Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
319 FOUS30 KWBC 210045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 845 PM EDT Thu Jun 20 2024 Day 1 Valid 01Z Fri Jun 21 2024 - 12Z Fri Jun 21 2024 ...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR SOUTHEAST SOUTH DAKOTA... 01Z Update... Recent hi-res guidance and observation trends did not suggest the need for any large-scale changes to the previous outlook. Therefore, made only minor adjustments. This includes the Moderate Risk area that is centered over southeastern South Dakota and extends into southwestern Minnesota and northwestern Iowa. Neighborhood probabilities from the 18Z HREF indicate overnight rainfall accumulations of 3 inches or more are likely across this area. Pereira Previous Discussion... ...Upper Mississippi to Central Missouri Valley... The Moderate Risk remains in effect for much of southeastern South Dakota with this update, with some slight northward broadening out of deference to the 12z HREF. Gulf moisture surging northward across the region with the LLJ will be moving into the right entrance region of a 100 kt southwesterly jet. A warm front will develop across South Dakota this evening as the typical evening strengthening of the LLJ occurs. Storms will break out along that boundary from the Sand Hills of Nebraska northeast into South Dakota. The storms are likely to merge and train along the front as they`re forming, then shift off to the northeast with the best forcing through early Friday morning. The primary point of uncertainty remains exactly where the line of storms initially forms relative to where it has rained a lot recently. Should the storms form too far north and west, closer to central SD and the western Sand Hills, then it`s likely a cold frontal boundary serving as the back edge of the storms will sweep across the area as the much more persistent rainfall associated with the front falls over a drier area. This would significantly decrease the flooding potential in South Dakota. However, the trend on a national scale with convective forecasts for multiple inches of rain over the past week has almost unanimously trended south...meaning the heaviest rainfall totals occur south of where the guidance suggests they will. In this case that would support storms occurring over areas that have been much harder hit recently with heavy rain, resulting in soils being much less receptive to the heavy rainfall and therefore more widespread flash flooding. Thus, the Moderate Risk remains relatively uncertain in its location...though there`s been good agreement on the magnitude of the heaviest rainfall. ...Southwestern Colorado... Increasingly anomalous atmospheric moisture to the tune of up to 4 sigma above normal with PWATs approaching 1 inch will slam into the south facing slopes of the San Juan Mountains today into tonight. Upslope into the mountains will support the development of stationary cells of heavy rain. If these occur over any burn scars, then flash flooding will be locally significantly worsened. The Slight Risk area inherited was left unchanged and looks solid for that area. Hourly rain totals to 2" and local amounts to 4" cannot be ruled out. The special update bridges the Marginal Risk between the Rockies and the Northern Plains, based on convective trends. ...West Texas into southeast New Mexico... The northern portion of the remnant moisture plume of the dissipating T.D. Alberto will move northwestward up the Rio Grande and into west Texas through the day today and into tonight. More widespread light to moderate rainfall than is typical for the desert of west Texas is expected. Given the low tolerance of most of the area to widespread rainfall -- burn scars -- the Slight remains in place...albeit lower confidence as the guidance does not appear to be overly wet in this region. However, upstream instability is supposed to build to the east of the designated risk areas, and it`s anomalously moist as inflow from the Gulf forces very high moisture around Albero`s periphery, so left in the Slight Risk as a course of least regret. In South Texas, diminishing of the convection in the area behind the main plume moving into Mexico has resulted in lessening concerns for more widespread flash flooding in the region, in favor of more isolated flash flooding. Isolated to very widely scattered convection today and tonight should greatly limit any resultant flash flooding, despite the newly saturated soils in the area. ...New York into New England... A Marginal Risk remains in effect across much of upstate New York and much of New England and has been nudged a little southward based on the 12z HREF. Lying on the northern side of the upper ridge will result in periodic upper level disturbances passing through and another afternoon and evening of scattered convection. Precipitable water values are 1.75-2", and ML CAPE is expected to build towards 2000 J/kg. Effective bulk shear is 20-30 kts, which should allow for some convective organization. The storms are likely to be slow moving, with some mergers expected between less and more organized thunderstorm activity. Hourly rain totals to 2.5" should be possible within this regime, with local amounts to 5". Widely scattered to possibly episodes of flash flooding are possible. ...Northeast Florida/Southeast Georgia... A new Marginal Risk was introduced for a small convective low that the National Hurricane Center is monitoring for tropical cyclone development. Precipitable water values of 2-2.5" are expected to move into the region ahead of the low due to easterly vertical wind shear. Hourly totals to 3" and local amounts to 5" are possible within this regime, but such amounts should be isolated to widely scattered at best. Soils are dry from a recent dearth of rainfall. The track and timing around 12z remain question marks -- whether it will be a day 1 or day 2 concern. Considering the above, added a Marginal Risk on this update. Roth/Wegman Day 2 Valid 12Z Fri Jun 21 2024 - 12Z Sat Jun 22 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTHERN MINNESOTA AND ADJACENT COUNTIES... ...Minnesota/Iowa border... A Moderate Risk of excessive rainfall remains in effect here, though it has shifted southward from continuity. This has been a very typical pattern with forecast convection in the guidance over the past week, possibly due to minimal mid-level capping aloft as hinted at by the modest 700 hPa temperature field. Convective complexes early and late are expected in and near southern MN, with the flow backing mid-period in preparation for the second convective round. While the guidance shows a break in activity, this is no guarantee as guidance tends to dissipate overnight activity too quickly in the morning. There are signs in the HREF for local 5-8" amounts. Hourly rain totals to 3" are possible as PWs rise at or above 2". ML CAPE should be north of 2000 J/kg in the warm sector. Effective bulk shear should be more than sufficient for organized convection -- mesocyclones in particular. Cell training is anticipated. The biggest question continues to be where does this MCS move as compared to the track of the heaviest rains from the past week, which were 8-10" to the west-southwest of Minneapolis and south of St. Paul. Soils are mostly saturated here, and rivers are forecast to be in significant flood from the anticipated heavy rainfall. For now...think there may be just enough misalignment and just enough uncertainty to refrain from a High Risk category, for now. Numerous local instances of flash flooding are likely, some significant, so the Moderate Risk remains in effect. ...Four Corners/Southwest... Anomalous moisture will remain in place across the Four Corners, especially Colorado and Utah. Meanwhile a shortwave trough moving into the area out of California/Nevada will result in more widespread showers and thunderstorms, especially during peak heating Friday afternoon. The Slight risk area remains in place and has seen expansion based on the available ingredients and the 12z HREF probabilities of hourly rainfall of 0.5"+. The Marginal Risk was equally expanded to cover much of AZ. Widely scattered to scattered instances of flash flooding are more likely Friday as compared with today due to the much improved upper level support. Temperatures at 700 hPa appear too warm -- 15-18C -- implying a bit of capping across western UT and NV -- so an effort was made to leave those locations out of the risk areas. ...South Texas... Continued east to east-southeast flow across the Gulf will bring any embedded convection inland into south Texas Friday. The convection should remain disorganized, but due to recent wet soils from Alberto, isolated instances of flash flooding can`t be ruled out. ...Portions of the Northeast... A front moving south across New England and NY will support organized convection Friday afternoon as the hot and humid air mass ahead of the front is uplifted by the front. Since soils across New England remain at or below normal for moisture content, any convection should only result in isolated to widely scattered flash flooding. Good continuity was kept here. ...Northeast Florida/Southeast Georgia... A new Marginal Risk was introduced for a small convective low that the National Hurricane Center is monitoring for tropical cyclone development which should be slowly recurving inland of the Southeast Coast. Precipitable water values of 2-2.5" are expected near the low. Hourly totals to 3" and local amounts to 5" are possible within this regime, but such amounts should be isolated to widely scattered at best. There is a range of QPF in the guidance, and it`s unclear which areas will see overlap with any heavy rainfall the previous day, so left the risk Marginal for the time being. Roth/Wegman Day 3 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... ...Upper Great Lakes and Upper Mississippi Valley... Heavy rain may cause flash flooding from the Twin Cities east Saturday morning when added to rainfall across Wisconsin from the overnight period. By this point however, the MCS should be picking up eastward speed as it moves into Wisconsin. Thus...rainfall amounts and flash flooding risk will decrease as the heavy rain moves into areas of WI and MI that are somewhat drier than areas to their west. Nonetheless...the MCS will remain intact as it moves across the area, becoming more linear in structure. Strong storms will develop to its south across IA/IL/MO...but the more progressive and shorter lived nature of the storms should minimize the flash flooding threat. The Slight Risk highlights where the warm front/MCS causes rain to be most persistent before the cold front pushes all of it to the east. Western areas across the Northern Plains were trimmed back, based on recent model trends. ...Southwest... As the remains of Alberto aloft continue chugging westward through the eastern Pacific, moisture pools across the Southwest to the southeast of a positively tilted upper level trough. There are distinct differences of opinion between the GFS and NAM moisture fields. Daytime heating should foster 1000-1500 J/kg of ML CAPE, which when combined with the moisture could lead to hourly rain totals up to 2.5". The guidance in general appears too dry across this region, based on the available ingredients. Issues with flash flooding are expected to be isolated to widely scattered. ...Deep South Texas... The potential for another tropical low to impact the Gulf Coast of Mexico could spread heavy rain north into Deep South Texas. Unlike Alberto, the rain with this system appears more confined, so only Deep South Texas has potential for resultant flash flooding. ...New England and NY... Continued passage of upper level disturbances would result in another round of afternoon showers and storms that may cause isolated flash flooding as tropical moisture returns to the area from the south as the front from Friday retreats back to the north. ..Northeast Florida/Southeast Georgia... A new Marginal Risk was introduced for a small convective low that the National Hurricane Center is monitoring for tropical cyclone development which may still be slowly recurving inland of the Southeast Coast. Precipitable water values of 2-2.5" are expected near the low. Hourly totals to 3" and local amounts to 5" are possible within this regime, but such amounts should be isolated to widely scattered at best. There is a range of QPF in the guidance, and it`s unclear which areas will see overlap with any heavy rainfall the previous couple days, so left the risk Marginal for the time being. Roth/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