Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Jan 29 2024 Valid 12Z Thu Feb 01 2024 - 12Z Mon Feb 05 2024 ...Large scale system likely to spread precipitation across much of the West this week and then the Gulf Coast by the weekend... ...Overview... Most guidance still shows the development of a blocky regime over the coming week. A Pacific upper trough reaching the West late this week will become negatively tilted by Friday, followed by the southern part of the trough elongating eastward and breaking off an upper low that undercuts a persistent Plains into Canada upper ridge during next weekend. This feature is likely to support a Gulf Coast storm that may be rather deep for a track that far south. Toward next Sunday-Monday, a southern stream Pacific system may approach California, with a lot of uncertainty over what role lingering trough energy near the northern half of the West Coast may play. Meanwhile most models/ensembles maintain the idea of a deepening Northeast upper trough with anchoring low. Strong and moist southerly flow ahead of the Pacific trough will support areas of heavy rain/mountain snow over central-southern California into Thursday. Then a broad area of rain/snow will likely extend farther inland over the West with rain developing over parts of the Plains by Friday-Saturday. This moisture, supporting potentially heavy rainfall, should then stream across the Gulf Coast and Southeast/Florida. The Northeast may see some precipitation mostly in the form of snow late in this week and into the weekend due to the deepening upper trough and associated surface system. ...Guidance/Predictability Assessment... Comparison of 12Z/18Z operational models/ensembles along with 12Z ECMWF-initialized machine learning models led to a guidance preference consisting of various weights of the 12Z/18Z GFS and 00Z-12Z/28 ECMWF during the first half of the period and some inclusion of the 18Z GEFS/12Z ECens by Sunday-Monday. There is still a fair amount of spread for exactly how quickly and deeply the Northeast upper trough evolves, with the flatter/eastward CMC-CMCens a notable extreme compared to other guidance (including the machine learning models). The overall average of most agreeable models/means has been trending deeper over recent days. 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. Latest trends seem to be navigating toward the middle or south-central part of the prior guidance spread, as the GFS has nudged a little north from earlier southern solutions while the previously northward CMC has adjusted to the south. Increased spread versus 24 hours ago in the 12Z ECMWF-initialized machine learning models tempers confidence in specifics for now. Along the West Coast by 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 12Z ECMWF-initialized machine learning models suggest the surface system could approach California more quickly than in some operational model runs--more similar to the GEFS/CMCens means. ...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. System progression should be sufficient to yield lower precipitation totals than forecast over northern California on Wednesday. However 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 new Day 4 Excessive Rainfall Outlook plans to introduce a Slight Risk area over that region within the existing Marginal Risk area. It is also worth noting that this setup has persisted well in the guidance over the past day and at least one first-guess guidance source suggests at least a Slight Risk threat level. As rain and higher elevation snow of varying intensity spreads through the West, there is some signal for relatively greater enhancement over south-central Arizona on Day 5/Friday but the corresponding ERO depicts no risk area as this activity should remain localized and fairly well below flash flood guidance values. The same is the case for developing rain/thunderstorms over Texas, but this region merits monitoring as a slightly faster trend in development would yield higher totals. 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 central-southern Rockies/High Plains. 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 but with low confidence in specifics. The eastern Pacific/California evolution by 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. 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. Upper troughing/embedded low over the Northeast and potential surface development may produce some precipitation over the region from late week through the weekend. Most of this precipitation should be snow from Friday onward. There will be a couple days with northern New England seeing 10-30 percent probabilities for seeing 0.25 inch liquid in the form of snow. 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 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