Where did Sunday's forecast go wrong?
Why were the "best-bet" forecasts of snowfall amounts for the Boxing day snowstorm (or No-mageddon) so poor in the Washington area? Despite expressing that this storm was a exceptionally tricky one, some readers are miffed or more by the mostly no snow result. The following paragraphs outline our thinking during the storm and where I think we might have done better.
The exact track of the storm was always uncertain as noted in the Dec. 20 blog. Through Dec. 23, we continued to stress there was considerable uncertainty in the forecast.
To get a major snowstorm in the Washington area, two streams of flow needed to phase (merge together) at the right time and, for that to happen, two conditions had to be met: 1) the storm would track far enough west to get us in the wrap around snow band, and 2) the low needed to develop quickly enough to our south to get some easterly flow to tap Atlantic moisture.
Essentially, if either condition was not met, the snowfall in the D.C. area would be minimal.
What made the forecast exceptionally difficult was that the various forecast models could never quite agree on the track and on how quickly the low would develop. To make matters worse, Christmas Eve and into early Christmas morning the models seemed to be approaching a consensus toward the snowier scenario.
The forecast difficulties magnified on Christmas eve morning when the GFS model jumped well west of its earlier forecast giving Washington more than an 10" of snow. The ensemble members (remember ensembles are lower resolution runs with the initial conditions tweaked to help assess the uncertainty) trended to the west and wetter. However, NOAA also issued a statement that the initial analysis of the GFS might have problems making it hard for us to assess what that run meant.
However, the 1 p.m. GFS run event trended wetter suggesting that DC might get a foot of snow and its ensembles again were supportive. The other higher resolution models were much less bullish on snow at that time. At that time, CWG bumped up the probability of an inch to 50%.
The real forecast dilemma developed on Christmas eve into Christmas morning as the models appeared to be converging toward a consensus. The 7 p.m. GFS model (to the right, top left panel) gave Washington between 0.75 and 1.00 inch liquid equivalent (approximately 7-10 inches of snow). That same evening the NAM model (top right panel) still forecast a miss but the run 6 hours later (not shown) brought the low track closer to the coast (bottom right) and gave Washington between .25" and .50" (3-5 inches or snow). The European model that night also predicted a 3-5 inch snowstorm over the area.
The 1 a.m. GFS model Christmas day held onto its very snowy forecast from earlier. By 5 a.m. Christmas morning, the National Weather Service issued a Winter Storm Watch for the area east of I-95 and they expanded the area mid-morning. At that point, CWG noted that accumulating snow was now likely. Based on this guidance and a quick peek at the NAM model run from 7 AM Christmas morning, I uttered the infamous quote that I was quite "bullish" on the event. However, within that article Jason noted "there remains a small - but not trivial chance - that the storm just skirts us."
The GFS Christmas morning (see above top right panel) started backing off giving D.C. around 5 inches of snow. In view of the slight eastward shift in guidance, the CWG issued the graphic below (to the right) indicating there was a 1 in 4 chance of the storm missing the area. Still at that time, the GFS and the SREF ensemble guidance were supporting the 3-6 inch "best bet" forecast issued at that time.
Even after the operational models started backing off, the ensemble guidance continued to support snow. However, the models were also indicating the precipitation gradient (how quickly the precipitation changed as you went west to east) was stronger than with many storms. Nonetheless, the SREF ensemble model guidance during Christmas day became even more adamant supporting the idea of getting 5 inches or more of snow. Every member of the SREF ensembles gave D.C. over 5 inches of snow on the 4 PM run (see the trend of the ensembles below). The forecast decisions though dinnertime on Christmas day were as good as one might expect given the model guidance that was available.
The most egregious fault with the forecasts of the event was how long meteorologists (not just CWG forecasters) held onto their initial best bet snowfall forecasts and how slow they were to knock back amounts in the face radar and satellite imagery that suggested a near miss. Without doubt, forecasters were late in making changes.
A simple extrapolation of radar and satellite trends did indeed suggest a near miss yet forecasters stubbornly held onto their forecasts. Why? Because sometimes extrapolation of clouds and weather even in 12 to 24 hour forecast can lead a forecaster astray. Extrapolation of satellite imagery has led to several major snowstorm surprises (busts).
The satellite and radar imagery 12 hours prior to the President's Day storm of 1979, Veteran's Day snowstorm of 1987 and the February 16, 1996 event all come to mind as storms that displayed rather innocuous precipitation field the night before the snow began. Satellite imagery of the President's Day storm indicated all the bright clouds and precipitation associated with the warm advection were shifting out to sea mostly south of DC and that the city might only add another inch of snow to the four that had fallen earlier in the evening. That imagery prompted one veteran forecaster to state that there would only be "another inch with the vort. [upper level energy]" There was, but it occurred every 15-20 minutes for several hours.
While no one thought Sunday's storm President's Day storm potential, the CWG crowd knows that sometimes the models have problems handling the smaller scale precipitation banding when mid-level circulation center closes off to our south. Hence the hesitancy to quickly change the forecast and instead to assert that the amounts probably would be on the lower end of the range shown in our graphic.
By late Saturday evening, the radar was still suggesting that the heaviest snow might stay just to our east. The conflicting guidance presented us with a dilemma, should we lower amounts and then risk having to raise them if a band of heavier snow developed over us. With such uncertainty, we opted to hold steady with our amounts.
Our biggest fault during the event was being a little slow to cut back amounts based on radar. However, extrapolation of radar doesn't always work. Obviously, in this case with the more rounded upper trough (dip in the jet stream) and with a low that deepened a little late to get us in the game, radar and satellite told the story. There were differences between this event and some of the events noted earlier in the discussion. Also, we probably should have given the NAM model, which showed the least precipitation leading up to the event, a little more weight as it does have higher resolution. However, it typically scores a little worse on average than the SREF ensemble mean.
In summary, we probably should have relied a little more heavily on the radar and satellite trends especially during the day of Dec 26. Even better, we should have paid better homage to the weather gods, they are a fickle bunch.
| December 29, 2010; 11:15 AM ET
Categories: Latest, Recaps, Winter Storms
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