Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1244 PM EDT Sun Sep 5 2021 Valid 12Z Wed Sep 8 2021 - 12Z Sun Sep 12 2021 17Z Update: The transition from an amplified to a more quasi-zonal flow pattern over the northern tier states and southern Canada is leading to an increase in model spread and thus forecast uncertainty compared to recent days. Overall agreement is still decent on Wednesday before greater differences become apparent by the end of the week. There are timing and amplitude differences with Pacific shortwave energy tracking eastward, with the past couple runs of the GFS indicating a more amplified solution over the Northern Plains and then the Great Lakes. The CMC is considerably weaker compared to the GFS and ECMWF across this same general area, although it is more amplified with the trough over the eastern U.S. by the weekend. Taking these factors into account, the WPC forecast was primarily derived from a CMC/ECMWF/ensemble mean blend through Friday, and then ECMWF/ECENS/GEFS for next weekend. Some previous WPC continuity was also maintained through the forecast period. The previous forecast discussion is appended below. /Hamrick ...Overview... Strong upper ridging will weaken as it drifts out of the Four Corners region midweek into the southern Plains as troughing gradually moves into the Pacific Northwest. Troughing in the Great Lakes will move into/through the Northeast later in the week, helping to keep Hurricane Larry out over the open Atlantic. The pattern is a rather dry one, with generally near to above normal temperatures over much of the lower 48. ...Guidance Evaluation and Preferences... The guidance continues to struggle with the flow out of the Pacific, with no clear consensus among the deterministic models. With heights lowering Wednesday, the disparate solutions grew farther apart thereafter, translating across the CONUS and southern Canada into next weekend. Questions remain as to how much troughing is left to slowly move east just off northern CA vs how much northern troughing may shear eastward through southern Canada, ultimately slipping into the backside of the eastern troughing if quick enough. Ensemble means generally followed their deterministic parent models but were a bit closer to each other (GEFS and ECMWF EPS) so that a trend toward that blend was preferred by the end of the period. To start, a deterministic blend sufficed for the first two days of the period, with a messy re-weighting in between. Confidence was lower than normal over the northern tier due to the inconsistent model signal (yet fairly consistent broader ensemble agreement). This is no surprise given the trend toward quasi-zonal flow vs amplified flow. Over the Southwest, the movement of the upper high southeastward may allow for an increase in moisture into the lower CO River basin later in the week. NHC is monitoring the potential for development off the Mexican coast that may try to lift northward, perhaps spreading moisture into the Southwest next weekend. Some GFS runs have been more aggressive in bringing significant rainfall to parts of AZ/SoCal, but continued to favor a drier solution rather than wetter until confidence increases. The daily 17Z NHC-WPC tropical coordination call will refine the surface progs and consequentially the QPF. In the Gulf, tropical development remains uncertain but moisture transport northward/northeastward continues to look more likely into the northern/northeastern Gulf. Maintained a modest QPF signal into the FL Panhandle and southeastern GA. Please see the NHC outlooks for more information. ...Weather/Hazard Highlights... Rainfall will focus along and ahead of a cold front from the Northeast to the Southeast before it exits/dissipates. Amounts will generally be light except over the northeastern Gulf where tropical moisture may support modest to perhaps heavier amounts. Daily showers/storms are likely for much of the FL peninsula to the south of the boundary. Some moisture will work back through the Southwest around the upper high as it drifts southeastward, focused over inland CA into the Great Basin and parts of the Desert Southwest. This may lift northward into the northern Rockies by later in the week into the weekend, ahead of another Pacific front. Temperatures will be near to above normal over most areas, especially in the interior West to the Northern Rockies ahead of the incoming Pacific trough Wed-Thu. Temperatures 5-15 degrees above normal are possible as the area of warmth moves into the southern/central Plains later in the week/weekend. Temperatures in the Great Lakes/Northeast will be near to a bit below normal via northwest flow behind the cold front this week, perhaps moderating a bit next weekend as the surface high approaches. Fracasso 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, 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