Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Jun 13 2021 Valid 12Z Wed Jun 16 2021 - 12Z Sun Jun 20 2021 ...Record to dangerous heat continuing through the week across much of the Interior West and through midweek over the northern Plains... ...Extreme heat possible for the Southwest/California through the end of the week... ...Overview... A very strong upper ridge centered over the Four Corners states will remain a dominant feature in the pattern through the period but should gradually weaken and drift a bit southward by next weekend. Long-term mean troughing over eastern North America will consist of a leading East Coast trough that departs after Thursday followed by moderately amplifying energy over western-central parts of southern Canada and northern tier U.S. (and possibly some trailing flow from Canada). This amplifying energy will push a front across the northern half of the lower 48 through the period, with the trailing part of the front likely stalling over the central Plains/Rockies. Pacific flow should bring another shortwave/upper low and leading frontal system into western Canada and northwestern U.S. next weekend. The western upper ridge will support a hazardous heat wave event over parts of the Interior West/Rockies and extending into California during the latter half of the week, with a broad area of temperatures that should approach/exceed daily record values. Some of this heat will extend into the northern Plains through Wednesday followed by a moderating trend. The upper ridge may weaken enough by Sunday to bring temperatures below record levels, albeit still well above normal. In the Gulf of Mexico there is continued potential for a disturbance to form near the Bay of Campeche and move northward late in the week and weekend, but with a large degree of uncertainty in its evolution. The National Hurricane Center continues to monitor this area in its outlooks. ...Guidance/Predictability Evaluation... Based on guidance through the 12Z/18Z cycles, the updated forecast started with an operational model blend for day 3 Wednesday followed by removal of the 12Z UKMET and in another couple days the CMC while ultimately trending to an even weight of the 12Z ECMWF/ECMWF mean and 18Z GFS/GEFS mean by day 7 Sunday. Within the initial East Coast trough there are still low-predictability shortwave uncertainties that will take additional time to resolve. Some models hint at enough flow separation to close off a small-scale upper low for a brief time somewhat south of the main closed low forecast to track just north of Maine. The 12Z UKMET held the upper trough west of consensus while the new run compares better to other guidance. A blend approach worked well to resolve detail spread for the southern Canada/northern tier shortwave that ultimately settles over eastern North America. By late week GFS runs stray a bit to the strong side with the upper trough and southward with the leading cold front. Guidance spread for North Pacific flow reaching western Canada and the northwestern U.S. by late week/weekend is less dramatic than it was 24 hours ago but some differences persist. The 12Z UKMET became exceedingly fast after Wednesday and then the CMC strayed on the fast side as well. By day 7 Sunday an average between the faster 12Z ECMWF and slower 18Z GFS for the arriving trough/upper low compared reasonably well to the GEFS/ECMWF means in principle. The new 00Z UKMET/CMC are somewhat less extreme than their 12Z runs and are close to the past couple ECMWF runs through late week. Today's guidance still signals the potential for tropical development to emerge from the Bay of Campeche and track northward over the Gulf of Mexico. The prevailing trend over the past day or more has been slower (with the CMC continuing to be on the faster side of the operational model envelope) and in some cases westward. Ensemble means are still fairly ill-defined due to individual member spread. Needless to say there is still considerable uncertainty. The updated manual forecast reflected only a very slight westward nudge of the surface system and extrapolation yielded an approach to the west-central Gulf Coast next weekend about a day later than was the case 36 hours ago. ...Weather Highlights/Hazards... Ahead of the northern tier cold front on Wednesday, expect a broad area of temperatures 10-20F above normal from California into the northern Plains. Continued progression of the front east of the Rockies will bring a pronounced cooling trend to the central U.S. after Wednesday/Thursday. The Southwest/California and the Great Basin should maintain similar anomalies into the weekend with the Northwest also rebounding to well above normal levels late in the week. The combination of an arriving Pacific front and gradual weakening of the Four Corners ridge may finally start to reduce the coverage of plus 10F and greater anomalies by next Sunday. Before then expect multiple days with numerous locations seeing temperatures approach or exceed daily record highs/warm lows. The combination of extreme daily temperatures and duration, as well as being rather early in the season for such an event, should make this a dangerous heat wave for sensitive groups. Excessive heat warnings are in place for many areas of the Southwest next week where temperatures may climb well into the 110s in the lower deserts with low temperatures only in the 80s and 90s. The upper trough near the East Coast should reach far enough eastward to allow for a drier trend over the northern half of the region mid-late week though shortwave energy/Atlantic surface waviness could still produce a little rain near the southern Mid-Atlantic. At the same time somewhat more rain may fall over Florida while a stalling/weakening front near the Gulf Coast may help to focus diurnally enhanced showers and thunderstorms. Areas near the Gulf Coast should continue to monitor forecasts/National Hurricane Center discussions over the coming days for the latest information regarding any potential tropical cyclone development by the latter half of the week into the weekend. Regardless of the strength of any possible system, the northward flow of tropical moisture would bring the threat of heavy rainfall to portions of the southern tier. The cold front pushing eastward from the northern Plains will produce some rain and thunderstorms from the Midwest eastward/southeastward into the eastern U.S. Some of this activity could be locally heavy but it may take into the short range time frame to determine any areas where it may be relatively more likely. Areas of rain may also be possible along the stalling front back over the central Plains/Rockies. Rausch/Fracasso 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