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Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
743 FOUS30 KWBC 031548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1148 AM EDT Wed Jul 3 2024 Day 1 Valid 16Z Wed Jul 03 2024 - 12Z Thu Jul 04 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS EASTWARD TO THE OHIO VALLEY|... ...16Z Update... Previous SLGT risk forecast remains on target with an elevated flash flood risk positioned across southern KS through southern MO with an emphasis over south-central and southeastern MO. Remnant outflow from nocturnal convection is currently analyzed across southern MO and will be the focal point for afternoon and evening convection with heavy rainfall likely given the antecedent environment. PWATs over 2" are aligned along and ahead of the quasi-stationary front across KS into MO, as well as within the confines of the aforementioned lingering outflow. 12z HREF neighborhood probabilities are favorable for at least 3" of rainfall across much of south-central MO to the KS/MO state line with values settling between 45-70% within that corridor. This correlates well with the elevated probabilities for at least 2"/hr rain rates suggested within the latest hi-res ensemble suite (25-40%). The above area will run on the higher end of the SLGT risk threshold with a non-zero chance at a targeted upgrade later pending how convection evolves. For now, the signals are strong enough for the current forecast, but a bit under what we would want to see for a higher forecast risk area. The SLGT risk was extended through portions of the Ohio Valley as the quasi-stationary front will be a target for convective development later this evening within a secondary tongue of elevated theta-E`s being advected northeast into southern OH. CAMs are leaning towards the possibility of scattered heavy rain signatures within areal convective development. The signal for at least 2" of precip within that area of OH has risen since the last update, indicative of the evolving pattern with favorable thermodynamics overlaid across that portion of the Ohio Valley. Rates between 1-2"/hr are plausible within this corridor, enough to be a signal for 1hr FFG exceedance within the latest hi-res ensemble. This was enough to necessitate the extension through portions of southern OH and adjacent KY. There are no changes to the MRGL risk area across the Southwest as a monsoonal setup is still forecasted to evolve later this afternoon with isolated flash flooding possible, especially in complex terrain and burn scars present. ...Previous Discussion... ...Mid Mississippi Valley and Great Plains... A cold front from a passing trough over the northern tier of states will stall in the vicinity of Kansas and Missouri later today, providing focus for renewed thunderstorm development from eastern Kansas and southern and central Missouri eastward into the Lower Ohio Valley. Except for a few adjustments based on the latest suite of global and ensemble guidance from the 03/00Z model runs...there Slight Risk area was largely unchanged from the previous issuance. The guidance still depicts strong instability (CAPE in excess of 2000 j/kg) and PWs in excess of 2 inches (above the 95th percentile) combining to support organized convection with rain rates exceeding 2 inches per hour at times combining with the potential for cell training or backbuilding. Elsewhere the Marginal Risk areas were generally maintained in the Upper Ohio Valley and Northern Plains where low chances of heavy rain rates and flash flooding persist. ...Southwest... Continued to trim the Marginal risk area as mid- and upper-level heights continue to build into the region....helping to suppress late day and evening convection even further. Bann Day 2 Valid 12Z Thu Jul 04 2024 - 12Z Fri Jul 05 2024 ...Upper Midwest to Mid Mississippi Valley... The environment should become increasing supportive of showers and thunderstorms as mid- and upper-level height falls spread across the Upper Midwest into a region with deeper moisture, The rain threat will be organized by a developing surface low pressure and an inverted trough axis extending to the north. There has continued to be multi- run shifts over the past 24-48 hours to the south with the cyclone, and QPF guidance has shifted south as well. Given a lack of convergence on the specific location of heavy rainfall thus far, the Slight Risk has been maintained with yet another shift to the south but not as much as before. Another key uncertainty is the level of instability. Models are only showing modest instability, generally with CAPE near or below 1000 j/kg, and PWs are not that anomalous, with the primary plume of high PW air pushed south by a cold front in previous days. Therefore, the threat of flash flooding in this case would likely come from two key factors: (1) sustaining moderate to occasionally high rain rates for longer durations (3-6 hours) and (2) the unusually wet ground conditions across the region increasing vulnerability. Therefore, pinpointing confidence in placement of the low-level cyclone and associated deformation axis will be important for any future shifts in the Slight Risk. ...Southern Plains and Mid-Mississippi Valley into the Ohio Valley and Mid Atlantic States... The plume of deeper and more anomalous moisture will be in place farther to the south, in the vicinity of the stalled front. PWs are likely to generally exceed the 90th percentile along the entire front from OK and KS eastward to the Mid-Atlantic coast. The Marginal Risk was generally maintained in this area, and expanded slightly in the Plains and Mid-Atlantic. No major changes were made at this time, as corridors of greater chances of flash flooding would be very dependent on convective evolution from the previous day (current Day 1 period). However, where sufficient instability exists, moisture levels will be high enough to support rain rates approaching or exceeding 2 inches per hour in the most organized thunderstorms and this may lead to flash flooding. Bann Day 3 Valid 12Z Fri Jul 05 2024 - 12Z Sat Jul 06 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN AND EASTERN UNITED STATES... Corridors of higher chances of heavy rainfall will continue to be aligned with a quasi-stationary front extending from the southern Plains into the Southeast US on Friday where higher values of precipitable water should be pooling in proximity to sufficient instability. With weaker mesoscale forcing in the Southeast US...exactly where convection develops is less certain so followed the better clustering shown by spaghetti plots of QPF from the GEFS/SREF as well as NCEP global runs and the ECMWF. Over the portions of the Southwest...a renewed risk of locally heavy rainfall develops late in the period as low level winds draw Gulf moisture upslope and is aided by some isentropic lift as the low rides up and over the tail end of the stalled front. 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