Extended Forecast Discussion NWS Weather Prediction Center College Park MD 321 PM 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. The latest round of guidance appear to have a little better agreement on shortwave energy/likely closed low over the Northwest by 12z Monday, progressing eastward as a compact closed low into the Ohio Valley by the end of the week. Some minor timing/placement differences remain, but at least for the first half of the week, a general model blend seemed to work well as a starting point. Thereafter, run to run continuity is fairly poor on the evolution/characteristics of the low, with low confidence on specifics, so a bit more lean towards the ensemble means helps to mitigate these differences. Just given the model trends over the past run or two of guidance, this resulted in a little faster/stronger depiction of the closed low as it moves across the northern U.S. as compared to previous WPC continuity. Meanwhile, the spread for the eastern Pacific and western U.S. flow becomes most noticeable by next Thursday-Friday with the 06z GFS continuing to amplify central Pacific ridging farther west than most of the other guidance. On the other side of the envelope, the CMC is the farthest east, with the energy reaching the West Coast by early Thursday. The 00z ECMWF has been consistent in a position somewhere in the middle of these two which is also the closest to the ensemble means (GEFS mean is a little slower, but not quite as much as its deterministic counterpart). Therefore, WPCs blend for later in the period blended more heavily towards the ensemble means as well as the ECMWF for some added definition and details where necessary. ...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. Guidance seems to better support an area of possibly heavy rain north of the surface low/warm front from the northern Plains into the Upper Midwest on Wednesday into Thursday. Some areas of locally moderate to heavy rainfall may also be possible within a broad area ahead of the cold front Wednesday and after 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 earlier in the week. Meanwhile the Pacific Northwest may see an increase of moisture during the latter half of the week but with continued uncertainty as to the magnitude. 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. Santorelli/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 Hazards: - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Mon, Apr 5. - Heavy rain across portions of the Upper Midwest, the Middle Mississippi Valley, and the Northern Plains, Wed-Thu, Apr 7-Apr 8. - Heavy rain across portions of the Tennessee Valley, the Southern Appalachians, the Southeast, and the Ohio Valley, Thu, Apr 8. - Heavy rain across portions of the Southeast, Fri, Apr 9. - Flooding occurring or imminent across portions of the Middle/Lower Mississippi Valley and the Ohio Valley. - Much above normal temperatures across portions of California and the Southwest, Mon, Apr 5 and Thu, Apr 8. - Much above normal temperatures across portions of Southern Texas, Wed-Fri, Apr 7-Apr 9. 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