When a forecast is about to bust normally you can see it coming. Monday morning was actually the strangest snowstorm bust I have ever worked through. At 6:30 a.m. everything seemed to be going well – an icy mix of sleet and freezing rain in northwestern Connecticut and a mix of rain and sleet in Hartford and New Haven with temperatures well above freezing. I felt pretty good about my “roads should be fine outside of the hills” forecast.
By 6:45 a.m. the forecast crashed and burned. I punched up our Bradley Airport skycam in our router and saw pounding down in Windsor Locks. Within 10 minutes Hartford had flipped to heavy snow and shortly thereafter New Haven did as well. You can see the progression on base reflectivity and correlation coefficient. Notice the heavy band of precipitation that develops in western Massachusetts and sinks south – at the same time the relatively low CC (indicating a mix of hydrometeors in the melting layer) becomes high when the precipitation changes to just snow.
Inside that band the atmosphere dynamically cooled and the >0C layer around 5,000ft AGL was quickly eliminated. Our computer models did not handle this mesoscale band particularly well (location, strength all fairly variable/uncertain) – think of it as a complex of summertime thunderstorms. It’s almost impossible to pin down the exact location and track of any given storm ahead of time!
The National Weather Service was also caught completely off guard by the rapid change to heavy snow.
CTZ010-312015- SOUTHERN NEW HAVEN- 734 AM EDT MON MAR 31 2014 .TODAY...RAIN...SNOW AND SLEET THIS MORNING...THEN RAIN THIS AFTERNOON. LITTLE OR NO SNOW AND SLEET ACCUMULATION. BREEZY WITH HIGHS IN THE MID 40S. NORTH WINDS 15 TO 20 MPH. CHANCE OF PRECIPITATION NEAR 100 PERCENT.
CTZ002-312000- HARTFORD CT- INCLUDING THE CITIES OF...HARTFORD...WINDSOR LOCKS 715 AM EDT MON MAR 31 2014 .TODAY...CLOUDY WITH RAIN...SLEET AND FREEZING RAIN LIKELY THIS MORNING...THEN PARTLY SUNNY WITH A SLIGHT CHANCE OF RAIN THIS AFTERNOON. LITTLE OR NO SLEET ACCUMULATION. HIGHS IN THE MID 40S. NORTH WINDS 15 TO 20 MPH WITH GUSTS UP TO 30 MPH. CHANCE OF PRECIPITATION 70 PERCENT.
These forecasts were issued after the change to snow had already happened! The 6z NAM did show the potential for a change to snow – though it produced only light precipitation (less than a tenth of an inch of QPF).
The GFS was way too warm for the event – by several degrees C even on a 6-hour forecast. The RAP did well – nailing the thermal profile.
What made this whole event odd was that while 3″ to 4″ of snow fell in central Connecticut virtually no snow fell in the Northwest Hills – the precipitation stayed all sleet and freezing rain up there! Keeping on top of short range/mesoscale modeling more closely like the RAP and HRRR could have helped in this event but at the end of the day this was one of those weird storms that will almost always surprise you.
Unfortuntately, that surprise came at the worst time -7:00 a.m. through 10:00 a.m. with snowfall rates of 1-2″ per hour in one of the most densely populated regions of the state.