Area Forecast Discussion
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FXUS64 KLUB 192043

Area Forecast Discussion
National Weather Service Lubbock TX
343 PM CDT Thu Apr 19 2018

Another nice day across the region today as surface winds have
shifted to the southeast and temperatures have remained in the mid
60s to near 70 degrees. Heading into tonight, wind speeds will
continue to increase as we start to see low-level moisture return
take place ahead of the closed low swinging east towards the Four
Corners region. The best moisture return will be across eastern
New Mexico but we will see slowly increasing dewpoints throughout
the day Friday. This will set up the region to see increasing
precipitation chances and possibly severe weather through the day

Unfortunately the picture for tomorrow remains somewhat uncertain
as we could see two rounds of precipitation. The first round will
develop late tomorrow morning into early afternoon across the
Rolling Plains which will be on the north side of a weak shortwave
swinging out in southwesterly flow ahead of the approaching mid-
level closed low. As this round pushes east and weakens, a second
round of thunderstorms will develop underneath a 100kt jet streak
swinging under the base of the low. The dryline will sharpen up
along or just west of the Texas/New Mexico state line providing a
low-level focus for storm initiation. Both the high-resolution TTU
and NSSL WRF models show a line of storms developing along the
dryline and then pushing east along with the jet streak through
Friday night. Model forecast soundings show that much of the
instability early in the day Friday will be elevated above an
inversion that tops out around 700 hPa. Lifting a parcel at this
level yields 500-750 J/kg CAPE which is not much but enough to
generate some thunderstorms. Combine that with the strong lift and
0-6km bulk shear values increasing to around 80 kts and we could
still see some severe wind gusts with this activity early in the
day. Heading into Friday night all models have a low-level jet
developing which pulls low 50 dewpoints rapidly northward after
sunset. This is enough to rapidly increase CAPE values to around
1000 J/kg along and ahead of the second line of storms late Friday
afternoon which could result in a higher chance of severe weather
mainly in the form of damaging winds and hail. Published studies
on environmental conditions and potential severe weather modes
shows that the environment remains favorable for QLCS development
due to the low CAPE environment and high shear values. The
complicating factor is such strong lift ahead of the storm system
may result in more widespread convective development that would
quickly rob the environment of instability.

This activity will move east through the early morning hours
Saturday as the upper-level low starts to push east. The dryline
will remain near the Texas/New Mexico border through sunrise then
start to mix east with time being followed by a cold front pushing
southwest behind the surface low. Cold temps aloft associated with
the core of the low and higher surface dewpoints could be enough
for another round of precipitation development along and behind
the front across the eastern half of the forecast area. This
activity will continue to shift east through the day so that the
Caprock will start to dry out by noon and all but the far northern
Rolling Plains drying out by sunset. Finally by Saturday night,
rain will have shifted east of the area with dry weather in storm
for Sunday and Monday. Low-level moisture will not be totally
scoured out behind the front Saturday so that dewpoints around 40F
remain in place which will both help moderate the diurnal
temperature cycle and keep things near normal for this time of
year. The long-range models have come into better agreement with
the timing of the front Tuesday with it almost through all of the
forecast area by sunset Tuesday. They continue to differ in
precipitation coverage and what the post-frontal surface flow will
be. The GFS quickly develops return flow by Thursday afternoon
while the ECMWF keeps northerly flow in place for an additional 12
hours before surface winds swing to the southeast Thursday night.

Most of the changes to the forecast focused on the next 48 hours
and PoPs were broken down into 6 hour grids instead of 12 hourly
in order to better reflect the evolution of precipitation.
PoPs were lowered for Friday morning into early afternoon to
reflect the uncertainty on where storms may develop but then ramp
back into likely by Friday night. Also increased precipitation
chances for Saturday morning for precipitation associated with the
surface front and passage of the low but quickly ramp back down by
Saturday night. Wind speeds were increased for tonight into
tomorrow afternoon over what the model blend placed in the grids
to take into account the deepening surface pressure gradient and
strong low-level jet mixing winds to the surface tonight and
Friday night. Also bumped up low temperatures tonight 3-5 degrees
over guidance to account for increased mixing and increasing
moisture levels. Last change was to PoPs for Tuesday and
Wednesday; model blend was placing 30-40 percent coverage in for
Tuesday into Tuesday night but this appears to be overdone so kept
PoPs at 20 percent.





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