Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Wed Mar 18 2020 Valid 12Z Sat Mar 21 2020 - 12Z Wed Mar 25 2020 ...Heavy Rain Threat across portions of the South... ...Overview... There is improving guidance agreement for the handling of the two primary features within the East Pacific/West Coast mean trough aloft. These are an initial upper low off the West Coast and descending Alaskan shortwave energy that should yield a trough with embedded upper low along the West Coast by early-mid week. This northern feature should serve to eject the first upper low inland after early Mon. The overall evolution during the period will bring increasing moisture to the western states during the first half of next week. Elsewhere some forecast specifics are more ambiguous aside from strong/chilly high pressure building into the East from the Plains/Mississippi Valley during the weekend. This is due to a combination of poor agreement with individual low-predictability shortwaves within northern stream flow across the northern tier U.S./southern Canada and uncertainty with timing/strength of shortwave energy ejecting from the West and carried along by progressive/low-amplitude west-southwesterly mean flow. There is a signal for some heavy rain potential over parts of the South while other areas over the eastern half of the country may see at least some precipitation during the period. ...Guidance/Predictability Assessment... Recent GFS trends toward most remaining guidance for the upper low offshore California as of early Sat have yielded much better clustering than seen in previous days. By the time the low starts opening up and approaching California by early Mon there are still some moderate speed differences with mixed trends over the past day. Up until 24 hours ago there had generally been a slower trend but then the ECMWF adjusted a little faster. On the other hand the 18Z GFS was on the slower side of the spread after that model had frequently been on the fast side, while the new 00Z run adjusted a bit faster to the middle of the spread. Meanwhile the latest GFS runs present some detail questions with the upstream shortwave around Sat-Mon but ultimately settle well into the model/ensemble mean consensus as the deepening trough evolves along the West Coast during the latter half of the period. Some earlier GFS runs had been too quick with the southward progression of this feature. Recent variability for the first upper low along with a fairly robust consensus for the trough that amplifies along the West Coast Mon-Wed recommends an average of recent GFS/ECMWF model runs (some 12Z UKMET included early) with gradually increasing 18Z GEFS/12Z ECMWF mean weight with time. The initial source of forecast uncertainty farther east is a short-range Alaskan shortwave whose amplification diverges into two primary clusters--a sharp and possibly closed feature reaching the Northwest by early Sun (12Z ECMWF and latest CMC runs) versus a broader trough reaching the Northern Plains at the same time (GFS runs and 00Z UKMET). The 12Z UKMET was a compromise of the two ideas. In addition there are detail differences for shortwave energy reaching the Southwest and southern Rockies Sat and continuing onward thereafter. Specifics of these shortwaves and possibly other weak features will ultimately determine evolution of low pressure that may develop along/offshore the East Coast around Mon-Tue. On the positive side there is at least a signal for this low pressure in the ensemble means and some operational runs such as the GFS/CMC. Surface evolution over the central/eastern U.S. by next Tue-Wed will depend in part on the details of energy associated with the initial upper low off California along with possibly other shortwaves. The aforementioned blend provides a conservative approach for dealing with low-confidence aspects of the central/eastern U.S. forecast. ...Weather/Hazard Highlights... Expect precipitation to be fairly light and scattered over the western U.S. during the weekend, with most activity confined to the Rockies and over northern areas dependent on uncertain specifics of shortwave energy dropping southeast from Alaska. Aside from some timing/detail issues there is good continuity with the overall idea of increasing coverage and intensity of rain/mountain snow over a large portion of the West during Mon-Wed as upper troughing descends along the West Coast and ejects the upper low initially west of California. Most guidance maintains highest totals over California, specifically along the Sierra Nevada, though the signals are mixed regarding the latitude of heaviest precip along the coast. A front lingering along or just south of the Gulf Coast should provide a focus for enhanced rainfall over parts of the South, especially in the Sat-Mon time frame. Uncertain details of shortwave energy aloft temper confidence in the timing and location of heaviest activity though. After the weekend expect one or more areas of precipitation over other portions of the East after the departure of strong central/eastern U.S. high pressure. Currently the details of a system that may evolve near the East Coast early in the week are very uncertain, with potential for some precipitation to be in the form of snow from the central Appalachians into the Northeast. Chilly temperatures over the central U.S. on Sat (generally 10-25F below normal) will moderate as supporting high pressure moves into the eastern U.S./Canada. Modestly below normal highs will persist over California and vicinity into early next week and then trend a little cooler toward midweek with the arrival of clouds and precipitation. At the same time above normal readings will become more common to the east of the Rockies by next Tue-Wed with most locations seeing plus 5-15F anomalies. 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