Extended Forecast Discussion NWS Weather Prediction Center College Park MD 235 AM EST Mon Mar 06 2023 Valid 12Z Thu Mar 09 2023 - 12Z Mon Mar 13 2023 ...A multi-day heavy rain event in the south-central and southeastern U.S. as well as snow over the north-central to northeastern U.S. are likely this week... ...Wetter pattern expected to develop along the West Coast with moderating temperatures... ...Overview... Model guidance continues to show considerable spread for important details of upper flow from the eastern Pacific eastward, but some primary common themes have been evident from the guidance. The first is that a vigorous West Coast upper trough should progress eastward and produce a system (with leading wavy front) that could produce areas of potentially heavy rain and thunderstorms across the south-central U.S. and southeastern U.S. into Thursday with some rainfall reaching into the East late in the week. Additionally, cold air farther north associated with a very strong Canadian surface high could support meaningful snow across the northern half of the Plains, Mississippi Valley, Appalachians, and eventually parts of the Northeast. This system should tend to have a generally eastward track across the East given the influence of ridging/above normal heights aloft over Canada. The second major theme of this forecast period will be the trend toward a wetter regime along the West Coast with moderating temperatures. This may lead to flooding concerns across California in particular where rain could fall in areas that have seen significant snow lately, but the impacts are still to be determined given the guidance spread. As the West moderates, the coldest temperature anomalies will shift more into the Plains by late week into the weekend. ...Guidance/Predictability Assessment... While the latest models and ensembles continue to have some difficulty in resolving the details of complex Pacific/western North America upper flow, involving initial Pacific through Alaska ridging along with a northwest Canada upper low and the flow between these two features, some aspects have fortunately been improving. Models now are more agreeable with pulling energy into a compact closed low into the eastern Pacific just offshore Washington State/British Columbia by Thursday/day 3. The 12Z GFS was the first of the GFS runs to catch onto this feature, after non-NCEP guidance has been showing it for at least a day. However, certainly some detail differences with this early in the period, including the 12Z UKMET perhaps holding onto a really energetic closed low spinning there and displaced while the 12Z CMC ejected this low quite fast. Preferred the ECMWF/GFS type of middle ground solution there. At least the newer 00Z CMC has come back toward the better consensus. This low feature and additional Pacific energy will help direct a late week West atmospheric river so the details will continue to be monitored, as this will impact precipitation distribution that have shown dramatic differences over the past couple of days. With energy forming the Pacific compact closed low, this leads to less energy available to eject eastward across the northern/central tier of the CONUS. The trend has been for more progressive flow and not as deep with still a potentially vigorous trough late week that looks to cause rain and snow threats. The WPC forecast followed this faster trend in the QPF/WWO and with the surface low/frontal systems as well as the 500mb heights. Variations still remain with the surface low track/timing that could cause sensible weather differences including amounts of rain and snow in the central to eastern U.S. as well as precipitation amounts and type across the Eastern Seaboard. While a lot of guidance now indicates a surface low in the western Atlantic over the weekend, confidence in the details remains fairly low. By early next week, somewhat flat upper level flow is forecast but any low amplitude troughs/ridges showed a lot of spread in the 12/18Z guidance cycle. The WPC forecast favored a mainly 12Z ECMWF/18Z GFS with some ensemble components early on, with increasing amounts of the GEFS and EC ensembles and subtracting the GFS by later in the period. ...Weather/Hazard Highlights... Areas of heavy rainfall and thunderstorms are expected to persist from the short range period into Thursday across the south-central U.S. and stretching east into the Tennessee Valley region, as subtropical Pacific moisture combines with low level Gulf moisture and streams into a frontal system. A Slight Risk is in place in the experimental Excessive Rainfall Outlook for these areas for day 4/Thursday--with the central and eastern parts of the Slight Risk likely to see the heavier rain and rain rates by then, but with any rain falling in the western part likely to cause flooding issues after a couple of days of heavy rain. The QPF looks to lessen by Friday as the system moves more quickly east for less of a flooding threat. Meanwhile farther north, snow will likely be ongoing across northern parts of the Plains as the period begins Thursday and expanding quickly into the Upper Midwest where Canadian high pressure spills colder air into the region. Probabilities for wintry weather remain broad given the uncertainty for timing and placement of notable snow, but continue to increase overall. The faster trend in the guidance meant the QPF and Winter Weather Outlook probabilities shifted a bit east for this forecast. Wintry weather is expected to extend into the Great Lakes and Appalachians around Friday and then into parts of the Mid-Atlantic and Northeast as well, depending on specifics of the expected central/eastern U.S. into western Atlantic low pressure system. The western U.S. is forecast to see more active weather into the latter part of the week. Though model guidance is still variable, the general trend is for a wetter pattern to emerge starting late Thursday in the Pacific Northwest and northern California with a considerable ramp up on Friday as an atmospheric river with anomalously high moisture takes aim at California. Snow levels look to rise late week, which could lead to rain falling atop snow, which may cause flooding concerns. Slight Risks of excessive rainfall are in place for day 5/Friday given this potential, but the areas may need to shift around in future forecasts as models vary in their axes of precipitation focus. Additional fronts coming into the West could provide more chances for precipitation there this weekend into early next week, but again with considerable uncertainty in the amounts and location of rain/mountain snow. Below normal temperatures over much of the West (up to 10-15F below normal) should persist through Thursday or so. The upcoming pattern change should lead to minimum temperatures rising to near/slightly above normal values by Saturday-Sunday while daytime highs may still remain a little below normal. Meanwhile the northern/central Plains will become the more prominent focus for cold temperatures with most days featuring highs 10-20F below normal with the best potential for locally colder highs mid-late week (and including the central Rockies during that time). Meanwhile above average temperatures at least in terms of highs will likely be confined to the Gulf Coast states Thursday-Friday, but even much of this area should drop to near or below normal over the weekend after the cold front passes through. However, the most recent forecast indicates that the southern High Plains may stay in the warm sector for slightly above average temperatures. In modified form, expect the chilly air over the central U.S. to spread eastward from the Plains after Friday. Tate 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