Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 PM EST Mon Jan 29 2024 Valid 12Z Thu Feb 01 2024 - 12Z Mon Feb 05 2024 ...Atmospheric River to spread heavy precipitation across much of the West this week, then reaches the Gulf Coast by the weekend... ...Another strong weather system increasingly likely to impact California late this weekend into early next week... ...Overview... The upper level pattern is expected to become increasingly blocky through this week as a Pacific upper trough becomes elongated along the West coast by mid week, slowing down and taking on a neutral to negative tilt. This system will bring a period of active weather and heavy precipitation to the West, eventually reaching the Gulf Coast by late week into this weekend. Meanwhile, downstream ridging will become more pronounced over the Northern Plains, allowing troughing to develop over the Northeast reinforced by a few embedded shortwave troughs. The resulting omega block will dominate the period into this weekend. By late in the weekend into early next week, an active southern stream system will begin to approach California though forecast uncertainty in the timing and magnitude of that potential system is low. ...Guidance/Predictability Assessment... The model blend preferences for this forecast cycle included mainly the deterministic GFS and ECMWF to start followed by a higher weight/inclusion of the GEFS and ECENS means by day 6-7. While there remains a fair amount of model uncertainty, the main model outlier was the CMC/CMCens and therefore wasn't included at all in the forecast blend preferences. Guidance continues to be well behaved with the system coming into the West through Friday-Saturday, but with latitude and timing spread persisting for the upper low that breaks off across the southern tier from the weekend into Monday. Overall trends to be a bit faster with the system reaching the Plains, and a nudge to the north across the Plains to Southeast compared to earlier cycles. Latest trends seem to be navigating toward the middle or south-central part of the prior guidance spread. Along the West Coast by Sunday into next Monday, there is decent agreement that a southern stream Pacific system should be coming into the picture but with a lot of spread for specifics. This is due in part from various ideas for how energy within the lingering upper trough along the northern-central West Coast could interact. In general the latest model guidance has sped up the timing of this system, reaching the CA coast by Sunday/Sunday night into Monday and could lead to a variety of weather impacts. ...Weather/Hazards Highlights... The upper trough and leading frontal system reaching the West Coast by the start of the period will support a strong southerly flow of enhanced moisture and bring a period of significant rainfall and high elevation snow to central/southern California on Thursday. Guidance continues to show some instability accompanying the activity along the southern California coast, which combined with terrain enhancement could produce some intense rain rates along the Transverse Ranges in particular as the system is moving through. Therefore the Day 4 Excessive Rainfall Outlook maintains the Slight Risk area over that region for possible excessive rainfall and its impacts including flash flooding, debris flows, mudslides particularly in areas of steep terrain or burn scars. The energy with the system then quickly moves eastward through southern Arizona late in the Day 4 ERO period and while some locally heavy rainfall may occur, the intensity and duration (along with some model uncertainty) of the rainfall and rates look to be low enough to limit any flash flood concerns as of now. However, once the system reaches the Southern Plains, it taps into some Gulf moisture with developing southerly low level flow. This will likely bring a period of showers and thunderstorms over TX into southern Oklahoma late in the Day 5 ERO period. Given the faster timing and increasing model agreement for possible 1-2" of rainfall, a Marginal Risk was introduced for this afternoon update. As the upper level energy crossing the Southwest/southern Rockies likely breaks off an upper low, there will be a period of easterly precipitation enhancement along the Rockies/High Plains with widespread accumulating snow for the terrain areas, some of which could be locally heavy/significant. As the upper low continues eastward during the weekend, expect heavier rainfall to spread across eastern Texas and continue along/near the Gulf Coast into the Southeast and Florida. The far northern periphery of the precipitation shield could contain a little wintry weather across portions of the southern Appalachians but with low confidence in specifics. The eastern Pacific/California evolution by Sunday into early next week will require close monitoring. The overall pattern appears to support the potential for a somewhat heavier and/or longer duration atmospheric river event for central-southern California than the one forecast late this week, with a somewhat more direct connection to lower latitude moisture with both heavy lower elevation rainfall and heavy mountain snow possible. However there are significant detail uncertainties that do not have great predictability a week out in time, so confidence is low in the details at this time. The persistent upper ridge over and near the northern Plains will keep the northern tier quite warm through the period, with morning lows up to 20-30F above normal and daytime highs 10-25F above normal. A few daily records will be possible for warm lows. Overall expect the coverage of warmest anomalies to decrease very gradually with time. The upper trough moving into the West will bring high temperatures down to 5-15F below normal over the West Coast states/Southwest from late this week onward, with the coolest anomalies most likely over southern California into Arizona. Farther east, the southern part of the Florida Peninsula may see temperatures up to several degrees below normal into Thursday followed by a rebound to normal for a time. Passage of the strong Gulf Coast system around Sunday-Monday should drop daytime highs to below normal levels across the southern tier. The Northeast will see above normal temperatures (especially for morning lows) late this week in the warm sector of an approaching front. Then increasing agreement on a developing deep trough/upper low over the region suggests high temperatures may decline to moderately below normal levels by the weekend. Rausch/Taylor 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