Christmas Weather

By: beell , 12:35 AM GMT on December 23, 2012

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College of DuPage-Severe Weather Warnings

Storm Prediction Center (SPC)
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The potential for some Christmas weather starts to come together tomorrow. The first act will be the passage of a moderately strong and compact shortwave moving east across the northern tier dragging a weak cold front extending from a surface low over the mid-MS and lower OH Valley, south to the Gulf Coast.

Isentropic upglide/warm air advection (WAA) in the warm sector will lift northward and bring some light snow to the midwest/Lower Great Lakes and a chance of a few elevated thunderstorms farther south along the frontal boundary back into TX Christmas Eve Morning.



A cold and dry surface airmass sitting atop existing snow pack over the central and northern Great Plains will be reinforced after frontal passage. The gates to an initially shallow arctic airmass to slide south will be open.

The second act will quickly take the stage during the day on Monday as a strong shortwave comes ashore on the west coast and amplifies into nearly a full-latitude trough as it digs south towards the Four Corners area,leading to development of a weak surface low/lee trough near the TX/OK panhandles.

Weak upslope flow and meager moisture should allow some snow over the front range of the Rockies, eastern, CO and WY Monday evening-spreading into western KS. Snow amounts should generally be on the light side-excluding the higher elevations.

Quite a bit of the snow that is produced over these areas will probably never see the ground-there will be a price to pay to overcome the lower tropospheric dry air and moisten the column enough for the flakes that will fall later to reach the ground. Snowfall over NW OK and south central KS could exceed the forecast a bit (2-3"?) if this process is more efficient than modeled. Additionally, once the system passes east of the area, strong and dry northerly flow on the order of 25-30 knots sets in. Daytime temps will fall into the lower teens on Christmas Day with sub-zero wind chills over the Texas panhandle and western OK. Show over for this part of the world.

Upslope flow, upglide, and moisture return gets kicked up a notch as surface ridging over the GOM slides east and adds a return flow from the gulf to the mix. The GFS closes off the mid-level low over the eastern panhandle of Texas with surface cyclogenesis starting to ramp up over central TX. With a plentiful supply of moisture, cold air and lift associated with the upper level system, the snow should begin to fall in earnest over NE OK and northern AR as the surface low continues to deepen as it moves across east TX towards the TN Valley. It may be somewhere in this general area NE OK/N AR) where the deformation axis will set up leading to 8-10" of snow-Those in the know suggest that forecasting mesoscale banding three days in advance is foolish. Word to the wise.

Precip should be all snow at least to the latitude of Oklahoma City with rain changing to snow over the Red River.



NCEP HPC Snow Accumulation-Day 3 Probability of at Least 4".
Valid 00Z 12/25 through 00Z 12/26

This won't be the end of the snow as the system matures and heads towards New England (by Thursday morning) but the winter side of this post has probably gone on long enough for now.

The potential for a notable severe event over the gulf coast may just end up being the larger weather story.

For the folks in southeast Texas, Christmas morning may bring the gift of severe weather as the surface low over central TX begins to deepen with a subsequent backing of the surface winds (from the SE) in response-with ample moisture converging along the frontal boundary/surface trough. At least a 30 knot southwesterly LLJ in place, and mid-level winds AOA 60 knots that may briefly assume a westerly component along the coast after rounding the base of the mid-level low over the eastern TX panhandle. The westerly component also extends up to the jet stream level with a broadly divergent flow over all of east TX.

All this clockwise turning of the shear vectors results in SRH values near 250-300 m2/s2 with 0-6km shear of 60 knots.

MLCAPE is currently modeled AOA 1500-2000 J/kg over SE TX early Tuesday morning with dewpoints in the mid 60's right along the coast. Convection should initiate near the surface low just east of Dallas and build to the SW along the boundary and east along a developing marine boundary.

With decently cold air aloft in place and more on the way courtesy of the mid-level low, lapse rates will be unseasonably high. Probably in excess of 7°C/km

Worth a mention that the normally reliable SREF does not show much in the way of severe weather-keeping CAPE AOA 500 J/kg. The GFS and NAM flirt with t-storms remaining slightly elevated-if true, damaging winds would be the main threat. The risk should be relatively short-lived for any particular point as the cold front undercuts the storms. I expect numerous tornado warnings Tuesday morning regardless of whether these storms can become surface based. The risk should continue east across Louisiana into the afternoon.

If you got that special someone a weather radio for Christmas, it might not be a bad idea to unwrap this one early.





All frames from today's 18Z GFS (top) and NAM (bottom)-valid 12Z (6AM CST) Tuesday.



2m Theta-e. Model run and forecast time as above.
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