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3.1 Data assimilation
3.2 HRES
The week before the main cold surge (8-14 February) temperatures started to drop. For several days a broad low stratocumulus deck expanded and persisted for several days over a hide area over the Great Plains and east of the Rockies. Several sites reported mist/drizzled/overcast conditions for few days in Texas and, Oklahoma. No indication of snow on the ground for most of Oklahoma, and Texas at the start of the period. Yet temperatures near surface were systematically warmer compared with the few sites checked. Temperature errors could be associated with cloud errors. The plot below show the VIS image at midday local time and the inset figure corresponds to a very short range forecast (H+18) simulated visible image. The cloud area is much reduced and "less thick"- pockets of open cells are visible in the HRES.
Checking the forecast errors of 2m temperatures over Texas it is clear that the onset of the cold was not well handled by the model. Forecasts ranges beyond three days had errors exceeding 15 degrees (model warmer).
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