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Area forecast discussion...updated 
National Weather Service Fort Worth Texas 
616 am CST Tue Feb 20 2018 




Aviation... 
/12z tafs/ 


A very complicated forecast is in store over the next 24-30 hours 
(and really through much of the week) as upper-level disturbances 
bring numerous chances for showers and storms to the area 
terminals. After the early-morning complex of showers and storms, 
a brief relative minimum in activity is expected through the mid- 
late morning hours within the somewhat more subsident airmass. 
Still, some showers and embedded storms will be possible. Skies in 
the wake of this activity have gone predominantly VFR. By late 
this morning and into the early afternoon, the next round of 
showers and storms is expected as another disturbance moves 
overhead. Was not quite confident enough in timing to introduce a 
tempo group for -tsra, but this may be necessary in the 15z 
amendments if high-resolution guidance continues to indicate 
similar convective coverage. The sharp cold front should arrive 
near or just after 21/00z at the metroplex airports with a quick 
north wind shift. While there may continue to be some embedded 
thunder as the front passes, the next batch of more widespread ts 
looks to arrive after midnight tonight as the next round of large- 
scale ascent overspreads the region. 


At Waco, periodic showers may develop and move across the 
airfield, with thunder becoming increasingly more likely after 20z 
ahead of the approaching cold front. As the front passes, around 
21/06z, increasing ascent atop the frontal inversion should result 
in an uptick in convective activity through the overnight hours. 


Overall, cigs are expected to favor a mixture of VFR and MVFR, 
with gradual deterioration behind the cold front late tonight. 


Carlaw 


&& 




Short term... /issued 352 am CST Tue Feb 20 2018/ 
/today and tonight/ 


A rather complex forecast will evolve today and tonight as a 
drawn-out active/unsettled weather pattern begins across the 
region. In the immediate term, the first wave of ascent is 
arriving early this morning out of the Concho Valley, and this 
has helped to initiate a cluster of showers and thunderstorms 
which will continue to rapidly push north and eastward within 
the fast southwesterly flow aloft. Effective deep layer shear 
values are running on the order of 40-50 kts, and this has been 
supporting persistent mid-level rotation in these storms with high 
velocities showing up aloft. Thus far, this enhanced momentum 
seems to be having a difficult time making it down to the surface 
and high-res guidance insists on a gradual weakening trend, but 
this activity may still pose a strong and gusty wind and small 
hail threat as it races towards the metroplex through the 4-6 am 
time frame. Anticipate that additional convection will fire in 
northeast to southwest oriented bands through the morning hours, 
aligned roughly along and just west of the I-35 corridor where the 
highest pops have been painted. Some locally heavy rainfall and 
associated isolated hydrologic issues are possible with this 
initial activity along the I-35 corridor, but most of the shower 
and thunderstorm activity should move along at a decent enough 
clip to preclude any substantial issues. 


During the afternoon hours, the sharp Arctic front, which is 
currently draped just north of I-40/44 in Oklahoma (and actually 
this front is slowly easing back north and west as a warm front at 
this hour!) Will begin to make its notable southward push. The 
faster NAM, 3km NAM, and rap output was generally followed for 
the front timing given the much colder (and denser) airmass 
analyzed across far northwest Oklahoma and southern Kansas than 
earlier guidance had indicated. With this faster guidance, the 
initial north wind shift is forecast to arrive across our 
northwest counties around 4 PM, into the metroplex around 6-8 PM, 
and then through our southeastern counties by daybreak Wednesday. 
Additional showers and storms should Blossom ahead of the surging 
cold front late this morning and through the afternoon, and storm 
training may begin at this point as upwind corfidi vector 
velocities decrease to under 5-10 kts along with boundary-parallel 
cloud-bearing flow. Precipitable water values are forecast to be in the 1.6 to 
1.8 inch range, which are easily records for the date, and would 
even be near-record values by April and may standards. The 
probability-matched mean quantitative precipitation forecast products from the href indicate the 
axis of heaviest rainfall into the early evening hours stretching 
from near I-35/35w to a Paris to Hillsboro line which seems 
reasonable and this appears to be well represented by the current 
Flood Watch. 


The other threat to contend with today will be the potential for 
some strong to perhaps severe storms in the warm sector this 
afternoon and evening. Forecast soundings ahead of the front show 
a thermodynamic/shear space which could support storms with a 
damaging wind or even an isolated tornado threat where surface- 
based instability resides (south and east of a Sulphur Springs to 
Hamilton line). Given the surging nature of the front, it's 
possible any initially surface-based convection gets undercut, but 
the presence of very strong 0-1 km shear is a concern, especially 
if the front hangs up a bit more than forecast. 


Finally, an additional wave of warm advection/isentropic ascent 
is forecast to materialize late tonight across the Concho Valley. 
This should lead to the development of additional showers and 
perhaps some isolated storms above the shallow frontal inversion 
across our western counties. Temperatures here will be falling 
through the 30s, and am growing a bit more concerned about some 
freezing rain potential developing towards daybreak on Wednesday 
north and west of an Eastland to Gainesville line. Each subsequent 
NAM/sref run is coming in colder, and forecast soundings are 
showing steepening 850-600 mb lapse rates facilitating increasing 
MUCAPES late tonight supporting the potential for elevated 
convection. With the exceptionally warm air just off the surface 
(+12 to +14 c at 900 mb), any heavier convection may drag this 
warmer air to the surface, reducing potential impacts from any 
freezing rain, but this threat will continue to be scrutinized 
very closely today. 


Carlaw 


&& 


Long term... /issued 352 am CST Tue Feb 20 2018/ 
/Wednesday through Monday/ 


The cold front will have moved through the southeastern parts of 
the forecast area as of 6 am Wednesday. Lift over the frontal 
boundary will be on-going. This along with the upper level trough 
remaining across the western United States, will result in 
continued chances of rain. The heaviest rain should shift to the 
east of the forecast area by Wednesday afternoon but some light to 
occasionally moderate rain will continue area wide Wednesday 
afternoon. With the cold advection behind the front, cloud cover 
and precipitation, temperatures will not rise much, if at all, 
during the day. After early morning lows in the lower 30s 
northwest to the upper 50s southeast, daytime highs will only be 
in the mid 30s northwest to the lower 60s southeast. 


Rain chances should decrease some Wednesday night, especially 
across the west as we await the next shortwave. However, some 
chances will remain and as temperatures will be near or slightly 
below freezing across the northwest, have included a mention of 
rain of freezing rain for the overnight Wednesday night into the 
early morning Thursday period for locations along and northwest of 
an Eastland to Bowie line. Fortunately temperatures are expected 
to be 30-32 degrees and given how warm we have been, no 
significant icing on roads is expected at this time. Rain chances 
will increase Thursday afternoon and Thursday night. Lows 
Wednesday night will range from around 30 northwest to the mid 40s 
southeast. Highs Thursday will range from the lower 40s northwest 
to the mid 60s southeast. 


Chances of showers and isolated thunderstorms will continue 
Thursday night through Saturday. As the upper level trough over 
the southwestern United States moves eastward across The Rockies, 
a Lee side low will develop. In response to the developing low, 
surface winds across the forecast area will come around the south 
Friday. Overnight lows Thursday night will range from the upper 
30s northwest to the upper 50s southeast and highs Friday will 
range from the upper 50s northwest to around 70 degrees southeast. 
The upper level trough will finally move east out of The Rockies 
and across the plains Saturday. As this system moves east, rain 
chances should end across all but the extreme eastern zones by 
Saturday night. 


58 


&& 


Preliminary point temps/pops... 
Dallas-ft. Worth 74 37 42 38 51 / 100 90 90 50 40 
Waco 74 42 50 40 54 / 90 90 100 50 50 
Paris 73 40 46 43 55 / 100 90 100 70 40 
Denton 73 35 39 36 49 / 90 70 90 50 40 
McKinney 73 36 43 39 51 / 100 90 90 50 40 
Dallas 73 37 44 40 52 / 100 90 90 50 40 
Terrell 74 39 48 42 55 / 100 90 100 60 40 
Corsicana 75 42 51 44 56 / 80 90 100 70 40 
Temple 74 44 53 41 54 / 90 90 100 50 50 
Mineral Wells 75 34 38 33 46 / 70 50 90 40 60 


&& 


Forward watches/warnings/advisories... 
Flood Watch through Wednesday evening for txz093>095-103>107- 
118>123-133>135-144>148-159>161. 


&& 



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