Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue Apr 9 2024 Valid 12Z Fri Apr 12 2024 - 12Z Tue Apr 16 2024 ...Overview... The vigorous storm system affecting the East at the start of the forecast period on Friday will continue to track over eastern Canada and push its trailing front eastward as the supporting upper trough lifts northeastward. Some areas of rain and snow may linger over the Northeast into Saturday. A trailing Rockies/Plains upper ridge will bring initial western warmth into the central U.S. with parts of the Plains seeing highs 15-25F above normal during the weekend. Guidance is starting to stabilize for the deep upper low dropping into the eastern Pacific during Friday-Saturday, bringing a brief period of precipitation into California during the weekend. However after Saturday there is still timing spread for the upper low between California and the Plains, with some influence from differences in upper troughing expected to push gradually into the Northwest. This upper low will most likely reach the Plains by next Tuesday and support a fairly strong surface low at that time. ...Guidance/Predictability Assessment... With guidance showing reasonable agreement for the departing eastern system, attention turns farther west to the upper low dropping over the eastern Pacific and then tracking inland plus northern stream flow that may bring a wave/frontal system across the northern/eastern U.S. A trailing trough may push into the Northwest early next week. Guidance is at least consolidating toward the idea of a deep upper low reaching offshore California by Saturday, with a general model composite looking reasonable up to that time. Then as spread develops for eastward progression, most of the recent runs of ECMWF- initialized machine learning models (MLs) offer the best support for continuity, which would bring the upper low center to the California coast by early Sunday and the High Plains by early next Tuesday. Relative to this scenario, the new 00Z GFS has trended slower while the UKMET has strayed somewhat faster. Recent ECMWF runs have tended to stray on the fast side (primarily by next Tuesday in the 12Z version) but the new 00Z run has nudged slower toward the GFS (which catches up by that time) and the CMC. Some of this timing spread is related to the upper trough expected to push into southwestern Canada and the northwestern U.S. by early next week. There has been a somewhat slower trend for the arrival of this trough (and a corresponding trend for the leading surface front) but most dynamical models/means and MLs suggest more Northwest troughing than forecast by the GFS--thus lowering confidence in the slowest side of the spread for the upper low. Meanwhile the guidance varies with less-defined details of northern stream energy that may support a weak wave and frontal system from the northern Plains into the East Saturday onward. The most common theme is fairly close to continuity, with this wave reaching the Upper Midwest/Great Lakes by Sunday and then the Northeast/eastern Canada by next Monday. The trailing cold front should extend into the Ohio Valley/Mid-Atlantic but with a moderate amount of uncertainty. Most guidance suggests that enough upper ridging should persist over central North America to support some lingering upper troughing over the Northeast early next week. Guidance comparisons through the 12Z/18Z cycles led to an updated forecast using a blend of operational models early-mid period and then a rapid trend to a model/mean mix late in the period. ...Weather/Hazards Highlights... The windy system tracking out of the Great Lakes as of early Friday will still support anomalous moisture over the Northeast in the strong southerly flow along/ahead of the cold front sweeping through the region during the day. A Marginal Risk has been maintained in the Day 4 Excessive Rainfall Outlook for some parts of northern New York through the northern half or so of New England given the potential for locally enhanced rainfall rates and sensitivity due to snow cover likely to melt over the coming days. Colder air will filter in with the passage of the front which may result in areas of mixed precipitation type or changeover to all snow by Friday night and persisting into Saturday. The upper low dropping southward over the eastern Pacific late this week should approach/reach California during the weekend. While moisture anomalies do not appear too extreme with this system, the depth of the upper system offers potential for some localized moderate to heavy rain bands. Such activity could lead to some runoff issues given fairly high soil moisture anomalies currently existing along the coast. Thus the Day 5/Saturday ERO depicts a Marginal Risk area along approximately the central half of the California coast where the best average of guidance suggests potential for rainfall enhancement. Some precipitation in the Sierra Nevada may be in the form of snow. As the upper low progresses eastward, it should produce precipitation of varying intensity across the Great Basin and Four Corners states, followed by increasing coverage/intensity of rainfall over the Plains by next Tuesday as it develops potentially strong low pressure. The Northwest may see some light precipitation late this week and perhaps another period of rain/mountain snow early next week with the arrival of an upper trough. Farther east, a weak wave/frontal system may produce some rain over parts of the Ohio Valley/Great Lakes through Mid-Atlantic during at least a part of the weekend. The Northeast will see unseasonably warm lows early Friday ahead of the cold front crossing the region during the day. Behind the front, parts of the Ohio Valley/Great Lakes into central-northern Appalachians will see a brief period of cool daytime highs Friday- Saturday. Farther west, from Friday through the weekend an area of above normal temperatures will shift from the West/High Plains into the central U.S. Highs should reach up to 10-20F above normal on Friday while parts of the northern/central Plains may see plus 15-25F anomalies during the weekend. Some of this warmth with lesser anomalies (plus 5-15F) will likely spread into most of the East from Sunday onward. Persistence of a mean trough aloft may hold daytime highs closer to normal over the Northeast though. The upper low tracking in from the eastern Pacific will bring below normal highs into California and the Southwest during Sunday- Tuesday while farther north an upper trough and leading cold front should promote modestly below normal readings over the Northwest by early next 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 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