Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 PM 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... An unusual amount of uncertainty in the guidance prompted a blend consisting of mostly 00z ECMWF/UKMET and 06z GFS. This is represented by our days 3 and 4 blends. The 00z deterministic Canadian is a bit more progressive and aggressive with that shortwave trough propagating through the Southern Plains/Lower Mississippi Valley on days 3 and 4, so it was only included at a reduced weighting. The 06z GEFS was introduced on day 4 followed by the 00z ECE/CMCE on day 5 in place of the 00z UKMET. The early introduction of the ensembles was to mitigate run to run inconsistencies in the GFS/GEFS. We leaned exlusively on the 3 ensembles for the days 6 and 7 blends due to continued run-to-run inconsistencies in the GFS. ...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. There's been notable convergence in the latest guidance suggesting a focused area of heavy rainfall setting up along the Oregon/California coastal border region on Day 5. 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. A mid-level closed low may influence convective activity over parts of central-eastern Oklahoma on Friday night. A Marginal Risk was introduced to account for a notable QPF clustering response by the guidance over this area. Wintry precip should be limited to the far northern U.S. tier and more earnestly into southern Canada as warm air spreads farther north. Kebede/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