Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1145 AM EDT Mon May 20 2019 Valid 12Z Thu May 23 2019 - 12Z Mon May 27 2019 ...Record/Dangerous Heat for the Southeast this weekend into next week... ...Heavy rainfall possible for portions of the central Plains... ...1600 UTC Update... The overall pattern evolution and forecast thoughts remain largely the same from the previous shift (see below). Deterministic/ensemble models continue to show relatively good synoptic agreement through the period, with differences confined primarily to timing and amplitude of smaller scale features. The blend for this update consisted of all deterministic guidance (between the latest runs of the ECMWF, GFS, and FV3-GFS) days 3 and 4, with increasing contributions from the EC/GEFS ensemble means beyond. This results in a forecast very close to the overnight shift and would continue to favor a stormy pattern for the central U.S. and potentially record warmth for the Southeast. Models appear to be trending towards stronger ridging in the Southeast (as has been consistently shown by the ECMWF), and so WPCs max temps in in that region were bumped up a few degrees. Santorelli Previous Discussion Below (issued at 0612 UTC)... ...Overview... Highly amplified upper level pattern will be dominated by deep troughing in the west and strong ridging in the Southeast. This is forecast to largely hold in place as reinforcing height falls sink into California on Saturday as the lead upper low lifts into the High Plains. Northeast/Mid-Atlantic will lie in the confluence between the northern and southern stream as a wavy boundary oscillates north/south. The pattern favors well below average temperatures in the west with rounds of precipitation but hot/dry conditions in the Southeast with record temperatures near 100F. In between, broad southwesterly flow aloft and south to southeasterly flow at the surface will favor rounds of convection and locally heavy rainfall with multi-inch totals over the period especially over the central Plains/southwestern Corn Belt. ...Guidance Evaluation and Preferences... Latest guidance proved to be a good consensus/starting point for the forecast, despite their differences right from Thu/D3 among themselves and between the ensemble means/deterministic models. Opted to rely entirely on the deterministic models to start due to a perceived lag in especially the ECMWF ensembles with the trend seen in the ECMWF/GFS/UKMET/Canadian to be slower with the system associated with the lead upper low in the Rockies Thursday moving into southern Canada late Friday into Saturday. Incorporated more ensemble guidance by next Sun/Mon as the shape of the upper low in the west loses focus as well as the strength of the upper high in the Southeast (ECMWF-led guidance stronger than the GFS-led cluster). Trend has been toward stronger upper ridging but temperatures are already forecast at record highs and in some cases tying monthly records, so caution was exercised for that lead time (6-7 days out) for now. ...Weather Highlights and Hazards... Summertime heat will become increasingly intense in the Southeast though humidity levels may be only moderate (dew points in the 60s F) which would allow overnight lows to fall to the upper 60s to low 70s. Temperatures will approach daily and perhaps monthly records in the 90s to around 100F over portions of northern Florida and Alabama/Georgia/South Carolina. The west, by contrast, will see well below average temperatures about 15-25F below typical values with abundant cloud cover and modest rain with high elevation snow (especially Thursday). Temperatures around the wavy boundary from the central Plains eastward will rise/fall as the front lifts northward or slips southward. This will also provide a focus for rainfall and some convection. Frontal boundary across the Plains into the Midwest will support rounds of rainfall with locally heavy amounts that may add up to several inches over the period, some of which may fall within a short amount of time over areas that have seen quite a bit of rainfall recently. Flooding/flash flooding may be a concern for several days, with locations to be refined over the next few days. Fracasso 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 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