Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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... Consistent with recent days, latest guidance suggests that a short-range Southwest/southern Plains upper low will be in the process of phasing with digging northern stream flow as of the start of the period early Thursday. From that time, 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. This system will spread a broad area of rainfall, some of it heavy, along the East Coast Thursday into early Friday before a drier trend for 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... The forecast of the system tracking northeast from the Ohio Valley through the Great Lakes into eastern Canada remains consistent for the most part. ECMWF runs have been the most stable recently 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. Some CMC runs including the new 00Z version start out slow/south but catch up soon thereafter. Thursday preference continues to place greater weight on the ECMWF versus other models while a more even weight looks reasonable Friday onward. Regarding the East Pacific into western U.S. evolution, the past couple days of ECMWF-initialized machine learning models (MLs) have generally been providing the lead on the rapid trends 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. Most of the 12Z MLs supported tilting the forecast at least somewhat toward the 18Z GFS/12Z CMC that tracked the upper low more offshore California by Saturday, versus the farther east 12Z ECMWF/UKMET. Later in the period the general trough axis in the 18Z GEFS/12Z ECens means also supported slower progression than the 12Z ECMWF, in favor of the GFS/CMC bringing the upper low only to Arizona or the Four Corners region by early Monday. The 00Z UKMET and ECMWF have in fact trended more offshore with the upper low and the new ECMWF is aligned better with the GFS/CMC and ensemble means by next Monday. Finally, there is indication of some upper troughing reaching the Northwest by early next week. The 18Z GFS appeared a little extreme with its upper low reaching the Northwest, while the 00Z GFS/CMC hold the upper low farther northwest. Ensemble means and the ECMWF have flatter flow, while the MLs have varying details but generally support some degree of troughing (but not an 18Z GFS upper low) Guidance preferences through the 12Z/18Z cycles led to starting with more ECMWF weight than other models for Thursday but then trending to slightly more total GFS/CMC weight versus the ECMWF thereafter, with some GEFS/ECens input after Saturday. ...Weather/Hazards Highlights... With continued detail differences for QPF details, there is still reasonable consensus that low pressure lifting northeastward from the Ohio Valley and the trailing cold front will spread a broad area of enhanced moisture northward across the East on Thursday. The Day 4 Excessive Rainfall Outlook valid Thursday-Thursday night maintains continuity from the prior Day 5 outlook for the Slight Risk area over parts of the parts of the northern Mid-Atlantic into central/south-central Appalachians. Aside from the drier GFS/GEFS scenario, 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. Other areas within the large surrounding Marginal Risk still have varying degrees of sensitivity and mixed messages from the guidance regarding location/timing of enhanced rainfall. By Friday most guidance still shows anomalous moisture over the Northeast along/ahead of the cold front sweeping through the region. The initial proposal is for a Marginal Risk area across northern parts of the region. Guidance remains mixed for explicit QPF, but the overall pattern appears supportive of some enhanced rain rates and this region should be more sensitive given current snow cover likely to melting over the coming days. Colder air behind the front could change some precipitation to snow by Friday night, with some areas of rain/snow possibly extending into Saturday. This storm system may also produce brisk to strong winds over some areas. 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. Continued pronounced trending for the eastern Pacific into western U.S. pattern, specifically the closed upper low now expected 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. Upper troughing that may reach the Northwest could produce a little more precipitation by late weekend/early next week. Meanwhile the latest consensus West Coast evolution allows for 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. An area of above normal temperatures will move from the West into the Plains Thursday into the weekend, corresponding to upper ridge progression. Expect daytime highs up to 10-20F above normal over the West during Thursday-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). 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