Monday’s Surprise Snowstorm

snow

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.

10:14 UTC to 11:30 UTC OKX BR/CC 3/31/14

10:14 UTC to 11:30 UTC OKX BR/CC 3/31/14

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).

bufkitprofile

bufkitprofile

 

bufkitprofile

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.

The Deluge Has Ended

After nearly 4.5″ of rain in parts of southeastern Connecticut the heavy rain has come to an end. Here’s a look at some of the rain totals across the state – many of which are quite impressive!

  • Westbrook – 4.71″
  • Norwich – 4.46″
  • North Haven – 3.49″
  • Bridgeport (ASOS) – 3.15″
  • North Grosvenordale – 3.12″
  • Hampton – 3.08″
  • West Hartford – 2.22″
  • New Hartford – 2.06″

2014033014_metars_alb

The rain was focused along a warm front that managed to sneak into southern Connecticut. The winds were out of the south and east in Groton, Westerly, and Westhampton Beach while Providence, Willimantic, and New Haven had northerly winds. An impressive theta-e gradient as well with mid 50s for temperatures and dew points in Groton and low 40s elsewhere in Connecticut.

The Yantic River managed to reach moderate flood stage with 4.5″ of rain through the basin while other rivers in northeastern Connecticut reached minor flooding benchmarks.

ytcc3_hg

cvtc3_hg

Not a surprise to see minor flooding given the setup with a strong low level jet on top of a stalled warm front near the south coast. A nice surge of high precipitable water air moved in overnight all in a region of strong quasi-geostrophic ascent and even a bit of elevated instability. Classic “sultan of sandbag” signal for southeastern Connecticut, Rhode Island, and southeastern Massachusetts.

Winter 2013/2014 – Not as Bad as it Seemed

For the Hartford area you’ll probably be surprised to hear that (barring an unusual spring snowstorm) we’ll end the season just about average for snowfall this year. As of today, 50.1″ of snow has fallen at Bradley International Airport which is just about the long term mean snowfall for the Hartford area.

For snow lovers this winter has gone out with a giant thud. March has been very cold but basically snowless. Opportunity after opportunity has managed to miss us either to the north or south. It’s been uncanny! I can’t remember a month with so much potential ending with so little to show for it.

Through March 22nd the cold this month has been notable. In fact, we’re tied for 7th coldest since 1905.

  • 24.3º – 1916
  • 25.6º – 1960
  • 28.9º – 1941
  • 29.0º – 1950
  • 29.0º – 1984
  • 30.0º – 1906
  • 30.2º – 2014 (also 2005 and 1993)

The Challenge of a 7 Day Forecast

Every morning and every night we provide a 7 day forecast to our viewers. When I started at WVIT in 2005 it was a 5 day outlook. Why did we switch? Because viewers wanted the heads up!

Unfortunately, a day 6 or 7 forecast can be wildly inaccurate at times while perfectly spot on others. So why is this? The answer comes from West Hartford native Ed Lorenz – the famous meteorologist who discovered “chaos theory”.  Here is a quote from his landmark work Deterministic Nonperiodic Flow from the Journal of Atmospheric Science.

Two states differing by imperceptible amounts may eventually evolve into two considerably different states … If, then, there is any error whatever in observing the present state — and in any real system such errors seem inevitable — an acceptable prediction of an instantaneous state in the distant future may well be impossible….In view of the inevitable inaccuracy and incompleteness of weather observations, precise very-long-range forecasting would seem to be nonexistent.

 

And therein lies the problem. While we can do a very good job at initializing what the atmosphere looks like (from remote sensing, weather balloons, surface observations, etc.) that initial representation will never be perfect. The less than perfect initialization will grow into larger and larger errors as time moves on in our computer models solving equations of motion for the atmosphere.

Some storms are more susceptible to those small initialization errors on the models and some areas of the earth are relatively data-poor and initialization errors are larger there just by a function of geography.

Lorenz’s work is something that everyone sees time and again with weather forecasts. A day one forecast is more accurate than a day 4, 5, or 6 forecast almost all the time. The reason why is that the errors in our computer models grow at a non-linear rate with time.

About 7 days before this Wednesday’s storm (on March 5th) here’s what I posted on Facebook.

f1

 

Now is that post of any use to someone? I’m not sure. Some say yes and some say no. I view a day 6 or 7 forecast as a “head’s up!” or a “stay tuned” kind of deal. No hype involved just telling people that there’s a possibility of something. Some people commented with “Then why say anything at all?” or “When an accurate prediction of what the weather will be in 2 days can start happening then maybe I’ll believe a prediction 10 days out”

I understand both of those comments! Personally, I would rather post about a storm potential and be honest about the uncertainty associated with that forecast.

As it turned out we did get a storm. The storm was never really hyped (by us, not sure about the others) and it turned out to be rain. It still amazes me that 7 days out we can sometimes have a pretty good idea that yes there will be a storm or no there won’t be one. In that case we were only about 150 miles away from having a damn good snowstorm.

One thing that all meteorologists need to be better at is expressing uncertainty in our forecast and when we don’t know being able to say we don’t know. One way we can do this is through ensemble modeling which runs a given computer models many different times while each time the initial conditions are tweaked. That will tell us how susceptible a given storm is to model initialization. If all 50 tweaks on the Euro Ensembles still produce 45 “snow storm hits” then we can say a snow storm is a good bet! If 50 tweaks produce 50 dramatically different results than we need to have a spine to say “we just don’t know!”

That brings me to Monday. Our computer models over the last 48 hours have been all over the place. The operational run of the European model has very little snow on Monday though it has come substantially farther north since its overnight run.

ecmwf_apcp_f96_us

 

The European ensemble members as a whole, however, are substantially more “juiced” and farther north than the main operational run. Of the 50 ensemble members 40% of the produce >0.25″ liquid in 24 hours by Monday evening. The ensemble mean (take the average of all 50 members) is nearly 0.55″ of liquid in the 36 hours ending Tuesday morning.

This tells us that there’s a lot of spread in possible solutions! Many members showing no storm, some showing a big snowstorm, and others showing a glancing blow. The best thing we can do now is watch for trends and watch future runs. How will this storm evolve? Nothing to hype now other than say there COULD be a snowstorm on Monday. If we had more information we’d pass it along but at this point that’s the best I can do.