Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Oct 16 2023 Valid 12Z Thu Oct 19 2023 - 12Z Mon Oct 23 2023 ...Overview... Guidance continues to suggest that a strong upper ridge over the West at the start of the forecast period early Thursday will support eastern U.S. trough amplification and potential for deepening low pressure near the East Coast by the weekend. However important details of this development will rely on smaller scale and thus lower predictability shortwaves feeding into the overall mean trough, so it will take additional time to determine with some precision what the associated precipitation and wind effects will be. Behind this system expect the western U.S. ridge to weaken some and progress into the Plains/Mississippi Valley by the start of next week while a Pacific trough of moderate depth moves into the West and brings precipitation into the Pacific Northwest and northern Rockies. The forecast pattern will favor a broad area of above normal temperatures over the West and into the Plains late this week, followed by a cooler trend over the West as the upper trough arrives. With the exception of above normal readings over New England late this week, most of the East should see near to below average highs for most of the period. ...Guidance Evaluation/Predictability Assessment... While models and ensembles have been reflecting the amplified western ridge/eastern trough theme, there are still significant uncertainties with how the eastern trough energy will evolve and affect the surface evolution. Already at the start of the period, the latest GFS/GEFS mean runs show more southwestward amplitude of the initial trough crossing the Mississippi Valley/southern Plains versus other guidance (leading to greater southwestward extent of QPF on Thursday). Consensus shows the initial compact upper low crossing the Midwest ejecting quickly as upstream energy rounding the western ridge dives into the mean trough. The precise evolution of this energy as it reaches the East Coast, along with what influence there may be by another upstream shortwave (which could produce a separate surface wave/front), will determine the strength/track/timing of East Coast low pressure. Aside from the progressive 00Z/15 ECMWF run, recent GFS/ECMWF runs have been the most consistent with the general idea of a Mid-Atlantic wave becoming deeper as it reaches New England or vicinity by Sunday (when an upper low should be over or near the region). ECMWF mean runs have been hinting at this theme too, while GEFS means have tended to maintain greater definition of the initial inland surface low. The new 00Z GEFS mean is giving equal weight to a coastal wave though. Earlier/southward upper low formation in the 00Z UKMET leads to stronger Mid-Atlantic low pressure, while in contrast the 00Z CMC opens up its system to yield a weaker and progressive surface low. Prior CMC runs were closer to the GFS-GEFS/ECMWF-ECens cluster but faster. For the favored deterministic forecast based on 12Z/18Z guidance, a compromise among the 12Z/18Z GFS, 12Z ECMWF, and 18Z GEFS/12Z ECens means seemed to offer the best starting point. Confidence remains somewhat below average given the low predictability of important shortwave details. For the most part the upper trough entering the West late in the period has been much better behaved than the eastern system. The main consideration was to exclude the CMC as its latest runs have been straying to the amplified side of the guidance spread. An average of remaining model and ensemble guidance represents this feature well. ...Weather/Hazards Highlights... Currently there are no risk areas depicted in the Days 4-5 Excessive Rainfall Outlooks that cover the Thursday through Friday night time frame. The front extending south/southwest from Great Lakes low pressure late this week will produce areas of rainfall of varying intensity, with some continued uncertainty over precise coverage and amounts. As of Thursday, moisture and instability do not appear sufficient to yield a meaningful potential for intense rain rates. By Friday-Friday night moisture/instability could increase somewhat over parts of the Mid-Atlantic into southern New England just ahead of developing low pressure, with some guidance signals for locally heavier rainfall. Confidence in specifics is too low at this time to define any risk area though. Going into Saturday, the most likely forecast scenario would have increasing rainfall and strengthening winds over the Northeast with brisk to strong winds continuing through at least Sunday. Depending on system evolution/track, highest elevations could see a little snow. Lower predictability of important details of upper dynamics supporting this system suggest more time will be required to resolve precipitation and wind effects. Much of the West will be dry through late this week. The upper trough/leading surface front moving into the region should bring light to perhaps isolated moderate rainfall to the Pacific Northwest by the weekend, with this moisture continuing into the northern Rockies by Sunday-Monday. Some of the precipitation could fall as snow at high elevations of the northern Rockies. A broad area of very warm temperatures will extend across the West into the Plains late this week, followed by a steady cooling trend over the West leading to modestly below normal highs by Sunday-Monday as the upper trough moves in. The warmth over the central U.S. will also decrease in coverage by the start of next week with the northern Plains cooling toward normal for highs. Within the area of above normal temperatures, expect many locations to see one or more days with plus 10F or greater anomalies, and best potential for some plus 15-20F anomalies should be over the northern Rockies/High Plains late this week. Some daily records for highs/warm lows will be possible over the West late this week and temperatures could challenge record highs over parts of southern Texas Friday into the weekend. Meanwhile over the East, New England will see above normal temperatures Thursday into Saturday but the rest of the eastern U.S. should see a return of cooler than normal highs Friday onward with the amplified upper trough crossing the region. 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