Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Sun May 30 2021 Valid 12Z Wed Jun 02 2021 - 12Z Sun Jun 06 2021 ...Record warmth likely Wednesday and Thursday across parts of the West... ...Overview and Guidance Evaluation/Preferences... Most guidance shares the theme of a significant upper level pattern change over the course of the period with a mean trough reaching the Northwest U.S. and western Canada by next weekend--replacing the strong ridge over the West at the start of the period Wednesday--while eastern North America heights eventually rise and mean ridging prevails over the western Atlantic. However between the consensus developing Northwest trough and Atlantic ridge, there is considerable spread for what becomes of an initial Plains/Mississippi Valley trough and a potential High Plains upper trough/low toward the end of the week and next weekend. Specifics of these lower-confidence features will likely have considerable influence on the distribution and intensity of rainfall over the eastern two-thirds of the lower 48. The preference for resolving guidance spread continued to be with an operational blend early in the period followed by a transition to a model/ensemble mean mix to represent the most common aspects of the large scale pattern. If anything, latest guidance has become even more diverse with possible outcomes for the Plains/Mississippi Valley upper trough that may drift into the East. The last two CMC runs show the most dramatic change, from the 00Z version that brought the core of the trough to the Mid-Atlantic by next Sunday to the new 12Z run that keeps an upper low over the Mississippi Valley through next weekend (hinted at by the 00Z CMC mean). In contrast the 06Z/12Z GFS runs are quite rapid in ejecting a majority of the energy, though the 12Z run does hold back a compact upper low for a time. The 00Z GFS was somewhat closer in principle to latest ECMWF runs and GEFS/ECMWF means and thus was the run used in the updated forecast blend. The new ECMWF is still closest to the GEFS/ECMWF means. Solutions are also diverging/trending for leading shortwave energy that nears the West Coast by Thursday-Friday, with CMC/UKMET runs pulling off a closed low farther offshore California. The 12Z GFS/ECMWF pull off a little energy but to such a weak degree there is no closed contour, while being more progressive with the rest. Details of the possible trough/upper low over the High Plains next weekend are also ambiguous but at least the GEFS/ECMWF means and most GFS/ECMWF runs have maintained this feature in some fashion over the past day. Finally, the model/mean blend smooths out low-predictability detail differences within the otherwise agreeable late-period Northwest trough. ...Weather Highlights/Hazards... The best focus for rainfall around midweek or so will be along a wavy frontal system forecast to weaken ahead of the Central U.S. upper trough. There is some signal for one or more bands of moderate-heavy rainfall from parts of the Mid-Lower Mississippi Valley into the Ohio Valley/Lower Great Lakes. After the front dissipates, the combination of moist flow around Bermuda high pressure and the lingering upper trough over the east-central U.S. may support areas of moderate to heavy rainfall reaching into the Mid-Atlantic/Southeast by late in the week. Some diurnally favored showers/thunderstorms may persist over the southern half of the East through the weekend. Confidence remains moderate to low in specifics. Over the southern Plains and vicinity, a lingering front early in the period followed by possible development of a High Plains upper trough/low may promote episodes of showers/storms but again with fairly low confidence in the details at this time. Most of the West will be dry through midweek, followed by a gradual increase of moisture over the Northwest as flow aloft trends toward a broad trough. Some light to moderate totals will be possible over the Pacific Northwest with some activity also possible over the northern Rockies and eventually spreading eastward across the northern tier. Expect very warm to hot conditions over much of the West through Wednesday-Thursday. The most extreme anomalies should extend from California and the Northwest into the Great Basin where daytime highs of 15-25F above normal and possibly locally higher may meet or exceed daily records. Morning lows may also be warm enough to challenge daily records across this same region. The trend toward cyclonic flow aloft over the Northwest by next weekend and rising heights over eastern North America will gradually suppress the heat over the West and spread above normal readings across the northern tier into the Northeast. Western areas should still see decent coverage of plus 10-20F anomalies for highs through late week before a fairly narrow ribbon of plus 10F or so anomalies extends from central California into the central Rockies by Sunday. Also at that time the Northwest may see highs drop slightly below normal. Meanwhile highs reaching 15-25F above normal over parts of the northern Plains/Upper Mississippi Valley toward the end of the week could challenge daily records at a few locations. Portions of the northern Plains into New England should see highs 10-20F above normal next weekend. In contrast the southern Plains will tend to see highs up to 5-10F below normal due to clouds and showers/thunderstorms. The southern half of the East should see near-normal readings for most of the period. Rausch/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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml