Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Fri Apr 02 2021 Valid 12Z Mon Apr 05 2021 - 12Z Fri Apr 09 2021 ...Overview and Guidance Evaluation/Predictability Assessment... The two primary uncertainties of the forecast involve initial West Coast/Northwest shortwave energy that may progress through the western and central U.S. in the form of a closed low, followed by the specifics of eastern Pacific/western U.S. flow with guidance showing increasing divergence by late next week. The leading system should spread precipitation across the northern half of the West/Rockies and then onward through a majority of the central/eastern U.S. Late week flow details will determine how much precipitation reaches the Pacific Northwest at that time. Meanwhile an upper low just offshore New England and Nova Scotia at the start of the week will drift away only gradually due to a fairly blocky pattern over the Atlantic and eastern Canada. Over recent days guidance has been showing difficulty in resolving the character of eastern Pacific shortwave energy during the weekend, with resulting differences and variability affecting the forecast across the lower 48 for most of the period. In the past 24 hours the operational models have been gravitating toward the idea of West Coast energy forming an upper low by early Monday or shortly thereafter. This upper low would then continue across the western and central U.S. So far the new 00Z guidance is following through fairly well on the 12Z/18Z consensus, albeit with some spread for track and timing. This provides some increase in confidence but recent guidance behavior still provides some concern. Recent GFS runs appear to be on the deep side of the spread for the upper low and associated surface reflection so the preferred blend tones down that aspect its forecast. The spread for eastern Pacific and western U.S. flow becomes most noticeable by next Thursday-Friday as the GFS and more subdued GEFS mean amplify central Pacific ridging farther west than most other guidance, in turn leading to a farther west eastern Pacific trough/western U.S. ridge. In varying ways the ECMWF/CMC and their means are farther east with the upstream ridge, promoting a mean trough close to the West Coast and a more modest/eastward ridge to the east. Each cluster's trough position is internally consistent with their respective upstream ridge teleconnection-wise, and both clusters have maintained their solutions for a while, so an intermediate approach seems best at this time. An average of guidance looks good for the upper low initially near New England. The new 00Z GFS has trended a bit eastward from previous runs that were on the western side of the spread for a time. Between this low and the system emerging from the West, there are some lower predictability uncertainties for some southern-eastern Canada and eastern U.S. flow specifics that will affect where a surface boundary will be positioned on a day-to-day basis. Forecast preferences led to an operational model composite during the first half of the period transitioning toward a blend consisting of half 12Z ECMWF/ECMWF mean and half 18Z GFS/GEFS mean (but with much more GEFS mean than GFS) later in the week. ...Sensible Weather and Potential Hazards... Early in the week the system moving into the West will bring an area of rain and high elevation snow mainly over the northern half of the region. Coverage and intensity of precipitation should then increase to the east of the Rockies as the upper system continues onward and associated low pressure/frontal system evolve over the central U.S. Some areas of locally moderate to heavy rainfall may be possible within a broad area extending from the northern half of the Plains into portions of the East. A front ahead of this system may also provide a focus for enhanced rainfall over parts of the Midwest and Great Lakes. Meanwhile the Pacific Northwest may see an increase of moisture during the latter half of the week but with considerable uncertainty as to the magnitude, and there is still a less probable scenario that would have the region staying relatively dry into late week. Warm flow ahead of the initial Northwest system will bring well above normal temperatures to areas from the Southwest and parts of the Great Basin through the Midwest/Great Lakes early in the week. Best potential for plus 20F or greater anomalies for min and/or max readings will be over the northern and central Plains into Midwest. Some daily records are possible from the Southwest/southern Rockies northeastward. This warmth will moderate as it moves eastward ahead of the system that should reach into the Plains/Mississippi Valley, with late week eastern U.S. anomalies tending to be in the plus 10-20F range for morning lows and plus 5-15F for highs. System passage over the West should bring temperatures down only to around normal or perhaps briefly below, with southern areas tending to stay moderately above climatology. The Pacific Northwest should be near to modestly below normal for most of the week. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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