Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Thu Mar 12 2020 Valid 12Z Sun Mar 15 2020 - 12Z Thu Mar 19 2020 ...Heavy rain threat over the Southern Plains to lower MS and TN Valleys later this weekend into next week... ...Record cold likely for portions of the Northwest Sunday and Monday... ...Guidance/Predictability Assessment... An extremely strong upper ridge over the Gulf of Alaska (500mb height anomaly above +3 sigma) coupled with upper ridging over the Gulf of Mexico into the eastern third of the CONUS will be the drivers for the medium range period. This will favor a slowly moving closed low through California into the Great Basin with northern stream flow near the Great Lakes. Model/ensemble consensus was good overall through next Tue/Wed before uncertainty increased in how quickly the upper low ejects/weakens. Trend has been toward a slower evolution overall which has favored the ECMWF-led cluster. However, upstream Pacific flow suggests that at least by later next week a kicker system would move the flow along, though perhaps not move the longwave pattern. An amplifying northern stream shortwave across central Canada on Mon will drive a cold front southward into the north central U.S. through Tue and to the Eastern Seaboard by Wed. Models showed some timing/amplitude differences with this feature, but overall consensus was relatively good. Farther south and east, a wavy and quasi-stationary frontal boundary will extend from the Southern Plains to the Southeast through much of next week, eventually moving north as a warm front by Thu as the strong western trough approaches the central U.S. and cyclogenesis starts to occur across the central High Plains. Despite the time frame (Thu/day 7), ensemble members show relatively good clustering around a potential surface low developing to the lee of the Rockies by that time frame - and this has been the case for at least a few ensemble cycles). Overall, models showed relatively good consensus at the large scales through the medium range. A blend of the 00Z/06Z deterministic guidance formed a reasonable starting point for days 3-5 (Sun-Tue). Given gradually increasing spread during days 6-7, showed a gradual trend toward more ECENS/GEFS ensemble mean weighting for those days, with some continued, reduced usage of the deterministic ECMWF/GFS as the 00Z Canadian appeared too quick to release the upper low from the West. ...Weather/Hazard Highlights... The frontal boundary expected to linger from the Southern Plains to the Southeast is expected to focus showers and some thunderstorms, with potentially heavy rainfall, across multiple days. Several inches of rain are possible from portions of Texas and Oklahoma eastward to the Tennessee Valley, though specifics will be refined in the short range. Potential heavy rains across several days could lead to flooding problems for some locations especially where training occurs. Moisture lifting across a strong polar front in the western U.S. ahead of the incoming trough is expected to result in the potential for light snow across portions of the Northern Plains on Sun. The upper trough is also expected to produce widespread rain and mountain snow across much of the western U.S., with heavy snows possible across the Sierra over the weekend. A multi-day potential for one to several feet of snowfall is expected across much of the interior West from the Great Basin to the Rockies as the upper trough traverses the region. Strong arctic high pressure nosing down east of the Canadian Rockies will bring cold conditions to portions of the northern Rockies and northern High Plains, extending westward into the Pacific Northwest through the weekend. High temperatures will range from 20 to perhaps as much as 40 deg below average for some areas on Sun, and many areas remaining well below freezing through the weekend. Both record low and record low maximum temperatures may be tied/broken especially over Washington state, despite the much larger temperatures anomalies over Montana. This will modify somewhat through California but still some record cold will be possible toward the Colorado River Valley. Cooler air should also filter into much of the East Coast through early next week as a strong polar high pressure area behind the cold front slips across New England which will maintain an easterly component to the wind. The Gulf Coast region, remaining south of the front at least through the weekend, should see above average temperatures near 80 deg F or warmer along I-10 and into the low 90s over the lower Rio Grande. Fracasso/Ryan Hazards: - Heavy precipitation for the higher elevations of southern California, Arizona, Utah, and Colorado, Mon-Thu, Mar 16-Mar 19. - Heavy snow across the higher elevations of northern California and down along the Sierra Nevada, Sun-Tue, Mar 15-Mar 17. - Heavy snow across portions of the northern Rockies, Sun, Mar 15. - Heavy rain across portions of central California, Sun-Mon, Mar 15-Mar 16. - Heavy rain across portions of the central/southern Plains eastward into the Mid-Mississippi Valley, Sun-Tue, Mar 15-Mar 17. - Heavy rain across portions of the southern Plains northeastward across the Mid-Mississippi Valley into the Ohio Valley, Wed-Thu, Mar 18-Mar 19. - Heavy rain across portions of South Dakota, Thu, Mar 19. - Flooding possible across portions of the Great Plains. - Flooding occurring or imminent across portions of the Southeast, the lower and upper Mississippi Valley, and the northern Plains. - Flooding likely across portions of the Southeast and the northern Plains. - Much below normal temperatures across portions of the interior Pacific Northwest, the northern Rockies, and into the northern Plains, Sun-Thu, Mar 15-Mar 19. 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