Quantitative Precipitation Forecast
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Issued by NWS
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814 FOUS30 KWBC 192324 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 724 PM EDT Sun May 19 2024 Day 1 Valid 01Z Mon May 20 2024 - 12Z Mon May 20 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF KANSAS & OKLAHOMA... ...Central Plains... A shortwave is inbound into the central Plains. Convective initiation has begun across KS, with more activity upstream across eastern CO and less numerous activity across northern OK. The convection is quite well organized, with mesocyclones apparent. Precipitable water values approaching 1.5", effective bulk shear of ~50 kts, and MU CAPE of 4000+ J/kg will lead to organization, and eventual "squalling out" of the organizing convection into a forward propagating arc in several hours. Before then, hourly rain totals to 2.5" with local amounts to 4" are expected in this environment, particularly where cells train. The 3-hr FFG markers are generally around 2.5-3", which should be exceeded, so maintained the inherited Slight Risk but shifted it south based on radar reflectivity and 18z HREF trends. For southern portion of the depicted of the Slight Risk area, there could be multiple rounds, or worse, some hanging up of the southwest flank of the surface boundary which could prolong issues more than anticipated. This eventuality bears watching. Roth Day 2 Valid 12Z Mon May 20 2024 - 12Z Tue May 21 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS/CORN BELT, NORTHERN ILLINOIS, AND SOUTHEAST WISCONSIN... ...20Z Update... An additional MRGL risk was added to the western side of Lake Michigan extending from Green Bay down to Chicago and surrounding suburbs. More information on this setup included in the sub-heading below... As for the inherited MRGL, an expansion to the northeast was made to include more of northwestern IA and the southwest portion of MN. Strengthening upper level forcing and accompanying surface reflection will provide ample support for a swath of heavy precip to stream northeastward within a broad axis of diffluence. The biggest change was a bit more progressive evolution of the surface low with a strong vorticity maxima ejecting out of NE into the northern Midwest towards the end of the forecast period. Widespread convection is being depicted within many of the CAMs in the 12z suite with some hints of the potential already given via multiple ML algorithms over the past few days. A few of the models are still holding back on the threat within the area added, but this is becoming more of the outlier with the probabilistic data now supporting the expanded threat into IA/MN. These areas will have seen rainfall in the previous period as well, further priming the soils with streamflows continuing to be elevated based on the latest river data available. This area will ultimately be impacted further on D3, so this is just a quicker time scale that guidance is catching up on with the threat. Expect a general 1-2" with locally up to 4" possible across the outlined MRGL risk area. ...Northern Illinois and Southeast Wisconsin... Mid-level vorticity maxima over KS will continue to propagate to the northeast from the D1 period into the front half of D2 with some additional strengthening of the energy anticipated given the latest hi-res deterministic and ensemble suite. The trend has risen for more widespread convection to impact portions of the Midwest with its approach due to the vigor and increasingly favorable thermodynamics as a warm front lifts north of the region and places the area directly within the warm sector. HREF blended mean QPF has jumped from near 1" to above 1.5" now in spots with some individual CAMs signaling the threat for 2-3" for places within the urban corridor extending from Chicago to Milwaukee. 12z HREF neighborhood probabilities have also spiked with at least 1" total precip now over 90% for a majority of southeast WI into the northeast corner of IL along I-94. 2" probabilities are closer to 40-50% which gives you a general bound of what is to be expected through the course of the period with only some minor probabilities (10-15%) for up to 3". This grants a non-zero threat for higher impact, but generally looking at localized flooding within the urban centers along Lake Michigan. Considering the latest trends, have added a MRGL risk area along the western Lake Michigan area encompassing the region from Green Bay down to Chicago and surrounding locales with lower FFG indices. Kleebauer ...Previous Discussion... ...Central Plains and Corn Belt... Similar upper pattern over the central US on Monday but with changes upstream over the West. A southern stream shortwave will race through the southern Rockies just ahead of a digging northern stream trough into the northern Great Basin. Likely convection late Sunday/early Monday may leave the area rather stable for the daytime Monday, though the lingering frontal boundary will act as a focus for some rainfall into the afternoon. Any heavier rainfall may hold until overnight (00-12Z Tue) as the shortwave exits CO and the LLJ increases. Heavier rainfall may lie over the NE Sand Hills where FFG values are much higher, so the Marginal Risk outline is on the smaller side to the south and east of this region for any overnight isolated heavier rainfall as surface cyclogenesis starts in earnest. Fracasso Day 3 Valid 12Z Tue May 21 2024 - 12Z Wed May 22 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST... ...20Z Update... Current synoptic evolution and expected heavy rainfall footprint across the Upper Midwest is still on target with little run-to-run deviation from previous forecast issuance. Extended the area SLGT a touch further to the south over IL given the indications of a bit more warm sector convection that would allow for localized enhancement of QPF within any prominent convective cores that are established over the region. There`s some instances where the convective threat may extend further west into IA pending the eventual evolution of the surface low and track of convective activity shuffling out of the central plains. The highest threat for flash flooding will reside within the strong axis of deformation to the north and northwest side of the surface low, as well as more defined convective elements within the established warm sector extending from western and central WI down into IL. A cold front will move eastward out of the plains with more thunderstorms likely to form along the frontal boundary as we shift towards the end of the period. This will translate into another active setup for D4 and could be more prevalent for the period if the convective enhancement begins a bit earlier than currently forecast. For now, the MRGL was maintained for the southern edge of the cold front with the SLGT risk firmly outlining the deformation zone and warm sector. Kleebauer ...Previous Discussion... Robust southern stream shortwave over eastern CO Tuesday morning will head northeastward into southwestern MN by early evening as surface low pressure deepens below 990mb over northeastern MN at the end of the period. Strong southerly flow in the BL (850mb winds to 50kts) will help surge moisture northward into and around the low, with precipitable water values climbing to over 1.50" which is about +2 sigma and 850-700mb moisture flux anomalies > +5 sigma from MO northward to MN/WI. QPF mode will be deformation-driven on the NW side of the wrapped-up/occluded low where rates will be modest but perhaps exceeding longer-time FFG values vs warm-sector QPF (MO northward/northeastward) where instability will be present (CAPE > 2000 J/kg as far north as northern IA). The entire system will be rather progressive, but expansive. Focused the Slight Risk on the warm sector rainfall and nosed back toward eastern SD where FFG values are lower than points farther west (covered by the Slight Risk). Models show various axes of QPF tied to different forcings in the evolution of the system, which is covered by the broad Marginal Risk outline eastward to Lake Michigan. AI guidance was displaced a bit to the east of the dynamical parent models, but this was also reflected in the ensemble systems as well (GEFS/ECMWF/GEPS). QPF spread was largest well into the warm sector (MO into the Ozarks) where the flash flood threat may be more isolated depending on the convective evolution. Fracasso 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