Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Wed May 23 2018 Valid 12Z Sat May 26 2018 - 12Z Wed May 30 2018 ...Guidance and Predictability Assessment... The pattern over the contiguous U.S. looks to become fairly blocky/slow to evolve for a time, as an upper low forecast to be over California as of early Sat decelerates as it tracks into the Great Basin and a southern U.S./Gulf of Mexico upper trough/low evolves near or east of 90W longitude. A ridge in-between will build over the Plains and into the upper MS Valley. Flow may begin to open up somewhat toward the end of the period. Continuity and agreement are decent in principle for the large scale pattern evolution but guidance has had difficulties in resolving important details. There has been greater spread and day to day variability with aspects of flow over the eastern Pacific and across Canada/northeastern U.S. At least into next Mon guidance appears to have stabilized for the time being for the upper low expected to track into the Great Basin from CA, with enough separation from the westerlies to support a deceleration/possible stalling of the feature around the start of next week. By Tue-Wed guidance diverges regarding ejection of the upper low in response to upstream Pacific flow. ECMWF runs over the past day have been trending faster as more troughing takes shape off the Pacific Northwest coast--a nod to recent 12-hourly GFS runs. However some GFS runs have been somewhat exaggerated with the Pacific trough, possibly including the 18z/new 00z runs. Even if the trough arrives a little slower than in the GFS there is some teleconnection support for a trough developing near the West Coast based on recent multi-day D+8 means showing some positive height anomalies over the north-central Pacific south of Alaska and between Hudson Bay and the Great Lakes. An average among the 12z GFS, past couple ECMWF runs and GEFS/ECMWF means looks best for representing the most likely evolution. Confidence is still not especially high given how dramatically the spread increases in ensemble spaghetti plots late in the period. In some respects the forecast spread over the Gulf of Mexico and South/Southeast regions has not changed a lot over the past day, with the ECMWF still inhabiting the western side of the envelope for possible low pressure development and the GFS the eastern side. There are some noteworthy trends within that spread though. At some valid times recent GEFS runs, especially the 12z version and to some degree new 00z cycle, have adjusted westward albeit not yet to the ECMWF/CMC and their means. In addition the far eastward UKMET in the 12z run has adjusted to a slower/stronger version of the 00z GEFS mean in the new 00z cycle. Likely influence of low level flow on track for a while still seems to favor leaning away from the eastern side of the spread. UKMET/multi-day GEFS trends seem to be adding support to that idea. After significant adjustments in the guidance 24-36 hours ago, continuity looks better for the Sat-Mon part of the forecast covering the Great Lakes/Northeast with high pressure building across eastern Canada pushing a front into the Northeast/northern Mid Atlantic. Guidance has still been oscillating somewhat on the strength of shortwave energy brushing the upper Great Lakes plus amplitude/timing of Canadian shortwave energy by the latter half of the period. An emerging trend toward Tue-Wed is for eastern Canada trough energy to push another front into the northeastern quadrant of the country. This upper trough is plausible as it would feed into a teleconnection-favored trough just to the east (based on the positive height anomalies between the Great Lakes and Hudson Bay). Guidance comparisons/trends led to the forecast including components of the 12z GFS/ECMWF and their means along with the 12z CMC, as well as slight 00z/22 ECMWF input late. Operational model weight decreased from 75 percent during the weekend to 25 percent late. ...Weather/Threats Highlights... The system that may track into the South from the Gulf, and more broadly a multi-day period of moist southerly flow, will generate the potential for heavy rainfall over an area extending from near the lower MS Valley through the Southeast/TN Valley and possibly into the Mid Atlantic. Monitor forecast details as they become more clear over the coming days, as locations that have received considerable preceding rainfall will be quite sensitive to another heavy rain episode. The other area of focus for heavy rainfall potential will be ahead of the western U.S. upper low as guidance is suggesting a period of fairly slow movement for the feature. The best signal for highest rainfall totals during the period extends across the northern Rockies/Plains with some enhanced activity also possible over the Great Basin and central Plains. Across the Great Lakes/Northeast expect periods of rainfall of varied intensity with a couple fronts providing the focus, the first one likely stalling for a couple days or so. Expect multiple days of very warm temperatures across much of the northern U.S. Anomalies over parts of the northern Plains/upper MS Valley may reach plus 15-20F on one or more days. Readings could be nearly as anomalous over the central-southern Plains especially Sat-Mon. Some daily records are possible. On the other hand the upper low moving into the West will bring some well below normal highs across California and the Great Basin during the weekend. Temps should moderate next week as the upper feature gradually weakens. Over much of the East, moist flow from the south will promote warm mins but clouds/moisture will likely keep daytime highs below normal especially over the Southeast. One or more fronts dropping into the Northeast may lead to one or more days of below normal temps over that region. 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