Rare Snow Storm Impacts the Deep South

 

 
 
     
 ArcGIS 9.2 display of Snowfall Swath, Saturday Morning January 19, 2008

Click to enlarge
graphic courtesy of Joe Maniscalco, NWS Mobile, Alabama

Saturday January 19, 2008 - A rare snow storm impacted the deep south from northern Louisiana east through central Mississippi and central Alabama early Saturday morning. Snow even fell over portions of the northern zones of the Mobile National Weather Service forecast area. The graphic above shows bursts of heavier snows of 3 to 4 inches were reported over northeast Wayne County Mississippi...extending into Choctaw and northern Clarke Counties of southwest Alabama. Lighter snowfall amounts of 1 to 2 inches were reported over Wilcox and Northern Washington Counties in Southwest Alabama.

Forecasters were watching a deep upper level trough of low pressure extending from the nation's mid section into Texas. Ahead of this trough...strong upper level southwesterly winds of 100 to 120 knots helped to bring enhanced lift and Gulf moisture northeast into the southern states. Initially...the deep south experienced a cool environment as the area was north of a cold front draped over the northern Gulf. Weather maps showed a wave of surface low pressure forming east of the southern Texas coastline. The low tracked east northeast...parallel to the northern Gulf Coast.

The graphics below show the evolution of the surface pressure features overlayed with radar composites. Note...the large moisture field spreading northeast over the southern states.

Surface Features/w Radar Midnight 1/19/08
Surface Features/w Radar 6 AM CST 1/19/08
Surface Features/w Radar 9 AM CST 1/19/08
Surface Features/w Radar 12 PM CST 1/19/08

The concern for forecasters was the low level dry air and dewpoints below freezing... forecast to also spread into the interior of southeast Mississippi and southwest Alabama late Friday night into early Saturday morning. See the following upper air soundings:

JAN Upper Air Sounding - 00Z Sat 19Jan08/6 PM CST Fri 18Jan08
BHM Upper Air Sounding - 00Z Sat 19Jan08/6 PM CST Fri 18Jan08

Jackson Mississippi 00Z Sat 1/19/08 / Fri 6PM CST 1/18/08
Birmingham Alabama 00Z Sat 1/19/08 / Fri 6PM CST 1/18/08

Note the large spread between the temperature and dewpoint profiles...dry air in the lower levels of the atmosphere. Also note the subfreezing dewpoint temperatures. The stage was set for precipitation falling through this layer to enhance evaporation...which would result in a rapid cool down of the temperature profile by Saturday morning.

After diagnosing the large scale flow...ascent...moisture...and forecast low and deep layer temperature profiles... along with coordination with adjacent offices...forecasters at the National Weather Service in Mobile issued a Winter Weather Advisory on Friday evening January 18, 2008. Winter Weather Advisories are issued when a mixture of light freezing drizzle or light freezing rain, light sleet and or light snow is expected within 12 to 24 hours. The complexity of deep south winter weather events have several unknowns/questions that present problems for forecasters. Some of these in this case were:

* The degree of cooling which would take place due to evaporation.
* Timing of the reinforcing surge of colder air.
* How much moisture would be present for the cold air to work with.

As the surface low pressure system moved eastward Saturday morning...an arctic airmass surged southeast. With the help of evaporative cooling in combination with the reinforcement of arctic air into the region...the temperature profile did indeed fall to below freezing. Look what happened to the soundings:

JAN Upper Air Sounding - 12Z Sat 19Jan08/6 AM CST
BHM Upper Air Sounding - 12Z Sat 19Jan08/6 AM CST

Jackson Mississippi 12Z Sat 1/19/08 12Z / 6AM CST
Birmingham Alabama 12Z Sat 1/19/08 12Z / 6AM CST

With the temperature/dewpoint profile much closer...indicating high moisture content air and the deep column of subfreezing temperatures in the low levels... these soundings represent an atmosphere supportive of snow.

With snowfall being observed and looking to meet advisory criteria: (A light accumulation of snow of 1/2 inch to less than 2 inches of snow is expected within 24 hours)...A Snow Advisory was issued after sunrise Saturday morning for portions of the interior of Southwest Alabama.

Click on the link to see the (PNS) Public Information Statement issued by the National Weather Service in Mobile to see what happened in the local area.

With snowfall being highly reflective...the following links of high resolution visible imagery taken late Saturday morning...shows the snowfall foot print exceptionally well.
Zoomed Out
Zoomed In




Acknowledgments: Text and page development by Joe Maniscalco, Meteorologist/Forecaster NWS Mobile, AL . Thanks to Ray Ball, ITO for making the page available on the www. Special thanks also goes to Jeff Garmon (Senior Forecaster) for gathering weather information revolving around this unusual winter weather event. Thanks to the NWS Mobile, AL management and operations team for their encouragement and support of this project.


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