fxus64 kfwd 191227 aaa 

Area forecast discussion...updated 
National Weather Service Fort Worth Texas 
727 am CDT sun Aug 19 2018 

12 UTC taf cycle 

Concerns---near term windshift and convective potential this 
afternoon and tonight. 

Outflow from rain/thunderstorms and rain will briefly interrupt southerly winds 
across the metroplex tafs. Accompanying this windshift will be the 
potential for a few instances of rain showers and i've included both of 
these elements (windshift and shra) in a tempo group for metroplex 
tafs. The windshift should be light enough such that area 
terminals remain in a south flow. More continuous southwest to southerly 
winds will likely resume during the late morning into the early 
afternoon hours across the metroplex. Southerly winds will prevail 
at Waco through the entire 12 UTC taf cycle. 

The next challenge will be timing the potential bouts of rain showers 
and/or thunderstorms and rain at the metroplex. This morning's convection should 
leave behind a boundary which should act as a focus for renewed 
convective development this afternoon. I'll maintain the inherited 
afternoon thunderstorms in the vicinity with low confidence, but if the aformentioned 
boundary is shunted south of d10 airspace and/or vanishes beneath 
the ambient flow, it's possible that little to no new ts develops 
across the metroplex this afternoon. This evening offers a better 
opportunity for convective development as low level ascent 
increases from the west and southwest. I've appended a 3 hour 
window for thunderstorms in the vicinity between 03-06 UTC Monday with modest confidence. 
Depending on the location of convection, south flow operations 
could be briefly disrupted due to numerous convective outflows. 
Confidence in timing/placement of these chaotic wind shifts is 
too low to explicitly mention in the taf. Waco will remain far 
enough to the south such that shower or storms appear unlikely 
through the 12 UTC taf cycle, though the potential is non- 
zero...especially between the 6 and 12 UTC Monday time frame. 
Outside of rain/storms, VFR is expected at all terminals. 



Short term... /issued 407 am CDT sun Aug 19 2018/ 
/today and tonight/ 

The main challenge in the short term will be convective trends 
and consequently the high temperature forecast. 

This morning, a large shield of rain with embedded thunderstorms 
continues to slowly advance eastward across southwestern Oklahoma 
and adjacent parts of North Texas. Regional vwp sampled 20-30 
knots southerly flow around 1km this morning. Despite this flow, 
convective outflow continues to race east and southeastward from 
this rain/storm activity suggesting that its overall progression 
through the rest of North Texas may be limited. For now, i'll 
advertise a 40-50 pop across the Big Country and western reaches 
of texoma during the pre-dawn hours. This complex should dissipate 
completely as its convective outflow becomes further displaced, 
limiting the ability for the regeneration of new convective cells. 
With this in mind, pops will decrease through noon with highest 
chances for rain near and north of I-20. 

Widespread cloud cover from this complex should slide southward 
across much of north and central Texas through the day. Beneath 
the large cirrus canopy, low level flow late this morning and this 
afternoon should once again veer some to the southwest. While the 
clouds may hinder diabatic heating and keep temperatures 1-3 
degrees lower than Saturday, it'll still be another hot day with 
afternoon highs climbing into the upper 90s to near 101 degrees. 
The exception will be along the immediate Red River valley where 
temperatures may only climb into the low to mid 90s. Heat index 
values across the area may climb into the 100 to 105 degree 
range...especially across East Texas. At this time, I don't see a 
need for a heat advisory as the areal coverage of heat indices 
above 105 appears patchy in nature. Regardless, please exercise 
your heat safety rules! 

Model derived wind fields indicate that front/convective outflow 
should slide southward towards the I-20 corridor in the 
evening. As a result, convection is expected to re-develop late 
this afternoon and into the evening hours ahead of this feature. 
There will be a risk for strong to possibly severe downburst winds 
given steep 0-3km lapse rates (nearly 9 c/km) and dcape values in 
excess of 1100 j/kg. Though the exact coverage of showers/storms 
is unknown, it seems reasonable to increase pops during the 00-03 
UTC time frame Monday (or Sunday evening) as the front/convective 
outflow slides southeastward. I'll keep a 60 pop across northeast 
Texas, but given the decent cam consensus, i've elected to 
increase pops up to around 50% further southwest towards the d/fw 
metroplex and into the eastern Big Country. Storm motion should be 
a little faster today, but there will still be a heavy rain/flood 
potential with showers and thunderstorms. 



Long term... /issued 407 am CDT sun Aug 19 2018/ 
/Monday through Saturday/ 

Although the main forcing associated with a departing upper low 
will be confined to the Central Plains, the tail end of a final 
spoke of energy will skirt The Ark-la-tex on Monday. At daybreak 
Monday morning, showers and thunderstorms may be ongoing along 
the associated cold front within North Texas, but as the boundary 
moves deeper into central and East Texas, it will become 
increasingly separated from the upper support. Minimal low-level 
convergence along the boundary will both reduce compressional 
warming at peak heating and limit the potential that surface 
parcels will be able to overcome lingering inhibition, 
particularly if morning cloudiness is slow to erode. Warming 
southwesterly winds ahead of the front may help to overcome the 
reduced insolation, but even with extraordinary MLCAPE values 
Monday afternoon, limited shear and marginal lapse rates aloft 
should prevent any updrafts from persisting long enough to become 
strong. Thus, only low-end pops will accompany the front, with a 
conditional severe threat. Strengthening northwesterly flow above 
the frontal layer should assure its forward progress and preclude 
any postfrontal precipitation. However, if the slower NAM/European model (ecmwf) 
solutions pan out, there may be a wider window (both temporally 
and spatially) for thunderstorms Monday afternoon into Monday 
evening, with an enhanced severe threat. In any event, any 
showers and storms should exit to the southeast Monday night. 

Milder and drier air will filter into the region on Tuesday, but 
with abundant sunshine, high temperatures may be within a degree 
or two of normal values. But after the oppressive heat and 
humidity of preceding days, the middle of the week will feel 
noticeably more comfortable in comparison. The low-level advection 
will wane Tuesday night, but with an unseasonably mild start to 
Wednesday and weak northeasterly surface flow, similar daytime 
temperatures are expected. Southerly winds will return on 
Thursday, and Gulf moisture will steadily increase late in the 
week. The GFS has come into agreement with other extended guidance 
with late-season ridging reasserting itself over the Lone Star 
state. As a result, the extended portion of the forecast will 
remain free of rain chances, and temperatures will climb into the 
upcoming weekend. 



Preliminary point temps/pops... 
Dallas-ft. Worth 97 77 93 74 93 / 20 50 10 5 0 
Waco 101 78 99 74 96 / 10 40 20 20 5 
Paris 91 74 91 68 89 / 30 60 20 5 5 
Denton 96 74 92 69 92 / 30 50 10 5 0 
McKinney 96 75 92 69 91 / 20 50 10 5 0 
Dallas 99 78 94 74 94 / 20 50 10 5 0 
Terrell 98 77 93 71 93 / 20 50 20 10 5 
Corsicana 99 78 97 74 94 / 10 40 30 20 5 
Temple 98 77 99 74 97 / 5 20 20 20 10 
Mineral Wells 94 73 93 70 94 / 30 40 10 5 0 


Forward watches/warnings/advisories... 

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