Verification of ETA-12 Forecast After 36 Hours

We now compare the Eta-12 model prediction beyond 36 hours to the verification, subjectively provided by the Buffalo NY WFO in its storm summary for the "Bald Eagle" lake snowstorm. What should we expect out of the Eta-12 at this point?

Single band lake effect snow bands are at the small end of what we might expect the Eta-12 model to be able to resolve. They are very sensitive to

• Boundary layer wind direction and shear and
• The degree of synoptic-scale forcing,

both of which are increasingly likely to be in error as the forecast moves forward in time. Also, the model may have preferred locations for the band based on how well it resolves:

• Topography,
• Surface roughness variations, and
• The planetary boundary layer,

among other effects. These preferred locations may differ from the verification. Finally, some surface elements which are held constant in the Eta-12 forecast (e.g. LSTs) may be affected to some degree by air/lake interactions, which in turn may affect the lake effect snow band location and intensity.

As we can see by clicking on the "Display Graphic" button below, the single lake effect snow band was forecast to continue at varying degrees of strength through the remaining forecast period.

36-84h Eta-12 fcsts from 00 UTC 24 Dec., valid 12 UTC 25 Dec. - 12 UTC 27 Dec. 2001

Qualitative comparison with WFO Buffalo NY storm summary

Maxima in strength occur during the three hours ending 15 UTC 25 December, while the simulated snow band was moving northward. It then weakens by about a factor of two by 21 UTC, only to strengthen again along the north shore of Lake Erie with maxima in 850-hPa vertical motion and 3-hourly precipitation rates at 03 UTC 26 December. This is represented by panels 1-7 in the animation.

This behavior does not agree with what was qualitatively reported by the Buffalo NY WFO in its storm summary, which states:

"Winds backed again on Christmas Day, the 25th, but the band was much weaker and pushed north across the area dropping another 1 to 3 inches before lifting way north by late Christmas evening. Some of this activity even reached Toronto Christmas night. The action pretty much shut off early Wednesday...."

In the model forecast for early Wednesday (panels 7-10, valid 6:00-15:00 UTC, 26 December), the lake effect band shifts slowly northward from the northern part of the lake, crossing the Buffalo area north of the observing station, to the southeastern Niagara Peninsula and Niagara County, NY, and slowly strengthens.

During the day of the 26th the Buffalo WFO reports that "the snow band began to redevelop" over the Niagara Peninsula of nearby southern Ontario, and brought heavy snow to that area through the day. The model (63-69 hours valid 15:00-21:00 UTC 26 December, panels 10-12) has a relatively weak and broad (compared to the maximum model strength) snow band in about the right place. It is reported that the snowband shifts south again to hit Buffalo during the evening of the 26th, bringing 8" of snow in 6 hours, finally settling over the areas immediately south of Buffalo. This corresponds to panels 13-17 of the forecast animation valid from 00:00 UTC to 12:00 UTC 27 December, which indicates a single snowband to the north of the reported verification.