Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sun Jan 05 2020 Valid 12Z Wed Jan 08 2020 - 12Z Sun Jan 12 2020 ...Heavy rain potential Fri into the weekend from the Lower Mississippi Valley to the Ohio Valley... ...Overview/Guidance Assessment... Latest guidance continues to advertise an amplified mean trough aloft settling over the western U.S., between strengthening ridges over the east-central Pacific and off the Southeast U.S.. The Climate Prediction Center 8-14 Day forecast still suggests some variation of this pattern may persist well beyond the end of the medium range period. Evolution aloft and associated surface features will lead to a heavy rain threat most likely from the Lower Mississippi Valley through the Tennessee Valley and Ohio Valley. Teleconnections relative to the positive height anomaly center which multi-day means show near or offshore the East Coast later in the period also target this area for greatest precipitation potential. Meanwhile North Pacific energy feeding into the mean trough will bring episodes of enhanced precipitation with best focus over the Pacific Northwest. Near the start of the period on Wed the models and ensemble means have gravitated to the idea started by some solutions yesterday that deepening low pressure off the New England coast will trace a wide enough arc to keep a majority of the associated moisture just offshore. Guidance has not behaved well for this system thus far due to evolution being dependent on important small scale details aloft so further adjustments could still be possible. Farther west there is reasonable agreement that a shortwave will reach the western states by early Wed, progressing eastward while additional energy arrives Thu. Ejection of the leading energy and possible interaction with southern Canada flow should lead to a surface wave that tracks from the Plains northeastward into Canada Thu onward. The latter shortwave will progress across the Southwest/southern Rockies late in the week with solutions diverging for timing especially after early Fri. Although the new 00Z GFS has slowed a bit, it continues to bring the shortwave more aggressively into the eastern U.S. than most other guidance including the GEFS mean. Preference remains with a non-GFS evolution that maintains greater integrity of positive height anomalies near the East Coast. On the other hand yesterday's 00Z and 12Z ECMWF runs may become a bit slow and confidence is low that the ejecting energy will consolidate in a way that ECMWF/CMC runs show by day 7 Sun. As a result the ensemble means become the best option for the manual forecast valid next weekend. At least into day 6 Sat the guidance is becoming better clustered in principle for upper trough energy that drops into the Northwest by early Sat and expands over the West through Sun. Interestingly by day 7 Sun the new 00Z GFS shows less amplitude on the southwest side of the trough versus consensus, in contrast to a number of previous runs that had been digging the trough a bit westward of the majority. GEFS/ECMWF means, the 12Z ECMWF, and new 00Z CMC cluster fairly well. At the surface there is still a moderate amount of model/ensemble spread (similar to yesterday) for low pressure that tracks into southern British Columbia and/or the Pacific Northwest Fri/Sat--favoring maintenance of an intermediate path. Early in the period a model blend with more 18Z GFS/12Z ECMWF weight than the 12Z CMC/UKMET best represented consensus for significant features with day 5 Fri being the transition day that started to incorporate the 18Z GEFS/12Z ECMWF means. Days 6-7 Sat-Sun trended increasingly toward the means given questionable attributes of the GFS after early Fri and the ECMWF by late in the weekend. The overall theme of the forecast maintains good continuity with some typical run-to-run detail adjustments. ...Weather Highlights/Threats... In spite of lingering uncertainties in the specifics, confidence is increasing for the potential of a heavy rainfall event from the Lower Mississippi Valley into the Tennessee Valley and Ohio Valley late this week into the weekend. The cold front trailing from late week Plains-southern Canada system should have one or more embedded waves and may progress fairly slowly for a time while awaiting upstream western energy aloft. Two or three days of Gulf inflow ahead of this wavy front would provide considerable moisture to enhance rainfall amounts and some embedded convection is possible. This area encompasses some locations that recently received significant rainfall so effects from this upcoming event will require close monitoring. There is still a signal for some wintry precipitation on the northern periphery of the moisture shield but so far with fairly low probabilities of significant accumulations. Over the West, continue to expect highest totals of rain/mountain snow over the Pacific Northwest coast/northwest corner of California as well as the Cascades, with a secondary maximum over the northern Rockies. Some moisture will extend southeastward but with lighter amounts in most cases. Heaviest precipitation should occur from Fri or Fri night into the weekend, though amounts are likely not to be extreme given fairly modest precipitable water anomalies. The aforementioned Plains-southern Canada system may produce some precipitation over parts of the Upper Midwest and Great Lakes but with some dependence on system strength that has yet to be fully resolved. Some activity should accompany the trailing front as it reaches the Northeast. Earlier in the period, the system tracking off the New England coast on Wed will strengthen as it heads into the Canadian Maritimes. Based on current forecast most areas should see modest amounts of precipitation but there will be a brief period of breezy conditions in its wake. Expect highs to be 5-15F below normal on average from the northern Plains through much of the West during the period. Colder air may drop into Montana toward the end of the period next Sun. Warmth will overspread the eastern half of the country with some anomalies likely to exceed plus 20F, especially for morning lows. 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