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Sunday, March 18, 2007

1/4SM +SN SNINCR 02/14 P0007

METARs taken Friday evening and into the early morning hours of the Saturday at KALB...records the occurrence of two inch per hour snowfall rates at 05z and again at 07z.

The 'dP' notation at the end of Albany/s observations is the one-hour pressure change. Falling surface pressure means vertical motion in the column above the station is rising. The quicker the pressure falls...the greater the upward vertical motion (UVM); therefore...more snow.

METAR KALB 170151Z 01012G17KT 1/4SM SN FZFG OVC002 M07/M09 A3007 RMK AO2 SFC VIS 1/2 SLP187 SNINCR 01/07 P0004 [...]
METAR KALB 170251Z 01010KT 1/4SM SN FZFG OVC002 M07/M08 A3001 RMK AO2 SFC VIS 1/2 SLP166 SNINCR 01/08 P0006 [...] dP = 2.1
METAR KALB 170351Z 01012KT 1/4SM SN FZFG OVC002 M07/M08 A2994 RMK AO2 SLP144 SNINCR 1/9 P0005 [...] dP = 2.2
METAR KALB 170451Z 01010KT 1/4SM +SN FZFG OVC002 M06/M08 A2989 RMK AO2 SLP124 SNINCR 2/11 P0006 [...] dP = 2
METAR KALB 170551Z 36012G18KT 1/4SM +SN FZFG OVC002 M06/M07 A2984 RMK AO2 SLP110 SNINCR 1/12 P0006 60031 [...] dP = 1.4
METAR KALB 170651Z 36011KT 1/4SM +SN FZFG OVC002 M06/M07 A2981 RMK AO2 SLP100 SNINCR 02/14 P0007 [...] dP = 1
METAR KALB 170751Z 36012KT 1/4SM +SN FZFG OVC002 M06/M07 A2978 RMK AO2 SLP087 SNINCR 01/014 P0003 [...] dP = 1.3

In six hours...the snow cover at KALB increased from 7" to 14".

SN:H2O ranged between 17 and 33 during the period. This suggests the presence of stellar dendrites in the crystal factory. It/s important to note the SNINCR report rounds the snow cover value to the nearest whole the SN:H2O conversion is not precise.

ALB launched a special rawindsonde flight at 06z on the 17th...right at the height of the storm.

Two interesting features are immediately evident. The depth of the region inside the cloud where the -12°C and -18°C temperature range is most favorable for dendrite growth and the deep layered veering wind profile indicating warm air advection (WAA) which is producing strong UVM in the 850-700 mb layer.

KALB was also in the right entrance region of a moderately strong upper level jet max...which contributed additional UVM through its transverse ageostrophic circulation.

Having a look inside the storm near its peak intensity with a well-time balloon flight reveals important details that can be applied when similar features are indicated on forecast soundings.

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