Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Jan 22 2024 Valid 12Z Thu Jan 25 2024 - 12Z Mon Jan 29 2024 ...Excessive Rainfall/Flooding Threat to continue late week from the Lower Mississippi/Tennessee Valleys to the Gulf Coast states/Southern Appalachians... ...Overview... A significant rainfall event is on tap with multiple days of heavy rain and runoff threats amid a slow to translate and amplified pattern this week, with primary focus from southeast Texas through the Lower Mississippi/Tennessee Valleys and Gulf Coast states to the southern Appalachians. ...Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of reasonably well clustered guidance from recent runs of the GFS/ECMWF/UKMET/Canadian for Thursday into Saturday whose solutions seem quite overall quite compatible with GEFS/ECMWF/Canadian/NAEFS ensembles and the National Blend of Models and WPC continuity in a pattern with above average predictability. Prefer to switch to the ensemble means to build a baseline forecast onward into early next week amid growing forecast spread but applied blend weighting emphasis toward ECMWF ensembles whose somewhat greater amplitude into these longer time frames seems more likely in this type of flow pattern. ...Weather/Hazards Highlights... The Pacific Northwest/northern California will see periods of moderate rains into mid-later week with multiple Pacific system approaches. Digging energies into a mean upper trough over the West should allow for some precipitation to work inland to the north-central Intermountain West/Rockies early this period to include some enhanced terrain/mountain snows. Upper ridge building up/inland through the West Coast heading into the weekend should increasingly limit most activity with a more northward to offshore focus. However in this transition, channeled/deepened moisture feed may focus locally heavy rainfall into favored southerly facing terrain of the Olympics and vicinity, prompting introduction of a Day 5/Friday Excessive Rainfall Outlook Marginal Risk area. This area looks to be a focus for more enhanced moisture and rainfall through next weekend. A significant rainfall episode will linger later week into the weekend as deep moisture feed in advance of several ejecting shortwave troughs and frontal waves should prove slow to spread heavy rain and thunderstorms through the Lower Mississippi/Tennessee Valleys and Gulf Coast states into the Southeast/southern Appalachians and vicinity. These will fuel growing flooding concerns given significant repeat/training potential and a signal in the guidance for locally heavy daily totals. WPC Day 4/Thursday and Day 5/Friday Excessive Rainfall Outlooks have Slight Risk and surrounding Marginal Risk areas from the Lower Mississippi Valley and Gulf Coast states to the southern Appalachians given slow pattern translation with favorable upper support, deep moisture return, and repeat activity signature. As confidence increases in both the axis of heaviest rainfall as well as potential overlap with rainfall on Tuesday and Wednesday leading to wetter antecedent conditions, a higher risk threshold may be needed. A swath of more moderate rains is also set to spread up across the Ohio Valley/Great Lakes and the East into later week as enhanced with frontal wave developments, and given much warmer temperatures and snowmelt as applicable, areal and stream flooding may become a concern. However, uncertainty remains with these waves as well as the extent and focus of seemingly modest coastal waves next weekend into early next week. Wintry precip should be limited to the far northern U.S. tier and more earnestly into southern Canada as warm air spreads farther north. 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 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