Quantitative Precipitation Forecast
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465 FOUS30 KWBC 190849 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 449 AM EDT Wed Jun 19 2024 Day 1 Valid 12Z Wed Jun 19 2024 - 12Z Thu Jun 20 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTH TEXAS... ...South Texas... Potential Tropical Cyclone One (PTC1) remains on track to move inland into Mexico late tonight based on the latest forecast out of the National Hurricane Center. For more details on the storm, please refer to their page www.hurricanes.gov. Very impressive moisture advection out of the Caribbean continues to the north of the developing low. Despite this, the storm has had the most difficult time developing convection around the center. There is an obvious swirl in the cloud formation on IR satellite, but relatively little convection. All of this to say, the storm has been struggling to organize. There is still time for it to do so, but the clock is ticking. The result of this lack of organization has been a disorganized area of showers and storms that is moving into the Texas Coast as of the time of this writing. Rainfall rates have significantly underperformed thusfar, and storm total rainfall amounts continue to come down and shift southward (closer to the center). Given that trend and in coordination with HGX/Houston, TX forecast office, the Moderate Risk was canceled northeast of Matagorda Bay, and the Slight Risk area was shifted southwest of Houston and south of San Antonio. Assuming the storm gets its act together soon...expect the still very impressive moisture advection to the north of the center of circulation on the order of 1,250 to 1,500 kg/m/s IVT values to move into the Lower Texas Coast today, resulting in storms that can produce prodigious rainfall rates as high as 2.5 inches per hour. Still expect isolated maximum rainfall amounts to exceed a foot by the time all is said and done Thursday. The heaviest rain will push westward with time, moving up the Rio Grande Valley tonight. The highest totals remain closer to the coast as that is closer to the moisture source (the Gulf), but rainfall totals of 4-6 inches are likely well up the Rio Grande through Thursday morning. Given the heavy rainfall rates, this will likely result in numerous, if localized instances of rapid onset flash flooding as the dry soils struggle to handle such a large amount of water all at once. ...Northeastern New Mexico, Texas and Oklahoma Panhandles, and southwestern Kansas... A snail`s pace moving front is crawling south across the Texas and Oklahoma Panhandles this morning. A robust LLJ is funneling plentiful Gulf moisture north up West Texas this morning. A strong ridge of high pressure over the western Dakotas is funneling slightly cooler and drier air southward through Kansas and into the front from the north. These clashing air masses have resulted in the front moving as slow as it is. Rainfall amounts have been nothing short of impressive, with several areas of 7 inch + totals just in the last few hours in the Oklahoma Panhandle and far southwest Kansas. By the start of the period, the storms now into the Texas Panhandle and northwest Oklahoma should be much weaker than they are currently, as diurnal heating begins to disrupt the moisture advection from the south. Nonetheless, outflow will continue rainfall into the morning, and with such large amounts of rain into the Panhandles currently and over the last few hours, FFGs are very low, so the additional rainfall will support continued flash flooding. The inherited Slight was expanded south and west to highlight these areas for the morning...though it`s possible isolated convection may re-impact these areas later this afternoon into this evening. The Slight continues to be expanded southwest to cover much of northeastern New Mexico. This is largely for expected dry line convection that will break out late this afternoon through this evening. The dry line has retreated well west into NM this morning, thanks in part to moisture advection from the easterly flow of PTC1. The typical eastward advance of the dry line this afternoon with unusual amounts of atmospheric moisture will make for scattered showers and thunderstorms capable of up to 2 inch per hour rates. Given NM is usually a desert and very dry, these high rainfall rates are likely to result in rapid-onset flash flooding in the impacted areas. ...Midwest and Great Lakes... Ridge running convection tracking westward around the western periphery of the heat dome bubble of high pressure over the southern Appalachians will cause storms to race eastward across the area today and into tonight. While the storms will be fast-moving, unidirectional flow may result in some localized training which may cause isolated flash flooding. Wegman Day 2 Valid 12Z Thu Jun 20 2024 - 12Z Fri Jun 21 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE RIO GRANDE VALLEY AND SOUTH TEXAS AS WELL AS FOR PORTIONS OF THE NORTHERN PLAINS AND UPPER MIDWEST... ...Rio Grande Valley and South Texas... Remnant rainfall from what will be rapidly weakening Potential Tropical Cyclone One (PTC1) Thursday morning will continue to push west with the full force of the open Gulf supporting it. Easterly flow will be well established across the entire Gulf. The rainfall will be moving into increasingly dry areas of west Texas and New Mexico Thursday. Thus...while the heaviest rainfall rates will be diminishing with longitude...what heavy rainfall does occur will be moving over areas less and less used to heavy rain. Thus, the flooding threat will continue into the Rio Grande Valley and south Texas. Further, increasing interaction with the terrain will locally enhance rainfall rates, especially on east-facing slopes. The storm is expected to be moving a little bit faster than inherited forecasts. This has resulted in a bit lower max totals, while spreading the heaviest rains further west. Most of the increases were in Mexico. For the Texas side of the Rio Grande, this has resulted in a drier forecast for much of central Texas. Thus...the Slight, has been trimmed from the north and east, now almost to the Rio Grande near Del Rio. For South Texas, the aforementioned broad easterly flow over the entirety of the Gulf will keep isolated to widely scattered showers and thunderstorms that develop over the Gulf moving westward across the area...with drier conditions expected to the north where areas to the east are not the abnormally hot Gulf, but rather unusually hot land. Thus...the threat through the day really is focused on areas due west of the Gulf...i.e. south Texas. The combination of upper level ridging and a slow moving trough over the Intermountain West will turn the prevailing flow more southerly, generally following up the Rio Grande Valley...Thus, the rainfall threat will expand northward into west Texas and much of eastern New Mexico through Thursday night. Terrain impacts will locally enhance rainfall rates resulting in isolated flash flooding, as well as any overlap from Day 1 rainfall in northeastern NM. ...Northern Plains and Upper Midwest... A stalled out front will be over eastern South Dakota, southern Minnesota and northwestern Iowa today. Convection may impact some of these areas to start the day Thursday morning from overnight convection tonight, but the bulk of the expected rainfall in this area will occur Thursday night. Developing troughing over the area and a departing area of high pressure will turn the front into a warm front that moves north through the night. Heavy rainfall rates are likely from storms as moisture convergence over the Slight Risk area interacts with the front. Much of this area from far northern NE, southeastern SD, and all of southern Minnesota have been hard hit with heavy rains in recent days. Thus, a repeat of the heavy rain over the area will likely result in widely scattered instances of flash flooding. Should the daily rainfall maximum continue to shift south as has been the trend with much of the nation`s forecast convection, increasingly hard hit areas will be increasingly impacted. Thus, a targeted upgrade is certainly not out of the question. ...4 Corners Region... Moisture from the remnants of PTC1 will track northwestward into the 4 Corners Region Thursday and Thursday night. Upslope flow, especially into the San Juans of southwest Colorado may result in isolated flash flooding, especially around burn scars. Increasingly anomalous moisture advecting into the area will result in increasing risk for flash flooding. ...Finger Lakes of NY and northwest PA... Convection capable of heavy rainfall rates will develop over this region today. Soils in the area are wetter than normal, so any training or stationary storms from the terrain may cause heavy rain over a concentrated area, resulting in potential isolated flash flooding. Wegman Day 3 Valid 12Z Fri Jun 21 2024 - 12Z Sat Jun 22 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN SOUTH DAKOTA THROUGH MUCH OF WISCONSIN AS WELL AS FOR MUCH OF THE NORTHERN COLORADO PLATEAU... ...Eastern South Dakota through Wisconsin... Multiple sources of forcing will support multiple rounds of heavy rain across the northern Plains into the Upper Great Lakes on Friday and Friday night. During the day on Friday, the warm front over the Slight Risk area will slow to a stall during the morning as the typical weakening of the LLJ occurs due to diurnal heating. This too should result in diminishing coverage and intensity of rainfall over this area through the morning. Then Friday afternoon and evening, a strengthening low that will develop as a shortwave ejects into the northern Plains will increase moisture advection ahead of the low and attendant cold front into the Slight Risk area. PWATs will increase with the storms into Friday night perhaps above 2 inches as the bulk of the moisture that was once Potential Tropical Cyclone One (PTC1) makes its way into the northern Plains. This tropical moisture and advection, strengthening low, and very favorable antecedent conditions over much of the Slight Risk area are all ingredients contributing to increasing concern for numerous and more widespread flash flooding in this region. The center of the low will move to central Minnesota by Saturday morning. Expect strong storms with both the warm front to the east of the low and the cold front south of it due to plentiful moisture. Uncertainty remains high as all of the aforementioned ingredients need to all come together at the same time. There have been distinct southward shifts with time in previous days in the placement of heavy rain from convection with all the convection impacting the nation. Thus, the primary point of uncertainty is where the heaviest rain will occur. For now, a higher-end Slight is considered in effect for the area of far eastern SD, nearly all of the southern half of Minnesota, and western Wisconsin. Should the southern portion of this area remain in the forecast for the heaviest rain of similar magnitude as the model consensus suggests, a Moderate Risk will likely be needed with future updates. Stay tuned. ...Northern Colorado Plateau... In coordination with SLC/Salt Lake City, UT and GJT/Grand Junction, CO forecast offices, a Slight Risk upgrade was introduced with this update. A potent shortwave moving into the West Coast will interact with some of the remnant moisture from PTC1, which will continue to be advected into the area from the Gulf. Atmospheric moisture will be a much as 4 sigma above normal. Interaction with the terrain, the shortwave increasing forcing, and local burn scars in both CO and UT will all increase the potential for flash flooding in this area, particularly in the south facing slopes of the San Juan Mountains. ...South Texas... Continued easterly flow over the Gulf will push additional convection developing over the Gulf into south Texas Friday and Friday night. The convection is likely to be mostly isolated to widely scattered with little in the way of organization. The potential for local training and the likely very favorable antecedent conditions/low FFGs from the recent passage of PTC1 in the area continues to support a Marginal Risk for this area. ...New England and Hudson Valley... A Marginal Risk area was introduced in this region for the third consecutive day of afternoon convection occurring as waves continue to run the top of the ridge that remains over the Southeast. A small area of high pressure will increase northwesterly flow into northern New England. Meanwhile the plume of moisture originating from the Gulf, moving north up the Plains, then turning west across the Great Lakes and into New England will be ever present. The front pushing south with drier air will provide a source of forcing for more concentrated convection, particularly over southern New England Friday afternoon. The signal remains weak, but isolated flash flooding is possible in flood-prone areas. 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