Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 AM 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... Low amplitude upper flow is forecast across much of the lower 48 next week, but a couple of features will cause impactful weather regardless. 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 and gradually drifting south. Persistent moisture will 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 snowfall accumulations, particularly across the higher elevations. Farther east, moisture return and a strengthening low pressure system 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. 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... While model agreement in the large scale pattern is reasonably good, differences on smaller scales are yet to be resolved and could have sensible weather impacts. The first model difference of note is the axis of the AR into California at the start of the period Tuesday, as models vary on the timing of the AR's southward shift. The GFS/GEFS suite was generally on the slower side of the envelope, maintaining strong IVT into areas of northern/central California that should see the heaviest rainfall focus over the weekend/Monday, aside from the 18Z deterministic GFS that was faster/farther south. Most other guidance showed rain focusing from the Bay Area south (though perhaps farther north of the Bay's latitude farther inland in the Sierra foothills) toward parts of southern California. However, the new 00Z ECMWF did shift north somewhat and aligns well with the 18Z AIFS precipitation, so these may be a good middle ground solution. The timing/placement of the AR axis will matter for flooding concerns. Energy from the eastern Pacific upper low perhaps combining with central Canadian energy will push a shortwave across the northern tier Wednesday-Thursday and draw a surface low northeastward into the Great Lakes. 12Z ECMWF guidance including the control and ensemble members were strongest with this low, while some AI/ML models and the 12Z UKMET hinted at similar low placement though were not as strong as the 991mb operational 12Z ECMWF. The 12Z CMC and many of its ensemble members were a bit east and weaker, but the 18Z GFS were west and weaker. A middle ground solution similar to the EC placement but not as deep seemed to work for the forecast. The incoming 00Z guidance is trending a bit southeast for the low position 12Z Thursday, but the 00Z EC has adjusted to a more reasonable central pressure. The evolution of the western upper trough pivoting south and east into late week continues to be a challenging aspect of the forecast. CMC runs consistently pull the upper low farther west/offshore and then end up slow as it pivots east. The 00Z GFS/ECMWF seem to be in better alignment with the trough as it moves into the interior West. Regarding an associated surface low, the 18Z GFS was a quite strong outlier near the CA/OR border early Friday. The WPC forecast used a blend of deterministic guidance favoring the EC and GFS early in the period. Quickly transitioned to a mostly ensemble mean blend by Days 6-7 given the increasing spread particularly with the western trough. ...Weather/Hazards Highlights... A strong and long-lasting atmospheric river taking aim at California will be ongoing as the period begins Tuesday. However, there is considerable uncertainty in when and how fast this AR shifts southward across the state, as discussed in the previous section. Maintained a Marginal Risk in the Day 4/Tuesday ERO because of low confidence in the placement of heavy rain. If the heavy rainfall axis ends up on the northern side over areas that receive multiple inches of rain during the short range period, or focuses over the urban Bay Area with heavy rates, either/both scenarios could warrant a Slight Risk upgrade in future cycles. Expect the AR to continue to weaken as it shifts 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. Have no Marginal Risk outlooked for this area in this forecast cycle because of the model spread (with some indicating no precip), but will continue to monitor. Additional rounds of coastal/low elevation rain could continue into later next week. Snow will also be likely across much of the Intermountain West with this system. Into Tuesday, the heaviest snow amounts are forecast across 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 last in the Sierra, Cascades, and northern Rockies. 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 is forecast to return late Tuesday and especially Wednesday-Thursday in the east-central U.S. as strengthening low pressure system and its attendant warm front lifts northward. As this system evolves, an expanding area of precipitation will develop, particularly across the Ohio Valley. There may be a nonzero threat for flooding as instability could reach pretty far north, but held off on any Day 5/Wednesday ERO because of uncertainty in the frontal placement and thus QPF placement, which varied across the Lower or Upper Ohio Valley. The Northeast and northern Mid-Atlantic have potential for wintry weather on Wednesday-Thursday where cold air hangs on. 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. Some slight cooling is possible into late week but temperatures should remain above average in the southern tier. 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