Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 AM EST Sun Feb 2 2025 Valid 12Z Wed Feb 05 2025 - 12Z Sun Feb 09 2025 ...Another round of precipitation could be heavy for California around Thursday... ...Overview... Guidance shows low amplitude upper flow across much of the lower 48 later this week into the weekend, but the overall pattern and embedded features will still produce areas of impactful weather. Precipitation over California is forecast to lessen somewhat for midweek, but heavy rain and snow amounts could get renewed around Thursday. Moisture farther inland will support precipitation across the Great Basin and Rockies, with significant snow possible for higher elevations. Farther east, moisture return and some degree of frontal wave development will lead to increasing precipitation chances across the eastern third of the lower 48 mid to late week, including possible winter weather for parts of the Mid-Atlantic and Northeast. Another round of precipitation is possible in similar areas by next weekend with another low pressure system. The quasi- zonal flow pattern with subtropical ridging near Cuba will promote above normal temperatures across a majority of the country, though a Canadian surface high nudging into the northwestern to north- central U.S. should lead to below normal temperatures there with coldest anomalies into Wednesday. ...Guidance/Predictability Assessment... From the large scale perspective, the ensemble means have been providing a fairly steady and agreeable forecast over recent days-- helping to anchor the forecast within some pronounced spread for various significant features. The medium range period starts with an upper low offshore of Vancouver Island/Washington State while there is some vorticity spinning farther east that promotes shortwave trough development (within broader cyclonic flow) Wednesday-Thursday in the north-central U.S., supporting surface low formation. There was still considerable spread in the position and strength of the surface low by 12Z Thursday somewhere in the Great Lakes/Ohio Valley, including a farther south low track from GEFS/GFS runs. The newer 00Z ECMWF/CMC runs are at least somewhat agreeable in a position over southern Ontario 12Z Thursday and moving quickly northeastward. Regarding the western upper low, at least some of the outliers like older CMC runs that pulled it well offshore are in better agreement now, but the exact shape of flow around the upper low and energy pivoting through the southern side are more variable for mid to late week. Models are at least agreeable that an AR will impact California on Thursday-Friday but surface low details moving from the eastern Pacific into California vary. Late week into the weekend, this trough should pivot eastward into the West and most ML models matched up fairly well with the ensemble means and an average of recent ECMWF/GFS runs for the overall upper trough. Upstream energy dropping down from Alaska could complicate the forecast with respect to specifics though. The overall shape and position of the trough by Saturday has decent support from teleconnections relative to the upstream Pacific/Alaska upper ridge. The trough axis moving quickly eastward could spin up another surface low this weekend over the south-central U.S. and quickly through the Ohio Valley or so. Similar to the late week system, the GFS was more suppressed with the developing low track compared to the ECMWF. Early in the forecast period, WPC used a blend of mainly deterministic guidance, and increased the proportion of ensemble means as the period progressed. ...Weather/Hazards Highlights... The long-lasting atmospheric river (AR) taking aim at California will finally weaken and move south by Wednesday, leading to reduced rain amounts. Light to moderate showers could linger over southern California, but these look to stay under any flooding thresholds even for the sensitive burn scars in that region. Thus do not have any risk area depicted in the Day 4/Wednesday ERO, but will continue to monitor. Meanwhile lower elevation rain/higher elevation snow could reach Oregon into northern California Wednesday and increase in magnitude in California on Thursday as another AR strengthens. This AR event should be shorter-lived than the near term/short range AR, but some locally heavy rain amounts are possible over areas that will have quite wet antecedent conditions by that time. Currently will start with a Marginal Risk in the Day 5/Thursday ERO for northern/central California, without any Slight Risk area due to the faster movement of the AR. Risk upgrades are possible in future cycles though. Precipitation looks to persist into Friday but abate into Saturday for California, but another round of precipitation is possible for the Pacific Northwest. Snow will also be likely from higher elevations of the Cascades and Sierra Nevada across much of the Intermountain West into the northern/central Rockies. The heaviest snow amounts are likely in higher elevations, but the northern Great Basin may see snow as well. The Thursday AR may produce a better defined period of enhanced snowfall in the Sierra Nevada. The pattern will also promote strong winds especially on mountain ridges like the Sierra and central Rockies, but some gusty conditions may reach lower elevations of the Great Basin and Four Corners too. Meanwhile farther east, moisture will be present by Wednesday- Thursday in the east-central U.S. as a developing frontal wave should reach the Ohio Valley/Great Lakes by early Thursday. An expanding area of precipitation will develop, particularly across or somewhat north or south of the Ohio Valley. There continues to be some threat for flooding somewhere within the Ohio Valley/central or south-central Appalachians as instability could reach sufficiently far north, and depending on surface wave strength (especially on the weaker side of the spread) there could be strong westerly flow at low levels just above the surface to promote some training and terrain enhancement. However the wide spread for surface details and QPF placement favor holding off with any Day 4/Wednesday ERO risk areas. The Northeast and northern Mid-Atlantic have potential for wintry weather on Wednesday- Thursday where cold air hangs on. A transition zone of sleet and/or freezing rain may exist between the snow over far northern areas and the rain to the south. Energy ejecting from the West next Saturday may promote development of another wave over the central U.S. with associated northern tier snow and an expanding area of rain over the east-central U.S. Colder than average temperatures are forecast for northwestern and north-central parts of the lower 48 through the period. Montana to the Dakotas will be particularly cold, with anomalies of 20-35 degrees below normal and highs barely reaching 0F in some locations into Wednesday. These areas will likely remain below average but with some moderation later this week. Meanwhile, other areas from the Four Corners states through the East should see above to well above normal temperatures during the period. Some areas will likely see one or more days of min/max temperatures reaching 20 or more degrees above normal and some daily records are likely to be set. Current forecasts suggest that daily records may be most numerous around Wednesday-Thursday. Some slight cooling is possible into Friday-Saturday but temperatures should remain well above average across the southern tier. Upper troughing and a cold front will likely finally cool the Four Corners to parts of the south-central U.S. by next Sunday. Tate/Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw