|Fig 1 -- SPC day 2 convective outlook from 1730Z, Saturday, Feb. 26, 2011.|
|Fig 2 -- 300mb height (contoured) and winds (shaded) at 12Z, Feb. 26, 2011.|
|Fig 3 -- GFS 36 hour forecast of 500 mb heights (contoured) and winds (shaded) at 00Z, Monday, Feb. 28, 2011.|
|Fig 4 -- ECMWF 48 hour forecast of 500mb heights (contoured) and winds (shaded) at 00Z, Monday, Feb. 28, 2011.|
However, from here on out things start diverging in the models (no pun intended...). Compare the GFS forecast for 500mb on Monday morning:
|Fig 5 -- GFS 48 hour forecast of 500mb heights (contoured) and winds (shaded) at 12Z, Monday, Fe.b 28, 2011.|
|Fig 6 -- ECMWF 48 hour forecast of 500mb heights (contoured) and winds (shaded) at 12Z, Monday, Feb. 28, 2011.|
But I'm not the only one having difficulty placing the surface low. We'll look at what the GFS forecasts to happen at the surface overnight. On Sunday night at 00Z, the center of the low is decently well-defined over western Oklahoma.
|Fig 7 -- GFS 36 hour forecast of surface temperature (shaded), sea-level pressure (contoured) and winds (barbs) at 00Z, Monday, Feb. 28, 2011.|
|Fig 8 -- GFS 42 hour forecast of surface temperature (shaded), sea-level pressure (contoured) and winds (barbs) at 06Z, Monday, Feb. 28, 2011.|
Here is the SPC's SREF forecast for low pressure centers from all of its different ensemble member models. At 00Z Sunday night, we see that there's generally good agreement in the placement of the low center in northwestern Oklahoma/southwestern Kansas.
|Fig 9 -- SREF 33 hour forecast of member surface low pressure centers for 00Z, Monday, Feb. 28, 2011.|
|Fig 10 -- SREF 39 hour forecast of member surface low pressure centers for 06Z, Monday, Feb. 28, 2011.|
So why this ambiguity in tracking where the low center is going to be? It all goes back into the feedback between the the temperature gradients below, which drive the winds aloft, which generate divergence aloft, which forms low pressure at the surface, which advects the temperature gradients around...and the feedback continues. Those subtle differences in the placement of the jet maxima and the dynamics of the winds aloft that we saw in the GFS and ECMWF models earlier have drastic implications for just where the surface low will be at any given time.
So what caused the divergence of the GFS and ECMWF upper air pattern forecasts? After all--they seemed to agree very well through early Sunday evening. Then they started differing. But by Sunday evening, we know that convection is going to start forming. Everyone agrees on that. The place where convection is likely to form would be along the fronts (the baroclinic zones) where the low-level convergence and lift are maximized. So we're likely going to have lots of storms firing along the fronts--which is also right underneath the jets. What do storms do? They tend to lift lots of warm, moist air into the upper atmosphere and can draw down cool air from aloft. This radically alters the thermal structure of the atmosphere in areas of convection. Since the position and strength of the jets themselves is tied to the thermal structure of the atmosphere, this in turn alters the jet pattern.
So, differences in how models handle convection can feed back into altering how the models forecast large-scale, upper-level patterns. If some models fire more convection or lift more warm air aloft in the convection, they can change the wind pattern aloft to keep things in balance. Models that aren't as vigorous in their convective prediction may not alter their wind patterns aloft as much. I'm guessing that differences in how the GFS and ECMWF forecast convection are leading to the differences in their upper air patterns. I would also suggest that the same thing is happening with the different SREF model members--and, since it all feeds back, that's why 's so hard to place the low pressure center...
Regardless, there's good agreement that moisture at low levels will be there:
|Fig 11 - - 33 hour forecast of 2m dewpoint temperature from the mean of the SREF model members at 00Z, Monday, Feb. 28, 2011.|