December 24, 2012: Surprise Christmas Eve Snow North and West of D.C.

Well, I do get it wrong from time time.

My call yesterday was for light precipitation this afternoon, mainly rain, mixing w/ pockets of sleet.  Instead...counties north and west of D.C. are being treated to an afternoon of light snowfall, and we will likely see 1"-2" of accumulation before the storm pushes out between 5-6 pm tonight.

I believe the NWS was also taken by surprise!  They were in "reaction" mode, rushing out winter weather advisories as the flakes began to fall.

First, the surface weather map (below) shows a weak region of low pressure approaching from the west.  A stationary front separates mild air and rain to the south, and cold air with snow to the north:

Intellicast

This is a fast-moving, weak system with limited moisture.  Early this morning, the cold (sub-freezing) air in the lowest 5,000 feet of atmosphere lay well to our north, across northern PA.   It certainly appeared that the precipitation, when it did break out over Baltimore-D.C., would be rain.

Alas, the early morning weather balloon launch at Dulles revealed very dry air in the lowest 20,000 feet of atmosphere, with temperatures hovering right around freezing at 5,000 feet.  When precipitation did begin to form in the cloud layer - perhaps as a mix of rain and snow at that level - it evaporated into the dry air.  Evaporation extracts heat from the air.  This chilled the air layer at the critical snow-making level (5,000 feet) several degrees below freezing.   The cooling continued, in spite of southerly winds at 5,000 feet pushing warmer air in from the south.  In the parlace of Meteorology 101:   Evaporative cooling won out over warm air advection.   Snow, not rain, was able to efficiently form in the cloud layer, and remain as snow all the way to the ground.

I've highlighted this pocket of evaporatively chilled air in the diagram below - showing isotherms of temperature at 5,000 feet.  Isotherms at or colder than freezing are shown in blue, those above freezing are shown in red:

Modified from NOAA

Here is a radar show that shows how the wedge of moderate snow (blue shades) line up with this cold pocket at 5,000 feet:

Weathertap

Now for the Wed-Thurs storm:   My thinking on this is basically unchanged from yesterday's blog post.   However, stronger cold air damming than forecast yesterday may lead to a more prolonged period of iciness (sleet, freezing rain) at the onset of this event.   I still see a strong push of warm air as the bulk of moisture moves through.   Upwards of 80%-90% of the precipitation in the metro will likely fall as liquid, but the percentage of ice will be higher farther west and north of D.C. - Baltimore.   Cold air damming, once established, can be slow to dislodge.  This means subfreezing air hangs tight in the valleys.