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.|
|Fig 2 -- SPC day 2 convective outlook for April 15, 2011. From the SPC.|
|Fig 3 -- 300mb analysis of winds (colors) and heights (contours) valid 12Z, April 14, 2011. From the HOOT website.|
|Fig 4 -- 12Z RUC initialization for April 14, 2011. From the HOOT website.|
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.|
|Fig 6 -- Same as figure 5, but annotated with approximate location of the center of the jet streak.|
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.|
|Fig 8 -- RUC 15Z dewpoint temperature analysis with wind barbs for April 14, 2011. From the HOOT website.|
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.|
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.|
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.|
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.