Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Wed May 06 2020 Valid 12Z Sat May 09 2020 - 12Z Wed May 13 2020 ...Record cold over the East especially this weekend... ...Heavy rain possible over southern Florida Sunday... ...Overview... During the weekend an amplified ridge-trough upper pattern will bring a more winter-like than spring pattern to many areas east of the Rockies and very warm temperatures over the West. Gradual arrival of a Pacific system will start to break down this pattern during the first half of next week. This weekend's deep upper trough over the East will lead to widespread record and near record lows/cold highs with only a slow moderation early next week. Areas west of the Rockies will see decreasing temperatures after a few lingering record highs/warm lows this weekend. Meanwhile one or more frontal waves crossing the southern Gulf of Mexico may bring heavy rain to the southern half of Florida around Sunday. ...Guidance/Predictability Assessment... The 00Z/06Z guidance cycles continued the recent theme of good consensus for the large scale pattern evolution but with typical embedded uncertainties. For the full forecast domain the 00Z ECMWF/06Z GFS (70 percent total weight days 3-5) provided the best comparison to consensus and continuity early-mid period. As has been the case lately the UKMET/CMC provided mixed comparisons to the majority depending on the feature. Typical decrease in confidence for details led to more 06Z GEFS/00Z ECMWF ensemble input by days 6-7. Late in the period the model/mean majority has adjusted a little faster with the overall trough entering the West. The idea of more progressive height falls reaching the West is plausible given the flow is undercutting the mean ridge over northwestern North America. On the other hand flow specifics over southwestern Canada are quite uncertain and there are still ways that the upper low (even if not the entire trough) could be held back some. Thus by day 7 Wed the updated forecast incorporated a 1/4 weight of prior continuity. Guidance continues to show low-latitude shortwave energy supporting a frontal wave over the far western Gulf of Mexico by the start of the period Sat, with one or more waves continuing across the southern Gulf and over/near the southern Florida Peninsula during the weekend and perhaps into early Mon. Agreement remains far from ideal for specifics but the overall signal for this waviness and associated heavy rainfall threat have been persistent to gradually increasing. Farther north the deepening system to affect the Northeast early in the weekend is reaching into the short range time frame. Check the Model Diagnostics Discussion PMDHMD for the latest info on 12Z model preferences valid through the day Sat. Guidance is still having trouble with resolving strength/timing details of the Northern Plains through Northeast wave Sat into early next week, related to the small scale of supporting shortwave energy. These issues affect the degree to which possible further development could affect parts of New England next week. The 12Z GFS and old 12Z/05 ECMWF are most pronounced in that regard. Perhaps not surprisingly the guidance has diverged for details of southern Canada and northern tier U.S. flow during the latter half of the period. Low predictability of small-scale individual features would favor maintaining a conservative blended model/mean approach for the time being. ...Weather/Hazard Highlights... Parts of the West will continue to see well above normal temperatures into the weekend. Expect highest anomalies to be plus 10-20F or so and centered north of Lake Tahoe. The approach/arrival of the eastern Pacific system will bring temperatures down to below normal values by next Tue-Wed, especially over California and southwest Oregon. At that time above normal temperatures should will reach the central/southern Rockies. Anomalies should be only in the plus 5-10F range but may still represent the highest warm anomalies over the entire lower 48. The pattern reminiscent of late winter east of the Rockies will be at its most extreme this weekend and overall centered over the Great Lakes and Northeast. Guidance has been consistent in showing the greatest negative temperature anomalies on Sat with highs 20-25F below normal from the eastern Great Lakes through the central Appalachians. Temperatures on Sat and/or Sun may approach or break daily records for lows/cold highs within the broader area of minus 10F or greater anomalies covering much of the eastern half of the lower 48. Also over some areas this cold surge would lead to a late frost/freeze where the growing season has already started. A trailing push of chilly air behind the wave tracking from the Northern Plains through the Northeast will reinforce minus 10-20F anomalies from the northern/central Plains to Mid-Atlantic/Northeast into the first half of next week. From Mon onward the South will start to see highs trend closer to normal but with lows still on the cool side. Finally by next Wed the double-digit cold anomalies should be confined to northern locations across the central/eastern U.S. The heaviest rainfall over the lower 48 during the period may be over the Florida Peninsula around Sun as one or more frontal waves bring enhanced moisture from southern Texas (rain potentially lingering into the weekend) across the Southern Gulf and through southern Florida. There is still a fair degree of uncertainty on the specifics but a steady to gradually strengthening signal for the overall event, especially for areas south of I-4. The system eventually reaching the West Coast should bring highest rainfall totals to northern California and southwest Oregon. Locally significant totals may be more from multi-day accumulations than intense activity but the latter could still be possible on a localized scale--to be determined in the much shorter term forecast range. Precipitation will also spread across the Pacific Northwest and northern Rockies. Guidance continues to suggest that leading moisture and shortwave energy interacting with a stationary (eventually becoming warm) front over the Rockies/Plains will promote areas of rainfall over the Central Plains and vicinity during the first half of next week with some locally heavy amounts possible. Ahead of these precipitation areas, the system departing from near Maine on Sat will bring some rain and higher elevation snow to northern areas before tracking farther away. Trailing lake-effect/enhanced rain/snow and a period of brisk to strong winds will add to the late-winter feel of the weather over the eastern Great Lakes/Northeast. The wave tracking eastward from the Northern Plains Sat onward should tend to have its light to locally moderate precipitation concentrated fairly close to its center. Most precip will be rain but an isolated pocket of snow may not be out of the question. Confidence is fairly low with precise effects of this system on New England by early next week. Rausch/Fracasso 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