Well, it doesn’t get much more succinct than this, from the SPC outlook for Friday through Tuesday: “An active stretch for severe weather will occur during the entire D4-8 period, mainly over the central and southern Plains.”I’m going to concentrate on the Friday/Saturday risk, but will include the SPC’s outlook for the next system Monday and Tuesday at the end of this post.
Friday, May 17
Here’s the setup according to most of the models forecasts. These images are Friday night at 7pm CDT (from Pivotal Weather):
This image shows dew points (the underlying colors), wind barbs (the flags) and barometric pressure (the lines with small numbers). It shows a fairly deep low pressure center just west of Goodland at 7pm Friday CDT. The dry line extends from near North Platte, NE down to the Midland, TX area — if you draw a line along US-83, you’ll be lined up with the dryline pretty well. Dew points ahead of the dryline are in the mid and upper 60’s (64 Dodge City, 66 Pratt and Hays, 61 Norton). Behind it, they are in the mid-30’s.
This image is at the same time, and shows the Energy-Helicity Index, a combination of the amount of energy in the air (from moisture) and the amount of twist in the first 15,000 feet or so of the air. Values are from 0 to 16, and the purples in this image are values right around 7…pretty healthy, from a severe weather generating point of view.
There are some big caveats to the apparent bullseye from Dodge City to about Ness City. The biggest one being that the model I chose to show the images from tends to be a few hours too fast this far out. The system that will cause this round of weather has not even come ashore yet; when it does get into more densely-sampled areas the computers will have better input data to work with and the output will likely move some. But I suspect it won’t be a lot.
SPC Outlook & Graphic for Friday:
Strong heating west of the dryline will result in a narrow, but expansive north-south zone of supercell potential, with very large hail and tornadoes possible.
Forecasters have upped the probability of severe weather to 30% — as high as they can go this far out — for a narrow corridor centered on a Grand Island, NE to Pampa, TX line.
Saturday, May 18
As always, there is some dependency Saturday on how Friday plays out. But I think this system, being somewhat more dynamic than the last couple, will make the day-to-day dependence somewhat less than it was last week.
Here are images from the same model, 7pm CDT Saturday:
This depicts a lee-side low centered in the Oklahoma panhandle south of Liberal. Wrap-around moisture brings higher dew points to the north of the low, 50’s and 60’s in central Kansas. The best moisture is out ahead of the dryline, with dew paints above 70 just east of I-35 in Oklahoma.
This image shows a bubble of EHI in Reno and Rice Counties, but the main strength is from just northeast of Oklahoma City to just east of Ponca City.
SPC Outlook & Graphic for Saturday:
While areas of ongoing storms may exist over northwest TX and OK Saturday morning, there will be a marked increase in storm coverage during the day, with a severe threat as far north as NE and IA. Widespread moisture (although likely weaker instability) will be sufficient for severe storms capable of damaging winds and a few tornadoes. Upgraded probabilities are possible in later outlooks as predictability increases.
Outlooks from affected NWS offices
NWS Dodge City:
As of now, models suggest storms to form along a dry line Friday [in western Kansas] with Central Kansas having the main threat Saturday. There could also be some wrap around storms with this system Saturday as this upper level low moves through the area. This system is still several days out future model runs will better depict exactly where this system will have the most influence.
Would anticipate storms to develop on dryline and roll east Friday night. [Models] show extensive precipitation throughout the day on Saturday with moist axis across the forecast area. Model QPF would suggest potential for an inch or so. If the widespread precipitation does occur as models forecast, it would likely diminish severe threat in the afternoon and evening. At best would be most likely in far southwest sections, closer to dry slot working northeast.
Friday will be very warm to hot out ahead of the system with high instability and increasing shear as the trough approaches. At least scattered thunderstorms could begin to form by late afternoon/evening off to our west and then move into our area as we head through Friday evening. This is a classic severe weather set up and if this pattern unfolds as currently modeled we will probably see some severe thunderstorms Friday evening either over our forecast area or at the very least just off to our west. The more southerly track of the upper low on Saturday would bring us thunderstorms with heavy rainfall and perhaps additional severe thunderstorms, but the best bet for severe thunderstorms on Saturday may be just south and southeast of our forecast area.
It is time to consider your storm preparations if you live anywhere in the western 2/3 of Kansas. Unlike previous storm systems, this system and the next are shaping up to be classic spring storm systems with all risks on the table. It’s climatologically just before the peak day of tornado reports each year, and there are a lot of events…high school and college graduations and the like…this weekend.
Quick look ahead to the Monday/Tuesday system (this is nothing more than SPC’s take…I’m not one to do much model analysis 6 and 7 days out).
An extensive mid/upper jet streak within a broad area of cyclonic flow aloft will nose eastward across the southwestern states and into the southern Plains on Monday/D7. Some model differences do exist regarding how amplified the trough will be, however, there will clearly be a substantial risk of severe weather over much of the central and southern Plains, as very strong wind shear develops with ample low-level moisture nearby and spreading north. Again, supercells producing very large hail and tornadoes are possible. Upgraded probabilities are possible in later outlooks as predictability increases.
Models suggest that a large portion of the upper trough energy will eject across the Plains on Tuesday/D8, with both the ECMWF and GFS deterministic models showing a 100kt 500 mb jet streak, anywhere from the OK/TX Panhandles into MO. Regardless of timing, a large area of strong wind shear with the upper trough, with substantial moisture in place, will lead to severe weather. It is too early for details but all modes of severe appear possible once again. Upgraded probabilities are possible in later outlooks as predictability increases.