Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Feb 1 2025 Valid 12Z Tue Feb 04 2025 - 12Z Sat Feb 08 2025 ...Atmospheric river event likely to last into Tuesday for California, with additional rounds of precipitation across the West through much of next week... ...Overview... Guidance shows low amplitude upper flow across much of the lower 48 next week, but the overall pattern and embedded features will still produce areas of impactful weather. A slow moving upper low drifting near/offshore Vancouver Island and the Pacific Northwest will sustain a strong atmospheric river into California as the period begins Tuesday. The moisture axis may drift south for a time but guidance is starting to cluster toward the idea of another system bringing a rebound of rain/snow after midweek. Persistent moisture will also lead to additional rounds of precipitation for much of the Intermountain West through most of next week. This includes snow with the potential for significant accumulations, particularly across the 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 the Northeast. Shortwave energy ejecting from the West next Saturday may support a central U.S. frontal wave with expanding precipitation. The quasi-zonal flow pattern with subtropical ridging near the Bahamas/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 much below normal temperatures there with coldest anomalies Tuesday-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. Latest model runs still show a lot of spread and variability for frontal wave development forecast to reach the Ohio Valley/Great Lakes by Thursday and continuing rapidly onward thereafter, but seem to be starting to converge somewhat for a potential system tracking into California around Thursday or so, along with mean troughing that should move into the West by Saturday. Most guidance initially shows good agreement on Tuesday, with a weak Pacific frontal wave tracking into California Tuesday-Tuesday night and helping to maintain enhanced precipitation over parts of the state. However solutions rapidly diverge for the developing eastern system as well as over the eastern Pacific/western U.S. Recent and new 12Z guidance shows significant spread and waffling for details of wave development from the east-central U.S. through and beyond the Northeast. UKMET runs continue to be on the strong side, while ECMWF runs have become more erratic and the GFS remains on the weak side. CMC/CMCens runs have been running fast but the 12Z run has nudged a little slower. Machine learning (ML) models have consistently favored the weak side of the spread, and seem to have trended toward more potential for secondary wave development that would track offshore New England. This offers support for including the GFS as part of a compromise, though the 12Z GFS appears overdone with how far southward the offshore wave tracks. Meanwhile, guidance has varied widely for the exact shape of flow around the initial upper low offshore the Pacific Northwest and Vancouver Island, as well as for another mid-lower latitude wave that may track to the south. Guidance had been split as far as whether a well-defined wave would track toward California after midweek, though latest ML runs are starting to show more support for an evolution like recent GFS runs in contrast to the slow/weak ECMWF. New 12Z runs including the ECMWF seem to be clustering better toward a well-defined system arriving around Thursday or so. Preference leaned toward depicting this system but in weaker fashion given the uncertainty through the 00Z/06Z cycles. Most ML models matched up fairly well with the ensemble means and an average of recent ECMWF/GFS runs for the overall upper trough that should reach the West Coast around early Friday and continue inland in positively-tilted fashion by Saturday. 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. In contrast, the 00Z CMC trended out of phase relative to consensus by the end of the period. Guidance comparisons/preferences led to starting the 500mb/surface forecast with half 00Z ECMWF and half 00Z/06Z GFS for the first half of the period, with a transition toward some incorporation of the 06Z GEFS/00Z ECens after early Thursday. ...Weather/Hazards Highlights... A strong and long-lasting atmospheric river taking aim at California will be ongoing as the period begins Tuesday. Guidance is still indicating that subtle detail differences at the surface and aloft, though with general agreement that arrival of a weak frontal wave will support enhanced precipitation, will affect the precise axis of heaviest rainfall over coastal/low elevation areas somewhere within north-central to south-central parts of the state. The overall combination of model guidance, excessive rainfall first-guess fields, and prior days of heavy rainfall seem to provide enough support to introduce a Slight Risk area in the Day 4/Tuesday ERO. This area extends from the coast 100-150 miles north and south of San Francisco into westward facing terrain of the Sierra Nevada below the snow level. The moisture feed into the West Coast should trend weaker/southward into Wednesday, leading to reduced rain amounts. There is some potential for rain to linger across areas of southern California over recent burn scars that would be sensitive to moderate rain, but model guidance varies. The Day 5 ERO depicts no Marginal Risk areas for this region because of the model spread (which includes potential for little to no rainfall), but will continue to monitor. Then latest guidance is showing a better signal for what could be a fairly vigorous system reaching California around Thursday/Thursday night with another episode of significant rainfall along with the possibility of some gusty winds. Snow will also be likely from higher elevations of the Sierra Nevada across much of the Intermountain West into the northern Rockies. Into Tuesday, expect the heaviest snow amounts from the Sierra Nevada into the northern Rockies, while the northern Great Basin also receives snow. Some snow should extend into the Wasatch and central Rockies for midweek and beyond while some moderate snows persist in the Sierra, Cascades, and northern Rockies. The possible Thursday system 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 likely return late Tuesday and especially 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 5/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 next week. 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 Tuesday-Wednesday. These areas will likely remain below average but with some moderation later next 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 next week. Current forecasts suggest that daily records may be most numerous around next Wednesday-Thursday. Some slight cooling is possible into Friday-Saturday but temperatures should remain well above average across the southern tier. Rausch/Tate 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