Extended Forecast Discussion NWS Weather Prediction Center College Park MD 317 AM EDT Mon May 19 2025 Valid 12Z Thu May 22 2025 - 12Z Mon May 26 2025 ...Overview... Hints of an omega block set up across North America later this week, as ridging is likely over the southwestern to central U.S. anchored by an upper high in central Canada, while rounds of troughing move through the Northwest and an upper low spins over the Great Lakes. The eastern trough will promote cooler than average temperatures and rain in the Northeast, while the ridge leads to warmer than average temperatures in the southwestern quadrant of the lower 48. Frontal systems emerging from the West toward the Plains and moist inflow ahead of them will allow for increasing rain and thunderstorm chances in the central U.S. from late week into the weekend. ...Guidance/Predictability Assessment... Model guidance shows reasonably good agreement with an upper low over the Great Lakes and energy transferring toward a coastal surface low in the western Atlantic as the period begins Thursday. A multi-model blend sufficed for these features, though minor discrepancies in the low track would yield QPF differences in the Northeast that could be notable. Models agree that the upper/surface lows should move slowly northeast and gradually lift the troughing out of the Southeast at least. The upper ridge behind this troughing also seems well predicted by the models, moving from the southwestern to south-central U.S. with time. By far, the most uncertain part of the forecast is with the flow pattern in the northeastern Pacific upper trough/low(s) coming into the Northwest. After a shortwave or two moving through, the main uncertainty is with the strength and timing of energy entering Friday-Saturday. The 12/18Z model cycle showed significant spread; the ECMWF was particularly weak with the energy and shunted it east more quickly than consensus, while the CMC (and new 00Z CMC) was deep enough to spin up a medium sized upper low in the Northwest. GFS runs, especially the 18Z, showed an in between solution with the timing of troughing, though perhaps on the deep side on Saturday over the Great Basin. This timing matched reasonably well with most EC- and GFS-based AI models. So favored the GFS with the ensemble means in general. The newer 00Z guidance may be coming to a better consensus on this, with the GFS and ECMWF with better alignment on timing and a weaker trough feature. All this energy arises from the Arctic/Russia/Alaska region and is thus uncertain, and further changes to the forecast are possible. The WPC forecast was based on a blend of the deterministic models early in the period. Given the spread in the West, ramped up the proportion of ensemble means more quickly than normal, reaching over half by Day 5. But hopefully the slightly better clustering in the newer 00Z model guidance will help refine the pattern. ...Weather/Hazards Highlights... Rainfall wrapping around the coastal low on Thursday could lead to isolated flash flooding in the Northeast Thursday, though instability should keep the threat pretty low end. A Day 4 Marginal Risk remains in place for parts of southern New England, but was able to be removed from the NYC metro area per model guidance that was a little faster to track the low northeast. Rain should linger in the Northeast Friday and Saturday as the low pulls away slowly. Precipitation, mostly rain except for the highest peaks, is possible across the northwestern U.S. underneath the rounds of upper troughing and as frontal systems move through. The details are uncertain based on the model spread with the supporting energies. The same is true farther east moisture streams into the central U.S. ahead of frontal systems. But in general, scattered thunderstorms are possible ahead of a dryline in the southern Plains and ahead of a warm front farther north across the central and northern Plains beginning Thursday. More organized convection may start to emerge by Friday and especially over the weekend in the central Plains/Middle Mississippi Valley. Left the Day 5/Friday ERO blank for now due to uncertainties in placement of any heavier rain amounts that could cause flash flooding, but Marginals may be needed in future cycles. The dynamical support may get better for heavier and more widespread rain in the central U.S. by the weekend. Upper ridging will raise temperatures above normal by around 10-15 degrees over California, the Central Great Basin, and the Desert Southwest Thursday. Highs will be generally in the mid-100s in the Desert Southwest, while back in the Los Angeles metro there may be record max/min temperatures. The warmer than normal temperatures will shift into the Four Corners states and the southern Plains late week, with gradual moderating into early next week while the Great Basin rises to a few degrees above average. Meanwhile, cooler than normal temperatures will take hold over the north-central to eastern U.S. late this week and into the weekend underneath upper troughing. Highs of 10-20 degrees below average will be common in the Appalachians to Northeast on Thursday in particular before slowly moderating. Tate 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