Just a brief evening update on the potential for severe weather tomorrow (Friday) evening. Earlier today, the Storm Prediction Center upgraded parts of north central, northeast Texas to an Enhanced (Level 3) risk of severe weather Friday afternoon and into the evening/overnight hours. In addition to the upgrade, the threats were shifted slightly west to encompass more of north central Texas including parts of the DFW metroplex. As it stands currently, the DFW metroplex is bisected by a Slight Risk (Level 2) for the western half and the Enhanced Risk (Level 3) across the eastern half. Threats within both the Slight and Enhanced Risk areas include damaging winds, large hail and the potential for tornadoes. Of course the big question remains…will it actually happen. The answer is both yes and no. Yes, the chances are much higher for seeing severe weather for points east of the DFW metroplex. That has been the forecast all along. What’s new is the threats further west which will likely turn into an “all or nothing” scenario depending on several factors including moisture return, the timing and strength of the approaching upper level system, and the amount of instability that can be realized by tomorrow afternoon. As mentioned yesterday, this may become one of those situations where forecasters will not have a good handle on what will actually happen until hours before. This morning, most all of the short-range models were developing storms east of the DFW metroplex with little to nothing within the metroplex itself. By late this afternoon, a few of the models have picked up on the chance for storm development along the dryline just west of DFW around the 5-7pm timeframe as the dryline/frontal boundary races into the region. However, even if we do see development to the west that moves into the metroplex, due to limited instability across the area, it could still take awhile for the storms to reach severe limits…which may not happen until the storms have moved east of the metroplex. This is a trend which will be monitored closely overnight and tomorrow morning.
Just for some fun reading, we’ve included the afternoon forecast discussion from the National Weather Service office in Fort Worth. This is a really well written and in-depth discussion of how complex tomorrow’s set-up is. A bit of a nerdy read, but most will pick up on the high degree of uncertainty over how tomorrow’s storms will play out across the area. The takeaways at this time are as follows: If you live east of the DFW metroplex, be on high alert tomorrow evening. Have your weather radios on and have a way to receive warnings if you’re out and about. Review your family’s severe weather safety plan. If you live in or around the DFW metroplex, know that this could be an all or nothing situation tomorrow, but be prepared anyway just in case.
The primary forecast concern is the severe weather potential on Friday afternoon/evening. Over the last 12-24 hours, model guidance has slowed down the speed and deepened the track of the potent upper level low that is now crashing onshore in California. This has implications on the severe weather risk area across North and Central Texas with more of the CWA in play for severe storms as the risk area shifts a bit westward. This upper level system will feature strong dynamic forcing for ascent as the trough rotates into a negative tilt and the diffluent exit region of an impinging 120 knot jet translates into the area. The degree of forcing this system will generate is on the upper end of what we typically see around here, and therefore it creates a great deal of uncertainty in the forecast. The reason for this is because dynamic forcing for ascent often primes the atmosphere for convection - meaning it creates a more favorable temperature and moisture profile for storms. This is critically important tomorrow, because the airmass in the morning hours will be rather unfavorable for convection, both strongly capped and only a very shallow layer of moisture. This probably means most of the area will experience low cloud cover for most of the day, thus limiting day time heating and keeping our highs in the 70s. However, as the forcing arrives into the western zones around mid afternoon we should see a rapid modification of the airmass. The capping inversion is forecast to lift and lapse rates will steepen as the thermal profile cools aloft. What shallow moisture is available is also lifted to moisten the profile and the strong low level flow off of the Gulf will result in good low level moisture flux and precipitable water values rising a third to half inch in just 6 hours. This of course is all what the models tell us, and we know they`re not perfect, especially when rapid airmass modification is taking place, which is why the uncertainty is higher than normal with this forecast. Still we do believe that conditions should become favorable for severe convection by the late afternoon hours over parts of our CWA, but this event may not evolve as a typical spring-time dryline scenario. For example, a dryline is evident in the models during the afternoon, but it appears to transition and behave more like a Pacific cold front after sunset and sweep eastward across the region. Convection will probably fire along the dryline/front, but it may be well after peak heating and just before the 520 pm sunset. In addition, the best moisture and instability should be east of I-35 and we may see a spontaneous eruption of scattered storms in that area by early to mid afternoon well ahead of any surface boundaries. In any case, PoPs will be highest in the east during the afternoon with 40 percent advertised there, tapering down to 10 and 20 percent in the western zones. After sunset, PoPs will increase remarkably along and to the east of the I-35 corridor, with rain almost a sure bet across the northeastern zones and far eastern zones. All of this activity should clear out from west to east by or shortly after midnight. For the severe weather potential, due to the strong wind shear and moderate instability (SBCAPE 1000-2000J/KG), all modes of severe will be possible as supercell structures and short line segments appear likely. The combination of wind shear and instability particularly highlights the northeastern zones as the most favorable region, where isolated tornadoes may occur. Wind damage will be most probable with line segments and large hail will be more probable with supercells, however the exact convective mode is in question. Strong forcing suggests the linear mode will be dominant, but the parameters point toward supercells. Farther to the west including the DFW Metroplex (where the severe weather risk area from SPC was adjusted to include) the severe weather threat may be an all or nothing type setup. What I mean by that is that there is a tendency for thunderstorms to take a while to mature into severe storms or supercells in cool season severe weather episodes here. This is probably due to the wind shear overwhelming the young updrafts and the more limited instability. As a result of the speed at which these storms move, they may initiate farther west, but may not produce severe weather until 50-75 miles farther east. However, it`s clear the parameter space supports a slight to enhanced risk of severe weather, but some storm scale processes may limit or inhibit this threat. At this time we want to stress that severe weather isn`t a sure bet in all outlooked areas, that this is different than the typical spring events, and confidence on the western extent of this threat is lower than usual due to the rapidly changing modification of the atmosphere in the afternoon. Please stay updated with the latest forecast data through the next 24 hours for any changes to our thinking or confidence.