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Area forecast discussion 
National Weather Service Fort Worth Texas
255 PM CDT Sat Apr 18 2015

the upper level level low that has spent the last couple of days
over the central rockies is now on the move eastward out into the
plains. A shortwave trough axis rotating around the base of the
upper low will track through North Texas this evening and provide
the lift required to generate convection. Already showers and
storms are forming across west central Texas...along an organizing
dry line. The approaching shortwave trough and partly sunny skies
has allowed the airmass over the region to begin the recovery
process from the convective overturning/stabilization that
occurred last night. Objective analysis has convective inhibition
eroding over most of the region...but about 50 j/kg of surface
based cinh remain. A few more hours of heating and increasing
southerly winds will continue to help prime the atmosphere for the
scattered storms as they move in from the west late this afternoon.

Surface based cape will range from 1000-2000 j/kg...which is
enough for a large hail threat especially given the steep middle
level lapse rates and relatively low -20c and 0c heights. Bulk
wind shear is not too impressive but is adequate for some
transient supercell structures. We generally expect the storm
activity to evolve into multi-cell clusters or short line segments
with an eastward motion by early evening. Damaging winds will be
possible...especially where these line segments organize. Not
everyone will be directly impacted by the heavy rainfall or storms
this evening...but we have increased probability of precipitation into the likely category
since most locations will probably see rainfall. A Severe
Thunderstorm Watch is in effect west of the I-35 corridor...where
the severe risk is greatest. However conditions across the I-35
corridor do look a little more favorable for severe weather than
yesterday so it is possible the box will be expanded eastward or a
new one will be issued in a few hours. As for timing...storms will
affect the western zones now through 8pm...the I-35 corridor from
7 to 11 PM...and the eastern zones from 9 PM to 2 am. Probability of precipitation will
end from west to east tonight as the shortwave trough moves east.

A surface low will track along the Red River valley Sunday and
send a cold front through the region during the daytime hours.
Winds will become southwesterly by morning...gradually veering to
the west and northwest during the day. Winds will be a bit breezy
from the northwest by late afternoon as deep mixing of the lower
atmosphere transfers higher momentum air down to the surface.
Some gusts to 30 miles per hour are possible by late afternoon. With the
veering winds and mainly sunny skies tomorrow...temperatures will
benefit from downslope subsidence and warm into the 80s areawide.
Across the eastern zones...the airmass will become very unstable
and weakly capped ahead of the front during the afternoon hours.
However convection will have trouble developing due to the drying
effects of westerly winds above the surface. Still we will hold
onto low probability of precipitation for isolated convection. Some severe threat does
exist...but it is likely the cells that do form in our County Warning Area will
have moved into East Texas before growing mature enough for
severe weather.

Much cooler weather is expected Sunday night with temperatures falling
into the middle 40s and middle 50s for lows. Another piece of upper
level energy will move through the northern zones early Monday
morning causing lift and saturation aloft. This will surely result
in middle level clouds and virga...and perhaps some light showers or
sprinkles if the low levels are moist enough. Will keep the
forecast dry for now but it is something to watch. These clouds
should dissipate by midday Monday with mostly sunny skies and
pleasant conditions. Highs will only be in the low 70s with
another cool night Monday night. Southerly winds return on
Tuesday...but moisture will be slow to arrive and model guidance
has backed off on quantitative precipitation forecast so will show a dry forecast.

Model guidance continues to display a lot of disagreement in the
middle to late week as they are struggling with the subtle features
in the west/southwest flow aloft. There is agreement that the cap
will be pretty tough to overcome Wednesday and Thursday and
therefore will keep probability of precipitation in the slight chance category. By
Thursday night and Friday the GFS/European model (ecmwf) are keying in on a
stronger shortwave trough that lifts and erodes the cap. In
addition...a cold front will probably accompany this
will show probability of precipitation increasing to 30-40 percent during this time.
Otherwise will show temperatures gradually warming from the 70s
middle-week to the middle 80s by Thursday and Friday. A slight cool down
is expected next Saturday in the wake of the cold front.



/issued 1249 PM CDT Sat Apr 18 2015/
the upper low now spinning over Colorado will move east across the
plains over the next 24 hours. A shortwave trough working its way
around the south side of the upper low will move across central
Texas this afternoon...aiding in the development of scattered
showers and thunderstorms west of the taf locations late this
afternoon. Convection will spread east across North Texas this
evening. Timing will be a challenge...but the latest hi-res
guidance once again pinpoints the 00-04z time frame as the most
likely at all taf locations this evening. Convection should move
east of the area and weaken around midnight local.

A cold front will push southeast across North Texas tomorrow.
South to southwest winds will increase to 10-15 knots around sunrise
ahead of the front. Winds will veer to the west shortly after 18z
while increasing to 15-20 knots. Northwest winds are expected at all
locations by 00z Sunday evening. At this appears that
any convection associated with the front will remain east of all
taf sites.



Preliminary point temps/pops...
Dallas-ft. Worth, Texas 60 83 51 72 50 / 70 5 0 5 5
Waco, Texas 61 86 50 73 47 / 60 10 0 0 5
Paris, Texas 59 82 50 70 46 / 60 20 5 5 5
Denton, Texas 59 80 49 71 47 / 70 5 5 5 5
McKinney, Texas 60 82 50 72 47 / 70 10 0 5 5
Dallas, Texas 61 85 54 74 52 / 70 10 0 5 5
Terrell, Texas 61 84 53 73 49 / 70 20 5 5 5
Corsicana, Texas 62 85 51 73 50 / 60 20 5 5 5
Temple, Texas 61 88 51 75 50 / 60 10 5 0 5
Mineral Wells, Texas 57 80 50 71 48 / 50 5 5 5 5


Forward watches/warnings/advisories...



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