Mesoscale Precipitation Discussion 0877
NWS Weather Prediction Center College Park MD
1005 PM EDT Fri Sep 21 2018

Areas affected...Dallas-Fort Worth Metro Area

Concerning...Heavy rainfall...Flash flooding likely

Valid 220204Z - 220630Z

Summary...Dangerous and life-threatening flash flooding is
developing in the immediate Dallas-Ft. Worth Metro Area this
evening. Persistent thunderstorms and heavy rain are likely to
lead to significant rainfall totals. Rain rates should reach (or
locally exceed) 3 in/hr in the strongest rain bands and
thunderstorms.

Discussion...Significant flash flooding is just beginning to
develop in North Texas, and specifically the immediate Dallas-Ft.
Worth metro area. An increasing concentration of thunderstorms and
heavy convective rain bands are expected during the evening very
near a developing surface low. Hi-res models had originally
projected this to be a little further west, but 01Z surface
observations showed the lowest pressures over Tarrant County, as
well as a coherent circulation to the surface winds. The hi-res
models were insistent that the heaviest rain would be concentrated
in the immediate vicinity of the surface low, and perhaps just to
the north-northeast, where low-level confluence will be maximized
(low -topped convection could thus be increasingly channeled into
this area). This would also be an area on the northern cusp of
broad southerly inflow and situated right in the middle of a
bubble of deep moisture. Therefore, the expectation is that the
hi-res models generally assessed the heavy rainfall potential with
this feature correctly, it will just end up being shifted further
east, and into the DFW metro area. GPS-PW observations in the
region were around 2.4 inches, significant values and above the
99th percentile in the regional climatology. The expectation of an
increasing concentration of convection in the immediate vicinity
of DFW metro is also implied by remote sensing data, with KFWS
radar and GOES-16 satellite showing these trends well.

Accounting for the low ZDR bias at KFWS still implies numerous
small droplets in the convective rain bands when examining the
dual pol datasets. The overall environment (very high PW and
MLCAPE over 500 j/kg) will support highly efficient rain
production, and this is confirmed by the dual pol data. This sort
of environment would make 3+ in/hr rain rates achievable in large,
relatively steady-state, and slow-moving convective bands. Several
mesonet sites in the vicinity of Plano have already reported rain
rates around 2 in/hr and some water rescues were reported in
Denton County. The expectation of significant rain rates
persisting for (at least) several hours over a large urban area
suggests that dangerous, life-threatening flash flooding will
continue to develop this evening. Radar trends also suggest this
flooding could affect a large portion of the DFW metro area and
surrounding counties, and thus travel could become increasingly
difficult with numerous flooded and impassible roads, particularly
dangerous at nighttime. Therefore, this focused mesoscale
precipitation discussion has been issued to highlight this
enhanced threat for dangerous flash flooding.

Lamers

ATTN...WFO...FWD...OUN...

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

LAT...LON   33819590 33549560 32799597 32079694 31829780
            32009847 32459867 33209787 33679674