Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Jan 25 2024 09:16:00 FOUS30 KWBC 250822 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 320 AM EST Thu Jan 25 2024 Day 1 Valid 12Z Thu Jan 25 2024 - 12Z Fri Jan 26 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL GULF COAST THROUGH THE SOUTHERN APPALACHIANS... A shortwave over OK/TX will lift northeast today, allowing for yet another round of heavy rainfall with somewhat organized convection moving relatively quickly through east TX and LA during the early mornings hours. Totals from this round of heavy rainfall look rather modest (localized 1-2" per 00z HREF PMM QPF), but much of the region has received 3-6"+ of rainfall over the past 24-36 hours. While the forecast 1-2" amounts would normally lead to little to no flooding in this rainfall-prone region, the antecedent conditions in this case may easily allow for scattered instances of flash flooding. While the bulk of the rainfall in LA will occur early this morning with rates as high as 1-2"/hr, farther northeast into MS/AL rates will be lower (peaking closer to 1"/hr) with additional localized amounts of 1-2" as well (over 3-6 hours ending midday to early afternoon). Antecedent conditions are also the concern through this portion of the Southeast, with prior rainfall totals a little more widely varied from 2-6" (highest over south-central MS). The end result is expected to be similar to the rainfall in LA, causing additional scattered instances of flash flooding (though a bit longer duration and areal in nature due to the lower rainfall rates). Even farther northeast into the southern Appalachians (northern GA and surrounding portions of East TN and western NC/SC), forecast rainfall is the highest with 12-hr localized totals of 2-3" expected. The bigger difference in this region is the antecedent conditions, which are relatively dry (with little to no rainfall having occurred thus far in this event). These higher localized totals will be due to persistent upslope component within the southern Appalachians as flow runs orthogonal to the terrain. With the grounds fairly solid considering the extended freezing period over the higher elevations, runoff potential may also be greatly increased due to the lower permeability of the soils. The expected impacts are similar to locations farther southwest, with scattered instances of flash flooding forecast (and mostly of longer duration affecting low-lying areas and small creeks and streams most significantly). While the above forecast and resulting impacts are relatively high confidence with good agreement among the guidance, the bulk of the activity is expected to end by 00z (late afternoon/early evening) as the shortwave lifts into the OH Valley and precipitation mostly comes to an end. The greater uncertainty comes late in the forecast period (after 06z) where the next disturbance is expected to lift northeast out of the western Gulf, potentially providing more large scale ascent focused over the central Gulf coast up through southern MS and AL. This will continue to be monitored closely, but the 00z HREF has backed off on the intensity of the convection (while also shifting it farther south into the Gulf), so the inherited Slight Risk area was maintained for this cycle. Churchill/Kleebauer Day 2 Valid 12Z Fri Jan 26 2024 - 12Z Sat Jan 27 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL GULF COAST AND SOUTHEAST... ....Central Gulf Coast and Southeast... The inherited Slight Risk was largely maintained from southeastern LA through much of southern MS/AL into portions of far western GA and the FL Panhandle, given the lingering frontal boundary and forecast ascent pattern during the day on Friday. While convection is largely only expected to result in localized totals of 1-2" (with the WPC QPF actually coming down since last cycle), there remains some potential for totals to locally exceed 2" (with the GFS and associated ensemble suite being the most intense of all the available guidance). This area along the central Gulf coast will continue to be the primary area of concern with a strong mid-level vorticity maxima attempting to pivot out of the Gulf, possibly impacting southeastern LA into southern MS/AL and the Panhandle of FL during the morning and afternoon hours Friday before pushing off to the northeast. This threat will continue to be monitored closely, as the full suite of CAMs comes into range later today. A much broader Marginal Risk (especially to the northwest) surrounds the Slight Risk area, given the much wetter antecedent conditions there (with localized totals of 1"+ possibly resulting in more isolated instances of flooding). ....Oklahoma and surrounding portions of North Texas and southern Kansas... Not much has changed farther west over the southern plains, as an upper low developing over the lee of the southern Rockies will slide eastward with large scale ascent providing a blossoming precip field over OK and into North TX and southern KS. Rates will be on the lower end overall, but some localized 1"/hr potential is possible given the favorable upper level dynamics with forecast soundings over OK indicative of elevated convective potential with deep saturation through the column. ....Pacific Northwest... An atmospheric river is forecast to spread inland across the Pacific Northwest coast on Friday, likely resulting in 1-3" localized totals along the coastal areas from northwest CA into OR. The area over southwestern OR and northwest CA has the best chance for an upgrade to a SLGT risk in subsequent updates with the heaviest precip occurring the back half of D2 when IVT's surge into that small corridor of the western US coast (and where antecedent conditions are relatively wet, given NASA SPoRT-LIS 0-100 cm soil moisture near the 80th percentile). Churchill/Kleebauer Day 3 Valid 12Z Sat Jan 27 2024 - 12Z Sun Jan 28 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST, MID-ATLANTIC, AND PACIFIC NORTHWEST... ....Southeast and Mid-Atlantic... An upper low/shortwave trough will slowly progress eastward on Saturday from the Southern Plains into the OH/TN Valleys. A relatively weak surface low pressure system is forecast to form over the Southeast in response to this feature, tracking along/near a frontal zone draped/dammed across the southern Appalachians into the Mid-Atlantic. Although forecast soundings depict little to no elevated instability, the combination of vorticity advection, moderate warm air advection, and upslope flow along the terrain could support some convective elements and localized rainfall rates to around 1"/hr. Guidance is in good agreement suggesting 1-3" totals (which also corresponds with the WPC QPF), and the best chance for these totals is in the vicinity of the southern Appalachians. An upgrade to Slight Risk may be justified in subsequent updates for this region (especially if forecast amounts of 1-3" pan out for the Day 1 forecast). For now, an inherited Marginal Risk was expanded more broadly across the region (to account for uncertainty as well as the wetter antecedent conditions across much of AL). ....Pacific Northwest... An inherited Marginal Risk was maintained for portions of the CA/OR/WA coasts with impacts from the aforementioned atmospheric river continuing into Saturday. The risk area highlights where guidance is in best agreement for additional 1-3" amounts, and subsequent expansions and upgrades are possible as the forecast comes better into focus. Churchill $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .