Extended Forecast Discussion NWS Weather Prediction Center College Park MD 226 AM EST Sun Mar 05 2023 Valid 12Z Wed Mar 08 2023 - 12Z Sun Mar 12 2023 ...Despite some remaining uncertainty, a multi-day heavy rain event in the south-central and southeastern U.S. as well as north-central to northeastern U.S. snow still look probable this week... ...Overview... While model guidance continues to struggle resolving important details of eastern Pacific through western North America flow, limiting confidence in the sensible weather specifics across the lower 48 this week into next weekend, there are some commonalities in the models to key on. In general at the start of the period Wednesday, western U.S. upper troughing will be reinforced and push eastward possibly in fairly vigorous form through the latter half of the week, though differences in the timing of the trough's ejection and its depth have been large. This trough combined with a wavy surface front could produce areas of potentially considerable rain and thunderstorms across the south-central U.S. through Wednesday-Thursday and shifting into the East late week. Additionally, cold air farther north associated with a very strong Canadian surface high could support meaningful snow across central and northern parts of the Plains, Mississippi Valley, and eventually parts of the Northeast. As the upper trough and surface high progress eastward, below average temperatures in the West will shift/expand into the Plains for the latter half of the week into the weekend. ...Guidance/Predictability Assessment... For multiple days the models and ensembles have been showing considerable difficulty in resolving the details of complex Pacific/western North America upper flow. This specifically involves the precise evolution of initial Pacific through Alaska ridging along with an initial northwest Canada upper low and the flow between these two features. Model solutions range from spilling Canadian energy southward into the western U.S. trough and taking the deep trough slowly eastward (most GFS runs, though backed off somewhat in the recent 00Z run) to energy splitting so that a compact upper low spins over the eastern Pacific near the U.S. Northwest/British Columbia while another shortwave trough/low tracks more quickly eastward across the northern or central tier of the CONUS. The latter is now more like the deterministic ECMWF/UKMET/CMC, with the 00Z ECMWF going even more toward this compared to the 12Z run that seemed to be more middle ground at the time. Overall the newer 00Z cycle of guidance trended toward having flatter flow across the CONUS, to various extents. This could lead to a weaker low and faster pattern in future forecasts if it persists. This is a tough forecast with low predictability and considerable differences between models and run to run variability within the same model suite. At the time of this forecast production, the 12/18Z model guidance was available and leaned toward a blend favoring the ECMWF, ECens mean, and the GEFS mean. These seemed to be middle ground solutions between the slower trough of the GFS/many GEFS members and the faster CMC/CMC ensemble mean. This approach also led to fairly good continuity with the previous forecast. But changes in the model guidance continues to lead to higher chances for more changes necessary to the forecast than normal. ...Weather/Hazard Highlights... The upper trough initially over the western U.S. should continue to support periods of rain along the northern/central West Coast and snow in the higher elevations for light to locally moderate totals. Confidence decreases in terms of coverage and intensity of precipitation across the West by the latter half of the week, based on the continued guidance spread and variability for details of flow aloft over the eastern Pacific and western North America. The most likely scenario would have approximately the northern-central portion of the West Coast seeing an increase of moisture by the late week/weekend time frame as a frontal system approaches. Farther east, a wavy frontal boundary looks to be in place across the south-central U.S. through Wednesday-Thursday while subtropical moisture aloft streams in from the Pacific and low level moisture comes in from the Gulf of Mexico. Rain and thunderstorms should increase Tuesday and persist through much of the week. Rain rates are likely to be high given the moist and unstable airmass. The experimental Days 4-5 (Wednesday-Thursday) Excessive Rainfall Outlooks both indicate Slight Risks of flash flooding across areas of the southern Plains to Lower Mississippi Valley, atop areas that have seen ample rainfall recently and should receive another round of heavy rain during the day 3 (short range) period Tuesday. The risk expands into the Mid-South by Thursday though there remains uncertainty in the details. Meanwhile farther north, snow will likely develop from the central Rockies expanding eastward into the central/northern Plains and Mississippi Valley where Canadian high pressure spills colder air into the region. Probabilities for wintry weather are broad for now given the uncertainty for timing and placement of notable snow, but are increasing overall. Wintry weather looks to extend into the Great Lakes and Appalachians and then into the Northeast as well, depending on specifics of the expected central/eastern U.S. low pressure system. Below normal temperatures over much of the West should persist through much of the period while also spreading into the northern/central Plains (lower confidence for the southern Plains) this week. There will be decent coverage of highs 10-20F or more below normal within this area, with 20-30F below average anomalies over the central Rockies and High Plains toward Thursday-Sunday. Parts of the West could moderate some by the weekend. Meanwhile above average temperatures at least in terms of highs will likely be confined to the Gulf Coast states Wednesday-Friday, but even these areas are currently expected to cool to near normal over the weekend after the cold front passes through. Tate/Rausch 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml