September 8-10 Severe Threat

[Quincy]

As a trough deepens across southern Canada, cooler air will begin to dive into the northern Plains in the coming days. With warm, moist air being drawn north up the Mississippi Valley and increasingly strong winds in place aloft, a series of disturbances will bring at least an elevated potential for severe thunderstorms for Monday through Wednesday.

The action begins on Monday as a shortwave swings through the northern Pains and and upper Midwest late in the day. It's Tuesday when a potentially more significant severe threat may unfold across the area.

With the Canadian trough digging south, surface cyclogenesis is modeled to take place over the central Plains. As the low ejects into the mid-Missouri Valley, the general vicinity of eastern Nebraska and Iowa (perhaps northwestern Missouri as well) currently appears to have the greatest threat for severe weather.

While wind fields aloft in terms of speed and shear will be more than adequate to sustain rotating updrafts, there are some other key elements that are a bit more uncertain. The NAM shows robust destabilization with dew-points into the mid-70s and MLCAPE values increasing to 1,500 to 2,000 J/kg. Some of the NAM-based holographs are very large and curving across Iowa, along with impressive STP and SCP values. While the GFS is a bit less ambitious, it still shows a fairly impressive setup in the same general area. It will be interesting to see how model trends continue to evolve with this setup. Should some of the higher-end instability solutions verify, this could become a significant event. However, it's still entirely possible cloud debris and leftover convection from Monday could mitigate the magnitude and areal coverage of the threat on Tuesday.

The setup shifts east into portions of the Midwest, Ohio Valley and Great Lakes on Wednesday. Here, although the wind fields will likely be as strong or even stronger than Tuesday, instability looks meager at best. Even the most robust solutions keep CAPE values on the low end around 500 J/kg or less with meager mid-level lapse rates at best. There may be more discussion of that threat in the GL/OV subforum as the bulk of that threat shifts into that region.

[MCS_hunter]

Based on how the NAM has performed for nearly event this summer and the fact that it is coming in as the strongest solution right now, I have no confidence in its forecast.  I'm leaning GFS.

[Quincy]

Based on how the NAM has performed for nearly event this summer and the fact that it is coming in as the strongest solution right now, I have no confidence in its forecast. I'm leaning GFS.

The 18z GFS and NAM are very close with synoptic features and the placement and strength of the surface low. I don't see any major differences and given that, I think Tuesday has the potential to be big. Will follow up after the 00z GFS in a bit.

[Minnesota Meso]

From the 07/12z ECMWF for western IA Tuesday evening. 

 

Dews 65-75°

Cape up to 2000

0-6km shear up to 50 knots

0-3km shear up to 40 knots

0-1km shear up to 20 knots

Crossover winds are favorable.

 

While the  08/0z NAM has shear values a bit stronger than that I think we get the idea.

 

If GFS follows suit I would expect most of IA to go a 30% risk.

[MCS_hunter]

18Z NAM has the surface low 6 mb deeper and in nearly the exact same place as the 18Z GFS at 00Z on Wednesday.

[Quincy]

18Z NAM has the surface low 6 mb deeper and in nearly the exact same place as the 18Z GFS at 00Z on Wednesday.

High res shows 993mb vs. 996mb from those points between the NAM and GFS. 00z data only has a 1-3mb difference Tuesday night.

[Quincy]

From the 07/12z ECMWF for western IA Tuesday evening.

Dews 65-75°

Cape up to 2000

0-6km shear up to 50 knots

0-3km shear up to 40 knots

0-1km shear up to 20 knots

Crossover winds are favorable.

While the 08/0z NAM has shear values a bit stronger than that I think we get the idea.

If GFS follows suit I would expect most of IA to go a 30% risk.

00z GFS for KAIO in western IA at 21z Tuesday:

Dew 73°

Cape up to 2289 (SBCAPE)

0-6km shear 42 knots

0-3km shear 26 knots

0-1km shear 20 knots

Good low-level backing.

[Chinook]

This may be a little over the top. Around Ames, IA, the 00z 4km NAM has up to 12" of rain in just 24 hours from 03z Tue to 03z Wed. This could be a signal for large amounts of convection in that region on late Tuesday night. This seems to indicate a similar coverage of rain, compared to August 31st. (the day before Labor Day)

[andyhb]

Extended snippet from a very long D2 outlook.

...CNTRL PLAINS/MID MO VALLEY INTO THE UPPER MIDWEST...
WHILE CONFIDENCE IN THE DEVELOPMENT OF SEVERE THUNDERSTORM ACTIVITY
IS INCREASING FOR LATE TUESDAY INTO TUESDAY NIGHT...A NUMBER OF
UNCERTAINTIES LINGER WHICH WILL TEMPER THE PROBABILISTIC
FORECAST...AND PRECLUDE A HIGHER CATEGORICAL OUTLOOK AND THE
INCLUSION OF 10 PERCENT SIGNIFICANT SEVERE PROBABILITIES...AT LEAST
FOR NOW. DIFFERENCES AMONG THE MODELS CONTINUE CONCERNING THE
TIMING OF MORE RAPID DEEPENING OF THE SURFACE LOW AND ITS
TRACK...WHICH COULD HAVE AN APPRECIABLE IMPACT ON THE SEVERE WEATHER
POTENTIAL. FURTHERMORE...MODELS CONTINUE TO SUGGEST THAT
CONSIDERABLE HIGH-LEVEL CLOUD COVER/CONVECTION ASSOCIATED WITH THE
LEAD UPPER IMPULSE WILL OVERSPREAD MUCH OF THE REGION DURING THE
DAY. WHILE LOWER/MID TROPOSPHERIC LAPSE RATES MAY BE INITIALLY
FAIRLY STEEP ACROSS THE DEVELOPING WARM SECTOR...MODEL FORECAST
SOUNDINGS INDICATE MOISTENING AND SATURATION FROM UPPER INTO
MID-LEVELS PRIOR TO THE ONSET OF EXPECTED VIGOROUS STORM
DEVELOPMENT. AND IT REMAINS UNCLEAR WHAT...IF ANY...IMPACT THIS
WILL HAVE.

STILL...WITH SURFACE DEW POINTS LIKELY TO INCREASE TO AROUND 70F
/ALONG WITH PRECIPITABLE WATER INCREASING UP TO AROUND 2
INCHES/...BENEATH REMNANT STEEP LOWER/MID TROPOSPHERIC LAPSE
RATES...SIZABLE MIXED-LAYER CAPE ON THE ORDER OF 2000-3000 J/KG IS
FORECAST TO DEVELOP BY TUESDAY AFTERNOON. AS THIS OCCURS AND
LARGE-SCALE ASCENT BEGINS TO INCREASE ACROSS THE MID MISSOURI
VALLEY...TOWARD THE UPPER MIDWEST...BOUNDARY LAYER BASED STORM
DEVELOPMENT MAY BEGIN AS EARLY AS THE 20-22Z TIME FRAME.

VERTICAL SHEAR IS EXPECTED TO ALREADY BE CONDUCIVE TO THE
DEVELOPMENT OF SUPERCELLS BY THE TIME THIS OCCURS...BEFORE SOUTHERLY
LOW-LEVEL JET STRENGTHENING /TO AT LEAST 40-50 KT AT 850 MB/
CONTRIBUTES TO THE EVOLUTION OF SIZABLE CLOCKWISE CURVED LOW-LEVEL
HODOGRAPHS SOMETIME LATE TUESDAY AFTERNOON OR EVENING. AT THE SAME
TIME...LIFT ASSOCIATED WITH STRENGTHENING LOW-LEVEL WARM ADVECTION
PROBABLY WILL SUPPORT A CONSIDERABLE INCREASE IN STORMS...AND THE
UPSCALE CONVECTIVE GROWTH OF AN ORGANIZING CONVECTIVE
SYSTEM...INITIALLY ACROSS PARTS OF EASTERN NEBRASKA INTO
IOWA...BEFORE SPREADING NORTHEASTWARD. HOW QUICKLY THIS OCCURS MAY
IMPACT THE TORNADO POTENTIAL...WHICH SHOULD BE PRESENT GIVEN THE
SIZE OF THE LOW-LEVEL HODOGRAPHS IN THE PRESENT OF HIGH
BOUNDARY-LAYER MOISTURE CONTENT/HUMIDITY.

A STRONG TORNADO OR TWO MAY NOT BE OUT OF THE
QUESTION...PARTICULARLY WITH ANY LINGERING DISCRETE STORMS EARLY
TUESDAY EVENING. HOWEVER...IT SEEMS THAT POTENTIALLY DAMAGING WIND
GUSTS WILL BECOME THE MORE PREDOMINANT SEVERE THREAT...AS HEAVY
PRECIPITATION LOADING AND THE DOWNWARD TRANSPORT OF MOMENTUM
INCREASE WITH THE DEVELOPING MESOSCALE CONVECTIVE SYSTEM.

..KERR.. 09/08/2014

[Minnesota Meso]

From DMX

 

MODELS IN GOOD AGREEMENT WITH BRINGING A DEEP LOW OFF THE ROCKIES
AND DIRECTLY THROUGH IOWA TUESDAY EVENING INTO WEDNESDAY MORNING.
COMPARATIVELY...THE 12Z NAM IS AN OUTLIER IN THAT IT IS SLOWER THAN
THE OTHER MODEL RUNS WITH THE PROGRESSION OF THIS LOW. MAGNITUDE-
WISE...THE NAM IS IN THE BALL PARK WITH CONVECTIVE TRENDS AND QPF.
SO WILL DISCARD IT FOR TIMING...BUT WILL KEEP IT FOR CONVECTIVE
PARAMETER PURPOSES. MODELS SHOW PWAT VALUES AT AN IMPRESSIVE +2 TO
+3 STD DEV...HINTING AT THE POTENTIAL FOR SIGNIFICANT HEAVY RAIN.
FORECAST SOUNDINGS SHOWING TALL...SKINNY CAPE PROFILE. MUCAPE VALUES
AROUND 1000 J/KG. MOST CAPE LOCATED IN MID TO LOW LEVELS OF THE
ATMOS. SOUNDING PROFILES SUCH AS THIS LEAN TOWARDS WEAK UPDRAFTS AND
A REDUCED HAIL THREAT. TORNADO THREAT...HOWEVER...WORTH WATCHING. 0-
1KM HELICITY IS BASICALLY OFF THE CHARTS. O-6KM BULK SHEAR
MAGNITUDES IN THE 50KT TO 70 KT RANGE WHICH IS ALSO SIGNIFICANT.
ADDITIONALLY...MODELS IN AGREEMENT WITH PURE DIVERGENCE AT H250 FROM
00Z WED THROUGH 06Z WED...WHICH WILL DEFINITELY SUPPORT LIFT. ALL OF
THIS SETTING UP FOR A LOW CAPE...HIGH SHEAR ENVIRONMENT WHICH IS
SUPPORTIVE OF TORNADOES.

[Quincy]

High res models (several HRRR runs in a row and 6z 4km NAM) peg southeastern Nebraska and far northeastern Kansas for initiation by mid-afternoon. Looks like a messy storm mode with clusters and a few discretes that all congeal relatively quickly as they move into northwestern Missouri and southern Iowa.

HRRR is handling precip decently over Nebraska so far. The difference between a low-end and a more robust event will likely be strongly influenced by how much clearing/destabilization takes place later today. I think a lean toward a less impressive event is warranted, but given the shear and wind fields in place, it won't take a lot to cause some significant severe. If the overall 2014 trend holds up, we'll just see spotty severe. (We can hope for some more Nebraska/Iowa magic though...)

[Quincy]

Impressive kinematic support in terms of shear and helicity in south-central Nebraska. Although thermodynamic fields are modest out ahead, there may be enough overlap near the KS/NE border in a few hours to light some fireworks.

[Jonbo]

Had some roughly 40mph gusts as the heavier part of the storm grazed me. I was more intrigued watching the rain change directions multiple times.

[Indystorm]

Tor watch now issued this Tuesday afternoon/eve for much of se Nebraska and ne KS.

[Quincy]

Tor watch now issued this Tuesday afternoon/eve for much of se Nebraska and ne KS.

50% probability of 2+ tornadoes

40% for at least 1 significant tornado

[Indystorm]

Helicity ramping up south of Hastings with CAPE of 3000 south of the border into KS.  Tor development possible as cells develop and become more discrete in this area in contrast to the line plowing through Omaha.  The cell approaching Nebraska City looks interesting.

[Jonbo]

That cell west of Nebraska City still looks interesting but I think it's getting caught up in the main line too much. Really trying hard though.

[Indystorm]

Tor warning for Nebraska City now.

[Jonbo]

TOR warning has been issued on it. Upper end of the line weakened and gave this a chance.

THE NATIONAL WEATHER SERVICE IN OMAHA HAS ISSUED A

* TORNADO WARNING FOR...

SOUTHEASTERN OTOE COUNTY IN SOUTHEAST NEBRASKA...

* UNTIL 500 PM CDT

* AT 433 PM CDT...A SEVERE THUNDERSTORM CAPABLE OF PRODUCING A

TORNADO WAS LOCATED NEAR LORTON...OR 32 MILES NORTHWEST OF

TARKIO...MOVING EAST AT 30 MPH.

HAZARD...TORNADO AND GOLF BALL SIZE HAIL.

SOURCE...RADAR INDICATED ROTATION.

IMPACT...FLYING DEBRIS WILL BE DANGEROUS TO THOSE CAUGHT WITHOUT

SHELTER. MOBILE HOMES WILL BE DAMAGED OR DESTROYED.

DAMAGE TO ROOFS...WINDOWS AND VEHICLES WILL OCCUR. TREE

DAMAGE IS LIKELY.

* THIS DANGEROUS STORM WILL BE NEAR...

NEBRASKA CITY AROUND 440 PM CDT.

THIS INCLUDES THE FOLLOWING HIGHWAYS...

HIGHWAY 2 IN NEBRASKA BETWEEN MILE MARKERS 504 AND 508.

HIGHWAY 75 IN NEBRASKA BETWEEN MILE MARKERS 42 AND 48.

[thewxmann]

NE City cell really ramping up in latest scans. Strong couplet just south of NE City.

[thewxmann]

Also, cell near Lenox, IA might need to be tornado-warned soon.

[Jonbo]

I started to notice that too. Cell near Lenox is getting more interesting.

[andyhb]

Funnel cloud reported on that fat HP on the Missouri River.

 

Edit: Velocities really picking up near Rock Port.

 

 

...A TORNADO WARNING REMAINS IN EFFECT FOR CENTRAL ATCHISON COUNTY

UNTIL 600 PM CDT...

 

AT 537 PM CDT...A SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO 

WAS LOCATED NEAR ROCKPORT...AND MOVING SOUTHEAST AT 10 MPH. TRAINED 

STORM SPOTTERS REPORTED A WALL CLOUD WITH RAPID ROTATION NEAR 

ROCKPORT.

 

HAZARD...TORNADO AND TENNIS BALL SIZE HAIL.

[Hvward]

Cell north of Mount Ayr, IA moving towards Tingley/Ellston is showing signs of rotation on velocity.  Could be one to watch.

[Jonbo]

CHASER.From-GA is live streaming on the again tor warned cell in NW Missouri near Maitland.

edit: Law enforcement confirmed tornado.

[NaoPos]

Cell just north of KC needs a tor warning ASAP;

[Chinook]

storm totals (I posted this in Great Lakes/Ohio Valley also). Omaha radar estimated over 8" at Maitland MO, (near reported tornado)

 

 

 

 

 

 

 

 

 

[Quincy]

That was one beast of a supercell yesterday that traveled from the Nebraska City area into NW Missouri. A tough area to chase with terrain and road layouts, but managed to get a few decent pictures. Here's a downscaled one from I-29 and Highway W last night at 5:56 p.m.

Very broad rotation, but for me it was the most prolific lightning producer I've seen. Toward dark some of the velocity scans were getting extreme, but the distance from radar beams was a factor. Either way, probably one of the most interesting storms I've chased to date.

[FarmerRick]

A friend of mine near Ft. Calhoun(just N. of Omaha) recorded 5.5" in his rain gauge.    

[Quincy]

That supercell that started in Nebraska actually produced 4 EF-0 tornadoes in northwestern Missouri, but none in Nebraska itself. The tornadoes were all relatively weak and short-lived and I'm not sure anyone got any close pictures. The image above was taken at the same time of one of the tornadoes. Who knows if it was rain-wrapped and/or if the terrain was blocking its view.

 

Anyway, this storm was really odd looking in person and on radar. (Probably because it was in a radar no-zone, too far away from EAX, TWX, OAX and DMX to be sampled properly. Right before sunset, along with intense cloud-to-ground lightning, there were several odd appendages coming out of the storm at different times.

 

Here is a photo of a mini rotating wall cloud-like feature that was lowering around 7:15 p.m. Looking back at the PNS, an EF-0 tornado was reported a couple of miles to the north between 7:10 and 7:15 p.m. Maybe I'll go back and watch video later, but nothing was all that conclusive.