Mesoscale Precipitation Discussion 1069
NWS Weather Prediction Center College Park MD
129 AM EST Thu Dec 27 2018

Areas affected...portions of central and eastern TX

Concerning...Heavy rainfall...Flash flooding possible

Valid 270628Z - 271225Z

Summary...The threat for training heavy rain will continue a
threat for flash flooding for parts of central and eastern TX
through 12Z. Rainfall rates peaking in the 2-3 in/hr range may
lead to 3-5 inch totals.

Discussion...06Z surface observations and radar placed a line of
convection, coincident with an outflow boundary from east of OSA
southwestward through LHB and just east of SAT. The northern
portion of the line has been averaging an eastward movement near
35 kt over the past 4 hours while the southern portion of the line
has averaged closer to 15 kt. Observed rainfall rates have been
generally 1-2 in/hr but locally between 2-3 in/hr as seen across
eastern Bell county through 0530Z where training convection ahead
of the main line allowed for increased rainfall to occur.

850 mb winds as sampled by VAD wind plots ranged from near 80 kt
(SHV) to 55 kt (EWX) from the south-southwest, generally in line
with or up to 10 kt weaker than RAP forecasts. Unidirectional flow
between the LFC and EL combined with the strong low level
southerly flow sets up an ideal environment for training if
convective activity is able to organize ahead of the forward
propagating main line long enough to support 2+ in/hr rates.

RAP forecasts have been consistent in the development of a 110+ kt
jet streak at 250 mb crossing the eastern TX/OK border between
09-10Z with would place an enhanced area of upper divergence over
northeastern TX combined with an already diffluent flow pattern
aloft. This region of larger scale lift may help to support more
focused convective activity ahead of the main line and 6-hr storm
totals over 3 inches.

The other area of concern is farther south where forward movement
of the convective line has been and should continue to be slower.
Hi-res guidance has struggled with the southern portion of the
convective line tonight and while low level flow is weaker to the
south and forecast mid-level height falls are near neutral over
the next 6 hours, localized areas of organized training have the
potential to support localized 4+ inch totals through 12Z. In
either location (north or south) the flash flood concern will be
where overlap of heavier totals overlaps with more urbanized
locations given dry antecedent conditions.

Otto

ATTN...WFO...CRP...EWX...FWD...HGX...LCH...SHV...

ATTN...RFC...LMRFC...WGRFC...

LAT...LON   33249424 33099342 32029327 30629405 29609533
            28989730 28819893 29089951 29639890 30499742
            31409659 32619534