Tonight's NAM Forecast - Severe Thunderstorms Monday Evening?



Dell City, Tx Forecast Sounding.
(Issued At 6 PM MDT).

Valid A6 9 PM MDT Monday Evening.

Something a little unusual with tonight's forecast sounding from the NAM model. For starters this sounding forecast is further west than what we would normally expect...about the Dell City, Texas area. As the dryline surges west around sunset this model run is showing the most favored area for severe thunderstorms to be to the southwest of the Carlsbad area by dark (9 PM MDT). Why? 

Because the NAM is forecasting the cutoff upper level low to be further to the south and southeast than previous runs by this time (9 PM MDT Monday night). In response its forecasting the dryline to be to the west of the Guadalupe Mtn's. Compare tonights forecasts with last nights runs (00Z/6 PM MDT) and the best favored areas for severe thunderstorms was northeast of the Pecos Valley in the Clovis and Portales areas at sunset. But other models disagree with this so this model's forecast is not set in stone. Use it as a rough guide only. 

500 Heights & Vorticity Forecast.

Valid At 9 PM MDT Monday.

By dark thirty Monday night a fairly decent area of vorticity or lift will be approaching Southeastern New Mexico from the southwest (red shaded area). Alas there is a fly in this forecasts ointment and that is that this model run is forecasting just enough capping or CIN to shut down anything that tries to fire up east of the dryline in Southeastern New Mexico. Unless its not surface rooted or based and is elevated in nature. Meaning that there may be a pocket of warmer air just a few thousand feet above the surface where thunderstorm development may occur. Elevated thunderstorms are notorious in this type of setup for being hail producers. 

Surface Dew Point Temperature Forecast.

Valid At 9 PM MDT Monday Night.

Surface dew point temperatures are forecast to be in the 50's and approaching 60 in Southeastern New Mexico by dark Monday evening. So plenty of surface moisture will be available for thunderstorms to work with.

Surface Based Cape Forecast.

Valid At 9 PM MDT Monday Night.

Surface based cape values are forecast to be approaching 4,000 j/kg southwest of Carlsbad just along the leading edge of the dryline...which is forecast to be as far west as Dell City by 9 PM MDT Monday night. That's a respectable values for this area any time of year much less that time of the night. So lots of energy will be present along the dryline.

Surface Based Lifted Index Forecast.

Valid At 9 PM MDT Monday night.

Forecast value for the Dell City sounding is -13ÂșC which is impressive to say the least. So lots of cooling as you go up in height through the atmospheric column. Meaning that the atmosphere will be very unstable because of the available cape, and the cooling with height. The more cooling you have the more unstable the atmosphere becomes. Think about having a warm muggy parcel of air at the surface, then lift it upwards. Its has lots of stored energy because of its cape value and it rapidly cools as it ascends. If there is no cap or cin (which is a layer of air above the surface that is warmer than the surface and acts like a lid preventing thunderstorms from forming) then that parcel of air rapidly rises, cools, condensing its moisture that it is carrying upwards, and boom you have a potent thunderstorms that develop. Surface to 6 km shear values are forecast to be in the 50 knot neighborhood so lots of wind shear to also foster severe development. 

Supercell Composite Forecast.

Valid At 6 PM MDT Monday Night.

This index forecasts a number of around 15 which is a respectable number that indicates that any thunderstorm that does manage to form could easily become a supercell thunderstorm. Or a thunderstorm with a bad attitude. Supercells are the wolves of the thunderstorm family. They are the strongest, longest lasting, and most violent. Producing large to very large hail, damaging wind gusts in excess of 60 mph, tornadoes, heavy rains and flash flooding. 

Tonight's model sounding also suggests that should any discrete supercell thunderstorms manage to form along the dryline late Monday afternoon and early evening they possibly be of the splitting type. Simply meaning that the storm could literally split in two with the left half moving more to the north or even the northwest, and the right half moving to the northeast. Left moving splits are famous for producing large hail and lots of it. While the right moving splits are noted for tornadoes and large hail. 

Whew that's a lot to chew on. After I gone and said all of this here is what our weather forecast boils down to. Thunderstorms are forecast to develop over the area tonight...mainly over and west of the mountains. Thunderstorms are forecast to develop over parts of the local area late Sunday afternoon and evening. Some of these could possibly become marginally severe. This continues into Monday night. 

Will our thunderstorms Monday afternoon and evening be severe...if they even manage to form? Possibly, but most of the models think that there will be a better chance of this Tuesday afternoon into early Wednesday morning. Move the dryline 50 to 100 miles to the east or west Monday evening and this could mean severe storms in the Pecos Valley or none at all. 

Some of us could see locally heavy rainfall (an inch or more) by the time Wednesday rolls around and the cutoff upper level low begins to lift northeastward out of the area. More on all of this Sunday and Monday.

Remember what I've been telling you for the past 37 years. Forecasting cutoff lows is one of the most difficult things there is to do in the weather world. I guarantee that this forecast by this one model will change between now and Monday. Be prepared for changes in our local forecasts and be prepared for the possibility for severe weather from Sunday night into Wednesday. Its spring in New Mexico and West Texas and just about anything (weatherwise) is possible! 

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