Convection in SW Texas

An isolated thunderstorm developed over Pecos County in southwestern Texas. While the storm wasn't producing a lot of lightning, it was interesting to see how the increase in lightning activity corralated to brief increases in radar reflectivity, resulting in the production of some small hail. This thunderstorm pulsed up and down a few times in intensity and was proceeded by brief increases in the flash rate of lightning.

Also of interest was watching the anvil of the thunderstorm blow off to the east-southeast pretty rapidly, while a field of surface cumulus clouds steadily moved to the north-northwest in an area just to the east of the thunderstorm. It will be interesting to see if development takes place this evening in the zone of higher shear to the east of the afternoon thunderstorm.

Gary Cannalte

CI regular vs severe

I have noticed on severe occasions where the CI (for regular convective initiation) is much lower than the probability depicted by the Severe CI product.  Intuitively, one would think the probability for convective initiation should ALWAYS be higher than the probability for convective initiation to lead to severe weather.

Fig 1. "Regular" CI product at 2037 UTC. Note the green 50% CI probability toward central AR.

Fig 2. Severe CI product at 2037 UTC. There is a 90% probability for severe convection in the same location as the above image.

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Comparing 1-min, 2-min and 5-min running FED totals

For the GLM products such as Flash Extent Density (FED), we are looking at looking at running accumulations over different time periods of 1 minute, 2 minutes and 5 minutes (and maybe others?). The 5-min (1-min update) loops are smoother, but may miss or be slower to show sudden changes. The 1-min loops tend to be rather jumpy and may obscure trends. The 2-min (1-min update) loops might be a good compromise, but we are still comparing.

The 3-panel  below shows a 30-min loop comparing all three accumulation times for a storm over northwest MS. Comparing the frame-to-frame variability among the three options, the 5-min loop is certainly smoother, but has higher max values than the 2-min or 1-min loops, which may require a different color scale.

Also here is a plot from the Tracking Meteogram which compares the max values from all three accumulation types. Note the 5-min accumulation is in the middle. The 2-min running accumulation (bottom curve) might be the best combination of smoothness and sensitivity.  A.Cope

RGB of convective initiation

I found this capture of convective initiation to be interesting and cool to watch. As dry air on the south side of the MCV advected east into far western AR, storms would fire near the state line. The Day Cloud Phase Distinction shows this cycle very well and provides a nice optical depth to the satellite loop.

Fig 1. Mid-level water vapor loop showing dry air advecting into western Arkansas on the south side of the MCV.

Fig 2. Day Cloud Phase Distinction RGB depicting convective initiation on OK/AR state line on the leading edge of the dry punch.

I was looking for any obvious signal in the lightning data for that point but nothing significant seemed to stand out for this case. That said, it is producting consistently high Flash Event Density with relatively high Total Energy values and low to moderate Average Flash Area.

Fig 3. Clockwise beginning with upper-left image:  Flash Event Density 2 minute data (1 minute update), Average Flash Area 2 minute data (1 minute update), Total Power 2 minute data (1 minute update), ENTLN 5 minute data (1 minute update).

It was interesting, however, that the AllSkyLAP CAPE for this area was quite muted. Makes me wonder if this isn't a function of the model first guess. Since the sky was cloudy and the satellite data relied more heavily on the GFS (which was convectively contaminated), I didn't find this data particularly useful.

Fig 3. AllSkyLAP CAPE product showing CAPE values < 350 J/kg over eastern OK and western AR. This is using primarily the GFS background information.

Fig 4. GFS QPF for 18Z. The convective contamination likely resulted in erroneous CAPE values on the AllSkyLAP product.

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AllSkyLAP CAPE missing frames

The AllSkyLAP CAPE product was coming in at sporadic times. A good frame would arrive at a give time (example 1758 UTC) but then an empty frame would populate in AWIPS at 1800 UTC. This really limited the usefulness of looping the product.

Fig 1. AllSkyLAP CAPE loop with missing frames.

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Large CAPE Spread Between AllSkyLAP & NUCAPS

Satellite loops showed a diminishing CU field across SJT CWA and surface dew points have been decreasing. AllSkyLAP CAPE has been diminishing over the past couple of hours, but was still near 1500 J/kg. A NUCAPS sounding (Improved Latency) around the same time had CAPE values well under 1000 J/kg and a distinct cap. The CAPE values over the past couple of days have been closer on AllSkyLAP & NUCAPS, and the NUCAPS values have typically been higher. NUCAPS  appears to be picking up stability changes more quickly than AllSkyLAP in this case.

1929Z AllSkyLAP CAPE with NUCAPS sounding point overlay

1941Z NUCAPS Improved Latency Sounding (North-central portion of SJT CWA)

Alex Brown

ProbSevere Color Table

The ProbSevere model color table doesn't make it easy to discriminate between very low probabilities and medium-high probabilities. With both ends having something of a purple/pink color, I had to look at the included table to determine where it fell on the scale. The high end of the table looks fine, but I think the lower end could use some modifications.

Fig 1. ProbSevere model overlayed on MRMS RALA product.

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Afternoon Forecast Thoughts for SJT

Quiet conditions are ongoing across SJT forecast area as of 20Z. Surface temperatures have climbed to near 90F, with accompanying dew points in the mid-upper 50s. Current CAPE values per AllSky LAP imagery are approaching 1500 J/kg which is in line with short-range guidance (these values were a few hundred J/kb higher than derived GOES CAPE).  Temperatures are forecast to climb into the mid 90s over the next couple of hours, which will help push CAPE values over 2000 J/kg. This should be sufficient to initiate convection, along with the arrival of a weak upper-level impulse and an increase in bulk shear values up to 35 kts.

Mostly clear skies at 20Z with weak West/East boundary across southern CWA

AllSky LAP CAPE near 19Z (1000-1500 J/kb over CWA)

AllSky LAP PW 4 Panel showing ample low-mid level moisture

Alex Brown

GOES/All-Sky Instability Images

The 4-panel loop below shows some of GOES/All-Sky LAP stability product, from about 1630z to 1900z. Much of this data comes from the GFS, which generally blends pretty well with the GOES data, but sometimes you can see the "seams". There is an area of relative stability around the MCV over eastern OK, and a nose of relative instability pushing into southern AR.  A.Cope

Forecast 5/16/18

An MCV from overnight convection is located over northeast OK and will drift eastward over the Ozarks later this afternoon. Moderate instability is observed over much of AR with AllSkyLAP CAPE values on the order of 1000-1500 J/kg.  The 12z raob from LZK indicated a fair amount of instability aloft, aided by relatively steep lapse rates from near 700mb through approx 550 mb. The AllSkyLAP PWAT was ~1.3" at 12z which compared very nicely to the 1.33" observed by the raob. Relatively dry air was observed aloft per the AllSkyLAP 0.7-0.3 sigma PWAT.  MLCAPE is progged to increase to around 2000 J/kg this afternoon with a DCAPE just over 1000 J/kg.

Fig 1. Split Water Vapor depicting MCV over northeast OK.

Fig 2. MCV evident in the high-level water vapor imagery.

Storms are ongoing over the Ozarks, but are expected to increase in coverage across the state throughout the afternoon in the weakley capped, low-shear environment. Shear will be limited today given the overall weak synoptic flow and vicinity of the aforementioned MCV. In general, anticipate strong to marginally severe, pulse ordinary cells but a few multicellular clusters are possible should storms develop strong enough cold pools. The primary hazards will be hail up to 1 inch in diamter and localized damaging winds.

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