Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Fri Jul 15 2022 Valid 12Z Mon Jul 18 2022 - 12Z Fri Jul 22 2022 ...Heat Threat to persist over much of the Western and Central U.S.... ...Overview... A strong upper ridge centered over the Four Corners region during the period will support a broad area of hot temperatures over a majority of the western/central U.S., with the greatest anomalies and potential for scattered daily record highs likely to be over or near the Plains. Flow around this ridge should continue to feed into a long-term mean trough over the northeastern quadrant of the lower 48, with surface systems/fronts generating episodes of showers/thunderstorms of varying intensity. Monsoonal moisture under the upper ridge will offer daily chances for rain/storms as well. Weak troughing will tend to prevail off the West Coast. ...Guidance Evaluation/Predictability Assessment... The large scale mean pattern anchored by the Four Corners ridge has good agreement/consistency and predictability, but the forecast of some specifics around and downstream from the ridge is more uncertain. Already in the first half of the period, significant differences develop for the combination of a trough crossing the southern half of eastern Canada and ejecting eastern U.S. shortwave energy. The most prominent difference involves the latest GEFS means not even depicting the southeastern Canada trough seen in other guidance by days 4-5 Tuesday-Wednesday. Meanwhile, ensembles show a very wide spread for the compact upper low/corresponding trough expected to track along the U.S. Canadian border and eventually reach southeastern Canada. Operational runs through 12Z/18Z showed better agreement in principle, with the main difference being that 12Z and especially 18Z runs of the GFS were a little faster than the 12Z ECMWF/UKMET/CMC during the first half of the period. It is not surprising to see the new 00Z runs offer some additional adjustments/spread given the ensemble diversity. Slightly faster trend in the UKMET/CMC and GFS continuity leave the 12Z ECMWF a bit on the slow side early in the week (00Z ECMWF is a tad faster Monday-Tuesday) while the UKMET strays south of the other guidance by Wednesday. Additional differences arise with a northeastern Pacific trough that progresses across the western and central parts of southern Canada, with some influence on the northern tier U.S. The 12Z GFS was the most extreme with this trough's amplitude. Finally, guidance offers a variety of ideas for how upper low/trough energy could eject from the mid-latitudes of the eastern Pacific. At the very least consensus suggests that the 12Z ECMWF is likely too quick. The new 00Z ECMWF is similar but other models still hold it back. The updated forecast based on guidance through the 12Z/18Z cycles emphasized the operational runs for about the first half of the period, with GFS input tilted toward the 12Z run for its better clustering with the northern tier upper low early in the week. General agreement on the mean pattern but increasing detail uncertainties led to a transition toward nearly half total input from the ensemble means by late in the period. GFS input reverted back exclusively to the 18Z run for its incoming northeastern Pacific trough while ensemble mean input was split among the ECens/CMCens/GEFS to downplay the less desirable GEFS evolution over eastern Canada and vicinity. ...Weather/Hazards Highlights... Expect the highest temperature anomalies to be over parts of the Plains where some locations should see one or more days with highs of 10-15F or so above normal. Some daily records will be possible for both daytime highs and warm lows. Monsoonal moisture and associated diurnally favored showers/storms with locally heavy rainfall may keep parts of the Four Corners/Great Basin region closer to normal for highs while other parts of the West should be somewhat warmer than normal. Exceptions will be over the far northern Rockies under the initial upper low/trough as well as along the immediate West Coast. A leading wave/frontal system may generate showers and thunderstorms over parts of the East early in the week. A trailing stronger system forecast to track across the northern tier/Great Lakes and into southeastern Canada may produce another episode of rain/storms from the far northern tier into portions of the East. In both cases some areas of heavy rainfall are possible but guidance signals are not yet agreeable enough to provide great confidence in resolving coverage/location/magnitude. A trailing front reaching the northern tier by late in the week could produce additional rainfall that is currently expected to be somewhat lighter and more scattered than with the two leading features. Except for a cool start to the week over the central Appalachians, the northern two-thirds of the East will tend to see above normal temperatures due to being in the warm sector of individual systems on most days. Persistent moisture will allow for multiple episodes of showers/thunderstorms over the Southeast/Florida and central Gulf Coast region. Temperatures should be near normal over those areas. Rausch 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