Quantitative Precipitation Forecast
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276 FOUS30 KWBC 190105 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 905 PM EDT Tue Jun 18 2024 Day 1 Valid 01Z Wed Jun 19 2024 - 12Z Wed Jun 19 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE LOWER TO THE MIDDLE TEXAS COAST AND FROM THE CENTRAL HIGH PLAINS TO THE UPPER MISSISSIPPI VALLEY... 01Z Update... The Moderate Risk was removed from the mid and upper Texas Coast. While localized heavy rainfall and flash flooding cannot be ruled out, the model consensus shows the majority of the heavy rain remaining offshore along with the better instability through the evening and overnight. However, there is good indication that the potential for better organized heavy rain and more widespread flash flooding concerns will begin to increase near the end of Day 1 as a mid-level shortwave and an accompanying slug of deeper moisture wrapping west on the north side of Potential T.C. One begin to move onshore. Elsewhere, made smaller adjustments based on current observation trends and recent hi-res guidance. This included maintaining the Slight Risk from the central High Plains northeastward into the upper Mississippi Valley. The signal for heavy rainfall and possible flash flooding is especially noteworthy from southwestern into central Kansas, where guidance indicates that redeveloping convection near the intersection of a cold front and dryline is likely to continue for at least the next few hours. Further to the northeast, training storms ahead of the cold front will carry the threat for heavy rainfall and localized flash flooding into the lower Missouri and upper Mississippi valleys through the evening into the overnight hours. Across northeast Minnesota, a deep moisture pool and favorable forcing associated with a wave lifting northeast across the region will continue to support training storms, raising additional heavy rainfall and flash flooding concerns for at least the next few hours before the wave lifts into western Ontario overnight. Pereira Day 2 Valid 12Z Wed Jun 19 2024 - 12Z Thu Jun 20 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN TEXAS AND PORTIONS OF THE MIDDLE AND UPPER TEXAS COASTS... ...South Texas... 21Z update... There was an overall southward trend with the tropical rainfall across Texas and portions of the Gulf Coast. The highest rain is expected to focus over south-central and southern Texas where areal averages of 3 to 8 inches are forecast with isolated maximums creeping into the double digits. With this southward shift in the QPF footprint all Excessive Rainfall Outlook thresholds were adjusted to the South. Campbell Potential Tropical Cyclone 1 (PTC1) is expected to turn westward across the western Gulf on Wednesday and make landfall in the southern Mexican state of Tamaulipas Wednesday night based on the 4am CDT update from the National Hurricane Center. While the center will be well south of Texas, a nothing short of impressive plume of moisture characterized by IVTs pushing the top of the scale at 1,500 kg/m/s will continue advecting northwestward out of the Caribbean, across the Gulf, and into south Texas on Wednesday. PTC1 will tap into this moisture plume as it tracks westward towards Mexico. This will greatly expand the associated precipitation shield north of the center of the storm. Thus, a prolonged period of heavy rain is likely across all of south Texas Wednesday and Wednesday night. Heavy rain will be ongoing across nearly all of the Texas coast at the start of the Day 2/Wednesday period at 7am Wednesday. With PWATs approaching 3 inches from Houston/Galveston southwestward down the entire Texas coast, the convection that will be embedded within the broader precipitation shield associated with PTC1 will be capable of extremely heavy rainfall rates as high as 3 inches per hour with the strongest storms. Rates this high will easily overwhelm smaller streams and creek watersheds with water falling close to all at once. Thus, rapid-onset flash flooding is likely across much of southern Texas with this storm. For most of the Moderate Risk area, the bulk of the rainfall with PTC1 will occur during this Day 2/Wednesday period. The rain will overspread south Texas from east to west. Thus, areas along the coast will see their heaviest rain during the day Wednesday, while towards the Rio Grande the heavier rain will be towards evening and into Wednesday night. Think the greatest rainfall rates will be with the parent easterly wave associated with PTC1, though heavy rain will be likely due to moisture availability well before and after the passage of the wave. As the bands of rain track westward and inland, they will very gradually weaken with time as they become separated from their moisture source...the Gulf. However, the extreme IVT will support them well inland from the coast. Nonetheless, the highest rainfall totals associated with PTC1 are likely closer to the coast, with only gradually diminishing rainfall totals as you move inland, as upsloping into the terrain plays an increasingly important role in the development and support of heavy rain. 5 to 10 inches of rain with isolated totals as high as 15 inches are expected across the Moderate Risk area. Many portions of the Moderate Risk area have been very dry of late, so empty rivers and streams will initially preclude much flooding...though as mentioned the extreme rainfall rates will locally exceed FFGs, regardless of how dry the area was before the storm. By the tail end of the event numerous instances of flash flooding are likely, so the Moderate Risk remains in effect with few changes from inherited. Note the antecedent dry conditions may lead many to a false sense of security as regards flooding...and rapid onset flash flooding will be common. The surrounding Slight Risk area was dramatically trimmed on the northern end out of north Texas with this update. As mentioned, the consolidated nature of the convection should hold much of the associated rainfall closer to the storm center over south Texas. Thus, much less rainfall is expected into north Texas, and the Slight Risk area was cancelled into the DFW Metroplex. ...Central Plains... 21Z update... The latest guidance suggested an increase in thunderstorm coverage and QPF amounts across portions of Kansas and northern Oklahoma during this period. As such a small expansion of the Slight Risk was made on the southern side across south-central Kansas and into portions of the Oklahoma and Texas Panhandles. Some the 1hr/3hr FFG guidance is as low as 1.5/2 inches in that part of the region. Campbell A small Slight risk area was introduced with this update across portions of southwest and south-central Kansas. Ongoing convection from the overnight period Tuesday night is expected across this region, with ongoing widely scattered instances of flash flooding ongoing. The southern/eastern end of the line of convection will track across the Slight Risk area through the morning Wednesday before dissipating. The inherited Slight risk area across portions of north-central Kansas and eastern Nebraska was downgraded with this update. This is in regards to the uncertainty with where the southern end of the front would be from Day 1/Tuesday. Since the general consensus in the guidance is for the southern end of the front to now be in southern Kansas, north Kansas into eastern Nebraska will miss out on the heaviest rain, so the signal for heavy rain in this area has diminished significantly. ...Midwest... Elsewhere the large Marginal from the Plains into the Great Lakes is largely in deference to the various waves of moisture streaming across the area on the northern end of the large high pressure area in the upper levels over the Southeast. Convection will likely be fast moving and largely disorganized, but since various sections of the Marginal risk area have seen heavy rains in recent days, the threat for isolated instances of flash flooding from these storms is there right through central New York. Wegman Day 3 Valid 12Z Thu Jun 20 2024 - 12Z Fri Jun 21 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE RIO GRANDE VALLEY OF TEXAS TO THE LOWER TEXAS COAST AS WELL AS FROM EASTERN SOUTH DAKOTA TO WESTERN WISCONSIN... ...Texas... 21Z update... The rainfall associated with PTC1 is expected to continue progressing westward during this period, primarily focusing over South Texas, Rio Grande, West Texas and eastern New Mexico. With the decreasing trend across portions of central and eastern Texas, the Slight and Marginal Risks reduced to the south/southwest. Campbell Residual rainfall from what will likely be dissipated PTC1 will continue across the Rio Grande Valley Thursday morning. The Slight was expanded to include the entire Lower Rio Grande Valley and Lower Texas Coast largely due to expected ongoing flash flooding at the start of the period Thursday morning. Additional rainfall during the day Thursday should be mostly light, but abundance of moisture will still support some widely scattered convection capable of producing flash flooding. The heaviest rain this period will be across the Big Bend region, where a higher-risk Slight is in effect. 2-4 inches of rain with locally higher amounts are likely in the Big Bend region. This is highly unusual in such a widespread manner from a tropical cyclone. Thus, this area has a low, but non-zero potential for further upgrades with future updates due to the infrequency of this amount of rain in this usually very dry area. There is a bit more uncertainty with how far north up the dry line into West Texas/southeast New Mexico the persistent heavy rain gets as the whole plume quickly weakens, so future adjustments to the Marginal Risk in this area are likely. ...Northern Plains and Portions of the Upper Midwest... 21Z update... There will be some of the tropical moisture from PTC1 riding over the ridge and combining with the moisture already in place over the northern tier states. This will result in a modest boost in rainfall rates and the overall foot print. Given the some of the increased trends, the Marginal Risk was expanded northwest Nebraska and far eastern Wyoming. The Slight Risk had small expansions south across South Dakota and southern Minnesota. Campbell A Slight Risk area was left largely unchanged with this update. A shortwave and right entrance region of a small 100 kt jet streak will track over a front over the area with attendant southerly flow of moisture up the central Plains and into this area. Recent heavy rain, including in some areas where its ongoing as of this writing will lead to wet soils that are more susceptible to flooding from heavy convection. The convection is most likely to impact this region Thursday night. ...Four Corners Region... 21Z update... A very small eastward bump of the Marginal Risk area was made across south-central Colorado to account for some of the guidance showing convection further east from the favored areas of terrain. Campbell The meeting of the remnant upper level energy from PTC 1 will interact with a slow moving trough over California. Residual moisture will interact with the terrain in this area. The result will be scattered showers and thunderstorms capable of heavy rain that may cause isolated flash flooding since this area is typically a dry area and not able to handle much heavy rain before flooding occurs. 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