Extended Forecast Discussion NWS Weather Prediction Center College Park MD 432 PM EDT Mon Mar 13 2023 Valid 12Z Thu Mar 16 2023 - 12Z Mon Mar 20 2023 ...Deep storm genesis to drive Ark-La-Tex/South severe thunderstorms with an excessive rainfall/flash flooding threat and a heavy snow/wind threat from a cooled central Rockies/Plains to the Upper Midwest/Great Lakes Thursday/Friday... ...Overview... Ejection of southern stream energy from California and the Southwest coupled with an amplifying northern stream upper trough/closed low over the north-central U.S. should favor deep surface low/frontal genesis to focus an excessive rainfall/convective threat across the Ark-La-Tex/South Thursday into Friday. The intensifying storm also offers a concurrent heavy snow/high wind threat in the wintry cold air on the backside of the deepening system from the central Rockies/Plains to the Upper Midwest/Great Lakes. System activity should eject across the East by Saturday. Additional but less certain Pacific system energies reach the West Coast then Intermountain West/Rockies later this weekend. ...Guidance/Predictability Assessment... The deterministic guidance is in good agreement in depicting the evolution of a quasi split stream upper-level flow pattern over the West, with only some differences in more subtle details in the location of embedded energy, before the pattern amplifies as a closed low develops downstream over the Midwest. This includes both run-to-run consistency with the latest forecasts from the GFS/ECMWF as well as across the model guidance between the 00/06Z GFS and 00Z ECMWF/UKMET. The 00Z CMC does lag a bit in the development of a closed low. In addition, the 00Z GEFS/ECens means also converge on the development of a closed low, albeit with an expectedly lower amount of detail than the deterministic guidance. Larger differences arise during the middle of the period in both the eventual evolution of the closed low into an open wave as it departs the East Coast and the timing/phasing of embedded energy withing the feature. There are also differences across the guidance in the evolution of shortwave energy upstream over the West in both the northern and southern stream. The timing/phasing of a weak southern stream shortwave moving through the Southwest mid- to late period is more in line between the 00Z ECMWF and 00Z/06Z GFS compared to the 00Z CMC, which is a bit more progressive. Additionally, another feature of interest will be the evolution of a closed low upstream over the Gulf of Alaska and the gyre-like rotation of shortwave energy that reaches the Pacific Northwest/West Coast. The prior runs of the GFS through 00Z as well as the 00Z CMC were a bit more aggressive in the depiction of a compact wave over the Pacific Northwest mid-period, though the 06Z GFS backed off this evolution and is more in line with the 00Z ECMWF/UKMET. All of the deterministic guidance disagrees on the next shortwave and potential for subsequent ridging over the West Coast at the end of the period, which is shown in the 00Z/06Z runs of the GFS but not depicted in either the 00Z ECMWF or CMC. Both the 00Z ECens and GEFS means show little detail in this evolution and a great deal of uncertainty into how this pattern will evolve. These differences will have implications for precipitation chances across the West later in the period. The WPC forecast suite blend began with a combination of the deterministic guidance with a bit stronger emphasis on the 00Z ECMWF/UKMET and 06Z GFS given the better agreement between this guidance compared to the 00Z CMC. The 06Z GFS was chosen over the 00Z run given the greater similarity/trend to the other guidance in handling shortwave energy over the Pacific Northwest as compared to the 00Z run. The contribution from the 00Z CMC was further reduced mid-late period given further differences in retaining a closed low over the Northeast longer than the other guidance, and the 00Z ECens and GEFS means were introduced as uncertainty rapidly grows with respect to the evolution of the pattern both over the Northeast and with shortwave energy over the West. While these details will eventually be important, there isn't enough confidence in any one of the deterministic solutions to reliably emphasize any one over the other at this time. ...Weather/Hazards Highlights... Favorable cyclo/fronto genesis across the central U.S. and deep pre-frontal moisture and instability return by a strong low level jet into the eastern Plains and Mississippi Valley and onward across the South and East later week will fuel an emerging/expanding area of showers and thunderstorm driven rains, including the risk for flash flooding. The WPC experimental Day 4 Excessive Rainfall Outlook (ERO) has a "Slight Risk" Thursday from the eastern southern Plains to the lower Mississippi Valley where there is favorable overlap between the potential for locally heavy rainrates with thunderstorms ahead of and along an eastward moving cold front over a region of wet ambient soils. In addition, the Storm Prediction Center has outlined a portion of the Southern Plains Thursday for the risk of severe weather. Strong deep-layer and low-level shear overlapping moderate instability will lead to the potential for organized supercells with the threat for high winds, hail, and perhaps a few tornadoes. The heavy rain threat shifts over the southeast Friday with a Day 5 "Marginal Risk" ERO in place. A higher flash flood threat could materialize where locally heavier rains/rain rates may occur over portions of southern Georgia and the Florida Panhandle, but the progressive nature of the front looks to keep the rainfall relatively distributed over time across the area for now. Otherwise, moderate rainfall chances will overspread the Midwest on Thursday and the East Coast into Friday/Saturday. Additionally, latest guidance preferences have continued to increase wrap-back snow potential from the central Rockies/Plains to the Upper Midwest/Great Lakes Thursday-Friday and eventually the Interior Northeast Saturday-Sunday following the surface low track as per the WPC Winter Weather Outlook (WWO). The highest chances for impactful snowfall currently looks to be over the Middle Missouri Valley Thursday and portions of the Upper Great Lakes Thursday-Friday. A broad area of blustery winds are expected Thursday following the intense cyclogenesis over the Plains with gusts upwards of 40-50 mph possible. These winds may also contribute to areas of blowing snow. A broad area of high pressure will set in bringing an end to precipitation chances for most of the eastern 2/3rds of the country by Sunday. Temperatures will tend to be cooler than average by 10-25 degrees, especially across most of the central U.S. following the passage of the system cold front and as a Canadian air mass robustly digs in. These cold temperatures reaching as far south as Texas may lead to the potential for some light snow over the Southern High Plains this weekend. Upstream, there looks to be a brief reprieve from the recent string of anomalously strong and wet storms over the West after Thursday and into Saturday before a series of uncertain upper-level waves/systems increases precipitation chances. More moderate lower-elevation coastal/valley rain and higher elevation snow will be possible, particularly along a southeasterly track from the West Coast across the Great Basin and into the Central/Southern Rockies Sunday-Monday. High temperatures look to remain around 5-10 degrees below average in general, with a pocket of colder temperatures 15-20 degrees below average lingering over the Rockies. Putnam/Schichtel 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 Hazards: - Heavy rain across portions of the Southeast, the Middle/Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Thu, Mar 16. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Fri, Mar 17. - Heavy snow across portions of the Central/Southern Rockies, and the Central/Southern Plains, Thu, Mar 16. - Heavy snow across portions of the Great Lakes and the Upper Mississippi Valley, Thu-Sat, Mar 16-Mar 18. - Severe weather across portions of the Lower Mississippi Valley and the Southern Plains, Thu, Mar 16. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Central Rockies, the Northern/Central Plains, and the Middle/Upper Mississippi Valley, Thu, Mar 16. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Thu-Fri, Mar 16-Mar 17. - Much below normal temperatures across portions of mainland Alaska, Thu-Fri, Mar 16-Mar 17. 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