Extended Forecast Discussion NWS Weather Prediction Center College Park MD 322 PM EDT Mon Apr 8 2024 Valid 12Z Thu Apr 11 2024 - 12Z Mon Apr 15 2024 ***Areas of heavy rain likely to reach the East Coast by Thursday with strong storms possible over the Southeast*** ...General Overview... A Southwest/Southern Plains storm system will be trekking across the central U.S. midweek before advancing through the Ohio Valley, Great Lakes and East later in the week. The Ohio Valley surface low pressure should lift into eastern Canada as upper energy consolidates into a Great Lakes/eastern Canada upper low while the trailing trough continues through the East Coast/Northeast. Broad areas of rainfall with embedded heavier downpours, will accompany this system. Drier air will filter in the wake of this system by the weekend. A trailing upper ridge progressing from the West into the Plains/Mississippi Valley will support similar movement of an area of well above normal temperatures. Behind this ridge, guidance continues to show pronounced trending for what a growing consensus shows will be an upper low that should drop south from just south of Alaska to offshore California and then track into the West, spreading precipitation ahead of its path. ...Guidance/Predictability Assessment... Several runs of the guidance depict a low pressure system advancing northeast across the Ohio Valley, Great Lakes, New England and the Canadian Maritimes. There are some timing differences that have yet to be resolved that affect the placement of QPF and amounts. The model that continues to be the most consistent from run to run is the ECMWF with an intermediate surface low position over western Kentucky/Tennessee as of early Thursday, while persistently faster GFS runs have started to nudge slower toward the ECMWF in recent runs. The CMC has waffled between slow initially to being on the faster side of the cluster. Like the previous forecast, leaning toward a heavier weighting of the ECMWF for the weekend and beyond. Guidance is trending toward slower and deeper troughing, and now a pronounced signal for an upper low to drop southward to a position offshore California and then progress inland reaching the Southwest/Four Corner region by Monday. The Pacific Northwest may have some upper trough enter the region near the start of the week which would support an increase in cloud cover and possibility of light precipitation moving onshore ...Weather/Hazards Highlights... A low pressure system will likely lift northeastward from the Ohio Valley Thursday onward and the associated trailing cold front will usher in a broad area of enhanced precipitation along the Eastern Seaboard and Great Lakes region. There is a broad Marginal Risk for excessive rainfall spanning from central Florida north to Michigan and then east to the Atlantic Ocean. A Slight Risk area that was already in affect for parts of the northern Mid-Atlantic into central/south-central Appalachians was expanded with this forecast package to span from northeast Georgia to southeast New York. This region shows the best guidance overlap and continuity for relatively higher rainfall totals as well as higher antecedent soil moisture and current stream flows. General model consensus has 1 to 3+ inches along the Appalachians or vicinity. The system will continue to advance north/northeast so by Friday anomalous moisture over the Northeast along/ahead of the cold front sweeping through the region. A Marginal Risk is already in effect for portions of northern New England and continues to reflect the level of threat for region on Day 5 given sensitivity of snow cover likely to melting over the coming days. Colder air will filter in with the passage of the front which may result it areas of mixed precipitation type or change over to all snow by Friday night and persisting into Saturday. Strong gusty winds will accompany this system. The Storm Prediction Center is monitoring the potential for severe weather with the convection forecast to track across the Southeast on Thursday. Check their latest outlooks for more information. For the West, the overall pattern suggests a closed upper low to drop to a position offshore California and then track inland, has led to a lighter trend for what rain/mountain snow may reach the Northwest/northern Rockies late this week but a brief increase of precipitation focus for California by the weekend, with some moisture possibly coming into the Southwest thereafter. The Pacific Northwest may see some light precipitation this weekend or near the start of the coming week with the upper troughing. The latest guidance maintains consensus for a faster progression across the northern tier, potentially bringing a frontal system and some rainfall to the Great Lakes/Ohio Valley/northern Mid-Atlantic by Sunday or Monday. Above seasonal normal temperatures will spread into the Plains from the West into the weekend during which the daily values will be 10-20F above normal over the West through Friday, increasing to 15-25F above normal over the northern-central Plains and vicinity during the weekend. Some of this warmth with lesser anomalies will likely spread into the East by Sunday-Monday. Meanwhile, ongoing changes for the East Pacific system moving into the West have led to below normal highs becoming more confined to California and the Southwest during the weekend while some degree of upper troughing reaching the Northwest may bring slightly below normal readings to that region by Sunday or Monday. The cold front pushing through the East late this week will bring unseasonably warm overnight lows in the warm sector followed by a brief cooler period (especially over the Appalachians on Friday). Campbell/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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw