Sunday, May 8, 2011

A weak trough with conditional severe threat early this week

Back to a general synopsis of the weather conditions to expect in the coming week...

The SPC has a slight risk of severe weather out for various isolated spots in the upper midwest with even more conditional possibilities of severe weather in the southern plains over the next few days.

So, let's look at the overall setup.  Here's Sunday morning's 12Z 300mb upper air analysis:
Fig 1 -- 300mb analysis of winds (colors) and heights (contours) for 12Z, Sunday, May 8, 2011. 
Not the most amplified pattern at all--there is generally zonal (west to east) flow across the country with no deep troughs or ridges.  There is a shortwave trough entering the Pacific northwest, however, and this trough going to end up causing the severe weather chances throughout this week.

However, this isn't a typical kind of trough, at least according to the model forecasts.  Here's the GFS forecast down at 500mb on Monday morning.
Fig 2 -- GFS 18 hour forecast of 500mb heights (contours) and winds (colors) for 12Z, Monday, May 9, 2011.
That trough is forecast to deepen awfully quickly (though we're also looking at a lower level of the atmosphere in this image, but still...).  This seems like a rapid transition from a relatively flat, zonal pattern to an amplified trough-ridge pattern.  When such changes happen, we usually expect active weather.  With the curved jet streak visible on the southern side of the trough, we'd expect divergence in the exit region of the jet streak over the four-corners area and into southern Colorado.  With divergence aloft there, we might guess that there would be a surface low there...
Fig 3 -- GFS 18 hour forecast of surface temperature (colors) mean sea-level pressure (contours) and winds (barbs) for 12Z, Monday, May 9, 2011.
...but no! At least, not according to the GFS.  It has a pronounced surface low analyzed much further north over the Dakotas.  This doesn't seem to be very coherent--a low pressure center can't sustain itself without divergence aloft.  Therefore we'd expect that low to be weakening and we're still looking for a low to form under the area of divergence aloft.  Fortunately, the GFS model has these known physical relationships built into it, and by Monday evening the surface pattern seems to have corrected itself:
Fig 4 -- GFS 30 hour forecast of surface temperature (colors) mean sea-level pressure (contours) and winds (barbs) for 00Z, Tuesday (Monday evening), May 10, 2011.
The GFS has brought the center of the low back down further southwest.  But notice what has happened in the meantime.  In figure 3 (on Monday morning), there seemed to be a weak cold front trailing behind the low to the southwest.  However, with divergence aloft over southern Colorado, the strongest pressure falls at the surface were also in the southern Colorado area.  These pressure falls caused the surface winds across the southern plains and the southwest to back slightly, becoming more southeasterly rather than straight southerly as air rushed toward the region where pressure was falling.  This, combined with lots of daytime heating, seems to have caused warm air to surge westward, effectively wiping out that weak cold front we saw Monday morning.  As such, the GFS really doesn't have much of a cold front shown by Monday evening.

Of course, remember from the thermal wind arguments that I've talked about frequently in these blog posts that the upper-air winds, particularly the upper air jets, are dictated by temperature gradients below.  By effectively erasing that weak cold frontal boundary, the temperature gradient there has been erased too.  Without that temperature gradient, the jet aloft should tend to weaken.  Here's the GFS 500mb forecast for Monday night:
Fig 5 -- GFS 30 hour forecast of 500mb heights (contours) and winds (colors) for 00Z, Tuesday, (Monday night), May 10, 2011. 
It's rather subtle, but you can begin to see that the jet streak seems to be further retreating to the west around the base of the trough.  Now the exit region seems to be over eastern Arizona or western New Mexico.  It's still not in the best position to be supporting that surface low over eastern Colorado.

However, on Tuesday, the surface low and the upper-air trough finally seem to be getting their act together.  Cold air from eastern Montana seems to sneak in behind the surface low and re-establish a weak, but strengthening cold front.
Fig 6 --  GFS 48 hour forecast of surface temperature (colors) mean sea-level pressure (contours) and winds (barbs) for 18Z, Tuesday, May 10, 2011.
There's a feedback mechanism at work here--as cold air sneaks in behind the surface low and strengthens the temperature gradient, the upper-level winds increase along the temperature gradient.  This will strengthen the jet streak aloft and bring it further north, finally putting it in a position where its exit region (and associated divergence) is over the surface low and can support it to continue deepening.  But, a strengthening surface low is going to advect more cold air in behind it, further sharpening the cold front.  This strengthens the jet streak even more...and so on.  This is part of a phenomenon called baroclinic instability--an instability driven by temperature gradients.  Here's the 500mb map for Tuesday afternoon showing how the jet streak is forecast to move northward over the strengthening cold front.
Fig 7 -- GFS 48 hour forecast of 500mb heights (contours) and winds (colors) for 18Z, Tuesday,  May 10, 2011. 
Ironically, the GFS then forecasts the whole setup to weaken again after that--but that's already getting pretty far out in model time.

So what's the consequence of this slow-developing cyclone?  As you can see, over much of the central part of the country there will be southerly winds and lots of warm air advection from the south at the surface.  This will bring very warm air pretty far north--notice that high temperatures will probably get into the low 80s as far north as Iowa and northern Illinois.  Furthermore, this southerly advection will bring in more moisture, too. Here's the dewpoint forecast at the surface for Tuesday afternoon:
Fig 8 -- GFS 48 hour forecast of surface dewpoint temperature (colors) for 18Z, Tuesday, May 10, 2011.
That forecast is showing an area of over 70 degree dewpoints on Tuesday afternoon across much of the central Mississippi River valley.  That's a lot of moisture.  With warm, very moist air at the low levels, we're obviously looking at a lot of potential instability across a large swath of the country.

But remember--instability isn't everything.  First we'd have to check if the atmosphere was capped at all.  Here's a different way of looking at potential capping inversions.  This is the GFS forecast for 850mb temperatures on Tuesday afternoon:
Fig 9 -- GFS 48-hour forecast of 850mb temperatures (colors) and height (contours) for 18Z Tuesday, May 10, 2011.
 The temperatures are in Celsius, but you can see that at 850mb the temperatures are in the low-to-mid 20 Celsius range right over our area of 70 degree dewpoints.  That works out to around 70-77 degrees Fahrenheit.  But at the surface high temperature were forecast to be in the upper 70s to low 80s.  This does not represent significant cooling with height if the temperature only drops a few degrees from the surface to 850mb.  Thus, we can conclude that there's probably a fairly strong capping inversion in place over the region of warm, moist air at the surface.

With a capping inversion in place, we're going to need some sort of forcing mechanism to get storms going.  But what forcing mechanism?  We've already seen that the cold front is being washed out or weakly present way out west over the high plains--far removed from our area of greatest moisture and heating.  Furthermore, there's no wide-spread upper-air support.  That upper-level jet at 500mb is just barely keeping the low pressure center going--with no divergence at all suggested over the area of warm temperatures and high dewpoints. As such, there's nothing really to support thunderstorms except the warm moist air at the surface.

So, the chances for severe weather will be highly conditional over the next few days.  Yes, it's going to get warm and sticky over the central part of the country.  And there are a few areas where there seems to be some convergence of winds at the surface.  There looks to be a dryline forming in Oklahoma and Texas by mid-week.  There also will probably be a warm or stationary frontal boundary stretching across the upper midwest at the leading edge of all that warm air as this low tries to organize itself.  The Storm Prediction Center is focusing on any convergence along that warm/stationary front as the best chance for severe weather over the next couple of days.  However, there will be a possibility for thunderstorms anywhere that there happens to be enough convergence near the surface to cause lift that can overcome the capping inversion.  And with such warm, moist air to draw upon, any storms that form will have a high probability of becoming severe.

Furthermore, with how chaotically the GFS seems to be handling this low pressure center's development and the upper-air response, this is a highly uncertain forecast--bound to change as the week goes on.  So...we'll have to be alert.

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