Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Tue Apr 23 2024 Valid 12Z Fri Apr 26 2024 - 12Z Tue Apr 30 2024 ...Severe thunderstorms and heavy rain possible for the Plains and Mississippi Valley late this week through this weekend... ...Overview... The upper level pattern through this weekend will feature mean troughing over the west-central U.S. while a ridge builds over the East Coast. This pattern will result in varying degrees of unsettled weather over the western/central U.S. and fairly benign weather over the East. Of particular focus, a couple of potent shortwaves/embedded upper lows will travel through the base of the mean trough and eject northeastward across the Plains/Upper Midwest, triggering rounds of active and potentially hazardous weather across portions of the Plains and Mississippi Valley. The pattern will progress gradually early next week, with a northeastern Pacific upper low drifting toward British Columbia (with a broad trough to its south) while the trough associated with the second Plains system continues into the East and the East Coast upper ridge finally moves offshore. ...Guidance/Predictability Assessment... With guidance showing reasonable agreement for the leading central U.S. system, primary discrepancies involve the second system to emerge from the West into the Plains and then the Northeast Pacific upper low and surrounding flow. For the second Plains storm, the past couple ECMWF runs have been on the fast side of the spread while the UKMET has been slowest. With 00Z ECMWF-initialized machine learning models (MLs) also spread out for timing, preference remains with an intermediate timing that is closest to the GFS/CMC. The new 12Z ECMWF has adjusted slower to this favored solution. Meanwhile there is a fair amount of spread among dynamical models/ensembles and the MLs for the path and timing of the Northeast Pacific upper low along with its surrounding flow. The general theme is for moderately slow progression (but potentially some progression of leading energy), and away from the CMC/CMC mean that are quicker to bring additional energy behind this primary feature. Based on latest guidance comparisons among 00Z/06Z solutions, the updated forecast started with a composite of operational models early-mid period and then transitioned to a mix of models and 06Z GEFS/00Z ECens means along with phasing out the CMC by next Tuesday as it compared less favorably to other guidance over the Northwest. ...Weather/Hazards Highlights... Late this week, an upper level shortwave will move northeast across the Central U.S. and will be accompanied by a surface low pressure system. The surface low will move across the Central Plains and Upper Midwest on Friday, lifting a warm front north across the South-Central U.S. and pushing a cold front across the Central/Southern Plains and Middle/Upper Mississippi Valley. This system should produce showers and thunderstorms over portions of the Plains and Mississippi Valley, and the ingredients of instability and moisture will be in place for locally heavy rainfall totals. A broad Marginal Risk area remains in place for portions of the Plains and Middle/Upper Mississippi Valley in the Day 4 (Friday- Friday night) ERO. The southern tail of the trailing cold front may stall across the Southern Plains while the next system emerges from the West to develop another surface low over the Southern/Central Plains. The combination of this front's stalling, some signal for locally heavy rainfall, and wet antecedent conditions from either recent rain (eastern Texas) or rain forecast before Day 4 (Oklahoma/Arkansas) would appear to support a relatively greater flash flood threat from far northeastern Texas into Arkansas. Thus the Day 4 ERO plans to introduce a Slight Risk area over this region. Meanwhile the forecast pattern should focus another round of heavy rainfall across the Southern Plains and Middle Mississippi Valley. Within the Marginal Risk area being maintained for these regions in the Day 5 (Saturday-Saturday night) ERO, this afternoon's update proposes an upgrade to Slight Risk from north-central Oklahoma into northwestern Missouri. There is a reasonable signal from the guidance and general pattern evolution for heavy rainfall potential, albeit with some lingering uncertainty for specifics. The proposed Slight Risk incorporates the GFS/UKMET/CMC cluster that is slower than the 00Z ECMWF for the second Plains system, and is a rough middle ground among the ensemble guidance. In addition to heavy rainfall, severe thunderstorms will be possible ahead of the dryline in the warm sector of this system, and the Storm Prediction Center has highlighted portions of the Plains and Mississippi Valley in the Severe Weather Outlook for Friday through Sunday. Isolated to scattered severe thunderstorms will be possible and may bring a threat of damaging winds, large hail, and tornadoes. On the western side of the dryline, strong winds and low humidity will create and increased wildfire risk for portions of New Mexico, west Texas, the Texas and Oklahoma panhandles, and far southeastern Colorado. As the second Plains System lifts northeastward and the upper pattern shifts gradually eastward, the trailing cold front and its associated rainfall will push eastward as well. By next Tuesday the best focus for rain should be from the Northeast to the Lower Mississippi Valley. Totals should generally be lower than during Friday-Sunday but some locally heavy activity will still be possible. From the West into parts of the High Plains, mean upper troughing will allow precipitation chances to persist across much of the region through the end of this week into the weekend, but drier conditions will likely develop over the Southwest by Sunday. Precipitation will fall as rain at lower elevations and as snow at higher elevations. In association with the system emerging from the West, guidance is suggesting potential for anomalous moisture and low level upslope flow to produce a localized area of heavy precipitation (mostly rain) over the central High Plains. While this scenario is not unanimous, there seems to be enough of a cluster advertising significant totals to merit a Marginal Risk area centered over northeastern Colorado in the Day 5 ERO. The system approaching from the Northeast Pacific early next week may produce some light to moderate rain/mountain snow over the Northwest. Temperatures will trend above normal across most of the eastern half of the country late this week into the weekend as a warm front lifts northward and the anchoring system's cold front stalls ahead of the next system to emerge into the Plains by Sunday. High temperatures will likely reach the 70s and 80s over most areas, with 90s and potentially over 100 degrees possible for some locations in south Texas by Saturday. The warmth will be confined more to locations east of the Mississippi River by early next week as the second Plains system pushes its cold front eastward. Meanwhile cool highs of 5-15F below normal should progress across the West and into the northern Plains Friday-Sunday, followed by a warmer trend that may lift highs to 10-15F above normal over the Four Corners region and parts of the Plains by next Tuesday. Rausch/Dolan Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw