As the graphic above shows, we are dealing with a wintery system moving across portions of the Central Plains this afternoon that may cause some issues for the rush hour in places like Norfolk, Yankton, and Sioux City as rainfall changes or has already changed to snowfall. Some totals out of central Nebraska are already at 3”+ and climbing. The snow will begin to taper off across the SW this afternoon but is expected to continue across most of the major metro areas for quite some time this evening. Some 6” totals are expected across central sections of the state into SE South Dakota.
We have Chris Allington out in Nebraksa gathering some storm footage and we should have some available to show you shortly. Stay tuned…
A trough of low pressure is starting to move into the western plains as it ejects out of the Rockies overnight tonight. It will be positioned somewhere across MO/IL this time tomorrow. The main reason we are here is to discuss the potential for severe thunderstorm development tomorrow afternoon and evening. Let’s take a look at the regional radar currently…
Deep layer moisture streaming in from the Eastern Pacific and Western Gulf of Mexico is providing a large amount of rain and a few thunderstorms across much of Texas all the way into Michigan. Heavy rains and some areas of flash flooding have been noted in east Texas and into Arkansas this evening. Flood watches are up for portions of AR/LA/MS through the day tomorrow as additional heavy rainfall is likely with some training thunderstorms.
The latest run of the GFS and NAM both bring a powerful jet max by midday across AR/MO approaching 100kts. This would normally be a major signal for potentially significant severe weather. There are a couple of limiting factors that are likely to inhibit a major event this time though. Rather warm air aloft…which is related to Tropical Storm Raymond in the Eastern Pacific region…will keep lapse rates in the mid levels rather low. This moist environment will decrease the ability for the column to cool in an evaporative manner aloft…thus reducing CAPE values through the column. Most of the severe weather will therefore be right along the frontal boundary where the steepest pressure falls and shearing is located. Most of this will be shear driven. The area circled above will likely experience the highest threat (albeit marginal) of severe storms and maybe a few tornadoes during the afternoon into evening hours.
The same model above shows a rather strong convective signal just ahead of the pressure gradient/frontal boundary. This indicates that a squall line is likely the dominant storm mode with some cloud debris and shower activity ahead. Isolated storms are not likely due to a slight capping inversion with warm air and southwesterly flow aloft.
Finally, here is a visual look at the weak instability tongue located across the Midsouth by 3pm. The LI values are sometimes a better indicator of instability during the cool season and this is what we will use. You can see the weakly unstable environment runs along the 60+ degree isodrothermal gradient. Again, while there is some potential for severe weather along the linear system…the threat of tornadoes is not extremely large. I do expect tornado watches especially in the Memphis and Paducah areas. There is a likelihood that several severe thunderstorm warnings will be involved with some winds pushing greater than 60kts. This line will likely weaken quickly as diurnal heating processes weaken after the sun sets. We will continue to monitor the threat and update as needed. Several Live Storms Media crews are set to deploy tomorrow as well.
Good afternoon and good weekend everyone!
While the plains are looking at a severe threat and a blizzard develops just west of there, those along the gulf coast are uneasy as a storm approaches from the south.
Alot of uncertainty exists concerning Karen. While my previous forecast made on October 1st still seems to be the most likely scenario, considerable model disagreement is ongoing.
A few scenarios are possible:
- The storm completely decouples and moves steadily NW and causes little impact as a low level swirl (15% chance)
- The storm remains highly sheared and impacts SE Louisiana as a minimal tropical storm (35% chance)
- The storm persists and begins to turn as indicated in the NHC forecasts (50% chance)
The first two scenarios are fairly self explanatory. The third and most likely scenario is the most interesting and confusing.
This post will focus on that scenario and the current forecast is for this situation to develop, though there is aconsiderable amount of uncertainty.
As you can see, the storm is highly sheared with most of the cloud cover displaced of the low level center. However, over the past hour convection has begun firing on the northern edge of the low level center which suggests that atleast for now the storm is not on the verge of complete collapse.
As illustrated, the storm is currently experiencing about 20-25 knots of southwesterly shear. As the storm moves north, a small pocket of light shear may allow for some very slow strengthening in the overnight hours, but that is probably unlikely.
The 300 mb chart shows an approaching long wave trough. This is going to be the main steering mechanism in the longterm and provides a bit of divergence, aiding Karen, due to curvature. Depending on how close the TS and the trof get to each other will determine if a favorable (divergence) or negative (increased shear) effect is made on Karen.
Here, an analyzed (cold frontal placement) 1000-500 mb thickness chart with surface pressures cotoured shows the frontal interaction of Karen with the mid latitude cyclone. Assuming Karen can stay ahead of the front and avoid being absorbed, this scenario should cause the storm to being to move NE to ENE in about 36 hours from now (5 PM Central time)
700 mb RH continues to show dry air from the west being a problem and at this point we can probably expect that to be an issue that plagues the storm until dissipation.
While the potential threat of a significant storm appears to be diminishing, Karen is still a system worth keeping a close eye on if you have interests along the northern gulf coast.
Meteorologist Logan Poole