Quantitative Precipitation Forecast
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Issued by NWS
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785 FOUS30 KWBC 180847 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 447 AM EDT Tue Jun 18 2024 Day 1 Valid 12Z Tue Jun 18 2024 - 12Z Wed Jun 19 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE UPPER AND MIDDLE TEXAS COASTS... ...Upper and Middle Texas Coasts... Potential Tropical Cyclone 1 (PTC1) will continue to organize across the southern and western Gulf today. Associated moisture originating in the Caribbean will stream northwestward into the Texas coast around the northern and eastern periphery of the storm. IVT values are very impressive...building to the upper end of the scale at 1,500 kg/m/s. This means PWATs with the storm will consistently increase through the day, approaching 3 inches by the time the storm`s full brunt moves into the coast tonight. PWATs approaching 3 inches mean the atmosphere will be carrying about as much moisture as it can possibly carry...and storms that form in this environment will be capable of extremely heavy rainfall rates due to highly efficient rainfall processes. Moisture lost by the rainfall will be quickly replaced as moisture advection (signified by the extremely high IVT values) will be optimal. All of this to say, as the plume of moisture associated with PTC1 and supported by advection straight out of the Caribbean moves into the upper Texas coast tonight, it will contain convection capable of 3 inch per hour rainfall rates. These rates will easily overwhelm local streams and creeks as they track northwestward, no matter how dry/empty they were prior to the start of the rain. This will likely catch many off guard, so it`s important to avoid crossing flooded roadways. With the better-forecast organization of PTC1, it`s expected that the associated plume of moisture and heavy rainfall will be more consolidated as it approaches the Texas coast as compared with previous forecasts. This means the timing of the heavy rainfall has been slightly delayed, and is less likely to extend too far north from the coast. Thus...the inherited ERO risk areas have been trimmed from the north from inherited. It`s likely that it will take until after 06Z tonight before consistent and steady heavy rainfall begins moving into the coast, with only scattered convection expected prior to that. However, given the aforementioned heavy rainfall rates expected when the directly associated rainfall with PTC1 moves ashore, numerous instances of flash flooding are likely late tonight in the Moderate Risk area. Further west in the Slight Risk area, there will be slighly less time for the heavy rainfall to cause flash flooding since it will be starting later. For Louisiana, the forecast track of PTC1 and aforementioned consolidation of the precipitation shield should keep most of the heavy rainfall offshore, so the Moderate Risk area was trimmed out of the southwestern coast of the state with this update. ...Central Plains into the Midwest... Ongoing heavy convection across the Dakotas and northern Minnesota is associated with a low over western Nebraska. The low will track northward into eastern SD and eventually into the Red River of the North by tonight. The convection is likely to be ongoing into northern Minnesota by the start of the period at 7am this morning. The heavy rain will persist there and far northwestern Wisconsin for a few hours this morning, resulting in widely scattered instances of flash flooding. The inherited Slight Risk for this area is little changed, and will likely be able to be trimmed out of northern MN and WI with the midday update today. Further south, the cold front associated with the low is expected to stall across Kansas, eastern Nebraska, and western Iowa today. Late this afternoon, convection is expected to break out along the front, and increase in coverage and intensity through the evening. A small area of high pressure over the western Dakotas tonight will support northerly flow to the north of the front. Meanwhile, the typical strengthening of the LLJ and southerly flow over the southern Plains will advect plentiful Gulf moisture into the front from the south. The clashing of the 2 air masses will keep the front from moving very much tonight, allowing the clashing to continue over the same areas for multiple hours tonight. The resulting convection will also train and backbuild over the same areas along and immediately south of the front. This will support widely scattered instances of flash flooding across much of the rest of the Slight risk area from southwest Kansas northeast to western Iowa. The greatest risk of flash flooding will be over Kansas tonight, as this will be the southern end of the front, where the 2 clashing air masses will be most opposed to each other. This will keep any storms that form from moving very much, and the influx of moisture from the LLJ will support additional convective development. There has been the typical uncertainty and inconsistency in the guidance as to where the southern end of the front will be. 24 hours ago it was expected to be in north-central Kansas and into eastern Nebraska. Now there`s much better agreement across southwestern Kansas. Thus, this area is at highest risk of flash flooding. Meanwhile further north and east, amounts should come down a bit...but unidirectional southwesterly flow parallel to the front will still support training storms and potential for flash flooding. Thus, the Slight Risk area from Kansas north and east was nudged east a bit with guidance trends, but is largely unchanged. ...Central Appalachians... "Ridge-running" precipitation is expected to track across eastern Ohio through central New York today. The storms will be supported by an air mass with PWATs up to 1.75 inches. The storms will be capable of local rainfall rates to 1.5 inches per hour, which depending on where they form may cause isolated instances of flash flooding. There is somewhat better agreement for a bit better organization of convection in this area. Thus, a Marginal Risk area was introduced with this update. Wegman Day 2 Valid 12Z Wed Jun 19 2024 - 12Z Thu Jun 20 2024 This discussion will be updated shortly. Thank you for your patience. Wegman Day 3 Valid 12Z Thu Jun 20 2024 - 12Z Fri Jun 21 2024 This discussion will be updated shortly. Thank you for your patience. Wegman Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt