Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat May 04 2019 Valid 12Z Tue May 07 2019 - 12Z Sat May 11 2019 ...Overview... Various details remain uncertain but guidance is continuing to suggest that a strong eastern Pacific ridge will promote western U.S. troughing, while a broad/flat ridge will likely be oriented near or just offshore the East Coast. An ejecting Southwest U.S. upper low Tue onward and associated surface wave/frontal system, followed by slow movement/possible stalling of the trailing front as it becomes aligned with southwesterly flow aloft, will produce a heavy rainfall threat over parts of the Plains and Mississippi Valley. Some affected areas will likely already have wet ground thus elevating flash flood concern. A less extreme portion of this moisture should extend into the East, while the ejecting Southwest upper low and subsequent larger scale troughing should generate terrain enhanced precipitation over parts of the West. ...Guidance Evaluation/Predictability Assessment... In the Tue-Wed time frame there had been considerable uncertainty over details across the northeastern quadrant of the Lower 48 into Canada. Starting with yesterday's 06Z GFS there has been better agreement in principle on the more suppressed scenario which the majority cluster had shown 24 hours ago. There has still been somewhat of a southward trend with the northeastern front dropping into the Mid-Atlantic. As the ejecting Southwest U.S. upper low reaches the Rockies and beyond, guidance displays typical divergence for timing--due in part to differences that develop within the amplifying western trough. Overall prefer an intermediate timing at the moment, at least leaning away from the slow side of the spread (latest GFS runs) as the feature is carried along by progressive southwesterly flow aloft. On the other hand slower trend of the 00Z CMC leaves the UKMET as the fastest solution. Over the past day guidance has trended stronger for the surface wave reaching the Midwest by early Thu, reflecting better definition of the upper system. Also worth noting, recent GFS runs are stronger than other models with a leading eastern U.S. wave at the surface and aloft Wed-Thu. The rest of the forecast involves more broadly dispersed models/ensemble members for specifics. GFS/FV3 GFS runs are among the more western or southwestern solutions for the upper low that may close off within the western U.S. trough aloft. On the other hand the 12Z ECMWF mean is one of the most aggressive to knock down the upstream ridge toward the end of the period, leading to faster trough progression. The 12Z ECMWF is similar with the trough in spite of holding onto a little more ridge upstream. There is a general signal for some erosion of the ridge though. Toward the end of the period a compromise among the faster (12Z ECMWF/ECMWF mean) and slower (18Z GEFS mean/old 00Z ECMWF) solutions would provide a reasonable starting point--and is close to the 12Z CMC mean. Separate troughing (with possible embedded low) over southern Canada and extending into the northern tier of the Lower 48 will provide added forecast challenges, especially during the latter half of the period. Thus far guidance has not been very agreeable or consistent with the associated wave(s)/front(s) reaching the northern U.S.--maintaining low confidence in any specific solution. For about the first half of the period a multi-model blend emphasizes the most common elements of guidance. Then the forecast shifts toward a blend of the 18Z GEFS/12Z ECMWF means (more of the former due to aforementioned issue with the eastern Pacific ridge) and past two ECMWF runs. ...Weather Highlights/Threats... Continue to expect a threat of heavy rainfall over the central/southern Plains and Mississippi Valley. Less extreme but still significant rainfall may extend into the the Ohio/Tennessee Valleys and Northeast/Mid-Atlantic but with lesser confidence on specifics. The central/north-central Plains activity will likely take place during the first half of the period. A longer duration of heavy rain/thunderstorms will be possible farther south--in advance of the system emerging from the West into the Plains and then late week into the weekend as persistent low level flow from the Gulf interacts with a front that may stall for a time over the Lower Mississippi Valley/western Gulf Coast. The wave reaching the East late in the week will provide the primary focus for enhanced rainfall over that part of the country. Across the West, anticipate varying intensity of precipitation from California through the Great Basin/northern Arizona and into most of the Rockies as the upper low tracks through southern areas early in the week and the upper trough/possible embedded upper low develop over the West later in the week. Some of the precipitation should be in the form of snow over high elevations of the northern-central Rockies. Depending on exactly how features evolve and time of day, some snow could extend into the north-central High Plains. Highest precip totals are most likely over favored terrain within the Rockies from Wyoming to northern New Mexico. Some locations farther west may see significant amounts as well. Chilly air pushing southward over the Plains may bring highs down to 15-25F below normal over north-central/central High Plains on Wed-Thu, with a broader area of below normal readings over the central U.S./Rockies and Southwest during most of the period. The system tracking out of the Southwest will bring warmth into the East later in the week with low temperatures in particular 10F or greater above normal. The Pacific Northwest should see multiple days with highs 10-20F above normal. 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