Quantitative Precipitation Forecast
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Issued by NWS
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973 FOUS30 KWBC 202018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Mon May 20 2024 Day 1 Valid 16Z Mon May 20 2024 - 12Z Tue May 21 2024 ...THERE IS A SLIGHT RISK FOR PORTIONS OF EASTERN NEBRASKA AND WEST CENTRAL IOWA... ...16Z Update... ...Western Corn Belt... Following the special ERO issuance for the Florida Keys (see below, which remains on track), the Marginal Risk in the Central Plains and Mid Missouri Valley was expanded slightly farther south and east. In addition, a Slight Risk was added from eastern NE on east into west-central IA (in collaboration with OAX/FSD/DMX). By 06Z tonight, the region will be ideally placed beneath the left- exit region of a robust 120kt 250mb jet streak located over NM. This area will coincide at the nose as a 50kt LLJ that will overrun a stalled frontal boundary position along and just south of the Slight Risk area. NAEFS SAT for 06Z tonight shows an impressive plume of 850-925mb mean specific humidity topping the 99th climatological percentile over eastern NE and western IA, as well as a growing footprint of >90th climatological percentile PWs throughout the region. 14Z HRRR soundings along the NE/IA border between 06-09Z tonight show highly saturated atmospheric soundings along with as much as 1,000J/kg of MLCAPE and roughly 40kts of sfc-6km shear to sustain blossoming clusters of thunderstorms. The concern is for training along the front tonight in areas of eastern NE and west-central IA where NASA SPoRT-LIS shows elevated 0-10cm volumetric soil moisture. This suggests soils are a little more sensitive and may struggle to take in hourly rainfall rates topping 1.5"/hr. Latest 12Z HREF sports 25-35% probabilities of 3-hr QPF > 3-hr FFGs between 06-12Z Tues within the Slight Risk area where storms could produce hourly rainfall rates as high as 2"/hr. Given these factors, the Slight Risk was added for this forecast cycle. With heavy rainfall likely to continue in parts of the region into early Tuesday morning, morning commuters may have to contend with flooded roads, particularly in lower lying areas and along creeks/streams. Elsewhere, given radar and satellite trends, opted to trim the Marginal Risk out of northern IL given the fast storm motions in the region. The MCV is advancing directly into southern WI, where some additional thunderstorm development may ensue this afternoon in areas already seeing thunderstorm activity this morning. This led to maintaining the Marginal Risk in southern WI for the 16Z update. Mullinax ---1330Z Special ERO Issuance--- ...Florida Keys... It has been an active morning in the Florida Keys where rainfall gauges have been filling up to as much as 4-7 inches thanks to training convection aloft. Surface convergence oriented WSW-ENE and healthy upper level divergence is working in tandem with roughly 1,500-2,000 J/kg of MLCAPE nearly 40 knots of surface-6km vertical wind shear according to the 12Z KEY sounding. The 1km and 6km wind barbs are out of the WSW and W respectively, which will keep the orientation of line of heavy thunderstorms over the Keys for several more hours. Hourly rainfall rates of 2-3"/hr will continue through late morning but eventually, instability will be exhausted and storms should dissipate later this afternoon. While the Keys typically drain excessive rainfall well, high tide is approaching and can affect drainage. Given these factors, a Marginal Risk was issued for the Florida Keys. Mullinax ---Overnight Discussion--- ...Central Plains into the Mid Missouri Valley... The amplifying mid to upper level trof pushing east across the Central to Northern Rockies will ignite the next round of active Central Plains convection late Monday afternoon, continuing into the early hours of Tuesday. Overall favorable conditions for active convection with strengthening of the low level flow, strengthening boundary layer convergence and favorable upper difluence in the vicinity of the frontal boundary forecast to stretch from the lee of the Central Rockies, east northeastward into the Upper Mississippi Valley. There is spread among the latest hi res guidance as to whether this next round of convection will consolidate into one distinct line as per the ARW, or have more than one round of convection moving along the frontal zone as per the NAMNEST, FV3LAM and ARW2. However, there is consensus that either way, the convection will be fairly progressive to the east- northeast, and continue to be a detriment to very heavy rainfall amounts. We have kept the risk level at marginal given the expected quick movement. This is reflected in HREF neighborhood probabilities for 1"/hr amounts that are in the 20-45% range in the marginal risk area, but much lower, mostly less than 15% and more geographically spotty for 2"/hr amounts. ...Southeast Wisconsin into northeast Illinois... The well defined MCV moving into the Lower Missouri Valley early this morning is expected to remain intact as it pushes northeastward into the Upper Mississippi Valley and Upper Lakes region Monday into Monday night. While there may be weakening to the current organized convection associated with this MCV early day 1, there is the possibility for additional convective enhancement ahead of the MCV late Monday morning into afternoon across southeast Wisconsin into northeast Illinois. No significant changes made to the previous marginal risk area with concerns primarily for urbanized runoff issues from Chicago, north through Milwaukee and Green Bay. Oravec Day 2 Valid 12Z Tue May 21 2024 - 12Z Wed May 22 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MISSISSIPPI VALLEY... ...2030Z Update... Today`s 06/12Z ensemble and deterministic guidance has placed the axis of heaviest QPF a little farther north and west than where the 12Z HREF mean has its heaviest QPF. Recent trends have been for a more amplified solution, even on recent CAMs, that would suggest a slightly more westerly orientation of the heavier QPF. As the storm deepens Tuesday, the IVT will reach highly anomalous levels for late May, surpassing 750 kg/m/s at 18Z over western IA. PWs over western and central MN will top 1.5" between 18Z Tues - 00Z Wed and sampled soundings in the area sport warm cloud layers deeper than 9,000ft. While instability on the northwest flank of the storm will be hard to come by, the efficient warm rain processes involved can still produce steady 1-1.5"/hr rainfall rates beneath the deformation zone over multiple hours Tuesday afternoon. A similar situation occurred last week in southern New England where instability was lacking, but highly saturated/deep warm cloud layer environments still supported efficient rainfall rates that caused flooding. In that same case, over 4" of rainfall occurred in southern New England and those kind of totals are being modeled in both CAMs and deterministic guidance. The 12Z HREF does depict low chances (20-35%) for localized rainfall totals on Day 2 that could surpass 5" in portions of central MN. Should probabilistic guidance increase further in the coming shifts, there is a scenario where a Moderate Risk may need to be considered. At the moment, given most soils are not highly saturated and there remains some uncertainty on which areas see the heaviest rainfall, maintained a Slight Risk as the highest threat level. Did extend the Slight Risk south to include more of northern IA where their soils will be more sensitive following the thunderstorms rolling through Monday night and early Tuesday morning and another round of storms ahead of an approaching cold front could bring more flash flooding opportunities Tuesday afternoon and evening. Mullinax ---Previous Discussion--- A strong vort ejecting northeastward on the eastern side of the amplifying Rockies trof late day 1 into day 2 will become increasingly negatively tilted as it pushes northeast across the Central Plains into the Upper Mississippi Valley. This will help deepen surface low pressure across the eastern portions of the Central Plains early day 2, moving northeast into the Upper Mississippi Valley/Upper Lakes region later day 2. There is pretty good consensus for an axis of heavy comma head/deformation precip from far eastern South Dakota, across large portions of Minnesota and northern Wisconsin. Consensus is for two well defined areas of precip to push northeastward across the Upper Mississippi Valley into the Upper Lakes, comprised of a comma head/deformation precip area on the northwest side and a more progressive convective precip area on the southeast side. Similar to day 1, the expected progressive convection on the southeast side of this system will be a detriment to very heavy totals, while slower moving/pivoting comma head precip should support potential for heavy total rainfall amounts, although rates may not be very high. Stream flows as per the National Water Model, remain high where the heavier comma head/deformation precip is expected and correspond to the slight risk area. There were only some minor changes made to the previous slight risk area, decreasing it on the southeast side over southeast Wisconsin into northeast Illinois where the convective events are expected to be progressive. Oravec Day 3 Valid 12Z Wed May 22 2024 - 12Z Thu May 23 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST OKLAHOMA, ACROSS THE LOWER ARKANSAS VALLEY INTO THE MID MISSISSIPPI VALLEY... ...2030Z Update... Adjustments to the threat areas this forecast cycle were to expand the Slight and Marginal Risks areas farther south where greater moisture content (up to 2" PWs) and instability (1,000-2,000 J/kg MLCAPE) looks to be present. This setup also takes place beneath unusually strong WSW 250-500mb mean winds aloft, along with an IVT above the 90th climatological percentile that is intersecting the aforementioned frontal boundary Wednesday afternoon and into Wednesday night. With modeled soundings also depicting warm cloud layers as deep as 12,000-13,000ft Wednesday afternoon, this setup has a higher ceiling for potential flash flooding and will be closely monitored in subsequent forecast cycles. For now, no consideration of a Moderate Risk today given the lingering uncertainty on QPF and the axis on where the heaviest amounts occur. Mullinax ---Previous Forecast--- The trailing frontal boundary from the low moving northeastward from the Upper MS Valley into the Upper Lakes region day 2, will slow significantly on the southern end from the Southern Plains into the Lower Arkansas and Mid Mississippi Valley during day 3 as it becomes aligned more parallel to the southwest flow aloft. PW values expected to rise to 2+ standard deviations above the mean along this slowing frontal zone, supporting potential for heavy rainfall total from southeast Oklahoma, northeastward through the Ozarks of Arkansas and Missouri and toward the Mid Mississippi and Lower Ohio Valley regions. Shortwave energy embedded in the southwest flow aloft will enhance uvvs in this anomalous PW axis Wednesday afternoon continuing into the early hours of Thursday. Training of cells parallel to the boundary is a concern for heavy to locally excessive rainfall totals day 3 with the slight risk drawn for the max qpf consensus and across regions where soil moisture and stream flows are high. Changes to the previous outlook were to narrow the slight risk area to better match current qpf consensus and best region for potential training of precipitation areas. Oravec 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