Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
478 FOUS30 KWBC 240811 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 AM EDT Mon Jun 24 2024 Day 1 Valid 12Z Mon Jun 24 2024 - 12Z Tue Jun 25 2024 ..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF SOUTHEAST ARIZONA... ...Southwest... A continued pattern of elevated deep layer moisture will preside over much of the Western U.S as a mid-level ridge axis resides just east of the Four Corners with remnant tropical moisture situated on the western flank of the ridge. As a result, another afternoon and evening of scattered convection with access to substantial moisture across the Great Basin to the Mexican border during peak diurnal heating. Destabilization pattern is forecast during the typical climatological period between 17-01z before we see a decay in the overall environment. PWAT indices are running incredibly high across the aforementioned area with 2-4 deviations located over the central Great Basin and western CO interior with deviations closer to +4-5 within AZ leading to enhanced rainfall within any convective cells that develop. The best opportunity for more general storm clusters and organized thunderstorm development will be across Southeast AZ within the topography comprised of the Huachucas down into nearby Sonora. A pronounced theta-E ridge will extend north out of Mexico into the Southeast AZ terrain leading to targeted areas of convection that will be tied to the mountains in the vicinity. Considering the ambient environment, this will allow for local maxima exceeding 2" in some of the more prevalent storms, easily capable of causing flash flood concerns given the more complex terrain and flashy soils. The threat will extend close to the Tuscon urban center where any heavy rain threat could turn quickly into flash flooding within the urbanized setting. This prompted the continuation of the previous SLGT risk as the setup has deviated very little from the previous forecast. Further north and west, the focus for convection is a little more scattered, but the premise stands with such a moisture laden environment. Locally heavy rainfall could spell trouble for locations across the Desert Southwest with the best potential lying within the terrain centered around the Mogollon Rim into Northwest NM. Totals of 1-2" will be plausible in those affected by slow- moving convection tied to the terrain, enough to cause a flash flood concern within the terrain and surrounding locales. ...Upper Mississippi Valley/Upper Great Lakes... Flow will remain slightly backed across the Midwest ahead of an approaching front from the northwest. Moisture anomalies between 1-2 deviations above normal will encompass much of the Upper Midwest and adjacent Great Lake areas by this evening setting the stage for a primed environment capable of heavy rainfall. As the front approaches, a small mid-level perturbation will eject out of the Dakotas and help enhance large scale ascent somewhere over the Upper Mississippi River Valley. Steering flow aloft will take any convection and focus it southeast as it grows upscale and progressively moves into nearby WI. The progressive nature of the anticipated complex will limit the severity for flash flood concerns, however the local maximum is likely to be within the 3" range considering the available atmospheric moisture and prominent ascent pattern focused over the region. There is also some discrepancy in guidance in exactly where the forecasted complex will track as the deviation in proximity ranges from as far north as Green Bay to as far south as northern IL by the end of the period. This is also addressed within the probability fields as the 1" HREF EAS signature is uncharacteristically low around 25-30% for a large area between MSP down towards Milwaukee. Usually in a favorable setup like this, we would see some higher values reflected due to agreement within the CAMs. This is a red flag when it comes to this part of the ERO equation as the prospects for somewhere in the Upper Mississippi Valley to get flash flooding is decent, but specifically where is a question. This is also reflected in the latest SPC D1 Outlook and has been a factor in their higher risk potential. The area across southern WI is the most susceptible to flash flood concerns thanks to the previous periods of heavy rain that caused significant flood concerns all across that area. If there is growing consensus for that area in general, there will be opportunities for an upgrade. Until then, maintained the MRGL risk with emphasis on the prospects for a targeted upgrade somewhere across MN/WI/Northern IL. ...Northern New England.... A digging mid-level shortwave analyzed over Quebec will shift south into Northern New England, eventually closing off into an upper low centered across NH into ME by the afternoon hours. Focused mid- level ascent within an destabilized environment under the low will allow for a generation of convection across northern NH into ME during peak diurnal heating leading to scattered thunderstorms capable of heavy rainfall under any cell development. Storm motions will be weak thanks to being situated under the closed upper reflection, so cell cores will be able to espouse locally heavy rainfall that could bring totals upwards of 2-3" in the most prominent cell structures. Surface low pressure development over ME will drift into the Gulf of Maine creating an easterly low-level moisture feed inland which is part of the expected convergence pattern centered on the leeside of the higher terrain in central and western ME. This corridor is being depicted as the target for the heaviest precip potential with 00z HREF neighborhood probabilities for at least 2" signaling values between 60-80% encompassing the above area. The signal for 3" drops off a bit, but still has a central point of 25-35% situated across central and south-central ME. Considering the setup and convective mode for rainfall, a MRGL risk was kept from previous forecast. Kleebauer Day 2 Valid 12Z Tue Jun 25 2024 - 12Z Wed Jun 26 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDWEST, OHIO AND MID-MISSISSIPPI VALLEYS... ...Midwest and Mid-Mississippi Valley... A complex from the previous period will likely enter into portions of northern IL into MI by start of the D2 time frame with potential for heavy rainfall across these areas with the progression of the expected MCS. Rainfall totals of 1-3" will be possible within the impact zones in the complex`s path, much of the area comprised being urban focused corridors like Chicago, Southern Lake Michigan, extending towards northern IN. Considering the uncertainty of where the complex will motion, there was enough reason to have a broad MRGL, at minimum extend into Lower MI all the way out towards Lake Erie as some guidance suggests. When there is greater consensus on the eventual path of the complex, there could be some targeted changes in the risk areas, so stay tuned for those proposals in future updates. In the grand scheme, a cold front pressing from the north will begin to shift orientation more west to east as it crosses into the central Midwest and adjacent Ohio and Mississippi Valleys. Surface flow ahead of the front will allow for a pooling of moisture ahead of the boundary with a strengthening convergence signal located along the front. PWAT deviations around +1-2 will remain situated across the aforementioned areas with sufficient low-level buoyancy aligned within the same areas encompassed by the elevated moisture field. A round of convection will fire Tuesday afternoon and evening across IL and northwest MO into IN with locally enhanced rainfall capable of flash flooding as rates reach upwards of 2"/hr or greater in some of the better cell structures. Recent ensemble QPF output signals a widespread region of 1-2" of precip with associated deterministic signaling opportunities for totals greater than 4" in spots. These areas are situated as far north as the Chicago Suburbs, as far west as northern MO, and as far east as Northwest OH. Recent ensemble trends in the QPF with the expected setup and environment have led to enough confidence to expand the previous SLGT risk further west and east, aligning with the expected frontal positioning on Tuesday evening. The SLGT risk into southwest MI is for the prospects of the morning activity, as well as some chance for convection later in the period. Some guidance is also indicating a quick moving mid-level shortwave progressing southeast out of IA with a secondary enhancement of convection overnight into Wednesday across southeast IA into northern MO. The precip trends were much more pronounced from previous model output, so the SLGT was also expanded into that section of IA to account for the opportunity. ...Southwest... Mid-level ridge across the Southwestern US will strengthen with a more focused ridge axis centered over the AZ/NM border come Tuesday afternoon and beyond. This will shift the focus of the convective pattern across the west a bit further east with scattered storm development basically highlighting the Great Basin and much of AZ/NM into southwestern CO. PWAT anomalies will still be on the high side with a general 2-4 deviations above normal situated within the aforementioned areas of convective focus. Any storm development will exhibit weak storm motions with locally heavy rainfall upwards of 1"/hr in the most substantial updrafts. Slot canyons, arroyos, burn scars, and complex topography over the interior will continue to be the zones of highest opportunity for flash flooding with any urbanized areas also residing in the threat window. Considering a less organized threat with limited large scale ascent accompanying, a MRGL risk for the region was sufficient and maintained for continuity. Kleebauer Day 3 Valid 12Z Wed Jun 26 2024 - 12Z Thu Jun 27 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE NORTHEAST, OHIO VALLEY, AND LOWER MISSISSIPPI VALLEY, AS WELL AS THE SOUTHWEST U.S... ...East of the Mississippi River... A cold front located from the Central Plains into the Ohio Valley will make progress to the south and east through the course of Wednesday with scattered instances of thunderstorms likely across much of the region east of the Mississippi. The most prominent areas of precip will reside over the Lower Mississippi Valley where a combination of elevated moisture and plentiful surface instability will allow for heavy rainfall within any convective cores that permeate along and ahead of the approaching cold front. The second area of focus will reside over the Mid Atlantic and Northeast where the approach of the front, as well as the region positioned under the right-entrance region of an amplifying jet over Southeast Canada will create a widespread convective setup by Wednesday afternoon carrying through the remainder of the period. Recent NAEFS PWAT anomalies center the best deep layer moisture presence within the Mid Atlantic from WV up through the Delmarva, including the Northeast Megalopolis from DC to Boston. A MRGL risk is currently forecast across much of the Northeast and central Mid Atlantic with potential for a SLGT risk upgrade in future updates as the pattern evolves and details become more concrete on convective specifics. The current setup is more than capable of dropping some localized amounts over 2" in places where convection is most prevalent, especially in any areas that see repeating cells before the cold front passes. The MRGL risk extended southwest through the Ohio and Lower Mississippi Valleys to account for the scattered storm threat with isolated flash flood concerns within more urbanized settings and/or training convective signatures along the approaching cold front. ...Southwest into the Central High Plains... Scattered to widely scattered thunderstorms will persist across the Southwestern US, expanding into the Central Rockies and adjacent High Plains as our mid-level ridge orients more southwest to northeast with an extension over the above areas. PWAT indices will still be elevated as noted by the vast expanse of 1-2 standard deviations above normal PWATs within the latest NAEFS and ECENS output. This is a far cry from what we have experienced recently however, so the threat for organized convection will be subdued across the interior Southwest with a general flash flood threat relegated to the usual complex topography, burn scars, and any urban settings as convection will be slow-moving in nature. A more organized convective scheme is plausible over the portions of the Central High Plains as a mid-level shortwave is forecast to round the northern extent of the ridge and enter Southwest WY into western NE by Wednesday evening. A focused ascent pattern under the guide of the mid-level energy will help spawn a complex of thunderstorms across the High plains with a forward propagation to the south and southeast as it scoots into portions of the Front Range and adjacent parts of NE/KS. Some guidance is more bullish on the potential and was enough of a signal to allow for the previous MRGL inheritance to be expanded northeast. The threat is currently on the lower end of MRGL for the setup, but will bear watching as we get closer with CAMs support. Kleebauer 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