Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Nov 04 2022 19:48:11 FOUS30 KWBC 041948 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 347 PM EDT Fri Nov 04 2022 Day 1 Valid 16Z Fri Nov 04 2022 - 12Z Sat Nov 05 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PACIFIC NORTHWEST... 16Z Update: The very strong atmospheric river is underway across portions of the Pacific Northwest and overall very little changes were needed for the ERO. The most recent analysis shows a plume of anomalous moisture, characterized by PWs of 1.2" to near 1.3" brushing the WA/OR coasts this morning and should slowly sag southward through the rest of today and tonight, reaching southern Oregon/northern California area by 12Z Saturday. Strong IVT values, impressive lift, and favorable orographic enhancement will contribute to high probabilities of hourly totals exceeding 0.5" with an isolated signals for near 1" hourly totals. Outside of the valley locations, totals of 3 inches to locally 6 inches through 12Z Saturday appear on track. The Marginal Risk area was extended southward across portions of southern Oregon based on the latest timing of the AR plume which may be trending a tad faster compared to earlier. Otherwise, the Slight Risk area in place highlights the greatest risk well incorporating the best environmental ingredients and potential flash flooding for the more sensitive locations like recent burn scars. For the Plains/Midwest area, not much change this update with just minor tweaks based on the latest radar trends and the previous discussion below remains on track. Taylor ....Pacific Northwest... Few changes were made with this forecast update in this area. The second atmospheric river event in a week is underway across western Washington State. As of this writing, 24 hour totals are approaching 2" along the coast west of the Olympics. There is good agreement that this atmospheric river will be significantly stronger than the first one this week, reaching Strong to even low-end Extreme strength, with IVT values in southwest WA/northwest OR exceeding 1000 kg/m/s in the GEFS. Since this area was already hard hit just a few days ago, this stronger AR will continue raising already high stream and river levels draining the Olympics and Cascades. This supports the inherited Slight Risk over the area. A piece of good news is that by tonight, there should be notable and significant southerly progress to the AR movement, reaching southern OR by Saturday morning. With the movement across OR much faster than over WA, the flooding threat will quickly diminish with lowering latitude. Most of the models are depicting an additional 3 to 6 inches of rainfall through Saturday morning falling over much of the same area as that on the previous day, especially through today. Increasingly saturated soil conditions and the magnitude of the rainfall totals will lead to a more notable threat of runoff concerns and flooding. This will especially be the case around the burn scar locations including the Bolt Creek, Nakia Creek, and Siouxon burn areas. The Slight Risk and Marginal Risk areas highlight the areas with an elevated threat for excessive rainfall and flooding concerns. ....Southern Plains/Lower Mississippi Valley to the Upper Great Lakes... A deep shortwave embedded within a longwave trough along the Front Range and northern Plains early this morning will become increasingly negatively tilted as it shifts northeastward across the Plains today. A strong 140 kt south-to-north oriented jet streak will precede the shortwave. Meanwhile, a deep cold air mass will follow behind this wave, resulting in a potent cold front that will start the day draped from the upper Great Lakes southwest to New Mexico. An LLJ of Gulf moisture will extend northeastward ahead of the trough. This front will act as a train track of sorts for moisture racing up the cold front with the upper jet. As the shortwave becomes more negatively tilted during the day today, a surface low will develop near KS, which will rapidly track northeast, reaching northeastern IA by Saturday morning. South of this low track, ample Gulf moisture being drawn north into the low will raise PWATs to near 2" in portions of AR/LA/MS, which is nearly +3 standard deviations above normal for this time of year. The fast movement of both the surface low and the attendant upper level shortwave will promote the rapid coagulation of scattered convection across eastern OK/TX into an intense squall line, which will race east to MS by Saturday morning. This scenario is well agreed upon in all of the CAMs guidance. As far as flash flooding potential is concern, this very fast movement across much of 3 large states over about 16 hours will dramatically reduce the possibility of flooding for most areas, despite the climatologically anomalous moisture being drawn into the squall line from the Gulf. Thus, while the squall line and any attendant convection are moving through any given area, local rainfall rates may exceed 2"/hr at times. However, if that intensity of rain lasts an hour or less, there simply will not be enough time for any one location to pick up enough rainfall to flash flood. Thus, in coordination with the TSA/Tulsa, OK; FWD/Dallas-Fort Worth, TX; and LZK/Little Rock, AR forecast offices, the previous Slight Risk for this area was dropped down to a Marginal Risk. Along and north of the storm track from KS to WI, meteorologically conditions will be somewhat more favorable for flooding, as periods of rain moving northeast along the front will follow similar tracks for much of the day, as the front will be much slower to move, thanks in part to opposing flow pushing the front north, impeding much eastward progress. Instability will be much lower here than points further south in the squall line, so there is very good confidence that rainfall rates will not be anywhere near as high. That may be overcome by the slow-moving nature of the larger scale front, but that front will be parked over an area of the country that is very dry, and in the middle of a long-term drought. Thus, much of the rainfall will be beneficial to these areas, especially since it won't be falling overly heavily. Certainly more sensitive and urbanized areas run the risk of an isolated flash flooding threat, especially any smaller creeks and streams, but this threat will remain isolated. Thus, the Marginal Risk in this area was maintained for this update. Expect a broad 1-4" of rain across this area today into Saturday. Wegman Day 2 Valid 12Z Sat Nov 05 2022 - 12Z Sun Nov 06 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSISSIPPI/TENNESSEE VALLEY... Convection ongoing at the start of the period will gradually move eastward across portions of the Lower Mississippi River Valley. The forcing associated with the storm system is expected to weaken during the period as the surface low lifts into the Great Lakes and the system is blocked by ridging off the Southeast. The diminishing forcing, lack of deep instability, and overall dry antecedent conditions will limit the potential flash flooding to areas mainly along the coast, perhaps northward into central MS/AL. Isolated rain rates over 1"/hr will be possible with a localized flash flood risk possible through about 18Z. Wegman/Taylor Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-3m_xcQ2XWD3wpCRyrMpqKF-z1ct6l5UXblKOYfodNgl= SEnoy9SxIlGOMrgdEOXSVmpVdm2z_8lSh4ZXjIQN584QPVY$=20=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-3m_xcQ2XWD3wpCRyrMpqKF-z1ct6l5UXblKOYfodNgl= SEnoy9SxIlGOMrgdEOXSVmpVdm2z_8lSh4ZXjIQNVPAqKJA$=20=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-3m_xcQ2XWD3wpCRyrMpqKF-z1ct6l5UXblKOYfodNgl= SEnoy9SxIlGOMrgdEOXSVmpVdm2z_8lSh4ZXjIQNYFoLY3o$=20=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .