Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
011 FOUS30 KWBC 192005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 PM EDT Sun May 19 2024 Day 1 Valid 16Z Sun May 19 2024 - 12Z Mon May 20 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN AND CENTRAL KANSAS... ...16Z Update... An additional SLGT risk was added to the current update across portions of KS. More information is provided in the "Central Plains" sub-heading below... No adjustments were made to the northern periphery of the inherited MRGL risk across the Upper Mississippi Valley as models have maintained relative continuity in a few convective bullseyes across southern MN up to the MSP area. The other area with no change is across coastal SC down to southeast GA where convergence pattern is well-documented within the past several runs of guidance where a small section of the coastal plain between Charleston to just south of Savannah will see locally 2-4" of rainfall with some CAMs approaching upwards of 5". Neighborhood probability of at least 3" is running at 80-90% within the entire zone encompassed by the MRGL risk, so the prospects that someone will see elevated rainfall totals is high with the urban corridors in Charleston and Savannah as the focal points in the current MRGL risk area. ...Central Plains... Current satellite and UA analysis pins a shortwave trough ejecting to the northeast across eastern NE this morning with some organized convective clusters within the confines of the mid-level ascent pattern. Another shortwave is currently moving out of northern NM with sights on the central plains later this afternoon and evening. Convective initiation along the dryline positioned over the KS/CO border will occur after 19/20z with expected forward propagation through KS as we move into the evening. The increased mid-level ascent and Southwesterly flow east of the Rockies in the mid/upper levels will carry waves of vorticity across the central Plains into the Corn Belt today/tonight, culminating in a more coherent/robust vort max out of NE into Iowa overnight. Surface frontal boundary through CO/KS will lift back northward today as a warm front with increasing moisture in the boundary layer (precipitable water values ~1.50" or +1.5 sigma), leading to a severe weather threat over KS late this afternoon. Additionally, warm front lifting through Iowa will help spur some rainfall (though generally beneficial) atop relatively lower FFG values (compared to KS). 00Z HREF probs for exceeding FFG values run about 10-50% from KS northeastward to southern MN, roughly in line with the Marginal Risk outline and weighted toward late afternoon into the overnight hours as the stronger vort max moves through. Question will be the details of the mode of convection and how things play out downstream.ar under the influence of the shortwave moving out of NM, along with better instability fields downstream over central OK/KS into southern NE will lead to expansive convective growth and congealing cold pools to aid in large, organized convective pattern that will move through KS and northern portions of OK. Latest HREF probability fields for 1-2"/hr rates are running much higher compared to the previous forecast iteration with a solid area encompassing northern KS down to the KS/OK border eclipsing 60-70% chances for those rates reaching towards the base of the current hourly FFG`s in place over the region (~1.5-2"/hr FFG indices). Taking it further, the 3-hr FFG markers are generally around 2.5-3" as well with the probability fields now signaling upwards of 70% chances to exceed that range with an areal average between 40-60% over northern KS. Despite the forward propagation of the complex, the environment is ripe with elevated PWAT anomalies between 1.5-2 deviations above normal anticipated for the afternoon and evening hours with the advection of warm, moist air occurring during much of the forecast period thanks to a modest LLJ. With the signals increasing for heavy QPF in the means and with the alignment of the probability fields showing an uptick in the potential, felt it was necessary to upgrade portions of the central and southern plains with a SLGT risk. The Slight Risk encompasses much of north-central and central KS down to about Wichita where the best probabilities align. There is some uncertainty on the southern extension into OK as guidances is less adamant on the potential as you move closer to the OK/KS border, but the cold pool propagation from the developing complex will be the primary component as a further south push on the southern flank of any convection would likely enhance local rates south along I-35 to the OK border. This will be a near term trend that will be monitored in real time during the convective evolution. Kleebauer ...Previous Discussion... ...Coastal South Carolina and Georgia... Vort max over northern GA this morning will slip southeastward across the coastal areas this afternoon, yielding another chance for showers/storms as it passes overhead. Maintained the Marginal Risk for this area of coastal GA/SC beneath its path where surface convergence is greater. 00Z HREF probs of exceeding FFG values (~3"/3h) climb as high as 40% in a very small area where CAPE will be sufficient. Urban areas will be most susceptible. Fracasso 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