Weekend Severe Threat: Tornadoes possible in southern MN

"High Risk" from SPC for severe storms and possible tornadoes in parts of Nebraska & Iowa Saturday

"Favorable area for strong tornadoes" near Omaha into western Iowa Satursday according to SPC

Full blown tornado outbreak possible near Omaha and into western Iowa Saturday

60% chance of damaging severe weather Saturday near Omaha into western Iowa according to SPC

Slight risk for severe storms (and possible tornadoes) Sunday in southern Minnesota

Stay "sky aware" this weekend - especially in northern Iowa and southern Minnesota

1.13" GFS model rainfall output for MSP by 1pm Monday

Source: Twin Cities NWS

Weekend Severe Threat:

I'm a little concerned about the threat for severe storms (and even possible tornadoes) bubbling into southern Minnesota this weekend.

Let's break down the potential severe scenario this weekend.

The System:

A powerful, "wound up" low pressure system will track near Omaha Saturday, then move into southern Minnesota by early Sunday morning.

Source: NOAA

Severe Weather Risk Factors:

This system has a few key factors going for it as a potential severe weather producer.

-Strong veering winds (low and mid level "directional wind shear") favorable for rotating storms that can produce damaging winds and tornadoes

-Strong upper level "dynamics" including fast moving "jet streaks" that can provide strong lift to build potentially violent storms

-A strong temperature contrast and warm frontal boundary that can keep storms going through Saturday night

Storm Prediction Center: "High Risk" for severe weather & tornadoes Saturday

This system has got the attention of the SPC in Norman, Oklahoma.

SPC has issued (what I believe is) the first "high risk" of 2012 centered on Omaha for Saturday.

Source: NOAA/SPC

Check out some of the alarming language in SPC's convective outlook for Saturday.

(My highlights in bold)

...CNTRL NEB INTO WRN IA...

"WITH THE SURFACE LOW DEEPENING JUST TO THE W...AND A BROAD WARM FRONT AND/OR OUTFLOWS IN PLACE...LOW LEVEL WINDS WILL REMAIN BACKED AND RH LEVELS SHOULD REMAIN RELATIVELY SHELTERED AS WELL...PROVIDING A FAVORABLE AREA FOR STRONG TORNADOES. CELLS WOULD LIKELY FORM OVER WRN SECTIONS OF THE HIGH RISK AREA WHERE HEATING WILL STEEPEN LOW LEVEL LAPSE RATES...THEN WOULD PROCEED RAPIDLY NEWD WITH AN INCREASING TORNADO THREAT. VERY LARGE HAIL WILL ALSO BE LIKELY. THE NRN EXTENT OF THE TORNADO THREAT WILL DEPEND ON THE ACTUAL LOCATION OF THE PRIMARY BOUNDARIES."

Minnesota Threat:

Looking at the maps, I am concerned about strong to severe storms firing in southern Minnesota late Saturday and Saturday night as the strong warm front slides north into Minnesota.

Sunday's severe weather outlook

Source: NOAA/SPC

The system appears to have enough wind shear and dynamics that could keep severe (even possibly tornadic) storms going right through Saturday night into Sunday morning as the system moves into southern Minnesota.

Check out SPC's wording on Sunday's severe risk in Minnesota.

(My highlights in bold)

...UPPER MS VALLEY/SRN GREAT LAKES...

AN UPPER-LEVEL LOW IS FORECAST TO MOVE INTO THE UPPER MS VALLEY SUNDAY AFTERNOON AS A 90 TO 110 MID-LEVEL JET NOSES IN THE UPPER MS VALLEY. AT THE SFC...A WARM FRONT SHOULD EXIST ACROSS MN AND WI WHERE THUNDERSTORMS ARE FORECAST TO BE ONGOING SUNDAY MORNING. NEW CONVECTION SHOULD INITIATE AROUND MIDDAY TO THE SOUTH OF THE WARM FRONT AS SFC TEMPS WARM AND AHEAD OF A COLD FRONT APPROACHING THE REGION FROM THE WEST. STORMS SHOULD PROGRESS EWD ACROSS THE SLIGHT RISK AREA DURING THE AFTERNOON. FORECAST SOUNDINGS AT 21Z SUNDAY FROM MADISON WI SWD INTO ERN IA AND FAR NW IL SHOW MODERATE INSTABILITY WITH SBCAPE VALUES OF 1000 TO 1500 J/KG. IN ADDITION...THE EXIT REGION OF THE MID-LEVEL JET WILL HELP TO CREATE STRONG DEEP LAYER SHEAR PROFILES FAVORABLE FOR ORGANIZED STORMS.THIS SHEAR ENVIRONMENT ALONG WITH COLD TEMPS ALOFT IN PROXIMITY TOTHE UPPER-LEVEL LOW SHOULD BE FAVORABLE FOR LARGE HAIL. FORECASTSOUNDINGS SUNDAY AFTERNOON NEAR AND TO THE SOUTH OF THE FRONT ALSO SHOW LOOPED HODOGRAPHS WITH STRONG LOW-LEVEL SHEAR. THIS MAY SUPPORT A TORNADO THREAT WITH CELLS THAT REMAIN DISCRETE AND HAVE ACCESS TO MODERATE INSTABILITY.

And late Friday afternoon's froecast discsuuion from the Twin Cities NWS seems to support my concern for southern Minnesota, and possibly the metro early Sunday morning.

(*My notes in lower case)

"THERE SEEMS TO BE A CREDIBLE THREAT FOR SEVERE STORMS DURING THE NIGHT FROM THE TWIN CITIES ON WEST AND SOUTH...WITH HIGH EMPHASIS ACROSS SOUTHWEST AND SOUTH CENTRAL MN. THE AMOUNT OF LOW LEVEL SHEAR INCREASES DURING THE EVENING WITH 0-1KM BULK SHEAR AT OR ABOVE 40 KNOTS ON SEVERAL SOLUTIONS FOR SOUTHWEST AND SOUTH CENTRAL MN. SEVERE WEATHER PARAMETERS ARE SUCH THAT TORNADOES WOULD BE POSSIBLE ALONG THE WARM FRONT IN SOUTHERN MN. RECENT RESEARCH DONE HERE BY DEVINNY AND HULTQUIST SHOW THAT THIS TYPE OF UPPER PATTERN (SOUTHWEST FLOW ARCHETYPE) WAS THE ONLY ONE THAT HAD STRONG NIGHTTIME TORNADOES IN OUR FORECAST AREA. MORE DISTRESSING IS WATCHING THE CIPS WARM SEASON ANALOGS. YESTERDAY THE NUMBER 5 ANALOG WAS MARCH 29 1998. TODAY THIS ANALOG HAS MOVED TO NUMBER 3. HENCE...SATURDAY NIGHT IS AN INTENSE PERIOD OF INTEREST."

(Translation: This pattern looks similar to the one that produced the Comfrey and St. Peter tornadoes in March 1988.)

"IN ADDITION...A DIFFLUENT 850-300MB THICKNESS PATTERN SPREADS INTO SOUTHWEST AND SOUTH CENTRAL MN BY 15/06Z ALONG WITH A LARGE INCREASE IN THE DIFFERENTIAL TEMPERATURE ADVECTION. THIS PUSHES THROUGH THE TWIN CITIES BY SUNDAY MORNING. THE 850-500MB WIND CROSSOVER IS RATHER SIGNIFICANT EARLY IN THE EVENING AND THEN BECOMES UNIDIRECTIONAL OVERNIGHT. AS THIS OCCURS MID LEVEL WINDS INCREASE TO BETWEEN 80 AND 100 KNOTS BY SUNDAY MORNING. ONE CONCERN IS THAT THE CONVECTION MAY BEGIN TO BOW DURING THE OVERNIGHT HOURS AS IT RACES NORTHEAST TOWARDS THE TWIN CITIES. YOU DEFINITELY GET A FEEL FOR THIS LOOKING AT THE ARW WRF."

(Translation: A "bow echo" with damaging winds is possible in the metro overnight Saturday night into Sunday morning.)

I would not be surprised to see SPC bump the "risk areas" further north into MN this weekend in response to the warm front and low pressure track.

As the models stand now, my view is that the most likely area for tornadic storms Saturday night and Sunday in Minnesota would be in and around a Worthington, Mankato, Albert Lea, Owatonna, Waseca, Rochester, Winona zone.

However, if the models shift the low and warm front even 60 miles north, we cannot rule out potentially tornadic storms very close to, and possible even in the metro early Sunday.

I hope I am being overly cautious here, but I calls 'em as I sees 'em.

Stay sky aware this weekend, and keep one ear/eye to developing severe weather trends and possible watches and warnings, especially south of the metro!

And please, after what we learned about people ignoring warnings in Joplin last spring, if you see or hear any "risk signals" for tornadoes or damaging winds (sirens, warnings etc) approaching you location, take cover 1st and then seek secondary information after you are in safe shelter.

PH