by Michael Strickler
A band of moderate snow developed over the far northeast Piedmont and northern and central Coastal Plain of central NC on the morning of 1/22/2014. The intensity of the snow band increased such that the visibility at KRWI was reduced to between 1SM and 2SM between 13Z and 16Z (Table 1). The snow band was particularly interesting in that it developed after the parent cyclone and associated moisture and forcing for ascent had moved well to the northeast of NC (Fig. 1), and consequently was a surprising and rather unexpected development for forecasters. In fact, the accumulations between 12Z and 18Z ranged from around a half inch to as much as one inch of snow, locally satisfying winter weather advisory criteria.
Figure 1. 1500Z WPC surface analysis.
It appeared that the band may have been influenced by Kerr Lake, located near the VA/NC border, since there was a plume-like feature in both satellite and radar data, which seemed to emanate from the lake (Fig. 2).
Figure 2. METAR and dual-imaged visibile satellite and regional 0.5 degree radar imagery, valid at 1345Z – 1400Z.
We remembered at least a couple of similar events in the past, where it appeared that the lakes in southern VA and northern NC were contributing factors to the development of small snow bands downwind of the lakes, well after the parent cyclone had passed. See the following links for details for those cases: March 2, 2009 and January 23, 2003. An additional horizontal convective roll cloud band originated over, and developed downwind of, Falls Lake in northern Wake County, evident in visible satellite imagery in Fig. 3 and the photo in Fig. 4.
Figure 3. METAR and dual-imaged visibile satellite and regional 0.5 degree radar imagery, valid at 1515Z – 1518Z.
Figure 4. Photograph of the lake band looking north-northeast from NWS Raleigh, located in central Wake County.
The development of the bands seemed to be the result of a unique junction of processes and properties on a range of scales in both time and space, such that without any one of them, the bands likely would not have existed. The first of these processes, and perhaps the most important one, was the meso-gamma scale sensible and latent surface heat and moisture flux from the respective lakes, whose temperatures were in the middle 40s, into the overlying arctic air above. The temperature differential between the lakes and the overlying 850 hPa temperatures was 18-20 C, which contributed to significant low level destabilization, on the order of several hundred J/kg of CAPE, which can be seen in the RAP BUFR soundings shown in Fig 5 and 6. Since the lake band development preceded any additional cloud street HCR development that occurred with diabatic diurnal heating of the boundary layer during the late morning to midday hours (Fig. 7), it is apparent that the presence of the lakes and their associated influence were vital to the band development.
Figure 5. KRWI RAP BUFR sounding, 00 hour forecast valid at 1100Z.
Figure 6. KRWI RAP BUFR sounding, 05 hour forecast valid at 1100Z.
Figure 7. METAR and visible satellite imagery, valid at 1500Z – 1515Z.
Another contributing influence for the band development was residual low level, meso-alpha to meso-gamma-scale moisture on the southwest flank of the departing parent cyclone, as seen in Fig 8. This nearly saturated air at 925 hPa, possibly a combination of upstream Great Lakes moisture and an orphaned portion of the deformation head attendant to the departing cyclone, produced a suitable environment/primed the atmosphere for condensation necessary to produce the cloud/snow bands.
Figure 8. RAP-analyzed 925 hPa RH and 11-3.9 micron satellite overlay, valid at 12Z.
Processes on the progressively larger scale also favored vertical motion necessary for the development of the bands. A series of low amplitude impulses/initially shear vorticity-dominated shortwave troughs on the back/west side of the mean trough axis aloft (Fig. 9), migrated through and briefly amplified into the trough base/across the Southern-Central Appalachians (Fig. 10), before the mean trough axis lifted up and away from the Middle Atlantic coast. Associated QG forcing for ascent/differential cyclonic vorticity advection shown in Fig. 11, was the larger scale process that acted upon and deepened the underlying moisture/instability, and which consequently resulted in the broader area of snow that developed to the south of the lake-induced band, shown in Fig. 12.
Figure 9. Water vapor satellite imagery and RAP-analyzed 400-200 hPa vorticity, valid at 0900Z – 0915Z.
Figure 10. Water vapor satellite imagery and RAP-analyzed 400-200 hPa vorticity, valid at 14Z.
Figure 11. 500 hPa height and vorticity, 700-500 hPa differential vorticity advection, and radar, valid at 14Z.
Figure 12. METAR, 850 hPa VWP data, and RAP-analyzed 850 hPa temperatures, valid at 13Z.
Table 1. KRWI METAR data valid 1153-1853Z.
METAR KRWI 221153Z AUTO 01006KT 9SM -SN OVC025 M06/M08 A2995 RMK AO2
SLP143 P0000 60000 70012 T10561083 11039 21056 53024 $
METAR KRWI 221253Z AUTO 35005KT 8SM -SN BKN028 OVC033 M06/M08 A2998 RMK AO2 SLP152 P0000 T10561083
SPECI KRWI 221310Z AUTO 34005KT 5SM -SN SCT026 OVC032 M06/M08 A2998 RMK AO2 P0000 T10561083
SPECI KRWI 221337Z AUTO 35007KT 2SM -SN OVC032 M06/M08 A2999 RMK AO2
SPECI KRWI 221339Z AUTO 34007KT 1 3/4SM -SN BKN028 OVC034 M06/M08 A2999
RMK AO2 P0000 T10561083
METAR KRWI 221353Z AUTO 34007KT 1 1/4SM -SN BKN024 OVC031 M06/M08 A3000 RMK AO2 SLP159 P0000 T10561083
METAR KRWI 221453Z AUTO 34005KT 1 3/4SM -SN FEW013 SCT017 OVC027 M06/M08 A3003 RMK AO2 SLP171 P0000 60000 T10561083 53018
SPECI KRWI 221505Z AUTO 01008KT 1 3/4SM -SN BKN011 OVC030 M06/M08 A3003
RMK AO2 P0000 T10561083
SPECI KRWI 221516Z AUTO 01007KT 2SM -SN BKN013 OVC021 M05/M08 A3004 RMK
AO2 P0000 T10501083
SPECI KRWI 221524Z AUTO 35006KT 1 3/4SM -SN SCT013 OVC021 M05/M08 A3004
RMK AO2 P0000 T10501078
METAR KRWI 221553Z AUTO 34007KT 1SM -SN FEW013 OVC019 M06/M08 A3004 RMK AO2 SLP176 P0001 T10561083
SPECI KRWI 221612Z AUTO 33005KT 1 1/4SM -SN FEW009 BKN013 OVC019 M06/M08 A3005 RMK AO2 P0000 T10561083
SPECI KRWI 221617Z AUTO 33006KT 2 1/2SM -SN SCT013 BKN019 M05/M08 A3004
RMK AO2 P0000 T10501078
SPECI KRWI 221619Z AUTO 33005KT 4SM -SN FEW007 SCT015 BKN022 M05/M08
A3004 RMK AO2 P0000 T10501083
SPECI KRWI 221628Z AUTO 33004KT 9SM -SN SCT015 SCT022 M05/M08 A3004 RMK
AO2 P0000 T10501078
METAR KRWI 221653Z AUTO 29005KT 9SM -SN FEW025 BKN036 M04/M09 A3003 RMK
AO2 SLP172 P0000 T10441094
METAR KRWI 221753Z AUTO 33010KT 9SM -SN SCT040 M03/M12 A3002 RMK AO2
SLP166 P0000 60001 T10331117 11033 21056 58004
METAR KRWI 221853Z AUTO VRB06KT 10SM CLR M03/M12 A3000 RMK AO2 SNE03
SLP159 P0000 T10281117