Sunday, March 3, 2013

Snow bow echo?

How many times has there been a bow echo in an environment cold enough for snow to reach the ground within the confines of its heavy precipitation shield?  The answer for my experience is – never! The closest for me was a snow thunderstorm and shelf cloud over Santa Fe ski area in a March event years ago.  But that was not a bow echo like this one.   This storm produced wind damage around Billings Montana in the late afternoon including snapped poles and an overturned tractor trailer.  The radar loop from IA State below shows the onset of the bow over Billings and then its progression east.  The local NWS office was on the situation and quickly issued a severe thunderstorm warning, later expanded to capture the growth of the bowing system.  

Later after passing east of Billings, I snagged a few radar images of the bow.  The bow echo appeared to show a velocity peak just south of Crow Agency though the area of high velocities was small.  The Differential reflectivity showed only heightened values east of the high reflectivity.  That's somewhat unusual for bow echoes of warmer environments where the heaviest rain and highest Differential reflectivities are colocated.  Differential reflectivity in light rain is typically small but in heavy rain the values are inflated up above 2 dB.  With high reflectivity and low Differential reflectivity, I am thinking that a large amount of ice makes up the precipitation type. The bottom right panel of figure 3 also shows the Kdp indicating lack of liquid.  Normally the Kdp would indicate elevated values in a bow echo in a warmer bow echo. But then the bottom right picture shows high Correlation Coefficient.  In a summer bow echo, any high reflectivity associated with low Differential reflectivity would mean falling hail and the Correlation Coefficient would be depressed.  Here it is not depressed.  My best guess is that this bow echo contained mostly ice in the form of snow and graupel.  

Figure 2.  Reflectivity (left panel, base velocity (middle panel), and Differential Reflectivity (right panel) from the Billings, MT WSR-88D lowest elevation scan at 2013-03-03 2246 UTC.

Figure 3.  Reflectivity (left panel, Correlation Coefficient (middle panel), and KDP (right panel) from the Billings, MT WSR-88D lowest elevation scan at 2013-03-03 2246 UTC.


A closer look at the Billings area shows that temperatures quickly fell from near 50 deg F to near freezing as a cold front swept through, quickly followed by the bow (fig. 4).  The webcam in figure 5 shows depressed visibility with heavy snow falling, and new snowcover on the ground, however the image was shot 30 minutes after the bow echo's arrival.  Winds were already strong immediately following the cold front which arrived at the Billings airport at 2129 UTC (fig. 6).  The bow arrived roughly 10 minutes later with a mix of rain, snow and graupel.  The precipitation changed to completely frozen forms only seven minutes later while the winds were still gusting to 50 kts.  Occasional lightning accompanied the heavy frozen precipitation.  This progression of precipitation agrees quite well with the Dual-polarization radar data in figures 2 and 3.


Figure 4.  A surface plot with a webcam image overlaid from near 2146 UTC courtesy of wunderground.com.

Figure 5.  An image taken at 2216 UTC 2012-03-03 courtesy of StormTeam webcam.  
KBIL 032147Z 31033G51KT 1/4SM R10L/3000VP6000FT -TSPLGSSN FG SCT007 BKN032CB OVC070 01/M01 A2956 RMK AO2 PK WND 31051/2135 WSHFT 2114 RAB34E47PLB39GSB42E43B47SNB43 TSB43 PRESRR OCNL LTGICCG TS OHD MOV E P0000 I1002 $
KBIL 032143Z 30036G51KT 1/2SM R10L/3000VP6000FT -TSRAPLSN SCT006 BKN032CB OVC070 01/M01 A2956 RMK AO2 PK WND 31051/2135 WSHFT 2114 RAB34PLB39GSB42E43SNB43 TSB43 PRESRR OCNL LTGIC TS OHD MOV E P0000 I1002 T00111011 $
KBIL 032140Z 30034G51KT 1SM R10L/5000VP6000FT -RAPL BKN020 BKN049 OVC080 02/M01 A2956 RMK AO2 PK WND 31051/2135 WSHFT 2114 RAB34PLB39 PRESRR VIS 12V1 1/2 P0000 I1001 T00221006 $
KBIL 032129Z 33027G43KT 10SM FEW030 SCT050 BKN080 09/M01 A2952 RMK AO2 PK WND 34043/2127 WSHFT 2114 VIRGA ALQDS VCSH W-N T00941006
KBIL 032053Z 08008KT 10SM SCT075 BKN110 12/M01 A2952 RMK AO2 SLP994 T01221006 58029
Figure 6.  A meteogram fro KBIL on 2012-03-03 including an annotation of the thunderstorm from 2140 - 2230 UTC.

The cold front (fig. 7) that forced the bow echo's formation was part of a very strong upper-level short-wave trough and accompanying jet streak passing through Montana.  Convective activity was occurring all along the front ahead of a dry slot.  Note that the majority of the frontal forcing was occurring behind the surface frontal location, a characteristic of anafronts.  This same system is forecasted to track to the southeast, merge with another southern stream wave and give the Mid-Atlantic states a late season snowstorm.

Figure 7.  Surface plot by UCAR/RAP overlaid on top of a GOES visible image taken at 2145 UTC 2013-03-03.  The blue curve represents the cold front in which the thunderstorm symbol marks the location of the bow.  
 With all of the convective activity, I would've expected to see some instability in the form of CAPE.  Finding some CAPE proved elusive.  According to the SPC mesoanalysis, the nearest surface-based CAPE could only be found in northern Wyoming (fig. 8).  However steep lapse rates in the lowest 3 km of the atmosphere pointed to the possibility that with a little more moisture than analyzed, CAPE could be found (fig. 9).  Even without much CAPE, the front appears to have been quite strong with a steep interface.  One could argue that the cold front was strong enough alone to cause a convective-like line, also called a narrow cold frontal rain band.  But if that's the case then why wasn't there one?  And why did a bow echo form?  Instability had to be there to produce this event.  However, the instability still led to relatively small amounts of CAPE, and I cannot ignore the strong vertical forcing of this front.  It is interesting that the MCS maintenance parameter painted high probabilities in the vicinity (fig. 10).  I suppose that any convective convective parameter that doesn't include CAPE would be ironically effective.

This bow echo was likely not very intense.  On its own, I doubt that the bow could've generated severe winds, or anything remotely close.  However, the cold front was very strong, and the overall system, dynamic.  Only a small convective supplement would be needed to push the winds into severe thresholds.

Figure 8.  Surface-based CAPE, low-level winds and mosaic reflectivity from the SPC mesoanalysis page.  

Figure 9.  0-3 km lapse rate analysis from the SPC mesanalysis page.

Figure 10.  The Mesoscale Convective System (MCS) maintenance parameter available on the SPC mesoanalysis page.

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