The US severe weather outbreak 23/4/2010

Hey folks,
Thought for a bit of hype before I make my chase trip in May, that I might look into the forecast dynamics in the US...now that there is something to forecast over there (See my friend Patrick Marsh's Blog for a better forecast effort, im still learning the ropes over there).

Yesterday (US time) saw a reasonable number of tornadoes develop from discrete supercells along the ejected trough. A cold front came through late in the evening to end the interest.

Now, to post some of the information for the next two days, which both have Moderate risks, and will like produce a large number of tornadoes, with discrete supercells, strong rotation and long/dangerous tornadoes a distinct possibility. However, the chase country for this outbreak is tough or more the case nearly impossible. The outbreak is now moving into Arkansas, and the Missippi valley, which makes chasing hard due to all the trees. There is still a potential for tornadoes in the alley, but the dynamics are far superior in eastern Texas and further east...particularly in terms of strong low level shear (a primary ingredient), and dynamics to allow the storms to persist until after dark (which is not the case in the alley as far west as Colorado.

You can see NWS is concerned for tomorrow:
CONFIDENCE IS INCREASING IN POTENTIAL FOR MULTIPLE TORNADOES OVER
CATEGORICAL MDT-RISK AREA...SOME POSSIBLY STG-VIOLENT. ATTM MOST
PROBABLE SCENARIO IS FOR AT LEAST TWO TEMPORALLY SEPARATED SVR
EPISODES WITH SOME SPATIAL OVERLAP...
1. AFTERNOON/EVENING FROM S-CENTRAL/ERN TX THROUGH ARKLATEX
REGION...AND
2. OVERNIGHT AND CONTINUING INTO DAY-2 PERIOD...ESPECIALLY OVER
PORTIONS LA/MS.
ONE OR TWO CORRIDORS OF RELATIVELY CONCENTRATED/STRONG-VIOLENT
TORNADO THREAT MAY DEVELOP WITHIN BROADER OUTLOOK AREAS NOW
DELINEATED. CONCERNS ABOUT COVERAGE/DURATION OF FAVORABLE SUPERCELL
MODES PRECLUDES GREATER CATEGORICAL OUTLOOK THIS SOON.

Basically the warning here is Eastern Texas, Arkansas, Missisippi, Louisana for the greatest risk of multiple tornadoes....why is this?
Strong Instability (CAPE above 1500 J/kg), combined with very strong deep shear (50-70 knots), more than sufficient for supercell thunderstorms. Furthermore, strong low level shear, and the depth of the shear means the rotation is deep and favouring the usual right moving supercell type. The rotational shear is also potentially strong enough to likely produce cyclical type supercells, meaning supercells that produce multiple tornadoes, with long life cycles, and generally are the source of the most potent and dangerous tornadoes. This is expected to continue overnight (meaning dangerous night tornadoes) into the MS valley. Interestingly for this early in the season there is significant capping, such that cells will remain isolated and separate.

And Saturday:
THIS SUGGESTS A HIGH
LIKELIHOOD OF MULTIPLE BANDS/CLUSTERS OF SUPERCELLS CAPABLE OF
DAMAGING WINDS...LARGE HAIL AND TORNADOES PERSISTING THROUGH THE
PERIOD.

THE GREATEST THREAT FOR CYCLIC SUPERCELLS CAPABLE OF SIGNIFICANT
TORNADOES WILL EXIST FROM THE VICINITY OF THE SECONDARY LOW TRACK
SEWD INTO THE WARM SECTOR /I.E. LARGELY DELINEATED BY MDT RISK
AREA/. HERE...ENVIRONMENT WILL BE CHARACTERIZED BY DEEP EFFECTIVE
INFLOW LAYERS YIELDING EFFECTIVE SRH IN EXCESS OF 200-300+
M2/S2...MODERATE INSTABILITY AND 50-70 KT OF DEEP-LAYER SHEAR.
SEVERE STORMS CAPABLE OF MAINLY DAMAGING WINDS AND TORNADOES WILL
CONTINUE THROUGH THE NIGHT WITH THE THREAT EXTENDING NWD THROUGH THE
OH RIVER AND AS FAR E AS THE CNTRL/SRN APPALACHIANS AND ERN GULF
STATES.

This sort of follows the previous as the trough moves, with a nasty potential for dangerous cyclical supercells likely to produce strong tornadoes in reasonable numbers.

The only question for chasers now, is what effect will the outflow boundaries from the last nights convection have on the system...and where things happen, not to mention where is the triple point (interesection of cold and warm fronts). And of course, where can you get the best chase country in tandem with the best possible conditions.

I'll continue to update as this event unfolds.