Merry Christmas Eve, folks! As most of you have heard in the chaos of shopping and wrapping presents, a significant severe weather and snow event threatens a large portion of the U.S. this week. This storm will have major impacts with heavy snowfall amounts, strong winds, and even a potentially major tornado outbreak in the Gulf Coast states on Christmas Day.
 |
| 12Z NAM run for 12Z December 25, 2012 courtesy of PSU Ewall |
We'll first begin discussing the formation of this potent weather system. An impressive upper-level trough is currently working its way across the Rockies, deepening as it progresses over the leeward side of the mountains. As this cold air moves southward and contacts warm Gulf air early Tuesday morning, significant amount of energy is present for the low pressure to intensify near the Mexico/TX border. Also, the presence of ample moisture will enable heavy snowfall and the tornado/severe weather threat.
 |
| Day 2 SPC Convective Outlook Categories and Probabilities |
The number one concern from this system is the severe storms to occur in
the Gulf states on Tuesday and into Wednesday morning. The Storm
Prediction Center's convective outlook for Christmas Day shows a
moderate risk area from central Louisiana extending eastward into
west-central Alabama. Extremely strong wind shear and considerable
instability in the warm sector of the low pressure system will be the
main drive behind the potential for isolated supercells supportive of
long-track tornadoes. Along with supercells capable of tornadoes,
possible bowing storms will also lead to very damaging winds. Based on
simulated radar reflectivities, the severe storms appear to begin in the
morning hours of Tuesday and continue through the evening. Residents of these areas should keep informed throughout the day on any severe weather watches and warnings issued by the National Weather Service.
 |
| 12/24 12Z NAM Snow Depths courtesy of WeatherBell |
On the opposite end of the weather spectrum, snowfall will also be a major concern for those traveling in the Midwest and Northeast this holiday. Excessive snowfall potential has already led to the issuance of winter storm watches and warnings extending from Oklahoma northeastward towards northwest Ohio and southern Michigan. Snow will likely start falling in Oklahoma early Christmas morning, and will track northeastward into Thursday. The 12Z NAM output shows snowfall depths up to 16" in some areas of the storm track. However, these snowfall amounts are likely overestimated. With the energy and moisture extracted from the Gulf by the highly-probable severe storms in the South, snowfall amounts should remain lower than what is currently forecasted by many models. Totals for much of the areas within the snow swath should remain below 12", with higher amounts possible in the higher elevations of the Appalachians. At any rate, people should exercise extreme caution if traveling on the roads on Tuesday through Thursday. At the airport, expect many delayed and cancelled flights due to this system. Also, be sure to stay tuned to any watches or warnings issued for your area by the National Weather Service.
As with any weather event, follow Stratustatus on
Twitter and
Facebook below for continuous updates throughout this significant Christmas storm. Happy holidays, and stay safe out there!
Current MSL pressure analysis from the SPC Mesoanalysis shows the deepening low pressure of 984 hPa located over north-central Illinois progressing northwestward. The black lines (known as isobars, or lines of equal pressure) along the western side of the storm are seen very close to one another; this indicates winds at the surface are beginning to strengthen. Wind speeds of 53 mph have been reported at the Davenport, IA airport. Of course, cold air associated with these gusty winds has allowed for heavy snowfall to occur, prompting blizzard conditions in Iowa and Wisconsin (MCD 2193).
