Extended Forecast Discussion NWS Weather Prediction Center College Park MD 324 PM EDT Tue May 07 2019 Valid 12Z Fri May 10 2019 - 12Z Tue May 14 2019 ...Heavy rainfall threat for eastern/southeastern Texas and the Lower Mississippi Valley late this week into the weekend... 15 UTC Update... A blend of the latest deterministic guidance was used as a forecast starting point through days 3-5 (Fri-Sun). The 00Z ECMWF/CMC and 06Z FV3 were preferred Fri-Sat as the GFS differed with its handling of the northern stream shortwave crossing the Great Lakes during that time frame. By Sun the operational GFS was used in place of the FV3 due to preferred slower timing with the closed upper low crossing the Desert Southwest and northern Baja California. Larger differences emerged in the models by days 6-7 (Mon-Tue) with additional northern stream energy amplifying across the Great Lakes/Northeast, and the resultant surface low off/along the northern Mid-Atlantic/Northeast coastline. Greater run-to-run inconsistency surrounding this system reduced forecast confidence, and greater weight was placed toward ensemble means (ECENS and NAEFS) by this time frame. Overall, the forecast remained relatively close to continuity, with only minor adjustments. Ryan Previous Discussion (issued at 0700 UTC)... ...Overview... Expect a stalled front near and northeast from the western Gulf Coast (anchored by low pressure over eastern Canada as of early Fri) to provide a focus for periods of heavy rain along and north of the western-central Gulf Coast region into the weekend. With continued uncertainty in the details, upper troughing forecast to amplify into the Plains/Mississippi Valley and then move into the eastern U.S. may draw some of this moisture northeastward later in the weekend/early next week while suppressing part of the front--reducing the heavy rain threat somewhat. Across the West the upper pattern will begin to look more like a Rex block as a warm ridge builds over the Northwest while a California upper low as of Fri eventually drifts along the western U.S.-Mexico border. This upper low and an impulse tracking to its south and east will promote multiple days of precipitation over the southern half of the West. Guidance shows signs of an amplifying trough developing over the eastern Pacific toward the end of the period next Tue. ...Guidance Evaluation/Predictability Assessment... Over and near the western U.S. the prevailing multi-day trend is for a stronger ridge to build into the Northwest and slower progression of the upper low expected to track near the western U.S.-Mexico border. In general recent GFS/GEFS runs have been somewhat ahead of the ECMWF/ECMWF mean with these trends. The 12Z ECMWF made a favorable adjustment relative to its faster prior runs so it could be included in the forecast through the period, while aforementioned trends seem to favor somewhat more GEFS mean than ECMWF mean for the ensemble input by the latter half of the period. The 12Z CMC mean looks quite suspicious with a rather fast moving eastern Pacific trough that reaches the West Coast by day 7 Tue. Some details remain complex, in particular for an impulse that likely tracks south of the California upper low around Fri and may continue into the southern Rockies/Plains--but with the potential for a portion of the energy to curl back toward the upper low. Evolution of upper troughing expected to amplify over the Plains/Mississippi Valley during the weekend--and the associated surface reflection--continue to be a significant forecast problem. Among guidance from the 12-18Z cycles there were essentially two clusters. The slower/sharper/more separated group of the ECMWF/UKMET/CMC aloft led to a weaker and more suppressed surface feature while the more consolidated GFS/GEFS mean/CMC mean had a stronger/northward surface system. The one thing in common is a multi-day trend toward the trailing cold front reaching farther southward. Operational GFS runs were a little suspicious in how they phased northern Ontario energy into the original shortwave so they might be too strong. The 18Z FV3 GFS did not show such phasing and the new 00Z GFS has trended away from that as well. Beyond that the details appear too fine in scale to resolve confidently at the extended time frame, thus favoring a blended approach. Behind this system additional shortwave energy (again with low confidence for specifics) will complicate the forecast within the overall mean trough. Specifics of the amplifying trough will have a significant influence on the duration of the potential heavy rain event over the South and how much moisture may stream farther northeastward. The ECMWF cluster described above would lead to more moisture/frontal waviness reaching northeastward and a shorter duration of rainfall over the Gulf Coast states. Again preferred an intermediate approach given lack of confidence in northern stream trough specifics. Note that the impulse tracking south of the California upper low on Fri may also play a role in promoting a frontal wave over the East. In spite of the differences over the South and East during the weekend, there is actually some convergence of solutions toward a wave off the Mid-Atlantic/New England coast early next week. Trends in the 00Z GFS and other incoming 00Z models suggest the 12-18Z GFS solutions are less likely. ...Weather Highlights/Threats... The best confidence in heavy rainfall potential over eastern/southeastern Texas through the Lower Mississippi Valley will be in the Fri-Sat time frame. Then latest trends show an increasing possibility that one or more frontal waves lifting through the eastern states may pull areas of significant rainfall into the Tennessee Valley and Mid-Atlantic while reducing precipitation over parts of the Gulf Coast region. However the front extending into the western Gulf and northern Mexico will still be close enough to maintain potential for rainfall of varying intensity through the first half of next week. Elsewhere over the eastern half of the country, the storm over eastern Canada early Fri will bring a brief period of enhanced rainfall to the Northeast while the system dropping into the Plains/Midwest/Great Lakes during the weekend should be accompanied by mostly light-moderate precipitation. Over the West, continue to expect terrain-enhanced precipitation from portions of California into the southern half of the Rockies. Highest 5-day totals should be over the Rockies. Isolated highest elevations of the Rockies could see a little snow but with much lower potential than will be the case before Fri. Western areas should gradually trend drier but overall expect a longer duration of precipitation given slower trends for the upper low tracking near the U.S.-Mexico border. Eventually the upper low should increase activity over the southern High Plains. Expect very warm temperatures over the Northwest for multiple days as the upper ridge builds over the region. There should be decent coverage of plus 10-20F anomalies with locally higher departures from normal. Daily records for highs/warm lows will be possible and some areas may approach 90F. Meanwhile the forecast pattern will keep the southern Plains through Southwest well below normal for highs. The southern High Plains should see the most extreme anomalies of minus 15-25F for highs on Fri. Areas from the Midwest to Northeast will be moderately cool during the period, 5-10F below normal and locally a bit cooler. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Southern Plains, Fri-Sun, May 10-May 12. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Tue, May 14. - Heavy rain across portions of the Mid-Atlantic, Sat-Sun, May 11-May 12. - Flooding possible across portions of the Central Plains, the Lower Mississippi Valley, the Northern Plains, the Great Lakes, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley. - Flooding occurring or imminent across portions of the Central Plains, the Lower Mississippi Valley, the Northern Plains, the Great Lakes, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley. - Flooding likely across portions of the Central Plains. - Much below normal temperatures across portions of the Southern Rockies, the Central Plains, and the Southern Plains, Fri-Sat, May 10-May 11. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml