Thursday Afternoon Forecast Thoughts

Here's the radar as of 4 PM this afternoon. We're actually noticing some showers and embedded thunderstorms to the east of I-55, north of I-20, and south of Highway 82 across east-central MS. I have to ask myself why this activity is here, and one potential explanation is this: 

Notice the relative higher pressures to our east, and how the lower pressures are kind of angled toward it. This is an inverted surface trough, and oftentimes can result in showers and storms over a limited spatial area, and in our case, it would be south-central into east-central Mississippi. 

Here's the latest water vapor imagery of Tropical Storm Karen. Notice the vast amounts of dry air on the north and west side of the system, while copious amounts of tropical moisture exists to the northeast and southeast. Tropical systems, for one, like lots of moisture because dry air disrupts their circulation...much like what Karen is experiencing now. How will Karen impact our weather in the GTR? 

Here's the latest national weather vapor imagery. Notice the large swirl of clouds over the Intermountain West region. This is associated with a large trough of low pressure that will also be influencing our weather pattern this weekend as well as influencing Karen's path. 

Let's jump to Saturday. I'm showing the GFS model here, but please remember there are several different models... 

For your Saturday, we have Karen approaching the central Gulf Coast while the aforementioned strong trough continues to lift northeast into the Great Lakes region. If Karen weren't in the picture, this in of itself would likely reduce any severe weather threat that might would otherwise be a possibility with a system of this stature. 

Here's the 500mb projection at 18z Sunday which is 1pm. Karen is in the process of getting picked up/absorbed by the trough, but the overall trough axis remains very deep. However, the precipitation forecast is interesting: 

Here's the same time, 1pm Sunday. We have a cold front approaching from the northwest, and Karen to our southeast near Dothan, AL (according to this model). If this were correct, dry air from the northeast being wrapped into Karen juxtaposed with subsidence often associated with the outer parts of a tropical system, would probably reduce our rainfall potential. 

This is slightly counter-intuitive. We still have uplift from the approaching trough/front, but when it is more or less "offset" by the dry air/subsidence, our rain potential MIGHT be lessened due to this. 

However, the European model has Karen significantly west relative to the GFS, but still has it passing to our south and southeast. However, given its closer proximity, our rain chances would be increased. The NAM has a similar track as well. 

Here's the NHC track as of the 4pm advisory. There's no doubt that the trough approaching us will pick Karen up and steer it northeast, but just how fast this happens is still uncertain. I think the best course of action is, for now, give all models equal credibility and watch how Karen interacts with the shear/dry air over the next 24 hours....

BOTTOM LINE: Rain still is a decent bet for us here in the GTR Saturday evening into Sunday afternoon, either in association with Karen or the front, or both. But given the potential fly in the ointment I discussed, our potential accumulation could be significantly reduced, or maybe enhanced depending on the ultimate track of Karen. 

The joys of forecasting...

IW