Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
755 FOUS30 KWBC 271558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Thu Jun 27 2024 Day 1 Valid 16Z Thu Jun 27 2024 - 12Z Fri Jun 28 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWESTERN U.S, AS WELL AS THE CENTRAL AND NORTHERN PLAINS... ...Central Plains... 16Z update... The greatest risk for excessive rainfall continues to focus across parts of Nebraska, Kansas, Missouri and Iowa where recent rains have raised soil saturation levels and lowered the threshold for flooding concerns. The Slight Risk that is already in effect required no changes. The latest hi-res guidance did show several hours of convection capable of rain rates of 1 to 2 inches/hour tracking across Nebraska and across central Oklahoma/southeast Kansas that were just outside of the area highlighted by the Marginal Risk. Therefore small adjustments were made in these locations to expand the Marginal. Campbell A cantankerous mid-level shortwave will eject east out of the central Rockies providing enhanced large scale forcing in-of the Front Range out through the Central Plains later this afternoon and evening. Return flow pattern on the backside of a surface ridge extension east of the Mississippi will allow for the advection of a moist, unstable airmass across much of KS/NE and points east with the help of a budding LLJ developing after 00z. Convective initiation across the Front Range through the western Sand Hills of NE will propagate eastward with upscale growth expected as it moves across the Plains, creating a swath of elevated QPF signatures in wake of the eventual complex. Recent QPF trends have favored a threat of 1-2" areal coverage with a maxima around 4" possible considering the environment in place. The primary focus is centered over north- central KS through the intersection of southeast NE and northeast KS where the current ensemble QPF footprint shows the heaviest precip axis, aligning well with the mean placement of the mid-level vorticity maxima as it propagates east through the above areas. Recent probabilities have showed a favorable axis of heavy precip potential, but capped in the higher potential due to the overall progressive nature of the expected complex. 00z HREF neighborhood probabilities for at least 2" were a general 20-40% with a max of 50% located around North Platte, NE and points southeast towards the NE/KS line. The 20-40% stripe runs west to east along the state line until out into northeast KS before the probabilities fall rapidly towards the converging state lines. There was less consensus in higher totals with a drop off to 10-15% for upwards of 3", so the threat will likely hit a relative maximum of 4" in those harder hit locations along the state lines. Some guidance also indicates another cell cluster forming across southern KS near Dodge City which could spell a secondary maxima if convection breaks properly. This is not shared amongst all CAMs, but was noticeable in the probability fields to warrant an extension of the SLGT further southwest, just to the north of DDC. The SLGT risk to the north was adjusted further west in its axis with the eastern fringes now just crossing the NE/IA/KS/MO state lines as more progressive deterministic members still hold the potential for the complex to outpace current thinking and make it towards the quad- state region prior to the end of the period. ...Northern Plains... 16Z update... The southward trend with the higher QPF has persisted with the latest guidance this morning. A very small adjustment was made to the Slight Risk area across southwest North Dakota to now cover extreme northern South Dakota. Otherwise, everywhere else is adequately covered by the appropriate level of risk for excessive rainfall. Campbell A robust shortwave trough is currently analyzed across western WA state with sights on the Northern Plains by later this afternoon. The aforementioned disturbance will pivot across the Northern Rockies, taking on a closed reflection along the Canadian Border around the longitude of Alberta and western MT. A strong diffluent signature downstream of the mean trough will create a focus of large scale ascent in-of the Northern Plains by Thursday afternoon creating a scattered to widespread area of convection sweeping across northeast MT through ND. The environment is somewhat favorable for bouts of heavy rainfall as pattern allows for moist air to be pulled poleward leading to PWAT anomalies between 1.5-2 standard deviations above normal centered over the northeast corner of MT through much of ND. QPF footprint across the northern plains to the Canadian border is stout with a swath of 1-1.5" totals being reflected in the blend and ensemble bias corrected QPF. Local maxima around 3" will be plausible in the setup as sufficient shear associated with the disturbance will lead to a primary supercellular mode upon initiation through the early evening with some upscale growth potential at nightfall as the LLJ ramps up and we see congealing cold pools across ND. Recent trends in the QPF have been more pronounced further south across the central and eastern ND plains with an axis of heavier rainfall now steadily making progress into west-central MN by the end of the period. The heaviest rates will likely occur further upstream where supercell generation will provide a significant punch in the areas they impact leading to more scattered high QPF outputs that will eventually be smoothed as the convective scheme shifts to more multi-cellular clusters and potential QLCS development. Recent probabilities for at least 2"/3-hrs were subdued over most of the region with the max around 30-40% centered over western ND where the supercell modes will be most common. This settled to 10-15% through much of the rest of the state as we move into the evening. This is another capped maximum evolution, but enough to warrant the SLGT risk from previous forecast with an extension further south and east to cover for the latest trends in more widespread convective coverage with embedded heavy rain signatures. ...Southwest... 16Z update...The Slight was expanded further west across southeast Utah has the latest guidance continues to show increase amounts and areal coverage of convection for this period. Elsewhere remains in good order at this time. Campbell Ridge will begin breaking down across the Southwestern U.S with the shortwave trough to the north squashing the height pattern creating more quasi-zonal flow across the Inner Mountain West. A litany of mid-level perturbations will ripple across the Great Basin towards the Central and Southern Rockies creating yet another period of convective initiation over the terrain under a relatively moist, unstable environment. Unlike previous periods, relative storm motions will be more prevalent, but the premise still stands for heavy rainfall likely anchored to the terrain with stronger cell cores capable of producing periods of steady rainfall running upwards of 1"/hr at peak intensity, in agreement with the 00z HREF signatures of scattered 15-20% probabilities for the inch per hour rates. Totals being depicted are within the realm of 0.5-1.5" with a maxima around 2" in any areas that see persistent thunderstorm impacts within the terrain. This is especially true for more of the interior west with the focus shifting across UT and western CO down through a good portions of NM. The primary concerns will be the complex terrain, slot canyons, and any burn scars remaining over portions of the west. The SLGT risk from previous forecast was expanded a bit to the west to account for the latest trends in guidance showing a heftier QPF footprint for those in eastern UT, and a touch to the east to encompass some of the harder hit locations in NM these past couple days, including near Ruidoso. Kleebauer Day 2 Valid 12Z Fri Jun 28 2024 - 12Z Sat Jun 29 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS, MID-MISSISSIPPI VALLEY, AND MIDWEST... ...Central Plains into the Midwest... Shortwave across the Central Plains from the previous period will eject eastward through the Midwest with a round of elevated convection likely over portions of IA/MO into IL through Friday morning. This is round 1 of flash flood potential as a secondary enhancement is forecast later in the day as another shortwave ejects east out of the Front Range and bisects the same areas that were hit the period prior. This next setup has more potential due to the addition of a cold front that will be progressing southeast out of the Upper Midwest thanks to the shortwave trough to the north perusing eastward with an associated low pressure and trailing cold front developing and sliding south and east through the period. The tandem of focused large scale ascent and surface convergence along the approaching cold front will aid in a re- development of convection across northeastern KS and northwestern MO, tracking slowly to the east-northeast up along the frontal boundary as the front trudges slowly through the Mid- Mississippi Valley. The mean flow will be shifting as well thanks to the primary trough being situated to the north. 850-700mb mean wind vectors are oriented close to, if not parallel to the boundary which would provide a higher threat for training/repeating convection within the best convergence location. This area has shifted within the last 12-24 hours with guidance now pinpointing the convergent focus around the KS/MO line, currently north of the Kansas City metro. The synoptic evolution currently depicted makes sense for that location to be the main area of interest as the nocturnal LLJ will be strongest across eastern KS with the northern apex of the jet converging right along the slow progressing cold front. The 850mb wind pattern between 00-06z on the latest deterministic yields a veering boundary layer profile with south- southwest winds orienting more southwesterly, running parallel to the height contours positioned over the Central Plains and Mid- Mississippi Valley. This is a classic signature for focused convergence with training potential within a surface boundary where there could be multiple hours of repeated development so long as the mid-level ascent pattern cooperates in tandem. The setup would eventually vacate to the east-northeast with some prospects further downstream, but there is more uncertainty on whether that will transpire. The setup is certainly there, so the SLGT risk inherited was maintained for a majority of the previous areas outlined. The biggest change was the removal of the Chicago urban area and surrounding suburbs as the best chance for flash flooding is now correlated with the morning round of convection and less so with the secondary enhancement. There is still a possibility this gets adjusted back, but the mean QPF has been cut back in recent ensemble depictions and was not enough to warrant a stay of the SLGT. The other change was the SLGT moving further west into KS to account for the initiation point of convection where repeating is possible Friday evening. ...Southwest... A weak mid-level perturbation will exit out of Sonora to the north with sights on the high terrain of Southeast AZ and the NM Bootheel. Sufficient diurnal destabilization with elevated mid- level moisture will allow for scattered thunderstorms to form near the Mexican border into the terrain adjacent and pivot into the above area. This setup is coincident to a Monsoon type setup where flash flood concerns are isolated, but non-zero and meet the low- end MRGL criteria for the threat. Totals up to 1" are plausible, but a few spots may see higher amounts due to overachieving convection. The previous MRGL was maintained with no changes necessary. Kleebauer Day 3 Valid 12Z Sat Jun 29 2024 - 12Z Sun Jun 30 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST, CENTRAL ROCKIES AND PLAINS, SOUTHERN PLAINS, OHIO VALLEY, AND NORTHEAST... Elongated MRGL risk is in place across a large chunk of the CONUS thanks to a sprawling surface ridge over the Northern Plains working in tandem with a surface low crossing through eastern Canada aligning a cold front and stationary boundary extending through the Northeast back into the Central and Southern Plains. The boundary will be the focal point of convection in whatever area it bisects with two main areas of interest. The first is across the northeast where a cold front will migrate eastward scattered to widespread showers and storms likely propagating through the Ohio Valley, eventually making headway into the Mid Atlantic as the surface low motions northeast into Quebec and the trailing cold front follows. This fairly reminiscent to what transpired yesterday with bouts of convection moving progressively through the region along and ahead of the surface front. This will be no different the setup will likely yield a SLGT risk somewhere if the current depiction holds. There is still a question of timing and location of the best large scale ascent, so decided a MRGL was sufficient for the time being with adjustments possible as we move closer. Over the Central and Southern Plains, differential heating with available moisture and a boundary overhead will be a focus of enhanced thunderstorm risks with locally heavy rainfall possible around the stationary boundary. A few mid-level perturbations sliding east out of the Western U.S will likely enhance some areas locally with a better risk of organized cell clusters and flash flooding potential. That seems to be centered over southern KS into northern OK currently, but that could certainly change pending the synoptic evolution of the frontal placement, and any mid-level energy. Another area to assess for future upgrades if the pattern holds. The remainder of the MRGL risk in place will be subject to a combo of shortwave placement and relevant instability available. That has the greatest potential over CO currently with scattered thunderstorms likely over NM. This is bordering on Monsoon-ish, something we will be getting used to in the coming days as the season is right around the corner. Fairly low to mid-grade MRGL for those areas currently, but will see if a targeted SLGT is necessary in future forecasts. 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