Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM 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 medium range period features a rather progressive flow pattern as a parade of storm systems traverses the country from West to East. The first will be move 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. 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 guidance continues to show above average agreement on the overall synoptic scale flow pattern, but plenty of uncertainty in the details. For the initial system out of the Southern Plains on Friday, guidance agrees that a surface low will develop along the central Gulf Coast and 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). Generally, there seems to be an overall trend towards a faster surface low, though the UKMET remains the quickest. 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. The WPC forecast tonight relied on a non-UKMET blend for this system. The next 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. Given the late period timing, the WPC forecast generally stuck close to the ensemble means, with a track farther south closer to the deterministic GFS and ECMWF. ...Weather/Hazards Highlights... Upper low/trough into the Southern Plains on Friday should tap plenty of Gulf of Mexico moisture to fuel a round of 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. 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. Exact amounts and precipitation type for some of the major East Coast cities remains highly uncertain at this point, but does need to be watched. 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. Above normal temperatures for this weekend should be mostly confined to the far northern tier 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 should see 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. 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