DFW: Tstorms Late Weds (4/22) May be Severe w/ Localized Flooding

Update #2 [Wednesday 4/22 10:40 pm]

***Severe Thunderstorm Watch***

A Severe Thunderstorm Watch has been issued for the thunderstorms that developed to the northwest as predicted. These continue to move southeast. The watch is in effect until 4 am.  Read my initial forecast discussion posted the day before, on Tuesday 4/21

Severe Thunderstorm Watch issued at 10:25 pm 4/22.
Severe Thunderstorm Watch issued at 10:25 pm 4/22.

Update#1 [Radar Update – Wednesday 4/22 8:49 pm]

Thunderstorms Have Formed As predicted – A Weather Watch May be Needed This Evening of Much of N. TX

Read my initial forecast discussion posted on Tuesday 4/21

Thunderstorms developed across the Panhandle and Northwest TX this afternoon as predicted.  They did form a little further west into the Panhandle than I thought; however, aside from a later-than-expected arrival time, it should have no bearing on anticipated impacts for North Texas (assuming the atmosphere continues to evolve as predicted).  These should continue to propagate quickly to the southeast, with additional development in advance (along the stalling front), across N. TX.  I still expect thunderstorms in North Texas this evening (particularly late this evening and overnight), primarily north of the metroplex (with D/FW on the southern edge of the greatest forcing).  As of the time of this post (8:49 pm), thunderstorms were developing rapidly downstream of the original thunderstorm cluster.

Thunderstorms in Northwest Texas moving southeast and expanding in coverage (8:49 pm Wed 4/22)

Previous Forecast Discussion Posted Tuesday 4/21

A cold front will move south through the Central Plains on Wednesday, April 22, as a surface low develops in the vicinity of southwestern Oklahoma/northwest Texas.  As the surface low develops, a dry line will propagate eastward toward Abilene.  Strong/deep moisture convergence in the vicinity of the surface low will be the impetus for convection by late afternoon. Significant atmospheric instability characterized by convective available potential instability (CAPE) in excess of 3,000 J/kg coupled with strong low-level directional wind shear in the vicinity of the surface front suggests the potential for severe thunderstorms, some of which may become supercells.  The CAPE alone supports very large hail, particularly over northwest Texas.

As the front slows and eventually stalls over North Texas, I believe that thunderstorms will propagate southeastward and expand in coverage Wednesday evening.  The greatest coverage of thunderstorms will be near the front, from D/FW northward.  If a complex of thunderstorms develops, a southward-advancing cold pool (outflow boundary) would support thunderstorm development further south (via cold-pool propagation).  I do have concerns about the potential for training thunderstorms and ultimately flash flooding (again, primarily D/FW northward).  The final resting place of the stationary front, and the formation of thunderstorm outflow, will determine the extent to which thunderstorms impact the metroplex and vicinity.

My Forecast & Interactive Radar

Animation — Cold Front, Dry Line, and Instability (CAPE)

This animation shows the progression of the cold front and the dry line through the day Wednesday.  I have used the 925-mb equivalent potential temperature ( θe > 300 K) to identify the front/dry line (solid contours). The color-fill represents CAPE.  There isn’t a color legend on this animation, but you certainly get the idea.  CAPE increases dramatically during the day, in excess of 2,000 J/kg, as the surface low and deep moisture convergence develop.  Thunderstorms should form in that area by mid-afternoon (northwest Texas), and then propagate southeastward Wednesday evening.

Highly Unstable Atmosphere – Forecast Soundings

This is a forecast sounding using 0z NAM 4/22 data, analyzed using RAOB.  These data are consistent with previous model runs, both intra- and inter-model.  This forecast sounding is based at the grid point for DFW Airport and indicates CAPE > 2,500 J/kg at 6 pm on Wednesday.  This level of instability is supportive of vigorous updrafts and very large hail.  In fact, hail stones to the size of baseballs can occur with this level of instability.  The forecast challenge is determining where the front will stall, because it will be the main contributor (forcing mechanism) for thunderstorms Wednesday evening.  Also, any outflow boundaries generated by early thunderstorms will reduce the severe threat for areas north of that boundary, and enhance the threat along the boundary itself.  Possible boundaries and their location at max heating remain yet to be seen.

00z 4/22 NAM Forecast Sounding for DFW Airport valid at 6 pm Wednesday. CAPE > 2,500 J/kg at this time.
00z 4/22 NAM Forecast Sounding for DFW Airport valid at 6 pm Wednesday. CAPE > 2,500 J/kg at this time.

My Expectations for Thunderstorms on Wednesday Evening 4/22

On this map, I’ve indicated my best estimate for the location of the surface low, dry line, and stalling (stationary) front by late afternoon Wednesday.  Thunderstorms should be developing to the northwest and expanding in coverage as they build to the southeast in the vicinity of the front Wednesday night.

Note: It is possible that thunderstorms could develop earlier in the day, along the cold front itself, as it moves southward through Oklahoma.  Again, in my opinion, the potential for very heavy rainfall, training thunderstorms, and possibly some flash flooding exists across North Texas.

My forecast for Wednesday 4/22/15 DFW/North Texas
My forecast for Wednesday 4/22/15 DFW/North Texas

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