Quantitative Precipitation Forecast
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326 FOUS30 KWBC 290117 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 917 PM EDT Tue May 28 2024 Day 1 Valid 01Z Wed May 29 2024 - 12Z Wed May 29 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS... ...01Z Update... ...Central High Plains through Texas... Semi-organized activity that developed along the dryline in west Texas has shifted east along with clusters of thunderstorms over the high Plains of CO/KS will continue to shift east overnight. Lack of instability over North Texas warranted removal of the Moderate Risk that extended over the Dallas-Ft Worth Metroplex. Despite vulnerability from heavy tstms this morning, confidence is great enough to limit the overnight risk to a Slight. CAMs continue to suggest overnight activity lasting past 12Z which may warrant a Slight Risk for the new Day 1 for after 12Z Wed. Organized activity over south-central Texas will continue shifting east and southeast overnight with enough progression to limit overall excessive rain risk. The Marginal Risk was expanded toward the lower Rio Grande Valley given the activity currently along and west of the Rio Grande. ...Northern Idaho and Western Montana... Trough axis over the PacNW will shift inland overnight with scattered thunderstorms continuing to develop ahead of the associated cold front from along the OR/ID border into western MT. Stretched the Marginal Risk a bit southwest to northeast given radar trends with cells along the OR/ID border producing 1"/hr rainfall estimates with some orientation in the direction of motion. 0.75" PW is about 1.5 sigma above normal which combined with MUCAPE of up to 1000 J/kg should allow continued redevelopment through this evening. Overnight activity should be largely west of areas over North and East Texas that had heavy rain earlier today. That said, the CAMs today have struggled with this western Texas activity so far, so a broad Slight Risk is still warranted from western Oklahoma down into south-central Texas. ...Southeast Florida... Isolated activity between Lake Okeechobee and the Miami metro should continue to wane through this evening, warranting removal of the Marginal Risk. Jackson Day 2 Valid 12Z Wed May 29 2024 - 12Z Thu May 30 2024 ... THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS, LOWER MISSISSIPPI VALLEY, NORTHERN UPPER HIGH PLAINS, AS WELL AS PORTIONS OF THE MID-ATLANTIC TO SOUTHERN NEW ENGLAND... ...2030Z Update... A new Marginal Risk area was introduced from central Ohio east to the southern New England coast, including much of the Mid-Atlantic and the DC to Boston I-95 corridor. Guidance shows several areas embedded within the Marginal where training cells may develop. However, none show particularly strong storms as part of the training. Given the sensitivity of much of the Marginal Risk area, and poor CAMs performance given decent instability expected Thursday afternoon and evening, think isolated flash flooding can be expected should training occur over any particularly sensitive area. No significant changes were needed to the inherited Marginal Risk areas. An expansion east to include more of the lower Mississippi Valley was added due to the latest model runs suggesting the heaviest rainfall totals may be centered around southern Arkansas/Arklatex region. Even there, 1 inch amounts appear to be spotty Thursday, and therefore given the high variability among the guidance, confidence still is not there for any upgrades. That said, this area could be considered a "high-end Marginal", in that should future runs remain consistent, a Slight may need to be considered given favorable antecedent conditions/low FFGs in the area. The Marginal over the northern High Plains remains largely unchanged, albeit with some east/southeast expansion based on latest guidance trends. Wegman ...Previous Discussion... ...Southern and Central Plains through the Lower Mississippi... Mid-level ridge axis across the Southern Plains will slide eastward over the course of Wednesday, but more convection will spawn upstream under the broad expansion of relatively prominent axis of theta-E`s that will advect as far northwest as the Front Range of the Rockies on the western flank of the surface ridge encompassing much of the eastern CONUS. Previous period convection will play a significant role in the prospects for more widespread thunderstorm coverage over the southern plains as remnant outflows and eventual positioning of the frontal boundary across TX will dictate where there could be more focused convection leading to better training and heavy rainfall potential. As of now, the coverage is relatively sporadic among most numerical guidance with a few outliers indicating some areas within east TX seeing sufficient rainfall that could allow for a targeted upgrade from the current MRGL risk. As of now, maintained continuity with enough of a general instability pattern situated from the Colorado Front Range down through the Lower Mississippi Valley to constitute some isolated flash flood threats with the best prospects likely closer to the elevated theta-E environment near the Gulf coast over into south-central TX near where the frontal boundary may lie. ...Montana and North Dakota... Diffluent pattern across the Northwest will allow for a continued threat of convection within the favored ascent pattern focused within a formidable jet coupling situated over northern MT back to WA state. The enhanced upper level dynamics in conjunction with sufficient surface forcing will promote a period of scattered strong to even potentially severe thunderstorms to propagate northeast out of western MT and southeast ID through central and eastern MT with the northeast expansion through northwest ND by the end of the forecast period. QPF forecasts are sufficient for flash flood concerns as recent ensemble means generally hover between 0.75-1.5" in the hardest hit locales. Considering the complex terrain of central MT, flash flood concerns are heightened, especially within the environment capable for rainfall rates breaching 1"/hr at times within the stronger cell cores, as indicated by the latest HREF probability fields (30-50% for >1"/hr). Recent NAEFS has come up with the regional PWAT anomalies situated over northeast MT into northwest ND with an area of 2-3 deviations above normal being depicted along the border. The evolution of the upper level pattern is formidable with the trough axis tilting negative during the period in question. It is plausible that guidance is underplaying the threat and there is an opportunity for an upgrade in future forecast updates, likely somewhere in eastern MT. For now, the previous MRGL risk was maintained with little deviation from the previous forecast. Kleebauer Day 3 Valid 12Z Thu May 30 2024 - 12Z Fri May 31 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... ...2030Z Update... There is a much more consistent signal for convective heavy rainfall across the central and southern Plains Friday as compared with Thursday. However, the placement of the heavy rain within the large Slight remains in poor agreement. A higher-end Slight is considered in effect for portions of North Texas, generally where the current Day 1 Moderate is in effect. While this area appears as likely to pick up heavy rainfall on Friday as much of the rest of the Slight, given the low FFGs around the Dallas-Fort Worth Metroplex, that area is under a higher threat due to the more favorable antecedent conditions for flash flooding. No significant changes were needed given the above described meteorological setup. Wegman ...Previous Discussion... Longwave pattern will remain highly amplified with a strong mid- level ridge positioned along and east of the Mississippi River with a quasi-zonal pattern situated over the plains back to the western US. A negatively tilted trough over the northern Rockies will pivot into Canada with a few mid-level vorticity maxima traversing over the northern plains off the southern base of the shortwave trough. The combination of relative vorticity advection to the north and several mid-level perturbations ejecting eastward out of the zonal pattern to the south will create a large area of favored ascent within the central and southern plains on Thursday afternoon through the end of the period. A large surface ridge over the eastern CONUS will expand back west of the Mississippi with the backside of the ridge allowing for a funnel of elevated theta-E`s to migrate poleward into the plains. The combination of the enhanced instability fields and incoming shortwave advection will generate a prominent areal extent of convection from the Front Range all the way across towards the Mid and Lower Mississippi Valleys. Congealing cold pools across the plains will likely develop at least one MCS in-of the central plains leading to widespread heavy rainfall and increasing concerns of flash flooding. Other areas within the confines of the central and southern plains will have other opportunities just based of general convective evolution within a favorable environment. Complexities arise when trying to decipher where the best cold pool convergence will reside, which is typical in setups like these where there is no one, true disturbance to key on. Regardless, ensemble means are downright bullish with quite an expanse of 1-3" totals spanning central and north TX up through the central plains as far north as eastern ND. A very large SLGT risk was inherited and maintained for now with an opportunity for an upgrade if there is a general consensus on exactly where the heaviest QPF footprint will reside. Kleebauer 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