Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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377 FXUS02 KWBC 140659 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue May 14 2024 Valid 12Z Fri May 17 2024 - 12Z Tue May 21 2024 ...Another southern tier heavy rainfall threat during the latter half of the week... ...Overview... Downstream from a reorienting eastern Pacific upper ridge, guidance is generally suggesting flat to broadly cyclonic mean flow late this week into the weekend, followed by a tendency toward more upper troughing over the West early next week. There are still a lot of lower-predictability detail questions about flow rounding the ridge and reaching the lower 48. There is also considerable spread for the details/timing of an upper shortwave and associated surface system crossing the eastern half of the country late this week into the weekend. Continue to expect the heaviest convective emphasis to be across the southern tier late this week into the weekend, with some areas of less extreme but potentially meaningful rainfall farther north. ...Guidance/Predictability Assessment... Over recent days the dynamical models/ensembles and machine learning (ML) models have been showing difficulty in resolving important but medium to smaller scale details within progressive Northeast Pacific into lower 48/southern Canada flow, and then how potentially better defined western troughing may evolve (influencing when an upper low drifting toward California accelerates inland). There has been plenty of spread and run-to-run variability over the East as well, with significant differences for timing/details of shortwave energy and associated surface reflection late this week onward plus what the larger scale flow will look like by early next week. Based on comparisons of 12Z/18Z guidance, a composite of operational model solutions provided the most palatable starting point. At the start of the period Friday the 12Z ML models offered more support toward flatter flow over the Northwest than the GFS/GEFS. By mid-late period, the ML models generally leaned toward the cluster of 12Z ECMWF/CMC and their ensemble means which were more progressive than the GFS/GEFS for the system crossing the East and moving into the Atlantic, and showed less pronounced digging south of an upper low that may reach southwestern Canada and the northwestern U.S. (but with spread for incorporation of the southern stream Pacific into California upper low) This majority cluster also suggested that by next Tuesday there should be some degree of upper troughing over eastern North America with ridging over the Plains, in contrast to a flat ridge reaching the East with southwesterly flow over the Plains. Thus the forecast leaned more toward the 12Z ECMWF-ECens/CMC-CMCens scenario. The one reservation was that incorporation of the Pacific/California low could have been a little slow. New 00Z guidance continues to highlight the volatility of the pattern, with the UKMET/CMC trending toward the GFS over the Northwest as of early Friday. Then the average of guidance trends slower with more separation for the eastern U.S. upper shortwave late week into the weekend, while the ECMWF has flipped somewhat in the GFS direction for the pattern over the East by next Tuesday. ...Weather/Hazards Highlights... Another southern tier convective event should be in progress at the start of the period early Friday. The Day 4 Excessive Rainfall Outlook that covers the Friday-Friday night time frame continues to depict a Slight Risk area that extends from Louisiana into Alabama, with some adjustment from continuity to reflect latest guidance. This area will be sensitive to additional rainfall given wet soil conditions. Frontal/wave focus with available moisture and instability along with potential for repeat rains may enhance totals. The surrounding Marginal Risk area accounts for some potential of a convective axis hanging back even farther to the west (still most pronounced in GFS runs). Farther north, latest ECMWF/GFS/CMC/ICON runs are starting to cluster toward a potential axis of less extreme but locally heavy rainfall over the Ohio Valley in association with a front affecting the region. Short-term rainfall should lead to somewhat wetter soil than initially observed. The Day 4 ERO introduces a Marginal Risk area to account for the developing guidance cluster. By Day 5 Saturday, the only coherent area of potentially heavy rainfall is over portions of the Southeast as the southern tier surface front/waviness continue onward. A Marginal Risk provides a reasonable starting point given the guidance scatter within this region. Confidence is much lower for rainfall coverage and totals farther northward over the East. Otherwise, some rainfall could linger along parts of the East Coast into Sunday if the slower side of the envelope verifies for the system affecting the region. Some areas of rainfall may develop over portions of the northern-central Plains into the Midwest by the weekend or early next week depending on shortwave energy/surface details, with low confidence for specifics of intensity at any particular location. Portions of the West may begin to see scattered precipitation by the weekend and early next week with the tendency toward more upper troughing and arrival of the southern stream upper low. The southern two-thirds of the West will likely see above normal temperatures continue through the weekend with some areas seeing highs 10-15F or so above normal. Northern areas will already be moderating close to normal Friday-Sunday while additional lowering of heights aloft over the West early next week should bring temperatures toward normal over the remainder of the West. Some of the western warmth will extend into portions of the Plains and Mississippi Valley with plus 5-15F anomalies. The Florida Peninsula and South Texas should see several days with highs up to 5-10F above normal, in a warm pattern to the south of a couple fronts expected to focus upon reaching more northern parts of these states. Some daily records for highs/warm lows will be possible. These temperatures may bring heat indices as high as 105-110 degrees for south Florida and upwards to 115 degrees for south Texas, highlighting a threat of hazardous heat. 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 (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 $$