Severe Weather Looming?

I thought I would put together some thoughts for next week's storm system that is scheduled to arrive here on Wednesday. Any time there are several consecutive days with temperatures in the 60s and 70s in February usually means there will be a price to pay at some point. That price to pay looks to be in the form of severe weather by the middle part of next week.

A +PNA pattern for much of January and frequent polar vortex intrusions lead to a colder than normal month. We've quickly reversed the pattern with a trough over the west and a ridge over the eastern half of the country. This pattern has brought stormy weather to California and the Desert Southwest and a mid-winter thaw to areas east of the Rockies.

The first system of note is currently off the coast of California and is delivering much needed rain to parts of Central and Southern California. Most models are fairly consistent with ejecting this system into the Southern Plains on Monday. A sustained 48-72 hours of southerly winds should advect low to mid 50s dew points northward in advance of this first system. Despite a long duration of return flow, the Gulf of Mexico is completely scoured of any true maritime tropical air. In fact, you would have to travel into the Northwest Caribbean just to find 70 degree dew points and almost to South America to find mid 70 dew points! Due to these factors, I'm not expecting a widespread severe weather threat with this first system. However, the SPC is forecasting a slight risk of severe weather across West Texas on Sunday, but this looks marginal at best to me. With strong dynamic and linear forcing, a weak squall line may develop and pose a low-end threat for gusty winds. The good news is that this system should provide many parched areas in the Great Plains with some much needed rainfall.

The big storm system I am concerned about is currently impacting shipping interests only around 160 degrees West longitude. After the passage of the first system, all eyes will quickly turn back to the west as another powerful storm system will be slamming onshore of California and the West Coast early next week. In advance of these two storm systems, our area should experience a southerly wind component from now until next Wednesday when our storm system moves through. Unfortunately, this will allow moisture to gradually increase for several days. The GFS and NAM both insist that lower 50 dew points arrive into our area by Sunday evening. I think this is likely too aggressive as we probably won't see lower 50 dew points until Monday.

By Tuesday morning, the GFS, NAM, NOGAPS, JMA, and ECMWF are all in reasonably good agreement with the timing and position of the trough. The Canadian is slightly faster but still agrees with the placement of a trough in the Desert Southwest. By Wednesday morning, however, model divergence begins to rear its ugly head. The Canadian is the fastest and furthest north of all the models and seems to be an outlier at this point. The JMA is the deepest and slowest of the solutions with the trough axis lagging back in Southwest Texas. The ECMWF and GFS are somewhat similar, but the GFS seems slightly faster. The NOGAPS is a compromise between the faster and weaker CMC and the slower and deeper ECMWF and GFS. The Canadian Ensembles are somewhat spread with the exact trough axis, but they argue for a slower and deeper solution.

While there is some disagreement between the models with the exact timing and strength of the system, I feel very confident that our area will experience severe weather with this storm system. I prefer the deeper and slower solution at this time and feel that the severe weather threat may be late Wednesday afternoon and Wednesday night. The GFS, ECMWF, and JMA all show an almost classic synoptic setup for a severe weather outbreak in our area for Wednesday. A powerful, negatively titled trough will move just to our north on Wednesday afternoon and evening accompanied by +90 knot winds at 500 mb. At the same time, the right rear quadrant of a +120 knot jet streak will be in place over our area. A very strong low level jet (+55 kts) will advect moisture northward rapidly through the day on Wednesday.

These impressive dynamics will initiate the development of a surface cyclone across Western Arkansas that will quickly deepen as it moves to the north and east. In advance of the cold front and low pressure center, a warm, moist air mass will be present across the lower Mississippi River valley. Just how moist the air mass will be remains in doubt due to the quality of maritime tropical air. At this point, I feel that lower 60s dew points will be advected into our area ahead of the cold front, which should allow for enough instability to support severe weather. Getting specific about the mesoscale environment is futile at this point, but the environment should at least be supportive of severe weather.


Let me go ahead and say that I strongly disagree with the SPC's 4-8 Day Outlook at this time, but I feel reasonably confident that they will outlook areas for severe weather on Wednesday in upcoming outlooks. Despite this, I've got several concerns and questions at this point that will be answered with time. Here are some of them:
1. If greater moisture return is realized, we could be in store for a much worse severe weather event. This would effectively increase the instability and lead to stronger and potentially supercellular storms.
2. If the track of the low is further north, this may tend to lessen the chances of a major outbreak. On the other hand, if the track of the low shifts slightly southward, this will increase the chances of a major outbreak. If the track shifts significantly southward, it will lessen our chances for severe weather.
3. If the timing of the system speeds up and pushes the cold front through during the peak heating of the afternoon, instability would be slightly higher. If the main threat of storms holds off until the nighttime hours, there will be slightly less instability available.
4. These dynamic, strongly forced, and negatively titled systems tend to produce more squall lines than discrete supercells. The reason for this is because most of the winds are parallel to the front rather than perpendicular. The parallel nature of the mean flow tends to produce more linear storms rather than discrete supercells. However, this system is somewhat strange in that the main storm system tilts to the southwest and the lower levels do not exhibit negatively tilted trough characteristics. In fact, the 850 mb chart argues for a positive trough tilt, and this pattern allows the mean wind to be more perpendicular to the front. This could increase the chances of more isolated storms if this pattern continues with successive models runs.
5. There's no easier way to blow a cool season Southeast severe weather event than to not properly forecast the stratus deck. As with just about every severe weather event in our area, the extent and strength of the low clouds will play a key role with this event. If clouds can break up and allow for more insolation, instability levels will be higher. A persistent stratus deck will severely limit instability and likely lead to a lesser severe weather threat.
6. Capping issues associated with the southwesterly flow may limit storms until strong forcing arrives. However, it may also allow energy to build throughout the day and be released when the main dynamics arrive. This is most likely to be a greater concern in Southwest Mississippi.

We're still several days away from this event, but I feel confident in the threat for severe weather for our area. Forecasting the mode of severe weather and the associated threats is extremely difficult, if not impossible at this point. Time will increase my confidence with this aspect of the storm system, especially by watching the track of the system and the moisture return over the next few days. Keep an eye on the blog early next week as I'll update my thoughts.