Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Thu May 07 2020 Valid 12Z Sun May 10 2020 - 12Z Thu May 14 2020 ...Record cold over the East through early next week... ...Heavy rain possible over southern Florida Sunday... ...Overview... An anomalously deep upper ridge/trough over the western/eastern half of North America, respectively, will slowly break down next week. Gradual arrival of a Pacific system will start to break down this pattern during the early part of next week as the flow tries to become more zonal across the CONUS. 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 on Sunday in the Great Basin. Meanwhile one or more frontal waves crossing the southern Gulf of Mexico may bring heavy rain to the southern half of Florida Sunday into early Mondays. ...Guidance/Predictability Assessment... The 12Z/18Z 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 18Z GFS/12Z ECMWF provided the best starting point for the first couple of days to the forecast (Sun-Mon) with a clipper system through the Ohio Valley into the Northeast. As has been the case lately the UKMET/CMC provided mixed comparisons to the majority depending on the feature at least early in the forecast. For Tue/Wed, the GFS was notably quicker/farther east with a small but perhaps potent sharp shortwave out of Hudson Bay that the consensus (including the ECMWF/UKMET/CMC) carries. This could bring a quick reinforcing cool/cold shot to the Great Lakes/Northeast before moderation begins around next Wednesday/Thursday. Favored the CMC a bit more over the GFS for the middle period days Tue/Wed with the incoming Pacific system and departing shortwave from New England very early Wed. Trended toward nearly 2/3rds ensemble weighting by next Thu given uncertainty in how the pattern unfolds into quasi-zonal flow--a pattern in which model agreement means less than normal. Details in the West regarding the upper low and inland push of lower heights (and a surface front) remain a bit murky. The model/mean majority suggests that the undercutting flow beneath the mean ridge over northwestern North America could progress somewhat quickly as part or even most of the old upper low decays just off the coast. Nudged the front a bit eastward but smaller-scale subtleties temper forecaster confidence. Regardless, system will head into the Plains in some fashion by the end of the period as heights rise east of the Mississippi. Guidance continues to show low-latitude shortwave energy supporting a frontal wave over the far western Gulf of Mexico during the short term, 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. ...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 over the Great Basin. 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 reach the central/southern Rockies then into the southern Plains next Thu. Anomalies should be only in the plus 5-10F range but may still represent the highest warm anomalies over the entire lower 48. The cold pattern centered over the Great Lakes and Northeast will only slowly east through the period. After a quick reinforcing cold shot Sun-Tue from the northern Plains to the Great Lakes (highs 10-20F below normal), temperatures will rebound to only slightly below normal levels by next Thu. Temperatures on 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. This also includes part of Florida where highs may stay in the 70s over central/southern areas, due in part to abundant cloud cover and rain. The heaviest rainfall over the lower 48 during the period may be over the Florida Peninsula around Sun into early Mon as one or more frontal waves bring enhanced moisture from southern Texas 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 broadly and perhaps more narrowly south of Lake Okeechobee. 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. Question on return flow out of the Gulf will determine farther south rainfall/convective potential--please see the SPC outlooks for more information. The wave tracking eastward from the Midwest Sun onward should tend to have its generally light 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 but quicker trend suggest minor affects. Fracasso/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