Dryline Initiation on the Cap: Lubbock, 24 May
Overnight shift, Lubbock, Texas, 24 May. At 00 UTC a 1004 hPa lee trough lies along the New Mexico–Texas border, a dryline bulges east near Pecos, and southeasterly winds of 12–15 kt carry surface dewpoints of 19–21 °C into the Permian Basin. The 00 UTC Amarillo sounding shows a well-mixed layer to 850 hPa, a 2–3 °C inversion between 850 and 800 hPa, and mixed-layer CIN of 60–90 J kg⁻¹ that erodes only where surface heating pushes the temperature above roughly 33 °C. The 850 hPa southerly jet is 35 kt. Site elevation is near 990 m, so station pressure is about 900 hPa. HRRR initiates isolated cells on the dryline at 22–23 UTC; the NAM nest keeps MLCIN above 75 J kg⁻¹ all evening. At 21 UTC visible imagery shows a cumulus field along the boundary with tops reaching the −12 °C level but no glaciation. Surface temperatures are 31 °C at Lubbock, 34 °C at Midland, and 29 °C at Hobbs under thin cirrus.
1. Estimate the LCL and convective temperature from the surface parcel, then say whether initiation by 23 UTC is more likely near Midland or near Lubbock. Show the parcel reasoning.
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Moisture and LCL
- Station pressure is about 900 hPa, so Td = 20 °C gives e ≈ 23 hPa and w ≈ 16.5 g kg⁻¹.
- The surface T–Td spread is about 11 K. Using the dry-adiabatic/dewpoint convergence rule of thumb (~125 m per K of spread), LCL ≈ 1.4 km AGL, near 760 hPa.
The cap, not the LCL, decides the outcome The 850–800 hPa inversion (roughly 0.5–1.0 km AGL) sits below the LCL. The parcel must cross it as a dry parcel, so all of the inhibition is spent there with no latent-heat offset. The convective temperature is therefore set by the dry adiabat that reaches the inversion top with enough residual buoyancy to carry on to the LFC.
Numbers
- Lubbock at 31 °C: the dry adiabat through 31 °C arrives at 800 hPa near 24 °C. Against a 2–3 °C inversion and the environmental temperature there, the parcel is still an estimated 2–4 °C short of the ~33–35 °C convective temperature. Three more hours of heating adds only 1–2 °C, giving a marginal 32–33 °C.
- Midland at 34 °C with comparable dewpoints is at or above its convective temperature now, and sits closer to the dryline bulge.
Conclusion: initiation is more likely near and south of Midland by 23 UTC. Lubbock storms require either eastward dryline mixing or a cold pool from an earlier storm undercutting the cap.
2. HRRR breaks out cells at 22–23 UTC while the NAM nest holds MLCIN above 75 J kg⁻¹ all evening. Which guidance do you weight, and how does the satellite cumulus trend change your decision?
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Why they disagree The two solutions differ far less in the synoptic pattern than in the boundary layer:
- Dryline position, which is sensitive to soil moisture and land-surface fields inherited from each parent model.
- Boundary-layer scheme mixing depth - a difference of a few hundred metres in mixed-layer depth changes the entrained moisture and therefore MLCIN.
- Cap treatment - a 1–2 hPa difference in inversion depth or strength shifts CIN by tens of J kg⁻¹, which is the whole forecast here. The nest is initialized from the coarser parent, so it inherits the parent's moisture and boundary-position bias; the hourly HRRR cycle ingests radar and has a better chance of placing the dryline correctly.
How to weight it Favour HRRR for timing and location of the boundary, but treat its storm initiation as a probability, not a deterministic yes. Convection-allowing models are known to initiate too readily along sharp drylines and to produce gridpoint storms that never verify.
The observation that outranks both A persistent cumulus field reaching the −12 °C level without glaciation means updrafts are reaching roughly 4–5 km and stalling. If tops keep rising and the first turrets glaciate and expand anvil, the cap has been breached - that is the real-time fingerprint, worth more than either model. If tops flatten and the cumulus detaches from the boundary or bases rise, mixing is eroding moisture and the cap wins.
3. Assuming storms do initiate, what hazards do you advertise for 00–06 UTC, and what wording fits the uncertainty?
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Environment MLCAPE in the 1500–2500 J kg⁻¹ range, 35 kt low-level jet, and effective bulk shear of roughly 30–40 kt put the mode in supercell territory. Two details shape the hazard mix:
- High cloud base. With LCL near 1.4 km (about 4,500 ft AGL), sub-cloud evaporation will be efficient. Expect outflow-dominant storms with dry microbursts and severe surface gusts, especially from the high-based cells near Hobbs.
- Hail. Steep mid-level lapse rates and strong shear support large hail; that is the primary severe threat.
- Tornado. Non-zero near the dryline/outflow intersection, but the high LCL suppresses the low-level stretching efficiency; keep the probability low but non-zero rather than zero.
Wording Because initiation is conditional, do not publish 'storms at Lubbock at 23 UTC'. Publish something like: isolated to scattered severe storms developing along and east of the dryline after 22 UTC, primarily large hail and damaging wind gusts; coverage uncertain, with the greatest chance south and east of Lubbock. Attach a probability graphic rather than a deterministic band, and state the failure mode (cap holds, storms stay south) in the briefing so the next shift knows what to watch.