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FOUS30 KWBC 182005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 PM EDT Sat May 18 2024 Day 1 Valid 16Z Sat May 18 2024 - 12Z Sun May 19 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST/CENTRAL GULF COAST... ...16Z Update... Maintained continuity through a majority of the previously covered areas within the SLGT and MRGL risk areas. The prospects for flash flooding remain highest over the Southeast U.S with convergence along the cold front slowly progressing eastward through the period. The primary period will be through the 00z hour before the threat wanes as the best ascent moves away from the region and we see more isolated to scattered rainfall coverage with less prominence in the hourly rates. Further north over AL, a remnant MCV will move east-southeast into northern MS/AL by this afternoon providing a focal point for convective initiation during peak instability. The PWAT anomalies are fairly tame for a higher end threat, but some isolated spots could see 2-3" over the course of the afternoon with a risk of flash flooding while cells move overhead. Given the probabilities for 1"/hr running between 40-60% and 2"/hr around 10-15% at peak, the threat is certainly capped for any prospects for widespread flooding, so the MRGl risk in place will suffice for the setup. Across NC, antecedent conditions favor a better opportunity for flash flooding concerns with streamflow anomalies relatively high after the recent active pattern across the region. A backdoor front will progress to the south during the afternoon and evening with a convergent low-level ascent pattern developing from western NC through east-central portions of the state. Instability and PWAT anomalies are generally modest with scattered convection likely to form within the next few hours across the terrain west of the Triad. Some isolated totals will likely hit the 2-3" marker with perhaps a max around 4" based on the latest HREFpmm and higher-end deterministic, so the threat is certainly within reason, but the isolated nature of the potential leans this more on the cusp of a true SLGT risk and a MRGL. Considering the downward trend of some of the probability fields from the previous iteration of the ensemble, as well as collaboration with the affected WFO's, decided to maintain continuity to allow for consistent messaging and the threat being truly isolated. There is a potential for a short term upgrade, but not at this time. Kleebauer ...Previous Discussion... ...Southern Alabama/Georgia into the Florida Panhandle... Mid-level trough/weak closed low over the Lower MS Valley/Mid- South this morning will continue eastward across southern TN today and into the southern Appalachians by early Sunday. On its southeast side, a stream of mid- level impulses will move across the FL Panhandle and southern AL/GA today ahead of a cold front that will finally clear through the area by tomorrow. SW flow out of the Gulf will maintain relatively high precipitable water values (~1.75" or +1.5-2 sigma) into the region ahead of the trough and surface front, with more impressive 850-700mb moisture flux anomalies into the region (+3 to +4 sigma). With ongoing convection over southern LA moving eastward, expect this to continue past 12Z along the I-10 corridor from southern MS eastward through early afternoon. Thereafter, models suggest a loss of mid-level vorticity dynamics to sustain heavier rainfall but this may be underdone given the approach of the cold front with still sufficient moisture available. Slight Risk outline was generally constrained to the higher probabilities of >2" QPF in the 12-18Z period this morning. Broader Marginal Risk covers much of the rest of the Southeast into the Carolinas where isolated heavier rain rates are possible. Trimmed away the northern/northeastern side of the Marginal Risk where instability is low (or non-existent) on easterly flow. Fracasso Day 2 Valid 12Z Sun May 19 2024 - 12Z Mon May 20 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS, UPPER MIDWEST, AND COASTAL SOUTH CAROLINA/GEORGIA... ...20Z Update... The previous forecast necessitated very little change across the central plains and corn belt with the synoptic pattern very much on track from prior NWP iterations. The only change was the expansion of the MRGL risk further north into MN and now the western portion WI, mainly the region north of LaCrosse. Recent trends in the hi- res convective evolution downstream are the reason for the addition with much of the CAMs showing fairly robust coverage within an upscale growth regime that originates over SD into western MN. A tongue of elevated theta-E's is forecast to be advected into southern MN with the multi-cell cluster out of the Dakotas to pivot eastward along the leading edge of the instability gradient. 12z HREF signal for rates >1"/hr have risen substantially from previous output with a swath of 40-60% probabilities and some bullseye's eclipsing the 70% marker, a signal indicative of greater potential overall. The rates are likely to be capped, however as the best instability will lie south of the MN border by the end of the period when the convection makes its way to the MN/WI border. Regardless, shortwave propagation within the broad southwest flow regime will be enough to maintain the organized nature of the convection and provide some locally heavy precip across the northern Midwest. The ECMWF is still holding on to the primary threat residing within IA, but there seems to be a split with one batch in KS and another in MN. Considering the bit of uncertainty and with the environment favorable for heavy rainfall from KS and points northeast, kept the MRGL risk with an opportunity for a SLGT risk upgrade if CAMs latch on to a particular area with greater impact (ie: Kansas or MN/WI). An additional MRGL risk was added portions of coastal SC/GA. More on that in the sub-section below... ...Coastal South Carolina and Georgia... Backdoor front will be pressing south through the Carolinas the beginning of the period with a surface wave riding up along a cold front present across southern GA. A combination of focused low- level forcing from the north and onshore development near the SC/GA border will create a time frame for prime convective development in the overlap of the onshore regime and frontal convergence over land. The instability fields depict this setup very well with a sharp inflection in the MUCAPE fields with little to no CAPE north and west of the coastal plain, but a skinny area of 1500-2500 J/kg located from Charleston down to just south of Savannah. This is where the latest CAMs are pinpointing the best threat of convection and rates bordering 1-2"/hr with very high neighborhood probabilities showing up within the latest HREF for at least 1"/hr (60-80%) and reasonable up to 2"/hr (20-30%). The biggest factor will be the speed of the front to the north and where does the intersection point occur. Some are further into GA and others closer to Charleston, which is a major factor in the flash flood potential as the urban corridors will undoubtedly be the most susceptible. After some collaboration with the local WFO, the threat was great enough to warrant a MRGL, but likely on the lower- end of the threshold necessary with a focus on the population centers. Kleebauer ...Previous Discussion... ...Central Plains and Upper Midwest... Broad SW flow over the central Plains across the Corn Belt will bring in increasing moisture to the region on Sunday with a surface frontal boundary hung up over Iowa. Mid-level vorticity will exit the central Rockies and moves across the central Plains as precipitable water values rise to around 1.50" (+1.5-2 sigma) with some instability in the afternoon/early evening (500-2000J/kg) that could yield some 1"/hr rates (per 00Z HREF thru 00Z Mon) which are near FFG values over Iowa and nearby areas. The region has been fairly dry the past week (most of the region has had about 50% of the normal rainfall) so much of the rain will be beneficial, but given 1-hr FFG values near 1"/hr (between MCI- OMA- DSM), maintained the Marginal Risk outline for the region. Higher probabilities exist farther southwest into central Kansas (to where the Marginal was extended), but FFG values are higher as well (2"/hr). 12Z CAM guidance through 12Z Mon may shed a little more light on the higher rainfall potential over the entire 24 hours. Fracasso Day 3 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... ...20Z Update... The MRGL risk was maintained, but the orientation was changed from the previous forecast issuance considering the latest trends within the QPF fields in the bias corrected ensemble and ML guidance. The premise for the heaviest rainfall over the Sandhills of NE remains the same with that area mostly void of the risk area due to very high FFGs. Further to the south, models are beginning to focus an initiation for heavy rainfall across northeast CO into southwest NE where the FFG indices are more favorable, albeit not significantly to favor a flash flood potential with the current precip forecast. We still reside outside of the CAMs window, but the pattern is ripe for elevated convective concerns and locally enhanced rates within the diffluent side of the sharpening shortwave trough entering the high plains. The MRGL was maneuvered towards the eastern fringes of the Sandhill's down towards the KS/CO/NE border where the latest ML guidance continues to hint a QPF maximum where the frontal boundary will be focused. This extends back into northeast CO where the convective initiation is being depicted, as well as north of the Sandhill's in southern SD where the best QPF footprint outside the higher FFG areas is located. Kleebauer ...Previous Discussion... Upper pattern remains fairly similar to D2 across the Plains, with broad SW flow over the region. One wave will exit through the Upper Midwest as troughing over the West deepens via northern and incoming southern stream vorticity. Upper diffluence east of the Rockies will support widespread showers/storms over the central Plains Monday into Monday night along the stalled surface boundary, with sufficient moisture in place and sfc CAPE 500-1500J/kg. Heaviest rainfall per the ensembles lie over western Nebraska (Sand Hills) where FFG values are highest, so focused the area to the southeast and east into Iowa where rainfall on D2 may lower FFG values a bit (from already somewhat lower numbers). 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