Here are some highlights of the latest NWS discussions for tomorrow.....
NWS LOUISVILLE: A strong low-level jet of 50-55 knots at 925 hPa will be present across the Ohio Valley starting late tonight and continuing through the day Sunday. This LLJ is stronger than what was depicted by guidance yesterday.
The amount of afternoon destabilization continues to be the critical factor. As mentioned yesterday, we only need a few hundred J/kg of CAPE to tap into the strong shear. Guidance continues to advertise at least a few hundred J/kg, given a surface temperature forecast of lower 70s and dewpoints in the lower 60s. The strong shear is also creating helicity values in the 400-500 m2/s2 range, which would support rotation with any individual cell or bowing segments within the convective line. The more we destabilize, the more individual cell development would be possible ahead of the line. Still expect most of our area to see a line of convection along the front, with possible bowing segments. This would be most prone across north-central Kentucky and southern Indiana, where the moderate severe threat has been posted by the Storm Prediction Center. At this time, still expect straight-line wind damage would be the main threat for this severe weather event.
NWS PADUCAH: Strong rightward turning hodograph and bullseye of Significant Tornado Parameter (index) suggest svr threat/with tornadic supercells possible thru pm hours as front enters/makes passage. Front may form into squall line with damaging winds/large hail as well.
NWS INDIANAPOLIS: VERY STRONG WIND SHEAR /40 KNOTS OR MORE FROM 1000-700 MILLIBARS AND 60 KNOTS OR MORE FROM 1000-500 MILLIBARS/ ALONG WITH LONG CURVED HODOGRAPHS AND VERY HIGH HELICITY NUMBERS OVER 300 METERS SQUARED PER SECONDS SQUARED WILL PROVIDE AMPLE SPIN IN THE ATMOSPHERE FOR TORNADIC ACTIVITY GIVEN ENOUGH INSTABILITY.
REGARDING INSTABILITY...TEMPERATURES IN THE UPPER 60S TO MID 70S WITH WARM ADVECTION ALONE...DEW POINTS APPROACHING 60 DEGREES...700-500 MILLIBAR LAPSE RATES TO 6 DEGREES CELSIUS PER KILOMETER OR MORE AND DRY INTRUSION ALOFT SHOULD RESULT IN AT LEAST SOME SURFACE INSTABILITY DESPITE EXPECTED THICK CLOUD COVER AND EARLIER OR ONGOING OVERNIGHT AND MORNING CONVECTION. EVEN THE LESS UNSTABLE...12Z DETERMINISTIC GFS IS NOW FORECASTING 500 J/KG CAPE OR MORE DURING THE AFTERNOON HOURS BUT WEAKENING SIGNIFICANTLY AFTER DARK. BASED ON THE ABOVE...AGREE WITH SPC DAY2 MODERATE RISK AND EXPECT A SEVERE WEATHER OUTBREAK SUNDAY MORNING THROUGH EARLY SUNDAY EVENING. WILL HAVE TO WATCH OUT FOR DISCRETE SUPERCELLS ESPECIALLY DURING THE MORNING NEAR THE SURFACE WARM FRONT AND AFTERNOON IN THE WARM SECTOR THEN...WITH ELONGATED FRONTOGENESIS OCCURRING NEAR THE COLD FRONT. THIS WILL BE MORE OF A WIDESPREAD DAMAGING WIND THREAT BUT WITH ALSO THE POTENTIAL FOR A FEW BRIEF SPIN UPS IN ANY CIRCULATION COUPLET.
NWS NORTHERN INDIANA: FLOW/SHEAR/LOW LVL HELICITY IS OFF THE CHARTS WITH A 120-130 KT MID LEVEL JET MAX NOSING INTO THE AREA. THE EXPECTATION FOR WEAK/MODERATE DESTABILIZATION AND SUCH A STRONG WIND FIELD (LINE NORMAL 0-3 KM BULK SHEAR MAGNITUDES APPROACHING 50 KNOTS!) HINT AT A POTENTIAL HIGH IMPACT SEVERE WX EVENT...WITH RIJ/BOWING SEGMENTS AND STEEPENING LOW LVL LAPSE RATES AUGMENTING THE POTENTIAL FOR DAMAGING WINDS AND MESOVORT TORNADOES. LOW LCL`S IN MOIST ENVIRONMENT AND SLIGHT BACKING OF LOW LEVEL WINDS IN RESPONSE TO PRESSURE FALLS ALSO SUGGEST AN ELEVATED RISK FOR TORNADOES WITH 0-1 KM SHEAR PUSHING 40 KNOTS. GIVEN THIS ADDED A TOR MENTION TO THE GRIDS. STAY TUNED AS THIS HAS THE POTENTIAL TO BE A SIGNIFICANT SVR WX/TORNADO OUTBREAK ACROSS THE LOWER LAKES AND OHIO VALLEY.
NWS CHICAGO: CLIMO SUGGESTS THAT NOVEMBER TORNADOES IN OUR AREA ARE VERY RARE...BUT WHEN THEY OCCUR THEY ARE FREQUENTLY STRONG. DAMAGING WIND THREAT WITH CONVECTIVE LINE LOOKS TO BE QUITE HIGH GIVEN THE ALREADY STRONG FIELDS IN PLACE...WITH ATTENDANT THREAT OF EMBEDDED MESOVORTEX DEVELOPMENT LEADING TO ENHANCED WIND DAMAGE AND/OR QLCS TORNADO THREAT.
NWS LINCOLN,IL: Wild card is the potential for discrete cells before the line develops and out ahead of the line. Any discrete cells will be able to quickly spin given the tremendous amounts of shear being tapped. Can not rule out tornadoes, possibly strong, in this scenario, Particularly across eastern forecast area.
Paducah NWS has been mentioning the STP...the Significant Tornado Parameter index which includes helicity, shear, CAPE and parcel LCL height.
On the index:
1 or higher is needed for tornadic storms.
On the chart on the left:
RED is over 7
This is an extremely high index.
The time frame is late afternoon or early evening.
This is from the NAM which has been the most bullish on the severe outbreak.
This does not mean a significant tornado (EF2 or stronger) is likely. Other atmospheric conditions come into play. Still, this is of concern.
Jared Heil has the temperature tomorrow afternoon for supercells to form ahead of the front at 72 degrees. Any sunshine tomorrow would allow this to happen.
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