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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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The plots below show the forecasts from 5 February 00UTC (left) and 6 February (middle) and 8 February (right). The lines represents HRES (red), ENS control (purple), ENS members (blue) and observations (black).
The two panels below show the ENS forecasts expressed by the latitude (x-axis) when 2MT and T850 hPa dropped below freezing (for a fixed longitude near Fort Worth, Texas) as a function of the forecast lead time (y-axis, in days) for a same verifying time, ie, 9th February 2021 12Z (~06 Local time, prior to sunrise). The ENS systematically is under-spread and warmer near surface but not near the top of the boundary layer. Vertical solid represents the outcome latitude (analysis).
Prediction of the peak of the event
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The plot below shows the forecast evolution for 1-day temperature mean averaged on 11 February inside the box outlined in the plots above (1x1 degree around Dallas). The plot includes observation (green),HRES (red dot), ENS control (purple dot), ENS distribution (blue boxes, median square and mean diamond) and model climate distribution (red boxes) plus the ensemble from DWD ICON model in evergreen and NCEP in light green.
The plot below shows the forecast evolution for 3-day temperature mean averaged inside the box outlined in the plots above (1x1 degree around Dallas). The plot includes observation (green), HRES (red dot), ENS control (purple dot), ENS distribution (blue boxes, median square and mean diamond) and model climate distribution (red boxes) plus the ensemble from DWD ICON model in evergreen.
4. Experience from general performance/other cases
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