543 FOUS30 KWBC 162017 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Tue Oct 16 2018 Day 1 Valid 1709Z Tue Oct 16 2018 - 12Z Wed Oct 17 2018 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF TEXAS & NORTHERN LOUISIANA... ...Southern U.S... A long-duration heavy rain event with embedded somewhat heavier rates occurred on the cool side of a frontal zone over the southern Plains early this morning, to the east of an upper level trough. Higher rain rates occurred with another heavy rain event across Acadiana in LA. Precipitable water values were well above average for the time of year, with model output and satellite-derived layer PW grids showing a moisture plume aloft from the eastern Pacific in which Tropical Storm Tara was embedded. GPS values in LA are in the 2-2.25" range and in TX in the 1.5-2" range. The main limiting factor to the rain rates in the cool sector of the front has been/will be minimal available instability across TX and northern LA, with heavy rains caused by frontogenesis above the surface and inflow above the sloped front. Since the upright convection was not surface based, outflows were and should not be a factor. After some possible backing to the low level flow late this morning into early this afternoon, there is some sense on the large scale that the confluent 850-700 mb flow and overall deep layer ascent will translate convection with higher rain rates eastward into more of Louisiana and Mississippi with time. Across TX and northern LA, expectations are that rain should continue off and on, with some increase of intensity again possible later today and tonight as the upper low in the Four Corners region draws closer. Slight Risk covers those areas where a significant amount of rain has fallen in recent days, thus lowering the Flash Flood Guidance values. A new moderate risk area was added in central TX due to the extremely low flash flood guidance caused by their exceedingly wet two week period. ...Eastern North Carolina... There is some potential for a combination of diurnally driven convection along a cold front and slightly elevated convection to the north of the front - to train from west to east, including the potential for a few cell mergers later today and this evening. With PW values above 1.75 inches and moderate instability in the vicinity of the front, we went ahead and introduced a small Marginal Risk area. The overall scope of the event is somewhat limited, at least in the guidance. Will have to monitor trends to see if the evidence grows for a somewhat larger scale heavy rain event. Roth/Burke Day 2 Valid 12Z Wed Oct 17 2018 - 12Z Thu Oct 18 2018 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL SOUTH TEXAS... ...Texas... The first area of concern for excessive rainfall is along the south coast of TX. The precipitable water values of 2- 2.25 inches are forecast to move onshore and into far south Texas on low level easterly flow. The low level trough and convergence in coastal areas enhances lift across the area. With pockets of elevated instability anticipate some convection to develop off the coast and maintain as it moves onshore. Any convective clusters will be heavy rainmakers, possibly training across the same locations given the mean flow may become aligned with the propagation vectors. This will result in rain rates approaching 1+ inch/hour which may produce some localized flood concerns across far south TX. While areal averages are low, around 1-1.5 inches, localized higher precipitation amounts are expected. Another area of concern is for ongoing precipitation across central TX as waves of moisture ride along the 850mb front. This combined with divergence aloft will help precipitation continue in this region. Given the recent rounds of rain and saturated soils in place, it may not take much precipitation to cause flood impacts. So while the areal averages are around 1 inch and a stratiform rain event is expected with modest rain rates, these values might be enough to produce localized flooding. Based on this, a marginal risk remains across central TX. Petersen/Pagano Day 3 Valid 12Z Thu Oct 18 2018 - 12Z Fri Oct 19 2018 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL SOUTH TEXAS... ...Eastern to Southern Texas... Southerly mid level flow develops which promotes warm/moist advection over the low level front in eastern Texas, and provides coastal convergence in southern Texas. The onshore flow promotes continuation of the above normal precipitable water values of 2-2.25 inches (2-2.5 standard deviations above the mean). This combined with strong surface convergence right along the coast and upper level support for lift from divergence maxima in the coastal waters and coastline will likely result in some convective clusters moving onshore and inland with possible training. Rainfall rates of 1-1.5 inches/hour are possible. Given prior rainfall from day 2 likely continuing into day 3, a slight risk was shown, particularly given the models trended a bit higher with day 3 rainfall totals than day 2. The UKMET/Canadian global/NAM/GFS have a forecast of several inches of rain just off the coast, and the UKMET shows heavier rain making it onshore during this period. Petersen/Pagano Day 1 threat area: http://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: http://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: http://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$