Severe Weather Outlook - March 8, 2011 C.E.

Bore da (which means Hello in Welsh, or something),

Well, it looks like that old trickster dryline in the southern United States is going to be at it again, along with a little help from his/her/its friends (aka a synoptic scale cyclone developing in the Southwest; okay so only one friend this time). Currently the SPC (Storm Prediction Center) down in Oklahoma is predicting a 5-10% chance of tornadoes forming in Lousiana later this afternoon. Their claim is that the combination of upper level jet streak dynamics (with the right jet entrance region, or upper (lower) level divergence (convergence) region over Arkansas and Louisiana, implying upward vertical motion), along with the dryline feature and frontogenetic forcing (in this order) will lead to severe weather in this region.

A look at the 15 UTC (9 am LST, so about an hour ago) NAM (initialized 00 UTC) 300 mb (plot of wind speed (fill pattern, with more purplely colors indicating stronger wind speeds) and wind barbs (knots) and geopotential height in units of dam (black contours)) shows that the right jet entrance region of a local jet streak is indeed over the Lousiana region. This will provide some initial upward motion in this region.

Interestingly though, a small jet streak feature off to the west will move into the region around 18-21 UTC (12-3 pm LST), with the right jet exit region over Lousiana. Since a thermally indirect circulation exists in the exit region of the jet, subsidence (downward vertical motion) will act to warm the the middle to upper troposphere. While at times this type of setup can lead to dynamic destabilization, allowing for the atmosphere to become unstable over time and fire

up some nice convection later in the afternoon, this can also inhibit severe thunderstorms by warming the air too much such that lapse rates decrease and are not as favorable for development of strong upward motion. My guess (along with Croix "Convergence", the man himself) is that it might be limiting thunderstorm development initially.

Forunately, the dryline comes to the rescue! During sunset hours (00 UTC Wednesday, March 9, aka 6 pm LST in Louisiana), the dryline moves into the region, along with the start-up of the nocturnal low level jet. The dryline acts as a lifting mechanism for warm, moist surface parcels to rise as the drier, more dense air forces the less dense moist air to rise. Along with this, the low level jet (see 925 dewpoint map, where color fill pattern is dewpoint, wind barbs are in knots and contours are height contours in dam) shows wind speeds reaching nearly 50 knots overnight over this region, and then near the Alabama-Mississippi border!!! Along with this, favorable CAPE (measure of instability) values > 1000 J/kg, Lifted Index less than -4 (a measure of lift; less than -4 is good for severe weather) and helicity (measure of the "corkscrewiness" or rotation more or less from 0-1 km (in this case)) values near 200 m^2 s^-2 reside in this region.

While strong helicity values are also present in Arkansas and Missouri, the instability and lifted index values are not quite there for a tornado outbreak, but organized severe weather (or at least strong convection) in the form of an MCS is still likely with high dewpoint values (55+ deg F), precipitable water (~1.5 in) and lift from the synoptic cyclone present.

Above: Lifted Index (fill), mean sea level pressure (mb) contours 00 UTC Wed 8 Mar 2011

Above: 0-1 km Storm Relative Helicity (fill and contour, m^2 s^-2).

Which leads me to the discussion regarding the forcing from the synoptic scale cyclone. A look at the 00 UTC Wed. (still from NAM 00 UTC Tuesday model run) 925 mb map (Wind speed in fill pattern similar to 300 mb map shown earlier, with heights in black contour and wind barbs in knots) shows the location of the cyclone over over the Kansas-Missouri border. This cyclone will provide frontal lift as it moves to the north-east, aiding in the generation of thunderstorms (and supercells farther south of the cyclone, near Louisiana) during the evening to overnight hours.

This blog post was sort of rushed due to my necessity to solve the mysteries of tropical meteorology today at home base, but the general idea is that the combination of upper level jet dynamics, the low level jet and dryline combo, along with the synoptic level forcing from a strong low level cyclone is going to create some interesting weather in Louisiana. Louisiana is in the perfect spot, since it is most affected by the three weather forcings described above, and also is in the zone for most instability and high values of lifted index. I'd say that a few tornadoes will form, mainly during the later evening hours and overnight. Those in Alabama and Mississippi should be on the lookout for tomorrow morning, since any generated convection will move east and possibly induce tornadogenesis in a region that is predicted to have high values of helicity and reasonable instability tomorrow morning.

Over/under on tornado reports: 5

Over/under on wind reports: 34

Over/under on hail reports: 9

Are these over/under's reasonable? I honestly don't know, but I've never done this before so it's a nice starting point.

Peace,

Helicity

NOTE: ALL MAPS FROM TWISTERDATA.COM.