Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Mar 30 2019 Valid 12Z Tue Apr 02 2019 - 12Z Sat Apr 06 2019 ...Midweek nor'easter continuing its gradual offshore trend... ...Overview... A short range Alaska ridge aloft (rebuilding into northeastern Russia next week) and then another ridge building into western Canada by late week will tend to favor cyclonic mean flow across southern Canada and into the northern CONUS east of the Rockies. Beneath the ridges expect a continued progression of Pacific shortwaves into the Lower 48. The series of features will likely support low pressure tracking off the East Coast Tue-Wed, a trailing system moving from the West Coast into the eastern U.S. Tue-Sat, and then a front reaching the West Coast to start the weekend. ...Guidance Assessment/Preferences... Models and ensembles continue to agree on the large scale evolution but not for embedded important details, including whether any stream interaction may occur over the eastern half of the country. A blend focusing more on latest operational models early in the period and about 2/3 toward the ensemble means by day 7 Sat captured the most agreeable aspects of the forecast while accounting for significant model/ensemble member spread for some features. The system expected to track over the western Atlantic from off the southeastern coast Tue onward is still problematic with track/timing spread leading to a variety of potential outcomes for the East Coast states. Over the past day or so there has been a notable trend in ECMWF-based guidance toward some other guidance that has been somewhat faster/eastward. This might well favor under-weighting the ECMWF scenario as it continues to be in the western part of the full envelope (especially through Tue). However some GEFS/CMC ensembles are not yet convinced of a track as far offshore as some operational models, while latest GFS runs are quite erratic for strength/timing--the 18Z run weak/fast and the new 00Z run slow and deep. Poor clustering and run to run variability thus far seem to suggest a modest potential for a trend reversal at some point, but some reflection of the offshore trend is reasonable. By Wed the character of a trough crossing the northeastern U.S. may play a part in system evolution/track--with current guidance split on how much interaction will occur. An upstream trough near the West Coast still has some question marks about specifics of a couple separate pieces of energy making up the feature. As the upper trough continues its way across the country the models/means agree fairly well for its existence and timing but there are a lot of differences in exact distribution of embedded energy at any particular point in time. This leads to ongoing difficulties in determining the path and strength of best-defined low pressure over the central/eastern states during the latter half of the week. As of day 6 Fri a slight majority of guidance through the 12-18Z cycles was indicating a farther south track than prior consensus but the 00Z cycle (aside from the new ECMWF) seems to be returning northward. During the latter half of the period the models and ensemble members diverge for the particulars of eastern Pacific shortwaves, with fairly low confidence for specifics. There is a general theme of some leading shortwave energy heading into a developing western North America mean ridge, followed by a potentially more energetic trough nearing the coast by early Sat. At the same time a southern stream shortwave may reach the southern Rockies/northern Mexico to start the weekend. ...Weather Highlights/Hazards... The forecast remains fairly well on track for areas of rain and higher elevation snow across the northern half to two-thirds of the West due to the system crossing the region Tue-Wed, with favored terrain seeing the highest amounts. Another round of moisture should spread into the West during the Thu-Sat period with precipitation extending from the northern/central West Coast into the northern Rockies. Within this area of late week rain/snow the northern California-Sierra Nevada region currently shows the best potential for highest totals. The system tracking out of the West around midweek will likely spread a broad shield of precipitation across the eastern half of the Lower 48 but with below average confidence for intensity at any particular location. Pockets of locally heavy activity are possible but so far there is coherent signal for their location and timing. Some of the precipitation may be in the form of snow over far northern areas. Behind this system another area of rain may develop by Sat in response to an approaching southern stream shortwave. Finally, Tue-Wed trends for the western Atlantic system continue to lower potential precipitation amounts/westward extent along the East Coast. Day 4-7 Winter Weather Outlook probabilities reflect some lingering modest ensemble support for snowfall potential over higher elevations from western NC into WV and parts of the Northeast--but marginal temperatures and 00Z guidance trends suggest further lowering of snow chances. The progressive pattern over most of the Lower 48 will keep temperatures variable, but with a general increase of above-normal temperature coverage with time. This is especially the case for min temps. Some localized anomalies of minus 10-15F are possible over the southern tier on Tue followed by a warming trend. The unsettled pattern over the West should keep morning lows well above average most days. 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