Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM 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... ...Wet pattern for southern Florida Sunday onward... ...Overview... An amplified western ridge/eastern trough upper pattern during the weekend into early next week will steadily break down, leading to a more typical springtime regime by the latter half of next week. This transition will start with the gradual arrival of a Pacific system into the West, with subsequent weakening leading to flatter flow downstream as well. This weekend's March-like 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. There will be three primary areas of significant precipitation during the period. Frontal wave(s) may bring heavy rain to southern Florida around Sunday-Monday and lingering frontal boundaries could focus additional periods of rain thereafter. The system reaching the West will spread moisture from northern California/Pacific Northwest through the Rockies while leading energy/moisture will support a multi-day period of rainfall potential over parts of the central U.S. ...Guidance/Predictability Assessment... Days 3-5 Sun-Tue continued the recent idea of good agreement for the overall pattern but with some embedded uncertainties, favoring an operational model blend that reflected consensus where agreement was best and toning down specifics where confidence was lower. The clipper system tracking from the Midwest into the Atlantic Sun-Tue has been troublesome thus far, with most models displaying some run-to-run variability for timing and/or strength--enough for even a consensus blend to adjust faster or slower depending on the run. Track has been more stable though. There is a decent cluster showing maintenance/further development off New England by early Tue but current solutions maintain enough progression for effects to be less meaningful than isolated GFS/ECMWF runs had shown in the previous two days. Meanwhile small scale maintains low confidence in southern Canada energy (possibly a compact upper low) that could near the Northern Plains/Upper Great Lakes by next Tue, favoring the blend approach. By next Tue the main considerations for the system nearing the West Coast were to lean away from the 00Z UKMET (low pressure farther south than consensus) and 06Z GFS (became flatter with the upper trough). With the southern Gulf waviness that extends northeastward, there is still spread for specifics but a continued tightening of guidance toward the heavy rain potential across the southern Florida Peninsula. Days 6-7 Wed-Thu transitioned toward a more even model/ensemble mean blend as the lower amplitude of the overall pattern leads to decreasing predictability. The primary question left to be answered is whether a portion of incoming western trough energy continues rapidly eastward (leaving the embedded upper low closer over/near the Pacific Northwest), leading to a fast moving wave over the eastern half of the lower 48 by next Thu, or the entire trough crosses the West at a slower pace and supports low pressure focused over the Plains next Thu with higher surface pressures to the east. Various GFS/ECMWF runs from the past day or so have depicted each scenario while the ensemble means have kept low pressure over the Plains into Thu. Preference sided with the Plains surface low in the means and 00Z GFS/ECMWF/CMC, with a leading warm front providing a boundary along which a wave could form if ultimately supported by shortwave energy aloft. Other uncertainties involve the aforementioned southern Canada/northern tier U.S. differences extending from Tue (but likely having less influence on the surface pattern with time) as well as trailing central/southern Canada flow which could have at least some peripheral influence on northern tier evolution late in the period. ...Weather/Hazard Highlights... The cold pattern east of the Rockies will persist from Sun into Tue with a broad area of minus 10-20F anomalies from the northern/central portions of the Plains through the Mid-Atlantic/Northeast. Pockets of minus 20-25F anomalies could exist within the Northern Plains Sun-Mon and even the Central Plains on Mon. The most numerous daily records in jeopardy should be for morning lows on Sun from southern New England into the South. With the next cold surge behind the Midwest into western Atlantic clipper system, parts of the Central Plains and eastern states may challenge record cold highs on Mon and the East could see some daily records for morning lows on Tue. Clouds/rain could also keep daytime temps near record cool high values over southern Florida on Sun. Wed-Thu will see a rapid moderation as heights aloft rise. By Thu any highs 5-10F below normal should be confined to the extreme northern tier and the South will likely start to see above normal highs. The West will see decent coverage of temperatures 10-20F above normal on Sun (with scattered record highs/warm lows) but the incoming Pacific system will limit such anomalies to locations over/just north of the Great Basin by Mon. After Mon an area of highs 5-10F above normal will drift from the eastern Great Basin into the southern Rockies/Plains. Coolest temperatures will be centered over California and extreme southwest Oregon Tue-Wed with highs 5-10F or so below normal. The highest rainfall totals for the five-day period may be over the southern Florida Peninsula and Keys. Heaviest activity should be around Sun-Sun night in association with one or more frontal waves tracking northeast from the southern Gulf. Exact details are still uncertain but over recent days the guidance has been strengthening its signal for the heavy rainfall potential across the southern Florida Peninsula and Keys. After the wave(s) depart, the lingering front to the south as well as arrival/stalling of another front--along with easterly low level flow--may lead to additional periods of rainfall through most of the week but with somewhat less intensity than the initial event. The system arriving from the eastern Pacific will spread precipitation from the central/northern West Coast through the northern Rockies. Highest amounts should focus over northwest California/southwest Oregon but other favored terrain over the Northwest/Northern Rockies will likely see significant totals as well. It is still likely that it will take into the short range time frame to determine location/timing of any more intense activity within this precipitation area. Highest elevations in the northern Rockies may see some snow from this system. Parts of the central U.S. will see a multi-day period of locally enhanced rainfall potential. Early in the period leading shortwave energy filtering through the western ridge aloft will generate scattered activity over the Four Corners states, with this energy and moisture then interacting with a front draped across the central Rockies/High Plains into the Southern Plains region. Additional upper support from the eastern Pacific system, yielding Plains low pressure with leading warm front, should continue central U.S. rainfall/convection potential into mid-late week. Highest probability for significant rainfall exists over the Central Plains with some eastward/northeastward extent also possible, with lower confidence on specifics given guidance spread for surface details. Check Storm Prediction Center outlooks for latest information on factors influencing any severe potential. The wave tracking eastward from the Midwest Sun onward should tend to have its generally 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 in the northwest part of the moisture shield. There is still some lingering uncertainty on precise influence of this system on New England by early next week but most trends suggest enough progression to support only modest effects. 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 Hazards: - Heavy rain across portions of California and the Pacific Northwest, Mon-Tue, May 11-May 12. - Heavy rain across portions of the Central/Southern Plains, the Mississippi Valley, the Great Lakes, and the Ohio Valley, Mon-Thu, May 11-May 14. - Heavy rain across portions of the Southeast, Sun-Wed, May 10-May 13. - Heavy rain across portions of the Southeast, Sun, May 10. - Flooding occurring or imminent across portions of the Middle/Lower Mississippi Valley, the Great Lakes, and the Northern Plains. - Flooding likely across portions of the Northern Plains. - Much above normal temperatures across portions of California, the Central/Northern Great Basin, the Pacific Northwest, and the Southwest, Sun, May 10. - Much above normal temperatures across portions of the Central Great Basin, the Northern Rockies, and the Northern Great Basin, Sun-Mon, May 10-May 11. - Much below normal temperatures across portions of the Plains, the Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Great Lakes, the Central/Southern Appalachians, the Mid-Atlantic, and the Southeast, Sun-Tue, May 10-May 12. - Much below normal temperatures across portions of the Northeast, the Central Appalachians, the Mid-Atlantic, the Upper Mississippi Valley, the Great Lakes, and the Ohio Valley, Sun-Wed, May 10-May 13. 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