*** Follow our latest live updates here: Heavy snow tonight, then mixed precipitation Thursday ***


Technical discussion

Yesterday I said those wishing for a boom scenario at their house should root for a “deformation” heavy snow band to sit over their neighborhood. What’s that all about?

Such bands often develop on the west side of a strong developing storm like this one.


In a 2003 presentation, Peter Banacas offered this slide (to the right) illustrating where such deformation zone generally form in relationship to strong cyclones that are associated with closed upper lows aloft. The scalloped area with the line of dots and x’s is the typical location where a deformation zone forms.

The bands usually form to the north of the 700 mb low near its ridge axis but oriented along the flow. Sometimes there can be more than one band depending on the stability of the air mass. With time, the bands sometimes wraps around the west side of the upper low. Usually mid-level frontogenesis or a squeezing together of the temperature lines is taking place along the deformation zone. The mid-level stability is also often reduced making it easier to get higher snowfall rates. On the back side of the low, drier air also starts being drawn east, these bands start to shrink and become relatively narrow.

Banacas also noted that the manifestation of the deformation zone was often somewhat tied to the stability. Early in the formation when the cold air is not as deep, moderate to heavy snowfall is possible. If there is an unstable layer aloft, you might get multiple narrow bands of heavy snow. Late in the storm when the cold, drier air has been drawn eastward, the snow rates lessen.


Today’s high resolution Canadian model (below) develops just such a band Thursday evening. I’m not showing the forecast because I think it’s right but rather to illustrate how small of a scale such bands occur on and intense the precipitation rates can be. In a three-hour period, the model predicts up to 0.80” liquid just southeast of D.C.


The European model develops the deformation band west of the city


Note how narrow the band is on the model forecast (above).

Last night’s European model developed such a band earlier much farther to the west. Note there is a dry area to the east where only light precipitation is falling on the European model. It’s almost impossible to predict with much certainty the exact location on one of these bands until they actually start forming. Note how different the two model forecasts are with their bands.

Such bands do shift and often press from west to east but often in an almost discontinuous manner leaving some folks with little additional snowfall while giving others several inches. If such a band parks over you house tomorrow early in the game, you might end up with a half foot from that single band. Later as drying starts taking place, such a band is more likely to lead to just a couple of more inches. Right now that looks like the most likely scenario for the city tomorrow evening unless somehow the Canadian model above is right.