Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1155 AM EDT Tue May 22 2018 Valid 12Z Fri May 25 2018 - 12Z Tue May 29 2018 16Z Update: The guidance is hinting at an omega block pattern trying to develop over the continental U.S. by early next week, with a very slow moving upper low over the Intermountain West and a broad upper trough over the southeast U.S., separated by an upper ridge over the Plains. In terms of the latest guidance, the 00Z UKMET appears too quick with height falls across southern Canada on Friday and Saturday, with the other guidance suggesting more in the way of ridging. The CMC continues to be stronger with the tropical disturbance over the Gulf of Mexico, although close to the model consensus in terms of placement over the weekend. Both the operational GFS and ECMWF have trended stronger with the western U.S. closed low over the past 36 hours, with the ECMWF indicating the most consistent signal. By the end of the forecast period, greater weighting was applied to the EC mean while still keeping some details from the operational guidance. The GEFS mean has lower heights over the eastern Pacific compared to the model consensus by the weekend and more ensemble member spread than the EC mean, and thus its percentage in the forecast is less. The previous extended forecast discussion is appended below for reference. Hamrick ...Guidance and Predictability Assessment... Guidance variability and spread have tempered confidence for various aspects of the forecast. Faster trends have been evident for a while with the upper low moving into the West, with 12z/18z runs continuing that tendency relative to prior days and new 00z runs thus far holding onto a similar timing into day 5 Sun. Upper low track has gravitated toward the southeast portion of the spread but not yet as far south as recent UKMET runs. Later in the period recent guidance diverged with this feature, the 12z ECMWF actually pulling it southwestward while the 12z CMC showed continued northeast progression. GFS runs were between the two ideas. Some of this late period spread is connected to the uncertainty in the character of upstream flow. Sporadic GFS runs have been quite eager to develop an amplified trough to the south of an upper low that settles into or near the Gulf of Alaska while other solutions tend to be flatter with cyclonic flow or farther west with troughing. Consecutive ECMWF runs have been very different though and result in two opposing solutions for the western low. Latest signals, reinforced by the 00z GFS/CMC, are toward the idea that by early next week the western upper low/trough may become slow to move in light of westerlies being sufficiently far removed from the feature. Farther east the past day of guidance has featured some significant adjustments. Most solutions become noticeably weaker with the shortwave initially over the northern Plains/southern Canada as well as with energy flowing into western Canada. The end result of these changes is a dampening of the associated surface low with time. Meanwhile consensus (not including the UKMET) showing higher heights over central Canada leads to a potential trough aloft crossing eastern Canada/northeast U.S., with its passage leaving high pressure over New England where low pressure/frontal system had been expected by Mon in previous guidance. By mid-late period the volatility of guidance at least favors leaning away from recent GFS runs that were strongest with the upper trough/surface high pressure, but consensus is sufficiently pronounced to incorporate the new scenario in the updated forecast. The forecast provides plenty of question marks from the northwest Caribbean into the southern/southeastern U.S. as well. There is decent agreement that some degree of low pressure should form but spread persists regarding whether the system will gravitate toward the core of an upper trough that deepens during the first half of the period before stabilizing or weakening a little, or track around the eastern side of the upper trough. Thus far it appears the higher probability is closer to the central/east-central Gulf track based on guidance comparisons to a feature initially analyzed over the northwestern Caribbean at 00z. This leads to leaning away from GFS/GEFS solutions. The National Hurricane Center is issuing special tropical weather outlooks for this potential system. Based on guidance comparisons through 12z/18z, the updated forecast incorporated the most common/favored ideas among the 18z GFS, past two ECMWF runs, 00z CMC and 12z NAEFS/ECMWF means early-mid period--followed by greater emphasis on the two means while phasing out GFS/CMC input but keeping a minority weight of ECMWF data. ...Weather/Threats Highlights... Through a combination of a potential system emerging from the northwest Caribbean/Gulf and the broad flow of moisture to the east of an upper trough/low becoming established along 90W longitude, heavy rainfall potential remains a concern for parts of the southern/eastern U.S. This is especially the case over areas that received significant rainfall totals over the past 1-2 weeks. Farther north expect some activity with a gradually weakening low pressure/frontal system with localized pockets of heavier rainfall. Meanwhile the upper low moving into the West will be responsible for another area of active weather during the period, from the central/north-central west coast into northern-central High Plains. Potential for slow movement of the upper low toward early next week could lead to enhanced rainfall over some locations. Expect the most persistent warmth relative to normal to be over the northern Plains/upper MS Valley with multiple days of plus 10F or greater anomalies for min/max temps. Elsewhere east of the Rockies min temps will generally be above normal but clouds/rain over the southeast will keep that part of the country somewhat below normal for highs. Onshore flow may bring a cooling trend for highs over portions of New England and the Mid Atlantic by early next week. The upper system moving into the west should bring a period of below normal highs to California and Great Basin especially from Fri through the weekend. WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities can be found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 Rausch