Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Sat Jan 27 2024 Valid 12Z Tue Jan 30 2024 - 12Z Sat Feb 3 2024 ...Large scale system likely to spread precipitation across much of the West next week with highest totals expected over California... 19Z Update: The main feature of interest next week continues to be the amplifying upper trough across the Western U.S. and the pattern change that will commence with it. The 12Z model guidance suite is generally in very good agreement on an atmospheric river event across the West Coast starting Wednesday and then moisture moving inland across the Intermountain West and Rockies later in the week. However, the 12Z CMC continues to be a more progressive solution with arrival of rain and mountain snow across California compared to the UKMET/GFS/ECMWF and the ensemble means, so a non-CMC blend was incorporated beyond Tuesday to account for this. By next Saturday, the low is expected to reach the western High Plains and begin tapping into Gulf of Mexico moisture that will fuel increasing coverage of showers and thunderstorms, some of which could be strong to severe across portions of Texas. Overall, the ECMWF is generally a little faster in bringing the surface low out across the southern Plains Saturday night, and the CMC slower, whereas the ensemble means suggest the low should be near the Texas Panhandle by 00Z Sunday. Across the Eastern U.S., model differences are more evident with shortwaves and closed lows to the east of the upper ridge axis. The CMC is much faster and weaker with the trough over New England by next Friday, whereas the ECMWF supports a closed low over the Ohio Valley and the GFS shows an elongated trough axis over the Great Lakes. Taking these factors into account, the updated WPC fronts/pressures forecast was primarily derived from a GFS/ECMWF/ECENS/GEFS blend along with about 20% of previous WPC continuity for the second half of the forecast period, with gradually increasing percentages of the means by Friday and Saturday. The previous forecast discussion is appended below for reference. /Hamrick -------------------------- ...Overview... Latest models and ensembles generally suggest that the large scale pattern will take on an increasingly blocky look with time. This evolution will feature an initial Rockies upper ridge settling over the central U.S. and extending north into Canada while a Pacific upper trough reaching the West eventually becomes negatively tilted and East Coast mean troughing will comprise one shortwave passing through around midweek and then reinforcing energy farther north toward the end of the week. Continue to expect strong and moist southerly flow ahead of the Pacific trough to support an atmospheric river event whose focus should progress southward along the West Coast over the course of the week. A broad shield of rain/snow will likely extend farther inland over the West after Wednesday and rain should develop over parts of the Plains by Friday or Saturday. Over the East, an area of light to perhaps moderate rain/snow should accompany a midweek system while the Northeast could see some snow late in the week as reinforcing energy aloft settles into the mean trough position. ...Guidance/Predictability Assessment... With respect to the larger scale aspects of the forecast, guidance has generally been more agreeable and consistent with the incoming western U.S. upper trough versus the two primary features comprising the East Coast mean trough. Continue to favor a consensus/average forecast over the West where there is a persistent spread for timing of the leading height falls (GFS slow, CMC fast) with no pronounced clustering thus far and differences well within typical guidance error at that time frame. What trends exist over the past day are just slightly slower as of midweek. The new 00Z GFS has added a new wrinkle in the surface forecast off the Pacific Northwest coast, ejecting the leading deep surface low north of other guidance and then tracking a trailing wave closer to the coast than other guidance on Wednesday. Model/ensemble consensus agrees upon the increasingly negative tilt of the upper trough by Friday-Saturday with typical detail uncertainties. Over the East, the leading system still exhibits significant spread but with recent trends gravitating more toward the slower/closed upper dynamics initially advertised by GFS runs. The new 00Z UKMET is a fast/open extreme while the new 00Z ECMWF has adjusted a little faster than the 00Z GFS/GEFS/CMC and 12Z ECens majority. Then guidance has been showing a lot of spread and variability with upstream energy likely to amplify into the Northeast and vicinity toward the end of the week. Through the 12Z/18Z cycles, solutions varied among open troughing or a closed low with most not notably deep. In contrast, 12Z ECMWF-initialized machine learning models generally suggested the potential for this troughing to develop up to a day earlier and in deeper fashion (and most with a closed low) than the GFS/ECMWF and their means (with the CMC/CMCens a little farther east). New 00Z model trends seem to be supporting a deeper upper low over or near the Northeast. Guidance comparisons led to updating the forecast with a 12Z/18Z operational model composite early with gradually increasing input from the 18Z GEFS/12Z ECMWF means to yield a half model/half mean mix by the end of the period next Saturday. ...Weather/Hazards Highlights... Guidance has made only incremental adjustments for the broad area of precipitation expected to spread across the West from Tuesday onward. Initial approach of the Pacific upper trough Tuesday-Thursday will support a strong southerly flow of enhanced moisture and likely bring a period of significant rainfall and high elevation snow to the West Coast states. During the period covered by the Days 4-5 Excessive Rainfall Outlooks (early Tuesday through Wednesday night), expect the heaviest rainfall to reach northern California around Wednesday when a Slight Risk area remains in the updated Day 5 outlook, with only minimal adjustments needed. This outlook area corresponds to the best overlap of guidance signals within persistent timing spread (GFS slowest, CMC fastest) that affects the southward extent of precipitation. The surrounding Marginal Risk area reflects an intermediate timing of significant but less extreme rainfall potential. The average of guidance has nudged a little slower with the arrival of this moisture late in the Day 4 period. This reduces confidence in the established Day 4 Marginal Risk area, but for now it is being maintained for the updated outlook. There is a Marginal Risk area over the Olympic Peninsula in western Washington on both days. Moisture anomalies and their persistence are better during Day 4 but guidance still shows some relative QPF maxima into Day 5 with existing wet conditions. From Thursday onward, moisture will push farther south through California with some heavy precipitation, as well as through much of the Interior West/Southwest/Rockies with rain and higher elevation snow being enhanced over southward facing terrain. Expect some rain of varying intensity to develop over portions of the central U.S. by Friday or Saturday. The system crossing the East Tuesday-Wednesday will produce an area of precipitation (exact rain/snow line somewhat uncertain at the moment) that should be mostly in the light to moderate range. Then upper troughing and potential surface development may produce some snow over the Northeast by Friday-Saturday. The northern-central Plains into Upper Midwest will see well above normal temperatures through the period, with northern areas tending to see the best chance for multiple days of min/max temperatures at least 20F above normal. Some morning lows could be up to 25-30F or so above normal and a few daily records will be possible. The West will be warm early in the week with somewhat less extreme anomalies (generally up to plus 10-20F), but that may still be warm enough to produce some daily records. The upper trough moving in should bring a cooling trend for the latter half of the week with highs over California and an expanding portion of the southwestern U.S. trending below normal Thursday-Saturday. Parts of the Southwest may see highs 10-15F below normal by the end of the week. The above normal min temperatures will take longer to trend toward normal due to clouds and precipitation. Over the East, the Florida Peninsula should see temperatures up to 5-10F below normal Tuesday-Wednesday and coastal New England should see similar anomalies for highs on Tuesday. Otherwise expect near to above normal temperatures, albeit with some uncertainty for highs over the Northeast by Friday-Saturday depending on depth of upper troughing. 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#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