Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sat Oct 19 2024 Valid 12Z Tue Oct 22 2024 - 12Z Sat Oct 26 2024 ...General Overview... The weather pattern across the continental U.S. on Tuesday will feature an upper ridge and surface high over the Eastern U.S., accompanied by above average temperatures. An incoming trough from the Midwest and a stronger trailing northern tier U.S./Canada trough will then dislodge this ridge while the associated cold front will herald the arrival of cooler weather towards the end of the week. The front will have limited moisture to work with so rainfall amounts from the Great Lakes to the Northeast will be generally light. Splitting elongated trough energy should produce a Pacific storm system expected to approach the Pacific Northwest late this week, but with uncertain timing/strength/track. This system and possibly a trailing front on Saturday should bring a return of rain and high mountain snow from northern California to Washington state. Ahead of the Pacific system, expect mean ridging aloft and above normal temperatures over the Interior West/Rockies and then into the central U.S. ...Guidance/Predictability Assessment... Models and ensembles continue to have difficulty figuring out some details resulting from the splitting of energy initially within a Canada into eastern Pacific elongated upper trough. While there are still differences with specifics of the northern energy that traverses the northeast quadrant of the lower 48 as a fairly vigorous shortwave around Wednesday-Thursday, this feature has generally been better behaved in the guidance than the closed low expected to form over the Pacific (then lift northeastward as it opens up) or smaller scale energy that may pinch off near Vancouver Island or the far Northwest U.S. and then drop southeastward. Behind these uncertainties, there is actually better than average agreement for Bering Sea/Aleutians flow that should quickly develop Northeast Pacific troughing toward the end of the week. At the moment there is no clear clustering and seemingly as much spread as in recent runs for where the eastern Pacific low will close off plus its northeastward timing as it opens up, as well as for one or more smaller scale pieces of energy that may originate from near/offshore the Pacific Northwest/Vancouver Island. CMC runs remain on the faster side of the spread for the Pacific system, with latest GFS runs tending to lean closer to the CMC than the slow ECMWF mean runs. Latest machine learning (ML) models reflect a lot of this spread as well, while the average surface low track has generally trended a bit southward versus 24-36 hours ago--but still within a range of the Pacific Northwest to Vancouver Island. There is also a little better agreement today that the opening of the upper low should support just moderate depth for the surface low. Overall prefer a solution closest to continuity, which is in the middle to faster half of the spread, while waiting for clearer clustering. This timing difference affects the pattern over the central U.S. by next Saturday. Interestingly, the 00Z ML models have become more defined with lingering Pacific Northwest/Vancouver Island energy that ultimately translates southeastward like ECMWF/UKMET runs. Such a solution would produce a more defined surface wave and even a trailing cold front over the central U.S. versus a broad southerly gradient as seen in most other guidance. The favored blend yields merely a wavy warm front at this time, with a more prominent consensus required to adjust toward the alternate scenario. The updated forecast started with a blend of the 00Z/06Z operational models (though splitting ECMWF input with the 12Z/18 run) with least weight to the CMC for the first half of the period. Then the forecast added some 00Z ECens/CMCens means (GEFS remaining questionably weak for the Pacific system) and WPC continuity to complement the 06Z GFS and two ECMWF runs for the rest of the period to maintain reasonable stability given persistent guidance spread. ...Weather/Hazards Highlights... The cold front crossing the Great Lakes and into the Northeast Wednesday into early Thursday will likely have just enough moisture and lift to produce some mainly light to moderate showers, mainly north of the Interstate 70 corridor. It should continue to remain dry and mostly sunny for areas farther to the south across the Mid-Atlantic and the Southeast U.S., where the very pleasant fall weather will continue for a while. There are no risk areas in the Day 4 and Day 5 excessive rainfall outlooks. Uncertain small- scale shortwave energy could produce some pockets of precipitation from the northern Rockies southeastward mid-late week but with very low confidence in existence or other details. The pattern will become favorable for the return of moisture to the Pacific Northwest late this week into the weekend. However there is continued spread for a system expected to develop over the eastern Pacific by midweek and lift northeastward thereafter. Rain and high elevation northern Cascades snow could commence as early as late Thursday per the fast side of the spread, or take until late Friday or so on the slower side. Behind this system, Northeast Pacific upper dynamics and an associated surface front should help to maintain precipitation through Saturday. Currently this looks more like a broad precipitation event rather than a focused atmospheric river, but some locally enhanced totals will be possible. Lighter precipitation may reach the Northern Rockies. Temperatures will feel more like September across areas from the central U.S. into the Northeast Tuesday into Wednesday, with highs running 10-20 degrees above average for late October and possibly even a little higher over parts of the Upper Midwest/northern Great Lakes on Tuesday. A cold front pushing rapidly eastward from the northern Plains will bring more seasonable conditions after midweek. From late week into the weekend, a building Western U.S. upper ridge that pushes eastward ahead of the Pacific storm system should promote a warming trend first over the Intermountain West and Rockies/western High Plains and then covering much of the central U.S. where many areas should see highs 10-20 degrees above normal. The West Coast states should be within a few degrees on either side of normal aside from a brief warmer period over parts of California around midweek. Rausch/Hamrick 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