insolation, with advancing warm sector conditions featuring temps
around 70/low 70s, with dew points into the 60s, and
aforementioned steep lapse rates driving moderate instability
(1000-1500 j/kg MLCAPE).

The impressive wind fields will support extreme deep layer bulk
shear and favorable sufficiently orthogonal orientation to likely
support discrete or semi-discrete supercells erupting from the
Pacific front. Supercells with time to mature south of the warm
front will pose greatest risk into the CWA, with strong low level
shear (ample speed shear) and sufficient veering from southerly
at the surface to south-southwest at 850 mb for hodograph
curvature that could support tornadoes, some possibly strong,
along with large hail and damaging wind risks inherent to
supercells. Storm motions will be very fast off to the northeast,
with right moving vectors over 60 mph. At this time, do have
concerns with how far north and east into the CWA the more ominous
severe risk will be realized given aforementioned uncertainties.
However, given the available data and recent trends, placement of
SPC Day 2 Moderate Risk into the western CWA appears reasonable
given the upper level jet core intersecting the advancing low
level theta-e axis favorably in western Illinois and into north
central Illinois. ENH extending east-southeast marks possible
warm front progress uncertainty, keeping higher probabilities
south of city of Chicago for now.

Exact timing of cold front will determine how long into the
early evening higher coverage of numerous storms extends, with
severe risk then quickly ending with frontal passage. While
discussion focused on potential for slower/farther west trend with
convective initiation, certainly can`t rule out a faster outcome,
such as was depicted on the 00z HREF CAM suite. Will continue to
highlight given the range in possible outcomes, a timing of severe
threat being roughly 4-10pm.

Posting LOT’s discussion for posterity because it is a beast.

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