National Weather Digest Forecasting SIMILAR CASES OF COLD AIR CYCLOGENESIS AND HEAVY SNOWFALL JUST EAST OF THE ROCKIES AND APPALACHIANS

Two synoptically similar situations are presented that involve strong surface Low development and heavy snow in response to strong SOO-mb vorticity maxima crossing the mountains. LFM performance is evaluated and found to be reasonably accurate at 500 mb but inaccurate at the surface in each case. The poor surface pressure forecasts apparently contributed to poor precipitation forecasts. Common features of each system are discussed with the intentions of flagging the situation for future recognition and identifying a possible characteristic model error. This study compares two synoptically similar situations that involve deep surface Low development and heavy snowfall in response to a strong 500-mb vorticity (vort) maximum crossing the mountains. In the February 6, 1984 case, the system was to the lee of the rockies. Each system was at about 350 North Latitude. Figures I thru 4 show the 24-hour limited fine-mesh model (LFM) 500-mb hgt/vort progs and the verifying analyses for the two cases. In each case, the 500 mb prog was fairly accurate. Note that each case featured a strong (greater than or equal to 22 per sec) vorticity maximum rounding the bottom of a long-wave trough, with a slightly less intense vort max about 100 Latitude to the north. 500-mb temperatures in each cases were cold •.• • Less than or equal to-30 C. Figures 5 and 7 show the 24-hr LFM surface pressure / I 000-500 m b thickness progs. Figures 6 and 8 show the verifying surface analyses, overlayed with the observed snowfall for the preceeding 12 hours. Despite the general accuracy of the LFM 500-mb forecasts, the LFM surface forecasts were fairly poor. In each case, the surface Low was forecast much too far east. Although only the 24-hour LFM progs are shown, a similar pattern of reasonably good 500-mb forecasts and correspondingly poor surface forecasts occurred in both earlier and later runs of the LFM and spectral models. The impact of surface Low development so far west was significant, as seen in figures 6 and 8, heavy snow fell north and west of the surface Lows. In the eastern storm, heavy snow continued to fall acros