Quantitative Precipitation Forecast
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Issued by NWS
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429 FOUS30 KWBC 211559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Fri Jun 21 2024 Day 1 Valid 16Z Fri Jun 21 2024 - 12Z Sat Jun 22 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST SOUTH DAKOTA... NORTHEAST NEBRASKA... NORTHERN IOWA... SOUTHERN MINNESOTA... AND WESTERN IOWA... ...Mid Missouri Valley and Upper Mississippi Valley... The primary change with this outlook update is to shift the Moderate and Slight Risk contours south about 30-40 miles. An ongoing band of elevated convection from northern Nebraska (near VTN) to southern Minnesota (near MKT) seems likely to be a reasonable proxy for the northward extent of potential extreme rainfall today. The atmosphere to the south is rapidly becoming unstable, with abundant sunshine, and the dichotomy should reinforce the surface front. This afternoon, new convection is expected to focus closer to the surface front. Overall, the front is slightly further south than models were projecting in some of their previous runs. The new data seems to have led to convergence in the hi-res model solutions with reasonably good consistency on the projected location of the highest totals. There was some consideration given to a High Risk upgrade within the Moderate Risk area. The pattern over the region is classic for flash flooding with an extensive WSW-ENE oriented front, strong instability and anomalous moisture available to the south, and increasing low-level inflow from a southwesterly direction to favor backbuilding and training. One key lingering open question for a High Risk upgrade is the degree to which today`s heaviest rainfall will overlap the areas most sensitive from previous rainfall; most of the hi-res models are just to the southeast, but it is very close. The area of greatest concern is from YKN-EST, or from Yankton County, SD to Emmet County, IA, and about 25mi either side of that line. These areas are in sunshine at the moment (indicating the surface front may be nearby or even just north) and overlap sufficiently with the extreme rainfall footprint from the past 24 hours. So it is possible the 12Z hi-res models have slightly overestimated the cold pool from ongoing convection; if this is the case, a High Risk may be issued on an unscheduled update this afternoon. Convective trends will be monitored closely. Either way, the potential exists for significant and life- threatening flash flood impacts in the region, and people in the area are advised to closely monitor for warnings later today. ...Four Corners Region into New Mexico... The Slight Risk area was expanded to the west into portions of the slot canyon region of Utah. Model consensus is for another round of afternoon convection to develop in a fairly unstable air mass ahead of an advancing shortwave in portions of UT, W CO, and N AZ. Generally, one of the biggest differentiating factors between active flash flood days and inactive or less active days in the Southwest U.S. is instability, and models are showing fairly widespread areas of CAPE above 1500 j/kg by this afternoon. Despite some pockets of morning convection, there is enough sunshine to support widespread destabilization, and precipitable water values should be above the 95th percentile for this time of year. Convection may also be more organized than is typical, with some backbuilding, as southerly low-mid level flow will be relatively strong. The IVT is above the 99th percentile for this time of year. Further southeast into New Mexico, there is more cloud cover associated with a deep moisture plume from the remnants of T.S. Alberto, which has since dissipated. However, where breaks in the cloud cover can occur, enough instability should support renewed convective activity this afternoon and evening, and the deep moisture (as seen in the CIRA ALPW product) should support efficient rainfall. One of the better signals is, unfortunately, in the vicinity of the Sacramento Mountains where there are several large burn scars and the HREF has reasonably high probabilities of 1 inch per hour rain rates. GOES day cloud phase RGB does show some low level cloud cover beginning to scatter out, so some instability nearby may be able to develop. ...New York and northern Pennsylvania into New England... A Slight Risk was introduced over southern New York and the lower Hudson Valley into southern New England -- particularly Massachusetts, Connecticut, and Rhode Island. Precipitable water values approaching 2 inches (above 95th percentile) will support very high rain rates in scattered convection that is already developing across the region. For more mesoscale detail, please reference MPD #480 about to be issued. Across the Slight Risk area, the 12Z HREF probabilities of 2 inch per hour rain rates peak at about 20 to 40 percent in any given hour, and this would be enough to generate some flash flooding, particularly in urban areas. ...North Florida and Coastal Southeast... A Marginal Risk was introduced for the potential of localized heavy rainfall around the tropical disturbance nearby in the Atlantic. Hi-res models show localized rainfall in excess of 2 inches per hour in some of the convection today. Lamers Day 2 Valid 12Z Sat Jun 22 2024 - 12Z Sun Jun 23 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER GREAT LAKES AND UPPER MISSISSIPPI VALLEY... The day 1 height falls supporting additional heavy precip across the Upper Mississippi Valley/Mid Missouri Valley will be pushing into the Upper Great Lakes day 2. Hi res models suggest a well defined MCV will persist into early day 2 as it pushes from the Upper Mississippi Valley into the Upper Lakes. With PW values expected to remain anomalous ahead of this MCV, 2-3 standard deviations above the mean, heavy rains are likely across the Upper Lakes on Saturday. This is reflected in the 12 hr HREF neighborhood probabilities ending 0000 UTC Sun that are high for 1 and 2"+ totals from northeast Wisconsin into the northern L.P. of Michigan. Convection also likely to become increasingly organized late Saturday afternoon in an axis of increasing instability, MUCAPE values 1000-1500, PW values 2-2.5" and area of enhanced upper difluence along the trailing cold front moving across northern Illinois, Iowa, into northeast Kansas and northern Missouri. Heavy rains are possible along and ahead of this trailing front, with the slight risk drawn southward into northern Illinois to cover areas of relatively lower ffg values. ...Portions of the Southwest... The upper trof over California into the Southern Great Basin during day 1 will weaken day 2 as the eastern upper ridge expands westward. The anomalous area of PW values will, however, expand farther to the north and west, encompassing areas from southeast California, into central to southern Arizona and western New Mexico, with values reaching 3 to 4 standard deviations above the mean. Not a lot of confidence on any scattered convection in this high PW axis, but locally heavy rainfall amounts and isolated runoff issues are possible. ...Far South Texas... The next surge of high PW values to the north of low pressure moving east to west across the southwest Gulf of Mexico/Bay of Campeche will move into South Texas day 2. Model consensus is for the heaviest precip to remain across northeast Mexico, with far South Texas on the northern edge of this precip. A marginal risk was maintained from the previous issuance given potential for locally heavy rainfall amounts across regions that received heavy rains recently from Alberto. ...Portions of southern Georgia into North Florida... The weak surface low approaching the Southeast coast day 1 will weaken further day 2 as it pushes into south central Georgia. Simulated radars from the NAM NEST and FV3LAM show potential for some slow moving cells in the vicinity of this weak low level circulation, which will be accompanied by an axis of above average PW values. Locally heavy precip totals possible near this weak low level center, southward into North Florida where a marginal risk was maintained from the previous issuance. Oravec Day 3 Valid 12Z Sun Jun 23 2024 - 12Z Mon Jun 24 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN NEW YORK STATE INTO NORTHERN NEW ENGLAND... The mid to upper level height falls pushing heavy precip potential into the Upper Lakes day 2 will continue to press eastward day 3 into the Northeast. The anomalous axis of PW values 2 to 2.5+ standard deviations above the mean across the Upper Lakes day 2 will also push into northern NY State and Northern New England day 3 ahead of surface low pressure moving eastward across these areas. The NAM is an outlier with a farther southward surface low track and subsequently farther south heavy precip axis compared to the remainder of the global guidance. Consensus is for an axis of heavy rains from far northern NY State into Northern New England, where a slight risk was maintained. ...Portions of the Southwest... Not a lot of large scale changes day 3 across the Southwest from day 2 with a broad marginal risk area depicted across areas from far southeast California, through much of Arizona and into northwest New Mexico. PW values will continue to remain 3 to 4 standard deviations above the mean in a region of generally weak forcing. Additional scattered convection likely across these areas, with locally heavy totals and isolated runoff issues possible. ...South Texas... PW values 2-2.5" and sustained east southeasterly low level flow is expected to persist for most of the upcoming day 3 period across northeast Mexico into South Texas. Similar to the day 2 period, model consensus is for the heaviest totals to be across northeast Mexico with far South Texas on the northern edge of this precip. Given no big changes overall to the pattern, the marginal risk was maintained over far South Texas for isolated runoff issues. Oravec 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