Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Fri Apr 12 2024 Valid 12Z Mon Apr 15 2024 - 12Z Fri Apr 19 2024 ...Deepening Plains surface low likely to bring severe and heavy rainfall threats ahead of it and strong winds and notable snow behind... ...Overview... A closed low over the Southwest U.S. to start the period Monday will progress eastward and support a deepening low pressure system over the Plains by Tuesday. Various hazards are likely with this storm system, including severe weather and bands of heavy rainfall on the warm side along with a broad area of gusty winds. Behind this storm, another upper trough digging down from British Columbia into the Northwest U.S. early next week should ultimately evolve into a broader positively tilted mean trough from south-central Canada through the West. Expect this trough to bring colder temperatures and multiple days of snow potential especially to the northern half of the Rockies/High Plains. Meanwhile upper ridging over the Mississippi Valley on Monday will shift eastward and settle near the East Coast Tuesday-Thursday, leading to a corresponding shift for above normal temperatures. ...Guidance/Predictability Assessment... Model guidance remains reasonably agreeable regarding the upper low moving from the western to central U.S. to the Midwest Monday- Wednesday, as well as the associated strong cyclogenesis at the surface. More model differences arise upstream with the upper trough/possible closed low dropping into the Northwest by Tuesday and east as the week progresses. By Wednesday 00Z CMC and ECMWF runs indicated this feature eastward closer to the first low, not amplified as south into the Northwest as GFS runs. EC and GEFS ensemble members generally fit their deterministic counterparts. However, the ECMWF-based AI models were more like the GFS/GEFS than the operational ECMWF suite and this provided some evidence to lean more toward the GFS/GEFS direction. These differences make the QPF somewhat uncertain. The GEFS/GFS suite provides more forcing for snow in Montana and the vicinity. Also, this forecast trended toward a slower movement of QPF with a southward displacement in the Midwest/Great Lakes/Ohio Valley compared to the previous forecast. Now in the newer 12Z runs, the GFS does not dig as far south as its previous run Tuesday-Wednesday, but the ECMWF is even farther north and by the latter part of the week has a negatively tilted trough/low as it evolves differently. There are also differences upstream with whether upper ridging comes into the Northwest or stays out in the eastern Pacific depending on how much energy lingers. Overall, there is still ample spread with the details of the pattern. Thus the WPC forecast used a multi-model deterministic blend early on but reduced the deterministic components (particularly the ECMWF and CMC) in favor of the GEFS and EC ensemble means, with the means over half the blend by Days 6-7 given the model differences. ...Weather/Hazards Highlights... Precipitation will be increasing in coverage across the Rockies/Central U.S. with multiple weather hazards as the upper low helps to deepen a Plains surface low Monday-Tuesday. The Storm Prediction Center is already highlighting areas of severe potential for the Plains on Monday and for the Lower to Mid- Mississippi Valley to western Ohio Valley on Tuesday given the abundant instability and moisture present with this system. These ingredients could also produce heavy rainfall with intense rates capable of flash flooding over parts of the Plains and the Midwest. Guidance continues to show a fair amount of scatter with the details, and some areas have dry antecedent conditions that could limit the potential, but the overall threat remains sufficient to show very broad Marginal Risk areas on both the Days 4 and 5 (Monday-Tuesday night) Excessive Rainfall Outlooks. These stretch from the south-central Plains/Mississippi Valley where potentially severe convection could produce heavy rainfall rates, to farther north across the northern half of the Plains into the Upper Midwest where convection pivoting closer to the low center could move more slowly for possibly a longer duration of heavy rain. An embedded Slight Risk could be added if and when models narrow in on a focus for heavy rainfall, especially across parts of the Upper Midwest on Tuesday, but for now there is still too much spread to be confident in the placement. Behind the low, the storm should produce a broad area of gusty winds. These winds will particularly focus over the southern/central High Plains Monday along with wildfire concerns, and stretch into the north-central High Plains Tuesday. The next upper trough digging into the western U.S. and eventually extending into the northern Plains early-mid week, along with another frontal boundary, should tend to focus precipitation over the northern half or so of the Rockies and High Plains for a few days. Thus meaningful snow will be possible over at least the higher elevations, and some snow may extend into the High Plains by midweek as colder air reaches the region. Blowing snow is also a concern that may cause impacts. Farther south, by around Thursday or so, the surface front ahead of this trough may become parallel to the upper low over the south-central to east-central U.S. The combination of decelerating progression and some Gulf inflow may lead to increasing rainfall near the front around that time. Plains/Mississippi Valley upper ridging will support well above normal temperatures across the central U.S. into Monday with some northern/central areas seeing highs and lows 20-25F above normal. Thereafter, the pattern evolution will lead to a broad area of above normal temperatures over the eastern half of the country into the southern Plains, with advancing frontal systems slowly trimming the western side of the warmth. Expect highs in the eastern U.S. to be up to 10-15F or so above normal with anomalies for morning lows tending to be several degrees higher. The southwestern U.S. will see below normal highs through Monday with the upper low crossing the region. Then the upper trough digging into the West and then including the northern Plains will likely start to bring below normal highs into the Northwest early next week, followed by readings 10-20F below average over the northern Rockies/High Plains by next Wednesday-Friday. Less extreme cool anomalies may extend farther southwestward. Tate/Santorelli 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