Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Mon May 13 2019 Valid 12Z Thu May 16 2019 - 12Z Mon May 20 2019 ...Overview... Mean troughing aloft will bring very active weather to a large portion of the western and central U.S. during the Thu-Mon period. One or more periods of heavy precipitation (including some high elevation snow) will be possible from the West Coast states through the northern 3/4 of the Interior West/Rockies, with episodes of heavy rainfall and severe weather possible over the central U.S. Meanwhile guidance spread yields very low confidence in specifics over the East, aside from likely existence of a strong front that will separate chilly air to the north from very warm air farther south. ...Guidance evaluation/preferences... Guidance is steadily trending toward resolving the leading strong trough reaching the West Coast by day 3 Thu and progressing inland thereafter. ECMWF/ECMWF mean runs were earlier to catch onto the separated nature of the trough with embedded upper low during Thu-Fri. By day 5 Sat operational GFS runs become questionable with their speed of height falls crossing the northern half of the Plains. The 06Z FV3 GFS compares better to remaining consensus. Some solutions including the ECMWF/UKMET/FV3 suggest that southern trough energy (whether from the initial upper low or otherwise) could form another upper low that reaches the Plains late Sat-Sun. In relative terms there is good agreement that upstream flow will bring another round of strong height falls to the West Coast by days 6-7 Sun-Mon. Interestingly, here the ECMWF/ECMWF mean have shown a bit of a trend toward the GFS/GEFS mean that have been deeper over the past day or so. Models/ensembles increasingly diverge for eastern U.S. flow details, highlighted by a deep New England/Mid-Atlantic trough aloft in the 00Z ECMWF versus a strong ridge in GFS runs. The 00Z ECMWF is fairly extreme relative to its ensembles but not an outlier, while the previous ECMWF run was less extreme and closer to the current ECMWF mean. Remaining solutions show flat to slightly cyclonic flow. Teleconnections relative to the negative height anomaly center associated with the West Coast mean trough favor modest cyclonic flow aloft--but one cannot rule out a transient ridge that could still yield slight troughing over a multi-day time frame. Thus preferences lean only slightly toward the ECMWF mean scenario relative to GEFS mean late in the period. Based on forecast considerations the updated forecast blend incorporated an operational model blend (plus the FV3 GFS) early in the period and then rapidly increased 00Z ECMWF mean/06Z GEFS mean input with time. ...Weather Highlights/Threats... Over the West expect two primary episodes of enhanced precipitation, one with a system reaching near the coast at the start of the period Thu and then another affecting the West Coast and points inland Sat onward. Greatest 5-day totals should extend from favored terrain in northern-central California into the northern Rockies--and at some locations could be quite high for so late in the wet season. Best potential for high elevation snow should be early and late period in the Sierra Nevada and Fri or Fri night onward in the Rockies. Progression of the initial western storm will generate areas of heavy rain/thunderstorms over the central U.S. by the weekend, with the Storm Prediction Center also indicating potential for severe weather for at least a couple days. Ahead of this activity, a wavy front draped over the Midwest could serve as a focus for locally heavy rainfall late this week. Over the East, a lingering low latitude front may bring periods of rain to the southern Florida Peninsula. Farther north one or more fronts may bring some rainfall but confidence is low in specifics. As the West Coast mean trough aloft becomes established, well below normal highs over California/western Nevada on day 3 Thu (minus 10-20F anomalies) will spread northeast with time and by Sat-Mon cover an area from the West Coast into the northern Plains. Clouds and precip will keep morning lows closer to normal over this area. Leading warmth will progress from the Rockies/Plains and likely moderate gradually as it reaches the East. Some locations over the north-central Plains may see highs 10-20F above normal Thu-Fri. Temperature distribution over parts of the East is very uncertain given a wide range of possible frontal location near the East Coast. Ensemble spread reaches at least 30F for highs at some locations, depending on whether the front is to the north or south. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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