Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1149 AM EDT Thu May 24 2018 Valid 12Z Sun May 27 2018 - 12Z Thu May 31 2018 ...Confidence increasing in potential heavy rainfall event for portions of the Gulf Coast and Southeastern states this weekend into early next week... ...Overview/Synoptic Assessment... Blocky flow pattern expected to persist across North America through the medium range, with persistent and relatively slow-propagating height anomalies. Negative height anomalies are expected to persist across northeast Canada and Greenland, and subtropical ridge remaining the dominant features off the U.S. Eastern Seaboard. A cutoff upper low initially across the Great Basin will be slow to move east during the Sun-Mon time period as it remains trapped beneath broad upper ridging across the Pacific Northwest/southwestern Canada. As heights fall to the north of the feature into early to mid next week, increasing influence of westerlies will allow the system to gradually move east into the northern plains/Upper Midwest. The lack of any significant influence of the westerlies early in the period across the southern tier will allow low pressure in the Gulf of Mexico to move northward toward the mouth of the Mississippi River by Mon, with perhaps a slightly increased northward pace by Wed-Thu in response to the shortwave moving across the northern plains/Midwest. Finally, another trough is expected to reach the West Coast by Tue-Wed, undercutting a ridge axis across western Canada once again. ...Guidance and Predictability Assessment... Model consensus with respect to low pressure across the northern Gulf of Mexico on Sun has improved a bit over the past day. The 06Z GFS made a significant shift farther west, and now looks much more like the ECMWF/CMC. Additionally, the number of GEFS members taking the low across the Florida Peninsula has dwindles to nearly zero. The biggest change among this guidance with respect to this feature is by next Wed-Thu when the consensus now carries the system farther north a bit more quickly, into the Tennessee valley by Thu morning. This appears to be in response to fairly minor changes with the shortwave moving from the Rockies into the northern plains, with most guidance showing lower heights compared to 24 hours ago, and perhaps stronger mid/upper-level flow digging a bit farther south across the central U.S., resulting in a greater degree of interaction between the two features. As of this morning, the eventual northern plains shortwave remains several hundred miles off the West Coast, and out of the North American RAOB network, thus would expect that any potential shifts among the guidance may not be over. Nonetheless, guidance spread surrounding this Rockies to Upper Midwest upper low/shortwave is relatively small, with fairly minor timing/amplitude differences. With additional trough expected to reach the West Coast by next Tue-Wed, the 00Z ECMWF/06Z GFS showed similar timing/amplitude, and the CMC was also similar but slightly slower. Consensus appears good that significant height falls should reach the Northwest by next Wed-Thu as this trough moves inland. The WPC forecast was based on a heavily deterministic blend, including the 00Z ECMWF/CMC/06Z GFS through much of the medium range. Some weight was placed toward the 00Z ECENS/06Z GEFS means by by days 6-7 (Wed-Thu) to account for increasing spread, although model spread for that time frame seems a bit below average. ...Weather/Threats Highlights... Confidence is increasing that the Gulf of Mexico low pressure, regardless of any potential tropical or subtropical development (see NHC products for more details), will bring the potential for a widespread, multi-day heavy rainfall event to portions of the Gulf Coast and Southeast states. Deterministic ECMWF and GFS solutions both show the potential for multi-inch rainfall totals from Sun onward. Ensemble probabilities suggest the potential for two areas of particular concern, one along the central Gulf Coast Sun-Tue, east of the mouth of the Mississippi (close to the surface low track), and perhaps another area Wed-Thu across the southern Appalachians/foothills, where upslope enhancement along with deep tropical moisture inflow may enhance rainfall amounts. PW standardized anomalies exceeding +2.5 or +3 standard deviations are shown by the ECENS across these areas, indicating deep tropical moisture spreading across the southern/southeastern states. At this time, confidence is increasing into the moderate-high realm that the Gulf Coast will see heavy rains. Confidence is a bit lower at this time across the southern Appalachians/foothills, and will ultimately be dependent on the eventual track of the low pressure. Elsewhere, additional heavy rains will be possible Sun-Mon from portions of the northern Great Basin/northern Rockies east into the High Plains, as deep moisture flux ahead of the upper low focuses precipitation along a quasi-stationary frontal boundary. PWs across portions of Wyoming/Montana east into the northern High Plains are near max values on ESAT percentiles, with values near +3 standard deviations possible. Thus, this appears to be another area of potential heavy rainfall, with flooding concerns enhanced a bit due to somewhat slow movement of the system and the persistent surface front in the vicinity. Additionally, temperatures are expected to be well above average across much of the central U.S. and the Great Lakes through much of the period. The largest max temp anomalies should occur Sun-Mon, when high temps may reach 10 to 20 deg F above average. Ryan 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