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Episode 4

When the Models Run Cold: Forecast Biases, CAPE, and Southeast Severe Weather

In this episode, we break down a research paper examining how high-resolution convection-allowing weather models handle near-surface temperature and moisture in severe storm environments. The discussion focuses on a key operational finding: widely used HREF model members often run about 1 K too cold in Southeast U.S. cool-season warm sectors, with some members closer to 2 K cold during daytime heating. The hosts explain why that seemingly small error can meaningfully reduce forecast CAPE, especially in high-shear, low-CAPE tornado environments, and what it means for forecasters using HREF and future RRFS guidance.

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When the Models Run Cold: Forecast Biases, CAPE, and Southeast Severe Weather

In this episode, we review a research paper on regional and seasonal biases in convection-allowing model forecasts of near-surface temperature and moisture.

Key Topics

  • What convection-allowing models are and why they matter for severe weather forecasting
  • How the HREF is used operationally by forecasters
  • Why 2-meter temperature and dewpoint are critical in storm inflow environments
  • The importance of warm-sector thermodynamics for tornado and severe thunderstorm forecasting
  • Two verification methods used in the paper:
    • Tornado-centered forecast errors compared against URMA
    • Warm-sector forecast errors compared against ASOS observations
  • The major finding: HREF members show a consistent cold bias in Southeast U.S. cool-season warm sectors
  • Why a 1 K cold bias can substantially reduce CAPE in high-shear, low-CAPE setups
  • Why the NAM nest and NSSL-WRF were often colder than other members
  • How Great Plains warm-season results differed, with less consistent temperature bias across models
  • Early findings suggesting RRFS may share some of the Southeast cool-season cold bias, though generally on the less-biased end

Why It Matters

Small temperature errors can have large impacts when severe weather potential depends on marginal instability. In Southeast cool-season events, where tornado environments often feature strong wind shear but limited CAPE, forecasters need to recognize when model guidance may be too cool and therefore underestimating instability.

Forecasting Takeaway

High-resolution model output is powerful, but it is not perfect. In Southeast cool-season severe weather setups, human forecasters should consider the possibility that CAM guidance may be underplaying low-level warmth, instability, and storm potential.