Thursday, January 17, 2013

Jan 17, 2013: Forecast Snow Totals For Metro Region Continue to Dwindle

This morning's radar tells much of the story with this storm:   At least for the next several hours, the moisture, uplift and energy are staying south of the D.C. - Baltimore region:

10:30 AM radar image of precipitation (green = rain, blue = snow);  Weathertap
And the light precipitation that is falling in the southern D.C. suburbs has been rain thus far.

The NWS has scaled back the accumulation threat to around 1"-2" of snow, and the entire metro region has been downgraded from a Winter Storm Watch to a Traveller's Advisory:

Snow accumulation forecast (NWS)
Winter Weather Advisory (NWS)

So what's happening with this storm in our region?    Super-dry air has invaded from the west, which is evaporating the precipitation before it reaches the surface.   Additionally, temperatures remain in the mid-40s across the metro regions.   We are essentially lacking the key ingredients for a heavy snow - cold air and moisture.

However, very high resolution forecast models that I have examined do suggest that light moisture will eventually overspread our region from the south later this afternoon.   Whether the temperatures can fall 10 F to be cold enough for all snow remains to be seen.  Certainly with the dry air layer, there is the potential that enough cooling could arise from evaporation - but the process of chilling the air may take too long - in other words, this storm is progressive and is expected to clear the region between 10 PM - midnight.   Now, this morning the surface winds have started to kick around from the north...meaning colder air may start filtering in...but the freezing line at the surface is way up in central PA, and I do not see it making any progress southward.

Bottom line:  The snowstorm may in fact be a nostorm.

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