Ensemble Forecasting. Brian Duddy. What is ensemble forecasting?. At its most basic level, ensemble forecasting is a method of using a mathematical model or models to predict the future behavior of a complex, dynamic system that cannot be predicted exactly
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This is the combined National Hurricane
Center forecast, with the white zone representing the “potential track area”
A note from the NHC accompanied their forecast: IT CANNOT BE REPEATED ENOUGH THAT FOUR- AND FIVE-DAY TRACK FORECASTS CAN HAVE SIGNIFICANT ERRORS…AND COMBINED WITH THE FACT THAT THE MODEL SPREAD IS STILL NOTABLE BEYOND 72 HOURS…ONE SHOULD NOT FOCUS ON THE EXACT TRACK.
This is an NOAA forecast from midnight, Apr. 6 2006 of convective available potential energy (CAPE), which measures the potential for large thunderstorms to occur. Each different model run, or “ensemble member”, uses a different color. Each model outlines an area at which the CAPE is above 1000 J/kg, a threshold above which large storms are likely to occur. The more bands a specific area is inside, the more likely it is (according the the models)
that the CAPE will be above the threshold. Thus Oklahoma, Kansas, and northwestern Texas are predicted to have a large CAFÉ, while Louisiana and Arkansas have a lower predicted probability (of course, the West and East are predicted to have essentially no probability of such a CAPE value)
We now have the two key components to the potential winter storm for Wednesday and Wednesday night over land in order to begin to get better sampling. As a result...we have seen a shift nwwd to the expected storm track as well as a timing adjustment to have the storm move in a bit quicker than previous model runs. This has occurred on all models...but there still remains a decent spread in storm tracks although the timing is roughly the same. The GFS(1) has been the most southerly track with this storm almost since it began tracking it several days ago. The NAM(2) has been the most northerly since the storm timing moved within 84hrs. The most consistent model run-to-run has been the Canadian Gem(3). This has consistently advertised a track through the middle of the forecast area and will be largely what this forecast is based off of...at least with the synoptic scale features. And it isn/T(sp) much surprise that the most consistent model now lies squarely in the middle of the 12z model spread.
After the models were a bit hesitant to build in the cold air a few days ago...they have all come on board with the idea that the cold front will move through Wednesday morning and early afternoon and will be followed by strong cold air advection at the boundary layer and cooling through the column due to evaporational cooling. These two factors have led to an increasing model certainty on an earlier transition from rain to snow...for KUIN-kcou(4) during early Wednesday afternoon...for kstl-kfam(5) during late Wednesday afternoon...and for kslo during early Wednesday evening.
Have taken a more aggressive approach on probability of precipitation than the previous forecast to acknowledge the model shift and a bit more increased confidence on track...but exact precipitation changeover timing is tricky and so snow forecast amounts are not a complete committment to the changeover timing until we see some consistency in this area.
(As of 9:30 PM on Monday, when these were the most recent messages, there is a 60% chance of snow tomorrow)