Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Tue Jan 02 2024 Valid 12Z Fri Jan 05 2024 - 12Z Tue Jan 09 2024 ...Major storm system to bring hazardous weather from the Gulf Coast northeastward Friday-Sunday... ...Overview... The extended period will be active as a series of storm systems track West to East. The leading system moves from the Southern Plains on Friday into the Eastern U.S. by this weekend, with increasing potential for heavy precipitation (rain to the south, snow to the north) from the Gulf Coast northeastward. In its wake the next trough drops into the West this weekend as well bringing another round of valley rain and mountain snows progressing from California into the Four Corners region. Cyclogenesis appears likely across the south-central Plains by Monday, bringing another round of heavy rainfall to parts of the South/Tennessee Valley region Monday-Tuesday, with increasing snow potential to the north and west of the low. ...Guidance/Predictability Assessment... The latest runs of guidance maintains above average confidence for the active large scale pattern during this period. The leading system will have a surface low develop along the central Gulf Coast that will track through the Southeast, deepening further as it exits the Mid-Atlantic early Sunday. There continues to be some key differences regarding timing and exact track of the low, which has huge implications for precipitation type, especially for more eastern areas (including major cities in the DC to NYC corridor). As noted with the previous forecast, the UKMET is on the faster side of consensus and was avoided for this forecast. There are also lots of uncertainty with the exact northward extent of the track too, and the medium to smaller scale shortwave details with lower probability may have a significant effect on surface evolution, so additional adjustments in the guidance appears likely. Meanwhile, another trough enters the Pacific Northwest late Friday/early Saturday and the guidance shows generally good agreement that it will amplify over the West before ejecting into the Plains around Monday. Guidance shows good agreement that a surface low spins up near the Texas/Oklahoma panhandle early Monday generally tracking east-northeastward into the Middle Mississippi Valley on Tuesday. There is significant uncertainty though on the exact track, with the CMC being north of the overall better consensus. The CMC, ECWMF, GFS and their respective ensemble means continue to provide a solid solution and this maintains WPC continuity. ...Weather/Hazards Highlights... The upper level low/trough over the Southern Plains Friday will have an abundance of moisture from the Gulf to aid in fueling moderate to locally heavy rainfall across the Lower Mississippi Valley/central Gulf Coast states and perhaps nearby areas on Friday. The Day 4 Excessive Rainfall Outlook maintains a Marginal Risk area extending from southeast Texas to the Florida Panhandle. There is some question about how far northward instability may extend, but anomalous moisture and a leading front forming near the Gulf Coast to provide a focus, plus fairly wet antecedent conditions from short-term rainfall, seem to suggest some flash flooding potential. Only minor adjustments were made to the inherited risk area for this period. Heavy precipitation will spread across the Southeast/Mid-Atlantic and possibly the Northeast this coming weekend, with increasing potential for heavy snow to the north and west of the low. Uncertainty persists on the exact amounts and precipitation type for some of the major East Coast cities given the spread in model guidance. The WPC winter weather outlook probabilities highlight the best potential for at least 0.25" liquid in the form of snow over the central Appalachians/interior Mid-Atlantic. Just to the east, guidance trends do show somewhat higher probabilities for accumulating snowfall near the DC-NYC corridor but highly dependent on the exact track of the low as sufficiently cold temperatures in this particular region remain marginal at best. It will continue to take some time to resolve important details given the current spread for surface low timing/track. A smaller-scale system may bring a brief focus of enhanced rain/mountain snow to the Pacific Northwest around Friday. Then expect broader coverage of moderate to locally heavy precipitation from the West Coast/California and into the interior West by next Monday. There is a decent signal for meaningful mountain snows across parts of the Cascades, Sierra Nevada, and interior mountain ranges/southern Rockies. Surface cyclogenesis across the south-central Plains early next week should allow precipitation to move into the central U.S., with potential for moderate to heavy snow to the north of the low track. This weekend above seasonal average temperatures can be expected across the far northern tier states with anomalies generally plus 5-15F for highs, and perhaps as high as plus 15-25F for morning lows over parts of Minnesota/Wisconsin. Above normal temperatures also look to return to the Southern Plains/Lower Mississippi Valley and eventually the East early next week. The southern two-thirds of the West will likely have below normal temperatures late this week, with some cooler anomalies at least 10F below normal for highs spreading across the West into the High Plains by Sunday-Tuesday as upper troughing moves over the region. Northern parts of the West should trend below normal as well. Campbell/Santorelli 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw