Case: February 24, 2003

Light snow fell through much of southern Wisconsin over night, with some banding present in southeast Wisconsin in the late morning. Some links to images and information about the storm are presented here, along with forecast ingredient maps.

Satellite images and
surface observations
by Scott Bachmeier (CIMSS, UW-Madison)

Forecast Discussion from NWS Sullivan:

3:00 PM 23 February 2003
Cris Garcia

3:30 AM 24 February 2003
Bob McMahon

KMKX Radar
1710 UTC February 24

Ingredient maps and discussion

For more information about the ingredients approach and these diagnostics see http://speedy.meteor.wisc.edu/~swetzel/winter

click to enlarge

Low moisture, light to moderate forcing, and low level instability combined in this event to produce light snow banding. Similar conditions without the instability would likely have not produced any precipitation.

Forcing and Moisture
Ingredient maps are shown in the table below from the ETA model forecasts initialized at 00UTC February 24 (the evening before the event), and at 12UTC February 24 (the morning of the event). Both forecasts valid at 18UTC February 24 show light forcing (QG and frontogenesis) throught the 850-600hPa layer over southeast Wisconsin. The atmosphere was relatively dry, with only 1g/kg predicted near the surface. The area that experienced snow at this time had a relatively deep layer of RH > 80% (through 600hPa). With some forcing in the presence of instability this atmosphere with meager moisture was able to produce precipitation.

Instability
The 18 hour forecast initialized at 00 UTC February 24 shows a small area of negative PVes (indicating some instability). Although this is not present over southeastern WI in the run initialized at 12 UTC February 24, a look at the cross-section shows that instability is still present in the later run. It remains just below the 800:850 hPa layer of the isobaric ingredient maps. Comparing the cross-sections valid at 1800 UTC shows the runs were remarkably consistent, with the exception that the layer of negative PVes dropped to slightly below 850hPa in the later run. In both forecasts, the strongest forcing was above the layer of instability (and thus not able to aid in releasing the instability); however some forcing did coincide with the area of instability.

Table of Ingredient maps

		850mb	700mb	600mb
00UTC 24 February 2003 ETA Model	18 hour forecast	Traditional PVQ & PVF Cross-Section	Traditional PVQ & PVF	Traditional PVQ & PVF
12UTC 24 February 2003 ETA Model	06 hour forecast	Traditional PVQ & PVF Cross-Section	Traditional PVQ & PVF	Traditional PVQ & PVF

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