Saturday, February 26, 2011

Moderate risk again...and model uncertainty

Another day, another moderate risk of severe weather in the southern US forecast for tomorrow.  Severe weather season seems to be here...
Fig 1 -- SPC day 2 convective outlook from 1730Z, Saturday, Feb. 26, 2011.
The evolution of this system seems to be straightforward--at first.  We start with the current upper-level conditions.  Like we saw with the last severe weather system, we start out with a deep trough off the coast of California this morning:
Fig 2 -- 300mb height (contoured) and winds (shaded) at 12Z, Feb. 26, 2011. 
This trough is forecast to move eastward and become more neutrally tilted (the axis will be pointed more north-south) by Sunday morning.  Both the GFS (shifting to 500mb here so that I can do a comparison...):
Fig 3 -- GFS 36 hour forecast of 500 mb heights (contoured) and winds (shaded) at 00Z, Monday, Feb. 28, 2011. 
And the ECMWF:
Fig 4 -- ECMWF 48 hour forecast of 500mb heights (contoured) and winds (shaded) at 00Z, Monday, Feb. 28, 2011.
Show similar upper-air patterns on Sunday evening.  The exit region of the jet around the base of the trough (particularly the left exit region since this jet seems to be straight and not very curved) would be the favored location for divergence aloft.  Divergence aloft means low pressure and lift at the surface.  So we'd generally expect widespread lift in the eastern Oklahoma-Arkansas-southern Missouri region on Sunday night.  This corresponds well with the western part of the SPC's convective risk above.

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.
To the ECMWF forecast at the same time:
Fig 6 -- ECMWF 48 hour forecast of 500mb heights (contoured) and winds (shaded) at 12Z, Monday, Feb. 28, 2011.
There are subtle, but significant differences in the wind field between the two models.  Notice the location of the maximum wind in the jet.  The ECMWF has the maximum over southeastern Missouri whereas the GFS has the maximum back over southeastern Oklahoma.  The ECMWF also places the critical coupling area between the anti-cyclonically curved jet over eastern Canada and the left exit region of the jet over the central US somewhat further west and north than in the GFS model.  These differences make it difficult to predict how the surface low is expected to move from Sunday night into Monday morning, since the areas of divergence aloft associated with the jet patterns are different in the two models.

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.
Note the strong baroclinic zone stretching east from the low center.  A baroclinic zone is just a fancy name for an area with a strong temperature gradient (that technically should be moving around as well, but...that's difficult to tell).  We would normally associate the temperature gradient stretching east from the low with the warm front.  But, compare the winds to the south and the north of that boundary--those are pretty strong winds to the north of it, blowing out of the north.  If the cold air to the north is advancing, this looks to be a cold front instead of a warm front.  But that wouldn't be consistent with our normal depiction of a surface low-pressure system--a cold front typically doesn't stretch to the northeast of the low center.  We'd expect to see a cold front in that location if the low center was actually further northeast --say, over Missouri or Illinois.  But here is what the GFS shows for the surface pattern just six hours later:
Fig 8 -- GFS 42 hour forecast of surface temperature (shaded), sea-level pressure (contoured) and winds (barbs) at 06Z, Monday, Feb. 28, 2011.  
The center of low pressure has been shifted to the northwest--now over Illinois and Indiana!  This can't be the same low pressure center--even driving on the interstate at 70 mph it takes 14 hours or so to reach Indiana from Oklahoma.  That would imply that the surface low would have had to have moved along at somewhere around 140 mph to move that far in that amount of time.  Highly unlikely.  So we have a reformation of the low pressure center further to the northeast along the baroclinic zone.  But this GFS model only gives us one perspective on where and when this transition will happen.

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.
Moving just six hours later, we can see that there is considerable spread among all the members, where they have the position of the surface low (or lows--some model members have two low centers at the same time) at various places along the baroclinic zone:
Fig 10 -- SREF 39 hour forecast of member surface low pressure centers for 06Z, Monday, Feb. 28, 2011.
So there is some ambiguity where the actual center of the low pressure is going to be.  As many storm chasers know, the further one gets from the low pressure center, supercell probabilities tend to weaken.  The low pressure center serves as a focal point for low-level wind shear and the further you get from that, usually the wind shear will tend to weaken somewhat.  Not always the case, though.

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.
The mean of the SREF models suggests 60+ degree Fahrenheit dewpoints as far north as southern Missouri and southern Illinois on Sunday night (note the strong gradient of moisture over central Oklahoma and Texas showing a classic dryline or cold front pattern).  There is some concern about an elevated mixed layer capping convection over Oklahoma--temperatures are somewhat warm at 850mb aloft.  But if that can be overcome, there is more than enough moisture to get some good storms.  Winds aloft are conducive for lift and shear also looks respectable.  So the potential for severe weather is definitely there.

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