19-20 December 2000 Snow Event Below are some charts depicting the thermal gradient moving across our region which evidently was the main contributor to the band of precipitation 19-20 December 2000. A brief discussion is included, though to a large extent, they are presented for those interested to peruse. They are named as such : 9251200t.gif The first 3 letters refer to the level : sfc - surface, 925 - 925 mb, etc.; the next 4, date & UTC time; and the next 1 or 2, the feature being shown : t - temperature, ta - temperature advection, p - sea level pressure, c - divergence (convergence), v - absolute vorticity, and va - absolute vorticity advection. For 2 maps "-lg" refers to larger scale surface plots (other being smaller scale). E.g., the chart above illustrates temperature contours at 925 mb at 19 December 1200 UTC. All of the charts are (approximately 1.6 MB) : Upper air 925 mb temperature, 12 UTC 925 mb temperature avdection, 12 UTC 925 mb divergence, 12 UTC 850 mb temperature, 12 UTC 700 mb temperature, 12 UTC 500 mb temperature, 12 UTC 500 mb absolute vorticity, 12 UTC 500 mb absolute vorticity advection, 12 UTC 300 mb, 12 UTC 300 mb divergence, 12 UTC 925 mb temperature, 00 UTC 925 mb temperature avdection, 00 UTC 925 mb divergence, 00 UTC 850 mb temperature, 00 UTC 700 mb temperature, 00 UTC 500 mb temperature, 00 UTC 500 mb absolute vorticity, 00 UTC 500 mb absolute vorticity advection, 00 UTC 300 mb, 00 UTC 300 mb divergence, 00 UTC Surface Surface temperature, 12 UTC Surface divergence, 12 UTC Surface temperature, 15 UTC Surface divergence, 15 UTC Surface temperature, 18 UTC Surface divergence, 18 UTC Surface temperature, 21 UTC Surface divergence, 21 UTC Surface temperature - large scale, 21 UTC Surface divergence - large scale, 21 UTC Surface temperature, 00 UTC Surface divergence, 00 UTC Sea level pressure, 12 UTC Sea level pressure, 18 UTC Sea level pressure, 00 UTC Sea level pressure, 06 UTC They all open to one window, and may be downloaded & used as desired - perhaps organized in a directory. Units should be obvious except for convergence. I actually have no idea what they are in Digital Atmosphere - they are not adjustable nor specified. Because of the light winds in the region, I don't think they can be greater than x 10^-6 /sec. Absolute vorticity is x 10^-5 /sec. Advections are probably mks units using the base values shown. Note that contours on surface convergence for 21 UTC are inconsistent with others - nothing I can do about this. Note that the links below open in groups of the same 2 or 3 windows for each topic discussed. Charts are primarily for the time period of 12-00 UTC, or 7 AM EST - 7 PM EST 19 December 2000. This is the time the precipitation band (see surface charts for band oriented SSE to NNW) - wish I could find radar charts that aren't copyrighted) primarily moved over central to eastern PA. Near the end of this period, the band began circulating around the developing cyclone to our ESE over the Atlantic. The strengthening temperature gradient during the period from the surface to 700 mb can be noted comparing the 12 & 00 UTC temperature charts for each level. Note that the gradient increased about 10-12 deg C quite consistently in the lower troposphere (e.g., 8501912t.gif & 8502000t.gif). At 500 mb in the mid troposphere, it increased slightly less than half that much. Note that much of the increase was simply advection - both cold air advection from the W (e.g., 9251912ta.gif) and advection of the stronger gradient to our SSW (e.g., 9251912t.gif & 8501912t.gif). The surface temperature gradient similarly strengthened. The difference among the 12 & 00 UTC charts seem to imply that convergence of warm, southerly flow with an easterly component at many locations near the Atlantic coast and colder westerly flow from our W was a great contributor. If so, the large amounts of convergence expected are not seen in the charts though. Convergence occurred at 925 mb at 12 UTC (e.g., 9251912c.gif), though definitely not strong; and it was noticeably absent at the surface (e.g., sfc1921c.gif) and at 925 mb at 00 UTC (9252000c.gif) - the band of convergence being to our S & E then. Divergence was actually present quite often at the surface. I include the larger scale charts to show that this was not only true here (perhaps surface heating under clearer skies S & E of the cloud band was quite significant for increasing the temperature gradient). Although surface convergence was large during the morning, the heavier precipitation generally did not occur until much later during the day and early-mid night. I had less than an inch of snow at 5 PM EST (22 UTC). Until then, many radar echoes which appeared very strong went overhead and only caused light snow - sometimes very light. Is that much time really required for the heavier precipitation to develop ? My snow total was about 2.3 inches at 7 PM EST (00 UTC), and the final total about 5.8 inches (the last half inch worth or so of light snow did not add to the accumulation because of compaction & blowing). The heaviest periods were around 7:20-8:10 PM EST (about an inch) and 12:15-1:10 AM EST the 20th (about an inch & a half). The point I am making is that the heaviest snow over the Poconos occurred while the band was wrapping around the coastal Low (sfc2000p.gif, sfc2006p.gif). Note that the Low is certainly W of the southerly winds with 70 deg F temperatures at 00 UTC - the analysis program does not consider wind direction for surface isobars - but the 06 UTC position is good. I included the 300 mb charts to illustrate that although strong winds aloft were present, it doesn't appear that a jet streak was. The 500 mb charts also indicate vorticity maxima & even in a banded type of pattern (5002000v.gif); yet because winds were rather parallel with the band, its advection was not large (5002000va.gif). It corresponded with the area of activity though, and is associated with some divergence near the top of the troposphere at 300 mb (3002000c.gif); so I suspect mid-upper toposphere dynamics also contributed - even if for no other reason than creating a favorable environment for sustaining the lower level activity. Yet too much should not be assumed with such sparse observations 12 hours apart.