Quantitative Precipitation Forecast
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696 FOUS30 KWBC 281602 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1202 PM EDT Tue May 28 2024 Day 1 Valid 16Z Tue May 28 2024 - 12Z Wed May 29 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTH TEXAS... ...16Z Update... ...Southern Plains... The Moderate Risk area was adjusted largely based on impacts from this morning`s convection across the Dallas-Ft. Worth Metroplex. CAMs guidance is not overly excited about this afternoon`s convection. Once the convection gets going, which is widely agreed upon, it quickly becomes fast moving. Only a brief opportunity for training appears when the storms first form across the eastern Texas Panhandle and western Oklahoma this afternoon. The Moderate Risk was trimmed on the southern end of the inherited area as that region was completely missed by this morning`s convection. Instead, it was extended east, in coordination with FWD/Dallas-Fort Worth forecast office, to include the Metroplex. With ongoing flash flooding from this morning`s convection in the area, should additional storms, if fast-moving, move over the Metroplex again this evening, then flash flooding is likely. The current slow moving convection along an instability gradient over northwest Oklahoma near Woodward suggests that should the second round of storms this afternoon form as expected, they could cause flash flooding in that otherwise dry area. Thus, while this first round appears unlikely to cause significant issues (though isolated flash flooding can`t be ruled out), the Slight was extended north to include that region. A higher-end Slight was expanded to include much of east Texas with the current activity potentially paving the way for any late night convection that may try to make its way that far east...though at the moment the current activity is by far the greater threat. ...Southeast Florida... A small Marginal risk area was added to southeast Florida for this afternoon as convection may develop along the sea-breeze front and may remain stationary due to opposing flow aloft. ...Northern Idaho and western Montana... No changes were made with this update as the axis of heaviest rainfall expected there from late this afternoon through tonight remains in that same area. Wegman ...Previous Discussion... ...Southern Plains... Current UA analysis across the west shows a developing mid-level ridge over the Four Corners that will shift eastward through the period, eventually residing over west TX by midday. Aloft, a few mid-level perturbations are currently making progress over the southern plains with convective initiation ongoing just over the Red River in southwest OK. This will be a theme to the period as these smaller shortwaves will ripple through the northern periphery of the ridge and eject over the plains allowing convective flare ups to occur south of I-40 through the morning. By the afternoon, a stronger shortwave will traverse the southern plains with rapid convective development in-of a dryline situated from southwest TX up through the Panhandle. Ahead of the dryline to the east, a stationary boundary will delineate the axis of greatest instability available come the second half of the period with the theta-E ridge from north TX down through the Gulf Coast pretty much outlining the stationary front bisecting the state. Convective initiation to the west will allow for locally heavy rainfall and severe weather to ransack areas of west TX up into the Panhandle, but eventual cold pool propagation from area cells will congeal allowing convection to grow upscale and slide to the southeast along the periphery of the sharp theta-E gradient laid out by the stationary front. A more widespread heavy rainfall threat will occur over portions of north TX down through the central TX Hill Country within the path of the eventual complex. Despite some discrepancy in hi-res with the overall evolution of the convective pattern, there was enough agreement in the threat of widespread totals exceeding 2" with several CAMs indicating upwards of 5-7" within the core of the MCS path over central TX. HREF probability fields were yielding some impressive signatures for at least 3" of rainfall with neighborhood probs around 50-70% located from an area just south of Wichita Falls down through the Hill Country southwest of the DFW metro. Spotty 10-15% probabilities for upwards of 5" were also positioned in that same corridor, a reflection of the potential with an environment conducive for significant rainfall given the elevated PWATs (+2 anomaly) and available instability (3000-4000 J/kg of MLCAPE) that are necessary for enhanced rainfall rates. The area across central TX has been the hot spot for recent rainfall with local FFG`s much lower than climo for all 1/3/6 hour indices, another lean towards a higher impact potential due to soil saturation and limited recovery. The DFW metro is also an area of interest given the impervious surfaces within the city limits and formidable coverage expanding the outskirts of the urban corridor. The second round of convection expected in the period is still looking to be just to the west of the metro and as a result will be on the cusp of more significant impact, but we`ll be monitoring the anticipated evolution of the complex in the evening, as well as the morning convection expected to dip south from the Red River and maneuver over the metroplex. Pending the impact from the initial wave of thunderstorms in the morning, a greater emphasis may be placed on locations closer to DFW, but for now, will maintain the SLGT risk from previous forecast over the population center. As for the area to the west and southwest, a Moderate Risk was introduced for portions of north TX down through parts of the Concho Valley into the northern and eastern Hill Country with an eastward advancement into the I-35 corridor south the DFW metro. Some adjustments to the MDT are plausible pending convective behavior in the morning, but ensemble means and associated probabilities highlight the area encompassing the upgraded risk area. ...Northwest... Surface low generation with a trailing cold front will be positioned over the Pacific Northwest today with attendant height falls occurring to the west thanks to an incoming, strong shortwave trough well documented on the latest WV satellite. Increased upper level ascent focused within a broad axis of diffluence on the lead side of the mean trough will take aim over the interior Pacific Northwest with an area of convection forming in-of the northeast OR and the central ID mountains, likely due to weak instability ahead of the surface cold front and ample large scale ascent over the terrain. Scattered thunderstorms capable of rainfall rates bordering 0.5-1"/hr will allow for localized flooding concerns within the complex terrain over central ID up through the western portions of MT. Latest HREF probability fields indicate a small opportunity for some convective cores to breach the 1"/hr threshold, a significant rate for the area in question as the terrain complexities exacerbate any heavy rainfall threats, posing a heightened risk of flash flooding. Total precip forecast is relatively modest with guidance settling between 0.75-1.5" over the aforementioned area with the latest HREFpmm signaling multiple maxes within the confines of the Salmon River Mountains up through the western chains of MT. A MRGL risk was maintained from previous forecast. Kleebauer Day 2 Valid 12Z Wed May 29 2024 - 12Z Thu May 30 2024 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS, LOWER MISSISSIPPI VALLEY, MONTANA, AND NORTHWEST NORTH DAKOTA... ...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 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... 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