Thursday, April 14, 2011

More moderate risks...and differences

We're definitely entering the heart of the spring severe weather season now.  In the wake of last weekend's moderate risks in northern Iowa and Wisconsin where several large and damaging tornadoes were reported, yet another potent shortwave is digging into the central part of the country.

The SPC has issued a moderate risk for eastern Oklahoma and western Arkansas today:
Fig 1 -- SPC day 1 convective outlook for April 14, 2011.  From the SPC.
And then as this trough and surface low move eastward, another moderate risk has been issued for an area in central Mississippi and Alabama with a slight risk area throughout much of the southeast.
Fig 2 -- SPC day 2 convective outlook for April 15, 2011. From the SPC.
Based on this morning's 12Z analyses, there is a sharp shortwave across the intermountain west at 300mb:
Fig 3 -- 300mb analysis of winds (colors) and heights (contours) valid 12Z, April 14, 2011.  From the HOOT website.
Now, compare this to the 12Z RUC model (the Rapid Update Cycle model) initialization:
Fig 4 -- 12Z RUC initialization for April 14, 2011.  From the HOOT website.
Overall the two are very similar--and they should be, as the RUC model should be initialized off of the 12Z observations.  But I do notice some small, but important differences.  The most critical is in the placement of the axis of that trough in the west.  In the objective (computer-drawn) analysis of the observations in figure 3, the axis of the trough is placed through far western Idaho and into central Utah.  However, in the RUC initialization, the trough axis seems to be located in eastern Idaho and down into central Colorado--further east than in the objective analysis.  Which do we believe?

One thing we can check is the water vapor imagery from that time.  Water vapor images retrieve radiation that is typically emitted from the upper-levels of the troposphere--around the 300mb level we're looking at in the maps above.  The strong jet streaks up there perturb and contort the water vapor field so that we can rather easly trace the location of most major jet axes aloft on the water vapor images.  Here's the water vapor image from 12Z this morning:
Fig 5 -- 12Z GOES-W water vapor image for April 14, 2011.  From the HOOT website.
See the sharp contrast between the drier air (the blacker shades) and the moister air (the white and blue shades) that stretches from eastern Nevada through southwestern Utah and into northern Arizona and New Mexico?  That roughly marks the center of the jet streak aloft at that time.  I tried to highlight it using my mouse that has a very poor response time on the image below:
Fig 6 -- Same as figure 5, but annotated with approximate location of the center of the jet streak.
If we assume that the winds are more or less parallel to the height contours (which isn't a bad assumption--we should be near geostrophic balance up there...) then we see that the actual trough axis agrees more with the RUC model initialization--it probably stretches into eastern Colorado at 12Z instead of being further west in Utah.  So...the model seems more believable after all.  This once again highlights the need to be cautious when looking at objective analyses--analyses that were done by a computer trying to fit contours to sparse data.  Using higher-resolution observations like the satellite imagery can help to correct any errors.

As the exit region of this jet streak aloft emerges from over the Rockies this afternoon, we've seen a surface low develop which is now in northwestern Oklahoma as of early this afternoon:
Fig 7 -- RUC surface analysis of temperature (colors), mean sea-level pressure (contours) and winds (barbs) for 18Z, April 14, 2011.  From the HOOT website.
 Note the wind field and temperature fields are a little different from what we usually see.  The warm sector to the east of the low is very expansive--the warm front boundary between colder air to the north and warmer air to the south extends northward out of the analyzed low pressure center before turning sharply eastward in northern Kansas and stretching across the midwest into western Pennsylvania.  This seems like an odd structure for a warm front, as usually we don't see them heading northward out of a low-pressure center.  Also the winds to the northeast of the low are more easterly than southerly--this doesn't do much for advecting in moister air needed to fuel thunderstorms.  Therefore, we might guess that the greater threat for severe storms would be further south to the southeast of the low where winds are southerly all the way down to the Gulf of Mexico.  This area probably has more moisture advecting in.  A check of the dewpoint analysis from the RUC shows that this is indeed where the higher dewpoints are located:
Fig 8 -- RUC 15Z dewpoint temperature analysis with wind barbs for April 14, 2011. From the HOOT website.
And we begin to see why the SPC has placed their moderate risk in that particular region.  Why not further south where the moisture is greater? The capping inversion seems to be stronger there and our jet streak aloft seems to be staying further north.  Without that upper air support that comes from the divergence induced by the jet streak exit region, it becomes much more difficult for storms to form.

Upper-level support is fine, but what about forcing near the surface?  We need something to provide lift.  Usually this is accomplished by a frontal boundary, but we saw that the warm front was way too far north and the cold front really didn't seem to have fully developed yet.  However, the dewpoint map shows the primary mechanism being looked to for lift--convergence of the winds along the dryline.  There is a notable dryline present in the dewpoint map from early this afternoon--the sharp boundary between moister air to the east and drier air to the west in western Oklahoma and north Texas is accompanied by winds shifting from southerly to the east and westerly to the west.  This means winds are converging near the dryline, and convergence at the surface typically forces air to rise.  As the dryline mixes eastward across the state of Oklahoma this afternoon, that convergence zone will move with it and storms will form as surface air is lifted.

Why haven't storms fired already?  Our old friend the capping inversion is still in place--but just barely.  The Norman forecast office launched a special sounding at 18Z this afternoon (around 1 PM CDT).  Here it is:
Fig 9 -- 18Z sounding from Norman, OK (KOUN) on April 14, 2011. From the SPC.
A small capping inversion remains right at around 800mb.  However, the lapse rate (the rate temperature is changing with height) below that inversion is very steep--nearly dry adiabatic.  This is a highly unstable layer--and that probably means that a lot of mixing of air is going on in that near-surface layer.  With such a small cap remaining, this mixing will almost assuredly erode the cap away in the near future.  The dewpoint is registered at 56 degrees Fahrenheit at the surface here.  Any small increases in moisture due to advection from the south will be magnified as increases in the convective potential energy.  Already there is a surface CAPE value of some 2064 J/kg.  Very unstable.

And I won't say much about wind shear, but it's rather strong--winds shift from 10 knots out of the south at the surface to 40 knots out of the southwest at 700mb in the sounding above.  Not the largest shear I've seen, but definitely there.  Furthermore, as evening approaches and the low-level jet kicks up, that shear will undoubtedly increase.  However, by then the threat will have shifted to eastern Oklahoma as the dryline will have already mixed through Norman.

One final interesting note about the movement of this low-pressure center.  The RUC model has the low pressure center moving northward along that north-south oriented warm front boundary before settling in at the point where the warm front suddenly turned east.  Here's the forecast surface map from the RUC model for 10 PM CDT tonight:
Fig 10 -- RUC 12 hour forecast of surface temperature (colors), winds (barbs) and mean sea-level pressure (contours) for 03Z, April 15, 2011.  From the HOOT website.
So the low is forecast to move almost due north throughout the day, according to the RUC model.  Also we can see that as the low moves north the cold front is finally beginning to show up, and it is beginning to overtake the dryline from central Kansas down into northern Oklahoma.  At least according to this model.

However, if we look at a map of surface pressure changes in the last three hours, we see something slightly different:
Fig 11 -- 3-hour pressure changes and wind vectors as of 19Z, April 14, 2011.  From the College of DuPage.
The area of strongest pressure falls over the last few hours has been in far eastern Kansas near the Kansas City area.  To me, this implies that the low pressure center is actually moving more easterly than the RUC model would suggest.  This would speed up the timeline of events a little bit as then the dryline would push through Oklahoma somewhat faster. If these strong pressure falls are a sign of the low pressure center's movement, then I wouldn't be surprised to see the risk area extended slightly eastward as the day wears on.  We can also watch the water vapor image to try and trace the location of the jet streak's exit region over time this afternoon.  If it seems that the jet streak is also moving further east, the surface low will probably try to stay with it as that divergence aloft is what's supporting the low.  We'll just have to see.

Regardless, I expect some impressive-looking storms in eastern Oklahoma today.  The tornadic potential will be high, so be alert and listen for warnings if you live in an at-risk area.

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