Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Mon Apr 27 2020 Valid 12Z Thu Apr 30 2020 - 12Z Mon May 04 2020 ...Much above normal temperatures spread from the West to the Plains late week... ...Stormy weather to progress across the eastern U.S. late week... ...Pattern Overview and Guidance/Predictability Assessment... During the Thu-Mon period expect flow aloft across the lower 48 to become less amplified which should correspond to decreasing predictability. Late this week initial Pacific energy will start to flatten and push eastward an upper ridge over the Rockies/Plains while a deep upper trough with embedded low drifts over the East. This eastern system will bring a heavy rainfall threat to parts of the Mid-Atlantic/Northeast. Then by Sun-Mon somewhat stronger Pacific energy will cross the western U.S., supporting precipitation from the Pacific Northwest into the Northern Plains, while flow to the east continues to lose amplitude and become more progressive. This evolution should lead to a gradual suppression/eastward spread of the very warm to hot temperatures initially over the West. The updated forecast incorporated a blend of operational model guidance from day 3 Thu into day 5 Sat followed by a transition to 45-60 percent total weight of the 06Z GEFS/00Z ECMWF means, leading to fairly modest adjustments from continuity. The main forecast consideration was to underweight the GFS somewhat. The 00Z/06Z runs offered slightly less confident eastern system details compared to other guidance and was stronger than consensus with energy coming into the West (both early and later in the period). For the late period energy in particular, teleconnections relative to the negative height anomalies/upper troughing in the Northeast Pacific favored some modest mean ridging over the West. This relationship suggested leaning away from the deeper side of the spread for the late-period western energy. The 06Z GFS at least trended weaker than the 00Z run and the 12Z version has furthered this trend. The new 12Z ECMWF is a little more amplified than the 00Z run but still not to the degree of the GFS. The late week eastern U.S. system has lingering detail uncertainties within the upper trough and for potential stream interaction which has been variable in runs from recent days. These specifics will affect evolution of associated low pressure. ...Weather/Hazard Highlights... The late week system affecting the East will bring the best potential for a period of heavy rainfall from near the Mid-Atlantic coast through the Northeast. Possible coastal low redevelopment and a tight gradient between low pressure/frontal system and high pressure tracking off the Canadian Maritimes will likely produce a strong flow of moisture to enhance rainfall intensity. Some locations within this region could experience runoff/flooding concerns due to wet ground from recent events. Leading shortwave energy and frontal system will bring one episode of precipitation across the Northwest into the Northern Plains late this week, with low level upslope flow behind the front possibly helping activity to persist over parts of the north-central Rockies/High Plains into the weekend. Another Pacific system should bring a brief episode of enhanced precipitation to the Pacific Northwest during the weekend with continued eastward progression across northern parts of the West and into the Plains. Some of the higher elevations may see a little snow from this event. As the first front settles into the south-central Plains and extends into the Ohio Valley/central-southern Appalachians by the weekend/early next week, expect increasing potential for some locally moderate to heavy rainfall in its vicinity. Energy aloft emerging from the West may enhance activity as well. Uncertain specifics by this time frame currently lead to fairly low confidence in determining precise coverage/timing/intensity of rainfall over this area. During Thu-Fri the much above normal temperatures will focus over areas from the Great Basin/Southwest through the Rockies into the Plains, with a broad area of plus 10-25F anomalies. By Sat-Mon the flatter trend for flow aloft will lead to the warmest temperatures extending farther eastward and being confined to the southern tier, with moderation of anomalies into the plus 5-15F range. Daily records for highs/warm lows should be most numerous through Fri but even with the moderating trend the temperature anomalies may be sufficient to produce some daily records from the Southern Plains into Southeast Sat-Mon. Northern parts of the West will see readings only slightly above normal after the first frontal passage and then a trend to modestly below normal highs after the second. The upper trough over the East late this week will produce a day or so of highs 5-15F below normal. Departure of this trough will lead to a warmer trend but another front may keep temperatures near to below normal over northern areas by early next week. 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