3/14/08 FCST: OK/AR/MS

[Rich Thompson]

I assume many folks are focusing on the following trough across the srn Plains by roughly Mon 3/17, but I see a potentially significant supercell episode on Friday 3/14 from ern OK across AR and nrn MS overnight. A very strong zonal jet streak and associated deepening surface cyclone are forecast to move across OK during the day. Meanwhile, a couple of days of return flow should result in 60s dewpoints beneath a plume of steep mid level lapse rates from the west. The net result should be a strongly sheared warm sector with reasonably strong instability and fairly cold mid level temperatures - a recipe for very large hail and tornadoes.

FWIW, I seem to recall that the previous operational GFS runs have been substantially faster with the following srn Rockies trough than either the MREF mean and ECMWF forecasts for the 3/17 system.

Rich T.

 

[Chris Wilburn]

I was just getting ready to post a forecast myself for Friday. This evenings run of the NAM looks very nice for eastern OK and into Arkansas. Plenty of moisture, instability and shear will be in place along with a very strong surface low of sub 990mb forecasted near central OK by 00z. I may stay close to home and focus on NE OK since I am not a big fan of the western Ark. area. Another line of cells may for in central Arkansas ahead of the dryline tha is progged to be in place in eastern OK at 00z. Still a few days to look at things I guess.

 

[Donald Giuliano]

Interesting that you should mention Friday, as I was just checking it out and thought it looked quite interesting. The NAM's 2000+ CAPE with zero CIN ahead of a nice dryline bulge underneath the leading edge of a strong jet nose certainly caught my eye, although I must say I'm generally not a big fan of round 850 mb lows (though my concerns about this lessen the farther east an event sets up). We'll see what happens, but the way this year is going I'm sure there will be about 107 tornadoes to get us that much closer to having 1000 by the end of April--and then nothing until October.

 

[Jeff Piotrowski]

Triple Red Alert

I like the new 72 Hour panels are coming together for a tornado’s NE-OK, SW-MO, NW-AR. SPC may have a moderate risk for Friday.

Here is Tulsa Skew-T click on the 72 hour.
http://beta.wxcaster.com/cgi-bin/parse_skewt_trace_all.cgi?model=NAM&STATIONID=KTUL

http://wxcaster4.com/nam/CENTRAL_ETA212_ATMOS_STP_72HR.gif

Update at 11:30 PM based on new 12KM ETA it looks like the warm front lifts north during the day to about PNC-TUL-FSM line by 0Z. Then the dryline punches NE into the warm front with tornadoic storms forming.

 

[Jim Bishop]

The 12z Wednesday WRF and GFS have the dryline further west across Central Oklahoma per the WRF and Western Oklahoma per the GFS. In both cases the veered 850mb flow is a big concern for both a strong cap and limited deep moisture.

But this is a strong upper system imbedded in the zonal flow with pretty good low level moisture being tapped and impressive 0-3km & 0-6km shear. So definitely something to key a close eye on as the models figure out exactly what this sytem will do.

 

[Heidi Farrar]

There has been a lot of talk around with regard to the GFS looking goofy for this Friday-ish event. ... Just looking at my area (north-central AR) ... It's hard to believe a model that is showing 15 knots on the 850mb winds at 6Z SAT (as the GFS is) when the WRF and others are showing winds in the 50-60 knot range for the same time. I don't think it's a matter of the GFS playing things faster or more slowly, either. The MAX the GFS shows with this system, suddenly, is still in the 15-25 knot range. Odd. It will definitely be interesting to see whether the GFS starts to look more like the WRF with the next run. One thing is certain at this point -- they can't both be right!

I think when tomorrow's Convective Outlooks come out, we'll know which models the SPC forecasters are believing the most when we see whether portions of OK/AR are in a Day-2 MODERATE RISK or not.

 

[Brett Adair]

I was taking a look at this setup, and it appears that a big chunk of the warm sector will remain capped. The main action will likely be along the warm front as the nocturnal LLJ cranks up across the LWR MS Valley and the upper level streak begins to come into play. Steepe plume of lapse rates that Rich mentioned will set the stage for massive hail if the inversion is overcome which it should be in AR/TN/N MS and further eastward along the boundary with time.

Looks like a high end SLGT risk at least from my standpoint at this point in time. If instability can be realized from the NAM and the forecast inversion is weaker, the warm sector would be a supercell playground with increasing wind fields.

 

[Bill Schintler]

Chase Target for Friday, March 14

Chase target:
Cleo Springs, OK (35 miles west of Enid).

Timing and storm mode:
A limited threat for diurnal severe storms is possible after 4 PM CDT, with large hail the primary hazard. Well after dark, a more significant severe event is likely in northeastern OK, western AR, and southwestern MO as a developing MCS tracks into an environment of increasing low level directional shear as the nocturnal LLJ strengthens.

Synopsis:
Earlier MDL runs had differed considerably with the track of the ULVL speed max and attendant SFC reflection, with the GFS well S of the NAM. The latest MDLs indicate a smaller discrepancy, with the UKMET having the largest SRLY track, and the NAM continuing to favor a NRLY track.

Discussion:
SFC low pressure will strengthen in response to forcing in the left-exit region of a 150kt H25 streak during the morning hours on Friday. This low should track from the OK panhandle ESE towards Clinton, OK during the afternoon hours while exhibiting a weakening trend late in the period. During the daylight hours, the favored area for marginally severe storms will be along a developing WF trailing E and NE from the low as a narrow axis of SFC moisture and attendant instability is pulled W along and just S of the WF. Hodograph curvatures will be moderate over a limited area as a WRLY 35kt LLJ surges over of SRLY 10kt SFC flow. SFC-6km shear will decrease rapidly from S to N as the strongest H5 flow spreads over SRN OK, and deep-layer shear should be around 40kts near the target area; sufficient for storm organization. During the overnight hours, areas of NERN OK, SERN KS, and SWRN MO should see a similar scenario as that which verified in SERN OK this evening; where the BL decoupled and SRH’s increased as a LLJ surged over backing SFC flow. As was the case this evening, a few tornadoes should verify despite little or no apparent SFC-based instability.

- bill
12:00 AM CDT, 03/14/08

 

[Gabe Garfield]

I have been monitoring the model output from the WRF and the GFS for a few days now, and the solutions seem to be pretty consistent. Currently, I like the area immediately (roughly 20 or 30 miles) N and NE of the forecast position of the surface low. I am optimistic that a few good storms could be in the offing; perhaps even a tornado, if the low-level shear can compensate for the lack of good moisture. Deep-layer shear should be more than adequate for supercells and a very nice EML should help to develop weak to moderate instability. The only caveats I see at this time are: the propagation speed of the wave (fast); I'm not sure what I think of WNW flow in March (storm mode?); and a relatively dry boundary layer.

 

[afischer]

I agree the probability for convective initiation looks decent tomorrow by 22-00Z near and east of the deepening surface low, as the upper wave now over Nevada slams into the area and some favorable jet dynamics come into play. I'm less concerned about the marginal BL moisture (MLLCLs ~1600-2000m) than I am about how exactly the convection will evolve. Interestingly, soundings from the last couple runs of the NAM-WRF and NAM-KF (and to a lesser extent the GFS) show a somewhat stout cap hanging on at the EML base even while the strong lift comes blasting in, which is a little unusual to see esp from the NAM-WRF. Maybe this is a signal that trashy convection might evolve farther north and then become undercut by the developing cold front... wouldn't surprise me given the "look" of the models. If something surface-based does go in advance of the low and the model thermodynamics are to be believed, it looks to be strongly forced through a high LFC which would hurt tornadic potenital. I have commitments this afternoon that I can't get out of or I'd probably roll the dice since it's fairly close to home. I'd settle for a supercell... and given this is NW OK, there will probably be a tornado regardless.

 

[Jim Bishop]

The strong CAP and limited moisture will be the main inhibiting factors this afternoon/evening across northwest/northcentral OK. However, given very cold temps aloft combined with upper 40s/lower 50s dewpoints and 850mb dewpionts in the 2-6C range, 500-1000 j/kg CAPE should develop across the warm sector.

Forcing ahead of the surface low near the triple point in northwest Ok, combined with strong upper support associated with a strong vorticity maximum forecast to enter the region by 00z, should be enough to initiate convection by 5 or 6pm.

Wind shear profiles will be more than adequate for supercells. But given somewhat limited boundry layer moisture these storms will initially be of the LP variety and somewhat high based. However, near and after dark storms will have some tornado potential as the bases become lower. But overall the main threat will be large hail.

In any case there should be some great LP supercells in northwest Oklahoma near dusk producing some big hail - with a tornado threat increasing after dark.