Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1133 AM EDT Mon Apr 22 2019 Valid 12Z Thu Apr 25 2019 - 12Z Mon Apr 29 2019 ...16Z Update... Guidance somewhat trended in opposite directions over the Southeast with respect to the exiting closed low (weakening to an open wave) with the 00Z ECMWF and many of its ensemble members quicker than the GFS/UKMET while the 00Z Canadian was mostly in between. Opted to rely on the previous forecast as it sat nicely in between the quicker/slower camps given no clear reason to change as of yet. Next system out of the Gulf of Alaska into southwestern Canada has mostly trended weaker in the guidance but the GFS remained stronger than the rest, which resulted in (likely) too much ridging into the Plains next Sun/Mon compared to the ensemble consensus. Upper low approaching California and weakening on Sunday remains rather well-forecast in the past two model/ensemble cycles though the ECMWF has had better continuity. Again, previous forecast with only very minor modifications proved to be a best starting point, but with a few changes to later periods in the temperature forecast based on the latest ECMWF ensembles and National Blend guidance. Previous discussion is below... Fracasso ...Overview and Guidance Preferences... Upper-level flow across the CONUS is not expected to change significantly during the extended forecast period. A progressive northern stream and less progressive southern stream, similar to the pattern seen in recent weeks, is expected to persist. Models/ensembles do hint at potential northern stream flow amplification from the North Pacific into Canada by next weekend, however. Development of a broad Rex block extending from Hawaii to Alaska is expected, which will reinforce upper troughing centered west of Hudson Bay. Conceptually, this setup seems to favor potential southward transport of polar air into central Canada and the north central U.S., and this bears out when teleconnections associated with the Pacific Rex block are examined. Models handle a southern stream trough initially across the Southern Plains on day 3 (Thu) similarly through Friday, before solutions begin to diverge on timing. Timing has been an issue with this system in the guidance for several days, largely hinging on the degree of separation between northern/southern streams and amplification of northern stream shortwave energy crossing the Great Lakes on Fri. In general, the trend has been toward more separation of streams and a slower progression of the trough/upper low across the Southeast through Sat. Farther west, timing of northern stream shortwave energy digging into the northwestern U.S. on Sat is a problem, with models showing a fair amount of variability. In general, a multi-model deterministic blend including the 12Z ECMWF/UKMET and 18Z GFS was used as a basis for the forecast during days 3-5 (Thu-Sat). After a few days of waffling, models/ensembles seem to have largely settled on the idea of another cutoff southern stream system approaching California Sat-Sun. The ECMWF/GFS are quite similar on timing of the feature, at least through Sun, which also has support from the respective ensemble means. By Mon, differences emerge, with confidence quickly decreasing. The culprit seems to once again be significant northern stream differences. The GFS has been persistent in digging an amplified upper trough into the Pacific Northwest by Sun-Mon, while the ECMWF/CMC show a more subdued feature with weaker height falls. Ensembles show a fair amount of disbursement, suggesting low confidence in this feature. The northern stream evolution by Mon also affects the southern stream feature, with a more amplified trough in the Northwest kicking the southern stream wave inland more quickly, and a less amplified trough keeping more stream separation and an upper low lingering across the Southwest for longer. The pattern in recent weeks would suggest that the idea of more stream separation is more likely, but the broader northern stream flow amplification described above suggests that the answer may be less simple. Thus, leaned heavily toward ensemble means (12Z ECENS/18Z GEFS) by days 6-7 (Sun-Mon), representing a compromise solution across the western U.S. ...Weather Highlights/Threats... Widespread showers and thunderstorms are expected from the lower Mississippi Valley to the Southeast Thu-Fri in association with the southern stream shortwave/upper low. Models suggest that at least locally heavy rainfall is possible. The frontal boundary associated with this system is expected to stall across the Florida Peninsula over the weekend and linger into early next week, which should keep scattered to numerous showers and thunderstorms a possibility, along with locally heavy rainfall. Farther west, rain and mountain snow will be possible across the Rockies Thu night into Fri as shortwave energy passes overhead. Precipitation should spread into the Northern/Central Plains Fri night into Sat in association where a couple stationary frontal boundaries will add to the lift generated by the upper-level shortwaves. This precipitation should spread east into the Midwest/Great Lakes by Sun, with colder air on the northern fringes potentially leading to some chance for snows from northern Minnesota east to northern Wisconsin and the Upper Peninsula of Michigan. The arrival of additional northern stream energy should bring another round of precipitation to the Rockies on Sun and into the Central/Northern Plains Mon. Temperatures are expected to be well above average from California east to the central Rockies Thu-Fri, with highs expected to reach 10 to 20 deg F above average for many areas. These temperatures anomalies may spread east into the Southern/Central High Plains for Sat. Meanwhile, a cooler polar air mass should begin to spread into areas from the northern Rockies to the Upper Midwest north of a surface front next weekend, with high temperatures at least 5 to 10 deg below average expected. The flow across central Canada by next weekend will play a significant role in temperatures across the north central U.S., with some model solutions much colder than currently forecast. Ryan 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