Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1137 AM EDT Mon May 21 2018 Valid 12Z Thu May 24 2018 - 12Z Mon May 28 2018 ...Heavy rain again possible from Florida into the Southeast & southern Appalachians... Guidance and Predictability Assessment ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Teleconnections with the strong positive anomalies southwest of the Aleutians and east of Iceland favor troughing in the West and the Labrador Sea, which the guidance agrees upon. Persistent upper troughing is expected to migrate northward from the central Gulf of Mexico into the lower MS valley. One issue lies near the Gulf coast, where the 00z ECMWF is decidedly northwest of the remainder of the guidance due to its more retrogressive upper trough -- went on the slow, southeast side of the guidance with this system as it has yet to form and track the low on an intermediate route between where a deep layer/cold core cyclone would track (central Gulf) and a more tropical/warm core feature would track (near the western FL coast). The big issue lies in the northern stream, where there is a sudden lack of clarity on whether or not a trough moves through eastern Canada. The main fly in the ointment flow pattern-wise for the Northeast is the latitude of the mid-level vortex towards the North Pole. More southerly guidance like the 00z ECMWF/00z Canadian allow this system to bridge southward to the system transiting the Great Lakes, using another shortwave moving across west-central Canada. The GFS/GEFS mean are more northerly and become out of phase with the Canadian/ECMWF guidance building a ridge over Ontario and Quebec. This leads to profoundly different pressure patterns across New England, with the 00z ECMWF and 06z GFS on opposite extremes. Since the guidance has shown a bit of variability with the Ontario/Quebec ridge, the preferred 500 hPa heights, surface winds, and pressures were based on the consensus. This led to an even blend of the 00z UKMET, 00z ECMWF, 00z Canadian, and 06z GFS through Saturday before using increasing amounts of the 00z NAEFS mean and 00z ECMWF ensemble mean thereafter, with some preference for the 00z Canadian amongst the deterministic guidance for Sunday into Next Monday over the 00z ECMWF/06z GFS solutions. This allowed for reasonable continuity and opens the window to later guidance shifts. Weather/Threats Highlights ~~~~~~~~~~~~~~~~~~~~~~~~~~ Areas from the northern Plains eastward will see rainfall/convection in association with the shortwave energy and surface low pressure/frontal system initially emerging over the northern Plains on Thu. Some activity may be locally moderate to heavy. Areas from the lower MS Valley through the Southeast including Florida will likely see a fairly wet period with the upper trough deepening along 90W longitude promoting a northward flow of moisture aloft and surface low pressure possibly tracking toward/settling over, near, or inland of the central Gulf Coast. Later in the period there will be a question of how much of the moisture over the Southeast spreads northward ahead of the front approaching the eastern states because of uncertain cyclone track/timing from the Gulf of Mexico. Heavy rainfall potential will require close monitoring given the significant totals already observed over some locations in the past week. Areas from the central/northern West Coast into the Rockies will likely see another episode of rainfall of varying intensity with the upper low forecast to come inland from the eastern Pacific. Much of the lower 48 will see above normal temperatures during the period. Exceptions will be over the Southeast/Florida where highs will be lower due to clouds/rainfall and California into the Great Basin under the eastern Pacific upper low that tracks inland. The most prominent warm anomalies should exist ahead of the system emerging from the northern Plains, and then from the Northwest through most of the Rockies and eventually into the Plains in association with upper ridging. Within both of these areas scattered locations may see daily record values for highs/warm lows. WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities can be found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 Roth/Rausch