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Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
005 FOUS30 KWBC 021855 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Tue Jul 2 2024 Day 1 Valid 1850Z Tue Jul 02 2024 - 12Z Wed Jul 03 2024 ...THERE IS A SLIGHT RISK FROM PORTIONS OF THE UPPER MIDWEST TO THE PLAINS... ...Upper Midwest through Central Great Plains... 19Z UPDATE: In collaboration with WFOs in the region, a Moderate Risk has been issued, primarily over southern and central Iowa. This is primarily driven by increased confidence in extremely high rain rates. There continues to be relatively lower confidence in a training configuration to convection that would sustain those rain rates for more than an hour or two (although that cannot be ruled out, and would lead to localized much higher totals). However, the combination of very strong instability, with CAPE in excess of 2000 j/kg, and PWs near or in excess of 2 inches should support rain rates peaking in the 2-3 inch per hour range in the strongest convection. This would be enough to cause flash flooding, and the Moderate Risk essentially covers the area where those rates are most likely. Latest runs of the experimental WoFS model back this up with 90th percentile hourly rates peaking in the 2-3 inch range, and 90th percentile totals peaking above 5 inches in south-central Iowa. This would be likely to lead to flash flooding. PREVIOUS DISCUSSION: Hi-res models, even the new 12Z runs, do not appear to have a great handle on the rather dense cloud cover, and band of rain and embedded elevated convection, that stretches from LNK-DSM-MKE. Overall, they are far too quick to diminish this in favor of rapid destabilization and a more organized round of convection this afternoon. Therefore, it is a real possibility that they are overestimating both rain rates and total rainfall on the northwest and northern periphery of the Slight Risk area from northern Iowa into central Wisconsin. Nevertheless, given recent reinvigoration of some of the elevated convection, especially in southeast Nebraska, and out of deference to the previous outlook, the Slight Risk was largely maintained. Areas further north will generally be more sensitive to rainfall impacts given the recent wet pattern. However, the risk areas were reduced from central Wisconsin into northern Michigan, where even elevated instability seems likely to be far more limited. Therefore that draws attention slightly further south, particularly in southern Iowa and northern Missouri, but also adjacent portions of northeast Kansas and western Illinois. Even though hi-res models have struggled with ongoing clouds and rain, they have increasingly focused the rainfall in this area. HREF EAS probabilities, which better assess model agreement on placement of heavy rainfall, of 3 inches of rainfall are maximized in the IA-MO border region. A limiting factor may be forward progression of convective lines and clusters, which is suggested by most hi-res models. However, there will be some potential for backbuilding with southwesterly low- level inflow and an upstream reservoir of strong instability in the central Plains. Even if convection earlier in the afternoon is more progressive, the resulting outflow may set up a boundary more favorably oriented for backbuilding and training, and this may be situated in the southern portion of the current Slight Risk area in the evening or overnight, perhaps down to Kansas City or into central Missouri. Precipitable water values near or in excess of 2 inches and strong instability would support very high rain rates in excess of 2 inches per hour in organized convection. ...Southeast... The only major change to the outlook was to extend the Marginal Risk down further into the western Florida Peninsula including the Tampa metro area. The overall environment will be similar to areas further north that were already in the risk area (Florida Panhandle into south Georgia), with PWs in the 2.1 to 2.3 inch range (above the 90th percentile) and strong instability supportive of intense rain rates. This has already been observed this morning just south of Savannah, GA with a slow-moving storm estimated to produce 4 inch per hour rain rates on MRMS. The key for any flash flood potential would be a storm that is moving slow enough or any storm mergers or brief training along sea breeze boundaries. ...Southwest... The Marginal Risk was trimmed out of far northeast New Mexico and the Texas Panhandle where the QPF signal on the latest guidance was much weaker, and there may be a bit of subsidence in the wake of an MCV evident on GOES-16 visible satellite loops. However, the Marginal Risk was maintained over a broad portion of eastern Arizona, and southern and central New Mexico with another day of monsoonal convection expected. A Slight Risk was considered for south-central New Mexico where there appears to be a boundary that showed up briefly in enhanced west-east oriented cloud cover, and that coincided with confluent 700mb flow on the RAP analysis. For now, we held off, but that area will be monitored for a possible targeted upgrade early this afternoon if convective trends warrant. Lamers Day 2 Valid 12Z Wed Jul 03 2024 - 12Z Thu Jul 04 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST TO THE GREAT PLAINS... ...Midwest to Great Plains... A cold front from a passing trough over the northern tier of states stalls over KS/MO by Wednesday morning, providing renewed focus for thunderstorm development from eastern KS through southern IL with potential for heavy to excessive rainfall...with only minor adjustments needed to the placement of the Slight Risk already in place. The frontal zone extends east over the rest of the Midwest to the eastern Great Lakes where a Marginal Risk is maintained with some areas of the Northeast trimmed with the 02/00Z QPF consensus being farther west. Over the northern Plains, the next trough axis enters the northern High Plains Wednesday night with activity over the Dakotas warranting a Marginal Risk. ...Southwest... A trough axis from the next reinforcing wave allows anomalous moisture over AZ/NM to spread northeast to the southern High Plains on Wednesday which warrants maintenance of the Marginal Risk with some trimming based on the 00Z consensus. Bann Day 3 Valid 12Z Thu Jul 04 2024 - 12Z Fri Jul 05 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL US... ...Upper Midwest to Mid Mississippi Valley... Mid and upper level height falls across the upper midwest dropping into a region with anomalously moisture rich environment should result in increasing chances of showers and thunderstorms on Thursday and Thursday night...with embedded shortwave energy and divergence aloft from a digging upper jet focusing and supporting areas of heavy rain and chances for excessive rainfall from intense rainfall rates during the latter part of the outlook period. ...Mid-Mississippi Valley eastward... Moisture should start to be transported from the Mid-Mississippi Valley eastward across the Ohio/Tennessee Valleys into the Mid Atlantic region as flow aloft responds to the digging trough over the nations mid-section. Some guidance generates convection that may challenge flash flood guidance. There was already a Marginal Risk introduced and saw little reason to make more than minor adjustments in placement. Bann 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