Extended Forecast Discussion NWS Weather Prediction Center College Park MD 323 PM EDT Mon May 01 2023 Valid 12Z Thu May 04 2023 - 12Z Mon May 08 2023 ...Progressively cooler and wetter pattern for the West Coast/Great Basin/Rockies this week... ...Overview... The upper level pattern should eventually transition away from the stubborn omega block configuration in place through the early part of the forecast period. However ridging will likely persist over central and northwestern Canada into early next week, favoring another upper trough settling over the northeastern Pacific/West Coast in the wake of an ejecting upper low/trough while some degree of northwesterly mean flow aloft may persist over New England. The rest of the lower 48 should see a gradual trend toward flatter mean flow, which may promote a surface system emerging from the Plains by Sunday-Monday depending on the details of ejecting western energy. The forecast pattern evolution will favor one or more days of precipitation across a broad area from the northern half or so of the West through parts of the Plains/Mississippi Valley and into the South. Very cool conditions over the Southwest and Northeast, plus very warm weather over the northern Rockies/High Plains, on Thursday should gradually moderate by the weekend and early next week. ...Guidance/Predictability Assessment... Latest guidance displays some similar ideas in the larger scale but there are also significant detail differences that have much lower predictability due to an unknown degree of interaction among separate features. The primary focus for these issues involve the relative influence of a southern Canada upper low retrograding under a central Canada ridge, the upper low forecast to track northeastward from California, energy entering the overall trough immediately to the west, and a larger scale northeastern Pacific trough that should descend toward the West Coast by the weekend. Flow over the Northeast also becomes a question mark toward days 6-7 Sunday-Monday. Broadly speaking, after an average of latest guidance (with more emphasis on the 06Z GFS/00Z ECMWF) reflected the prevailing themes of guidance for about the first half of the period, the GFS/ECMWF and their ensemble means provided the best template by days 6-7 Sunday-Monday. This solution showed a steady eastward progression of the shortwave emerging from the West, and by day 7 maintained a surface low track somewhat south of the 06Z GFS on its own (as recommended by the ECMWF mean and latest GEFS means). The 00Z CMC/CMCens held onto mean ridging aloft over the east-central U.S. into day 7, thus holding the surface reflection well westward, though the new 12Z CMC is trending some in the GFS direction. There is still low confidence in how this shortwave energy will look that far out in time. Meanwhile the ensembles help to provide an anchoring influence on the forecast with the late period West Coast upper trough, as individual operational model runs vary with shortwave details. GFS runs are generally on their own with how much ridging they build into the Northeast around the start of next week, with the ensemble means maintaining northwesterly or cyclonic flow. ...Weather/Hazards Highlights... Precipitation chances will steadily increase from the West Coast through the Great Basin and Northern Rockies mid- to late week as a closed low over the west ejects northeastward over the Great Basin and eventually northward into Canada. The precipitation may be locally moderate to heavy at times, with some scattered thunderstorm chances. Snow levels will begin high but are forecast to fall through the latter part of the week as the upper-low/trough progresses inland and provides cool air aloft. The highest snow chances will be for the Sierra Nevada on Thursday. The rainfall on top of ongoing snow melt may lead to some flooding concerns in terrain sensitive areas around the foothills of local mountain ranges as well as along local streams/rivers Thursday-Friday, but this will likely be driven more by local hydrological conditions harder to anticipate at this range. However there are gradually improving guidance signals for locally heavy rainfall potential over parts of the Great Basin/Northern Rockies, sufficient to propose adding Marginal Risk areas in the Days 4-5 (12Z Thursday-12Z Friday, 12Z Friday-12Z Saturday) Excessive Rainfall Outlooks over portions of this region. Moist, upslope flow settling in along portions of the northern/central High Plains and possibly a separate north-south convergence axis farther east may promote locally moderate to heavy rainfall late this week into the weekend. The Day 5 outlook proposes a Marginal Risk area over the north-central Plains to reflect the best overlap of guidance parameters for this potential activity. The next upper trough approaching/reaching the West Coast should bring additional precipitation to areas from the Pacific Northwest into the northern Rockies during the weekend and early next week, with some potential for the moisture shield to extend a little farther south as well. To the east, Gulf moisture flowing northward as well as additional moisture sourced from the Pacific will spread northeastward across the Central/Southern Plains and Middle/Lower Mississippi Valley into the Southeast mid- to late week leading to a broad region of showers and thunderstorms. The evolution of the pattern has remained rather nebulous the last few days as the guidance has varied on the timing and evolution of a 'backdoor' cold front sinking southwestward as well as embedded shortwave energy aloft, lessening confidence on the specifics of regions of heavier rainfall and potential flash flooding concerns. However, the guidance has shown the highest confidence in widespread convection and high QPF amounts over the Central Plains/Middle Mississippi Valley on Thursday, where a Day 4 Marginal Risk area continues in the Excessive Rainfall Outlook. This convection may trend downstream Friday into the Tennessee Valley, with the potential for training appearing sufficient to support a Marginal Risk area on the Day 5 outlook. Additional heavy rainfall chances look possible over the weekend more broadly across the central/southeastern U.S., but confidence remains low given the guidance spread in how the surface pattern and supporting shortwave dynamics will evolve. The persistent cool/wet pattern over the Great Lakes and Northeast due to the stubborn upper-level low will finally start to moderate later this week as the low departs over the Atlantic, with eastern U.S. highs finally reaching near to slightly above normal levels by the weekend and early next week after starting the period 5-15F below normal on Thursday. Thursday will also see some temperature extremes farther west, with highs at least 15-20F above normal over the northern half of the High Plains/Rockies and 10-20F below normal over the southwestern states. Thereafter, below normal high temperatures should spread across more of the West but with less extreme anomalies, while moderately above normal temperatures will settle over the central U.S. Rausch/Putnam 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 Hazards: - Heavy rain across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin, Fri, May 5. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Thu, May 4. - Severe weather across portions of the Southern Plains, Thu, May 4. - Flooding possible across portions of the Northeast, the Central Great Basin, the Northern Plains, the Northern Rockies, the Middle Mississippi Valley, the Northern Great Basin, and the Southeast. - Flooding occurring or imminent across portions of the Northern Plains, the Northern Rockies, the Middle Mississippi Valley, the Northern Great Basin, the Upper Mississippi Valley, and the Southeast. - Flooding likely across portions of the Upper Mississippi Valley, the Middle Mississippi Valley, the Northern Plains, the Northern Rockies, and the Northern Great Basin. - Much above normal temperatures across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin, Thu, May 4. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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