Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 PM EDT Tue Apr 26 2022 Valid 12Z Fri Apr 29 2022 - 12Z Tue May 3 2022 ***Heavy rainfall becoming more likely for portions of the Northern Plains for the end of the week*** 18Z Update: The 12Z model guidance initially has an excellent depiction on the large scale pattern across the nation, with a multi-deterministic model blend working well as a starting point in the forecast process for Friday and into early Saturday. By the weekend, some timing and amplitude differences emerge with the Pacific Northwest system moving inland across the Rockies, with the CMC a bit slower and the GFS a bit faster than the consensus. For the low pressure system and front reaching the West Coast early next week, the CMC/ECMWF/ECENS are generally similar with its progression across the Intermountain West, whereas the GFS/GEFS is faster and the GFS loses ensemble support by next Tuesday. Therefore, there was a gradual increase in use of the ensemble means for the second half of the forecast period, and also maintained some previous WPC continuity. In terms of excessive rainfall, a small Slight Risk area was added to the experimental Day 4 outlook across portions of central South Dakota and south-central North Dakota to account for a better model signal for 1-2+ inch rainfall totals over relatively saturated ground conditions from previous heavy precipitation events. The previous forecast discussion follows below. /Hamrick ---------------------- ...Overview... An upper trough with its axis over the Intermountain West as the medium range period begins Friday will push eastward into the north-central U.S. through the weekend as it forms a closed upper low. A surface low is likely to consolidate in conjunction with the trough/low, spreading rain to much of the central U.S. while snow is forecast on the backside of the low across the northern Rockies with some chance of wintry weather spreading into the northern High Plains as well, although the details of precipitation amounts and placement remain uncertain at this point. Meanwhile, surface and upper lows will move slowly away from the Northeast over the weekend after spreading some light to moderate rain and snow across the area for the latter part of the week. Periodic unsettled weather is expected in the West as a series of upper troughs and frontal systems move through. ...Model Guidance Evaluation... Model guidance remains quite agreeable in keeping a low pressure system (both in the upper levels and at the surface) moving slowly over the Canadian Maritimes through the weekend. Additionally, models have generally converged on a solution with a trough initially across the Intermountain West developing into a closed low as it moves over the central/northern Plains and Midwest over the weekend, but with still some slight wiggles in latitudinal position. The associated surface low track is fairly agreeable as well, with the ECMWF perhaps on the western side and GFS runs on the southeastern side of the spread in terms of the 12/18Z cycle, but within typical medium range spread. The newer 00Z ECWMF and GFS show good clustering with the low through at least Sunday. There is also better consensus in recent guidance for the upper low to weaken and lift into Canada early next week. The early part of the WPC forecast was based mainly on a blend of the 18Z GFS and 12Z ECMWF and UKMET. Upstream, most guidance has an additional trough/low approaching and entering the Northwest over the weekend. The major outlier with this feature was the 12Z CMC, which has a much weaker shortwave and thus by around Sunday-Monday produces a pattern rather out of phase with other guidance, with strong ridging in the West. The newer 00Z CMC came in closer to consensus with this feature. But deterministic models as well as ensemble members show differences in handling that shortwave Monday and beyond as well as what could be strong energy coming in behind it. Thus the WPC forecast phased in the ensemble means by the latter part of the forecast period given the differences, particularly favoring the 12Z EC and CMC ensemble means that were pretty agreeable with a slightly slower trough in the West compared to the GEFS mean. ...Sensible Weather/Threats... The surface low pressure system will spread precipitation across much of the central U.S. late this week that should spread into the East over the weekend. Precipitation totals are likely to be enhanced across northern/central parts of the Plains near the consolidating surface low. Conditions will likely be cold enough for snow in the northern Rockies on the backside of the low, and there is once again some threat of wintry precipitation spreading into the northern High Plains as well. In portions of the Dakotas, precipitation may begin as rain and transition to mixed precipitation or snow, though details of these threats remain somewhat uncertain. Additionally, rain could become heavy to locally excessive from parts of the Plains into the Mississippi/Ohio Valleys for the latter part of this week. Precipitation chances are expected to spread eastward into early next week as the low slowly moves east, but linger in the central U.S. as well as the front there may stall. Elsewhere, rounds of light to moderate precipitation (snow in the higher elevations and rain in the lower elevations) are forecast across the Northwest as a series of fronts cross the region along with upper-level energy. The Northeast could see lingering precipitation late this week before the coastal surface low and upper low pull away. Temperature-wise, the general trend over the lower 48 will be for cooler than average weather in the northern tier and cooler than average in the southern tier. The northeastern quadrant of the country can expect below average temperatures by 5-15F on Friday but moderating near normal by early next week. Highs will remain colder than normal by around 15-25F across the north-central U.S. underneath upper troughing. Meanwhile, the central/southern High Plains should see warmer than normal temperatures by up to 20F on Friday before the upper ridge promoting the warmth is suppressed over the weekend. Slightly above normal temperatures should then stretch across much of the southern tier. Tate/Hamrick 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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