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URMA Great Lakes change: GLERL pseudo-observations disabled in obsproc v1.2.3

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NOAA/NCEP disabled GLERL pseudo-observations in URMA over the Great Lakes as part of an obsproc v1.2.3 upgrade, citing low wind-speed bias impacting NBM guidance. Effective on or about March 24, 2025.

On February 24, 2025, NOAA/NCEP issued Service Change Notice 24-19 describing a change to the UnRestricted Mesoscale Analysis (URMA) over the Great Lakes as part of an obsproc v1.2.3 upgrade.

The change is effective on or about March 24, 2025, beginning with the 1200 UTC cycle.

What is changing

  • NCEP upgrades the observation processing package (obsproc) to v1.2.3.
  • As part of that upgrade, NCEP disables the generation of Great Lakes Environmental Research Laboratory (GLERL) pseudo-observations over the Great Lakes in URMA observation processing.
  • NOAA notes the change follows reports of a low wind-speed bias in URMA analyses over Lakes Erie and Superior, which was negatively impacting National Blend of Models (NBM) forecast guidance.
  • For consistency, the notice states that all types of GLERL observations (wind, temperature, and moisture) are disabled.

The notice explicitly says these changes do not impact RTMA or RTMA with Rapid Updates (RTMA-RU).

Why this matters

  • If you use URMA as an analysis baseline ("truth") for verification, you should expect a step change in URMA fields over the Great Lakes around the effective date.
  • If you rely on NBM guidance in Great Lakes nearshore/offshore wind workflows, NOAA's note about URMA wind bias is a strong hint that downstream behavior may change around the transition window.
  • NOAA also flags a general operational concern: decoders should be flexible about possible GRIB ordering/scaling changes around model/obsproc implementations.

Practical checks you can run

  • Split validation windows: When verifying wind/wind-gust over the Great Lakes, compare pre- and post-change periods separately rather than fitting a single baseline across the March 24, 2025 changeover.
  • Cross-check against RTMA: Because RTMA/RTMA-RU are listed as unaffected in the notice, they can be a useful reference point when diagnosing differences in URMA behavior during the transition.
  • Watch downstream post-processing: If you have thresholds calibrated on URMA/URMA-derived products, plan to re-check those thresholds (especially for wind-sensitive applications).

What this means for GribStream users

GribStream supports:

If you're backtesting or comparing guidance across this change, GribStream's historical access makes it straightforward to pull URMA/RTMA fields around the transition date and quantify whether any thresholds or bias corrections need to be updated.

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