Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 PM EDT Tue Apr 30 2024 Valid 12Z Fri May 03 2024 - 12Z Tue May 07 2024 19Z Update: The models remain in good overall synoptic scale agreement for the end of this week, so a general deterministic model blend works out well for Friday. This also holds true going into Saturday, with some gradual timing differences with the trough approaching the West Coast region. The GFS starts becoming more amplified with the trough/closed low over California, and thus leads to more enhanced precipitation across this region. More substantial differences become apparent across the West going into Sunday and beyond. The 00Z ECMWF was on the northern edge of the guidance with the upper low across the Northern Rockies. The AIFS from the ECMWF suggests a position farther south, more in line with the CMC, but not to the degree of the GFS. Looking ahead to next Tuesday, the CMC is stronger with the next trough approaching the Pacific Northwest. The incorporation of the ensemble means increased to about 40% for early next week, while still maintaining some previous WPC continuity. The previous forecast discussion, along with an updated ERO description, is appended below. /Hamrick ------------------ ...Overview... For late this week, lifting upper troughing and a couple of surface cold fronts moving east by Friday should push rain chances across the east-central and eastern U.S., but a subtropical jet and the western side of the front stalling will lead to additional threats for heavy rainfall in the south-central U.S. this weekend as well. Meanwhile, the West is likely to see some precipitation including some higher elevation snow during the period, but confidence in the details is still lower than desired given ample spread in the model guidance. ...Guidance/Predictability Assessment... As the medium range period begins Friday, a consolidating upper low should be located near the U.S./Canada border with a surface low nearby over northern Minnesota or so, while upper ridging is atop much of the East. Model guidance is within reasonable agreement with these features though with some detail differences. The bigger model diagnostics challenge is with an upper low dropping southeast through the eastern Pacific coming toward the West Coast this weekend. Deterministic models and ensemble members/means and AI/machine learning (ML) models vary with the positioning of this low. Even early in the medium range time frame, GFS runs have persistently been on the southwestern side of the spread, which then leads to the upper low farther south into California and the Southwest as it translates east early next week. Meanwhile, ECMWF runs do not dig the trough as far south, taking it across the Northwest over the weekend and quickly into the northern Plains. The 12Z EC ensemble mean was similarly shallow and fast with the trough. The 12Z ML models were generally in between these southwest and northeast extremes with the feature, though there were some variations. The CMC and UKMET have been in between, though the 12Z UKMET seemed to be a fast outlier. The incoming 00Z models seem to keep the same sorts of biases as the 12/18Z cycle. In going for a middle ground solution for the western low, which seems the most realistic result, using a blend combining the faster/shallower ECMWF with the slower/deeper GEFS mean (not quite as slow/deep as the deterministic GFS runs) was favored for the WPC model blend. With the preference for an intermediate solution, did not favor the GFS-type QPF, so backed away from the precipitation's southern extent into California and the central Great Basin by combining the NBM with the ECMWF/CMC. The GFS/GEFS mean/ensemble bias corrected solutions were farther south than preferred. ...Weather/Hazards Highlights... Scattered to widespread showers and thunderstorms are forecast from the south-central U.S. northeastward into the Lower Mississippi, Tennessee, and Ohio Valleys on Friday ahead of a frontal boundary. While the threat for heavy rainfall to cause flash flooding is lower than earlier in the week owing to waning upper level support, a Marginal Risk remains in place for the Day 4/Friday ERO for nuisance level flooding issues in some areas. The Marginal was expanded some to the southeast compared to the overnight issuance owing to a slightly faster frontal progression. By Saturday, the subtropical jet gets renewed across the Southern Plains and more focused convection is becoming likely in a moist and unstable environment. A Slight Risk area is now planned for portions of southwest Oklahoma and into portions of northwest Texas where a stronger model signal exists for an MCS with heavier rainfall rates. Many areas in the south-central U.S. will have quite saturated antecedent conditions because of recent and upcoming short range events, so this will be a factor for flooding as well. Additionally on Saturday, an instability axis is likely to advect northward across the southern/central Appalachians and into the Piedmont between the synoptic cold front and backdoor front in the East as a shortwave aloft moves through. Some heavy rain is likely for these areas so a Marginal Risk for flash flooding remains valid. Showers and thunderstorms should continue for portions of the central and eastern U.S. on Sunday. A round of light to moderate precipitation including higher elevation snow in Wyoming to north-central Colorado is possible on Friday. Then the Pacific upper low will provide support to spread precipitation into the Northwest and steadily eastward into the weekend and early next week. But the Pacific Northwest into the northern Rockies should see lower elevation rain and higher elevation snow, with precipitation reaching the northern Plains by Monday. A Marginal Risk area is now planned from northwestern California to western Washington for Day 4, and a Marginal Risk for western portions of the Sierra on Day 5 (and snow for the Sierra ridges). Warmer than normal temperatures by 10-15 degrees are forecast for the Ohio Valley to Great Lakes and Appalachians on Friday. Over the weekend a backdoor front may cool the Mid-Atlantic to a few degrees below normal for highs, while the east-central U.S. stays above normal but less so than on Friday. Farther west, the northern Rockies and Plains can expect cooler than normal temperatures through late week, but the Intermountain West to that area should warm up over the weekend to above normal. Warm temperatures should spread across much of the eastern two-thirds of the country next week. 90s are forecast to become more widespread across the southern tier, with temperatures over 100F into southwest Texas. On the other hand, the West Coast states are likely to see cooler than normal temperatures early next week, but with the details uncertain because of model spread with the trough aloft. Tate/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