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Area forecast discussion 
National Weather Service Fort Worth Texas
727 PM CDT Friday may 22 2015


For the 00z tafs...timing the onset of low MVFR and IFR stratus
returning to the region and then rain shower and thunderstorm
chances over the next 24-36 hours are the primary forecast

23z surface analysis indicated low-level moisture returning
rapidly to the region from the Texas Gulf Coast northwest over
west central Texas. This deep moisture return will result in cloud
heights lowering from the 2 to 3 thousand foot range to
approximately 800 feet above ground level tonight. These clouds are expected to
move over area airports from the west or southwest to the east.
Have low MVFR ceilings building in after 04z...then IFR ceilings 4 hours
later. Will monitor upstream trends and amend the tafs if this
timing looks like it will change. IFR ceilings are likely to hold
until the late morning hours of Saturday.

After noon on Saturday...large scale lift begins to increase over
Texas from west to east...resulting in an increase in rain chances
from Saturday afternoon through Saturday night. Scattered rain
showers are possibly anytime on Saturday...with thunderstorms
becoming more likely late Saturday afternoon. Widespread rain
shower activity with embedded thunderstorms is likely at all area
airports Saturday night. Did not place thunder in the forecast at
this time because it looks like we will have many hours of
precipitation with embedded thunderstorms possible.

Did not want to advertise 10 to 12 hours of thunderstorms at this
time...although this is a rare situation where this is actually
possible. Any rain shower or thunderstorm activity Saturday night
is expected to result in heavy rainfall and likely IFR
visibilities. Went ahead with a mention of heavy rain in at the
end of the dfw taf. Best window for widespread heavy rainfall at
area taf sites appears to be just before midnight to 15z Sunday at
this time.

A widespread heavy rainfall event is likely across the region this the tafs will likely be busy and amended frequently
as we move into Saturday afternoon and evening.



Previous discussion... /issued 327 PM CDT Friday may 22 2015/
a warm front is moving through North Texas at this time with
temperatures and dewpoints steadily climbing as the surface winds
attain a southerly component. Weak isentropic lift and warm
advection above the surface will continue through the overnight
hours...which will ensure low cloud cover and a few showers or
storms. This activity should generally be weak with no risk for
severe weather and limited rainfall. Temperatures tonight will
stay mild and humid with lows only in the 60s.

An upper level low currently near Las Vegas will lumber eastward
during the next couple of days and set the stage for a round of
heavy rainfall and some severe storms over the region Saturday
afternoon through Sunday afternoon. Precipitable water will
approach 2 inches which would be unusually high for may. Perhaps
more importantly the unidirectional southerly wind profile is one
that portends heavy rainfall events...because it Fosters cell
training or slow moving lines of storms. Also of note our low
level winds increase to near 40kt which would quickly replenish
the atmospheric moisture that is precipitated out. Unlike our
previous storm events during the last 3 weeks...directional wind
shear and instability will be much lower which will tend to
mitigate the severe weather threat. However the lower instability
and unidirectional shear would increase cell precipitation
efficiency. we saw...those slow moving supercells
had the ability to dump extreme amounts of rain...the highest
amounts were fairly localized. In this upcoming situation...there
is a greater potential for extreme rain amounts over a larger
area. Thus in addition to flash flooding...river flooding is
likely to become a significant impact...also exacerbated by the
fact that there is little room for water in area lakes.

As for the specifics...scattered to numerous thunderstorms will
develop to our west and head northeast into the western portions
of the County Warning Area during the afternoon hours. The severe weather threat
will be greatest from middle afternoon to middle evening...when
convection is more cellular and there is a little more
instability available. Generally expect strong winds and hail to
be the main severe weather threat...but there remains a low
tornado potential. Otherwise convection should start to
consolidate into a line or band that trains and dumps an average
of 2-4 inches of rain across the northwestern County Warning Area Saturday
evening. As time wears on a cold pool will develop Saturday
evening...which will help push the line slowly southeast toward
the I-35 corridor during the overnight hours and into Sunday
morning. Rainfall across the central zones and southern zones will
average 1 to 3 inches. Severe weather threat overnight for the
I-35 corridor would mainly be strong winds and perhaps a brief
weak tornado...but the overall limited instability will lessen
severe potential. The band of rain should continue to head
eastward through the morning hours Sunday...likely clearing the
County Warning Area by late afternoon. An average of 1-2 inches is expected over
the eastern zones. Some model guidance suggests more convection
will fire over the western zones in the afternoon...but the
airmass may be too worked over for another round so will keep
Sunday afternoon/evening probability of precipitation low.

A Flash Flood Watch is in effect for the entire area given the
potential for heavy rainfall and flooding...but again we are most
concerned about the region just north and west of the dfw
metroplex for significant flooding Saturday night. There still is
some uncertainty...and it is possible the training bands of
convection will set up in a different location...or move more
slowly Sunday morning across the central and eastern
the Flash Flood Watch is needed even though our deterministic quantitative precipitation forecast
forecast has lower amounts in the east.

Otherwise...the primary change in the forecast concerns Monday
afternoon and Monday night when another shortwave trough will move
through the state. Previously model guidance had focused convection
along the Texas coastline which protected US from robust
convection this far north. Convection along the Gulf Coast would
have intercepted or prevented Gulf moisture return...and would
have placed our region in a zone of subsidence. Model guidance
during the last day has weakened the Gulf Coast convection Monday
morning...and therefore brings the high moisture back into the
region Monday afternoon. If the 70 dewpoints do arrive back into
the would create an extremely unstable atmosphere over
the region. Both the NAM/GFS indicate widespread surface based
cape in excess of 5000 j/kg. This amount may be overdone...but
nonetheless there is a signal that high instability will be
available Monday afternoon as forcing from the shortwave arrives
late in the day. Thus have increased probability of precipitation to 60 percent...and we
are forecasting the potential for another round of heavy rainfall
and severe weather Monday evening and night. The high instability
would likely mean a mesoscale convective system or squall line would form...which would
move fairly quickly over the region and hopefully help limit
rainfall amounts to 1 to 2 inches. We will get more into the
specifics regarding this event during the next couple of days.

The region will remain in a weak flow regime with broad troughing
Tuesday into Thursday. Will continue to carry just a generic low
chance of convection...which will be diurnally driven. Low shear
will limit organization...but enough instability will be available
for pulse severe or locally heavy downpours where these storms do
pop up. Temperatures will continue to moderate during the week
with highs benefiting from more sunshine and reaching the middle 80s
by Wednesday and Thursday. The next system is slated for Friday
when the region will likely see another round of widespread rain.



Preliminary point temps/pops...
Dallas-ft. Worth, Texas 65 79 67 75 69 / 20 50 80 90 30
Waco, Texas 69 81 68 75 69 / 20 50 70 80 30
Paris, Texas 60 80 69 74 67 / 20 40 80 90 60
Denton, Texas 65 78 66 75 66 / 20 60 90 90 20
McKinney, Texas 64 80 69 74 66 / 20 50 80 90 30
Dallas, Texas 67 80 67 75 69 / 20 50 80 90 30
Terrell, Texas 68 81 70 74 68 / 20 40 70 90 50
Corsicana, Texas 69 81 70 75 69 / 20 40 50 90 50
Temple, Texas 69 81 69 76 69 / 20 40 70 80 30
Mineral Wells, Texas 66 77 66 77 65 / 20 60 100 70 20


Forward watches/warnings/advisories...
Flash Flood Watch from Saturday afternoon through Sunday evening
for txz091>095-100>107-115>123-129>135-141>148-156>162-174-175.



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