Severe weather potential

Interesting forecast shaping up for tomorrow evening. Somewhat strong trough will move into OK tomorrow bringing with i the chance for significant severe weather to central and eastern OK and continuing eastward overnight.

The more certain portion of the forecast is the later period late Sunday night when it appears probable that a severe MCS/squall line will tear up parts MO, AR, LA again. I think its likely as more time will allow for more moisture to advect northward in what amounts to me to be an odd linkage between a pre-existing low level jet well east of the effects of the trough and the trough. The apparent phasing of the two will occur later sunday evening. These highly dynamic environments lead to some interesting MCSs.

The early part of the threat in central OK is less certain. To me, at the moment, the most interesting part of the forecast is that the features of interest appear to slow down before entering it the Plains, then speed up once they do. This makes for a confusing scenario. The prominent features I see this morning are the dryline in western OK, the dryline warm front intersection in NW OK, and the rather strong 700 hPa cold advection over the dryline all at 00 UTC. Now, I have no idea what to expect in this type of slow then fast large scale regime. This fact alone adds uncertainty for my limited experience of significant severe weather in OK during Feb.

What is not clear is if the cold advection aloft will be enough to remove the cap, and if the instability will be large enough to be realized. We just had some rain followed by cold temperatures, and soil water fractions are high apparently. The last 10 and 30 day rainfall maps show that western OK is dry and eastern OK has seen around 2+ inches. So it is possible that strong sensible heating will take place on Sunday in areas that are drier ahead of the dryline. Will this actually matter? I will tell you on Monday.

The other major issue is how the dryline and shear align. At the moment the mean wind for sunday evening appears to be from the SW and the dryline is oriented more north-south. This angle difference will allow storms to move off the dryline. The shear vector should also orient itself across the dryline but the magnitude of that angle will be very important to storm mode.

I guess my inclination at this moment are that all the necessary ingredients can be found, it will be a matter of how they come together and when that will occur and for how long. For now, at least, it is important to get that moisture screaming northward. I could use a good storm chase.

A lot will change. But I think there is a chance for supercells. I am uncertain where convective initiation will occur, and where severe storms will get going, and what that initial mode will be. I will evaluate the Short range ensemble Forecast (SREF) when the 15z run becomes available later this afternoon.  Really this is just me getting my thoughts ramped up ... otherwise known as situational awareness.

MCS a plenty

 The storm track has been active enough to produce a number of MCSs. The movie above gives a fair depiction of the number of MCSs in the last 14 days or so. In fact over NE the 14 day rainfall departure has a large area of 4 inches above normal with peaks in the 8 inch range. The area shrinks if you go out 30 days but the numbers hold up. Pretty impressive narrow rainfall corridor.

Below are the 1000 hPa Reanalysis 2 temperature, 925 hPa specfic humidity, 850 hPa v wind, and 300 hPa vector wind anomalies maps. The low level warm anomaly centered over CO-NM is accompanied to its northeast by a large anomaly in specific humidity near 4 g/kg coincident with a 3 m/s v wind anomaly. The upper level jet is also anomalously strong and lies just north of NE where most of the MCS activity has been.

 [As an aside, this El Nino pattern for the interior Pac NW where I live has seen some abundant rainfall as well. My locale is 1" above normal as of June 10 which is the equivalent in climatology of October 1st! ]

The El Nino pattern and thus some of the seasonal outlooks  I have seen have suggested that the western Great Plains would be anomalously high in precipitation AND slightly below normal in terms of temperature. The funny thing about El Nino patterns for the Plains: the precipitation signal is weak but present, but the temperature signal is very weak. This is a new area of research for me so my discussion here is lacking. However, this particular El Nino emerged rapidly from a La Nina state {and is currently forecast to go back into a La Nina state}. This last happened in 2006 but previously did not happen since 1976.

In terms of the severe weather season we have been having, statistically it appears dull. The tornado count is hovering near the 25th percentile despite an active early May and an intermittent late May and early June.

This year has seen an active May (not compared to 2008) which is somewhat unusual on this graph but I dont believe is unusual over the last 20 years.  I will have to check that out.

MCS development in CO-NM

An MCS developed tonight in CO-NM in an impressively warm environment aloft.

At first glance, it appears upslope flow penetrated deep into CO and Northern NM, underneath westerly flow. The terrain served to initiate convection. The upslope flow was responsible for transporting some moisture approaching 50 F to the region.

The impressively warm atmosphere apparently did not stop convection along the NM-AZ border either. As with a previous study (Bunkers et al. 2010), this highlights that warm temperatures aloft do not necessarily reflect convective inhibition. The low level moisture, lapse rate aloft, and forcing are important. So why the reliance on these warm temperatures? Forecasters rule of thumb, that when the mid level temps get warm, convection becomes increasingly unlikely. Most likely this "rule" was the recognition of a process of warming by subsidence such that a cap was present and forcing was either weak or non-existent.

So, now that my script is done running here are some stats at 500 hPa:
STATION   T    T90
ABQ       -4C    -6C
DDC       -4C    -6C
LBF        -5C    -7C
MAF       -3C    -5C
AMA      -4C    -6C
top          -7C    -7C
oun         -3C    -6C
fwd         -4C    -5C
epz          -4C    -5C
oax          -6C    -7C
drt           -4C     -4C
crp           -4C    -4 C
bro           -6C    -4C
dnr           -7C    -7C
sgf           -5C    -6C
lzk           -4C    -6C

These are the ones I have checked and all but 1, have exceeded the 90th percentile. It would be interesting to see how often all these stations have simultaneously hit the 90th percentile of warm Temperatures.

warm front MCS

So I fell asleep last night and missed out on real time watching of the Mesoscale Convective Systems that formed. May 11-12 2010 .

I am always fascinated by the different evolutionary modes of convective initiation. Yesterday's mode was scattered convection under a moderate shear-high instability day. But it was effectively capped, given that multiple elevated supercells formed and quickly dissipated much to the chagrin of VORTEX2 participants. It was a classic Iowa warm front event, with little to no forcing but shear and instability abound.

at 2200 UTC, the first supercell was in the process of initiating in SW OK and after 1 hour falls apart. It skirts an area to the north just south of woodward I believe, and another set of CI takes place between 2345 and 0015 UTC. Again short lived supercells. However, the anvil aloft is cold and the cloud does not dissipate. So it looks like a storm is there, but on radar it was a useless blob of 30 dbz echo.

By 0145 the warm front in SE KS gets hot and explodes into the primary MCS. This initial convection gets going and after perhaps 45 minutes to an hour backbuilds to the west along the front while that old cloud shield drifts in by 0402. Not much longer later the old cloud shield is rapidly dissipated around the MCS as it swells to maximum size virtually covering MO by 1100 UTC.

at 0345 UTC SW KS sees another MCS initiate and suddenly from KS into NE convection is firing up. This MCS looks asymmetric almost half circle like but looks very similar to an MCC. Once these two systems get close enough to each other the eastern MCS appears to dissipate, at least its western end. The radar reflectivity pattern was chaotic in nature. This is a different breed of cloud system most notably MCC like. It doesnt come with much severe weather oddly enough given its environment. But the cloud shields are huge.

After looping tonights water vapor imagery .... the eastern MCS traveled from IL through VA out over the Atlantic. I noticed a pretty compelling anticyclonic gyre aloft. very classic long lived system.

Also note that CI occurred on both sides of the water vapor dry slot ... to the east failed and to the west got going.

The surface data doesn't look that compelling .. where the warm front convergence was occuring in SE KS is where CI took place along the front. beyond that moisture was slowly working north (implied moisture convergence). Both systems were on the anticyclonic shear side of the upper level jet which was just by position only since the wind field aloft was pretty homogenous.

Soundings from LMN indicated the cap had weakened boasting CAPE of between 2-3000 J kg-1 at 2100 UTC. The wind sbacked at 0000 UTC and the low levels became saturated as the cap layer seemed to strengthen a bit. though perhaps the lapse rate steepened under slight cooling aloft. 6 hours later the layer below 850 had a nasty 50 kt LLJ and warmed and moistened appreciably. moisture depth to the east was high, as the low levels appeared to be saturated.

visible imagery indicated some sort of cloud boundaries, like elevated outflow boundaries that might be implicated here. But I dont have access to the OK mesonet data to see if anything showed up there. In my experience it doesnt show up on the sfc winds, but it might be in the pressure field.

Interesting day. Maybe V2 will have some photogrammetry of these supercell initiations.