Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 12 2024 15:50:33 FOUS30 KWBC 121550 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1049 AM EST Fri Jan 12 2024 Day 1 Valid 16Z Fri Jan 12 2024 - 12Z Sat Jan 13 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC AND NORTHEAST... ....16Z Update... Few changes of substance were made to any of the ERO risk areas, but all areas are very low confidence that the expected rainfall can translate to impactful flash flooding. ....Mid-Mississippi Valley and Lower Ohio Valley... Starting with this area since this is the only one of the three risk areas where rain is ongoing. Thus, all changes were made to this area. The southwestern portion of the Marginal was trimmed to the expected back edge of the rainfall at 16Z. Rates across western TN (and everywhere in the Marginal Risk area) are a half-inch per hour or less based on radar imagery, and with decreasing instability is unlikely to change as the severe weather much further south saps the atmosphere of instability. The Marginal was trimmed to line up with the 12Z HRRR's depiction of rainfall through 12Z Saturday, largely following the Ohio River up to east-central Indiana. Soils before this event are very dry, and the very low rainfall rates are highly unlikely to result in much if any flash flooding, but a somewhat higher likelihood of training may allow the rainfall to last a bit longer in the Marginal risk area along a stationary upper level front immediately east of the low track, so this area is where the Marginal remains. ....Mid-Atlantic and Northeast U.S... The meteorology and forecast QPF footprint remain unchanged. A 6-hour period of rain is expected for most areas, perhaps an hour or so longer across southern New England, where the greatest flash flooding risk exists. Much of the area south of NYC is forecast to pick up between 0.75 and 1.25 inches of rain tonight. Very saturated soil conditions should mean much of that runs off into still-full rivers and streams, but in the 3 days that have elapsed since the last event, many of the rivers have come down enough that it appears unlikely this relatively small amount of rain should result in impactful flooding, as those rivers still in flood may either be delayed in their recession, or if they come up, won't be nearly to the level that was seen 3 days ago, thus significantly limiting forecast impacts. The one exception to this may be from northern NJ over the Hudson Valley and into CT and RI, where higher rainfall totals are expected due to upslope enhancement. Even there the rainfall amounts may just approach FFGs resulting in some limited flooding, but widespread impacts again are expected to be limited. To avoid "flip-flopping" the forecast the inherited ERO risk areas remain in place, but confidence in flash flooding is low. ....Pacific Northwest... The start of a multi-disturbance rainfall event will be today and tonight, with onshore flow and local precipitation already begun. Cool air in place has freezing levels low, so any rainfall is in the valleys where temperatures are holding in the 40s. This will greatly limit any flash flooding potential anywhere, including in the Marginal Risk area today as precip that falls at elevation will be in the form of snow. Warmer air that moves in with a stronger low tomorrow/Day 2/Saturday will increase the potential for flash flooding in this area. More on that in the Day 2 discussion. Wegman ....Previous Discussion... ....Mid-Atlantic and Northeast U.S.... Surface low pressure is expected to form over the Mid-Atlantic region later today along a boundary associated with deepening low pressure over portions of the Ohio Valley. The resulting inflow of low level moisture is maximized at about the same time that upper level divergence increases through a deep layer due to the approach of an upper level jet especially during the afternoon/evening over the Mid-Atlantic region and moreso into parts of the Northeast U.S. from this evening into the overnight hours. QPF amounts are not blockbuster values...due to the relatively short window of opportunity given the progressive nature of the system. However...models are beginning to pick up on a period of elevated convection and associated increase in rainfall rates which may prove to be problematic in areas still draining off water from the heavy rainfall earlier in the week.=20 Amounts alone are such that a Slight Risk area in parts of the Mid-Atlantic would probably not have been entertained had it not come on the heels of the heavy rainfall earlier in the week across an area that has been wet for at least 4 weeks. Confidence is higher for impacts farther north into parts of the Northeast U.S. where higher amounts from this event will overlap areas soaked a few days ago...in part given the higher magnitude afforded by strengthening low level flow and increasingly deep moisture.=20 ....Mid Mississippi Valley to the Lower Ohio Valley... Expanded a portion of the Marginal Risk area...mainly over northeastern Arkansas into adjacent areas of Tennessee...where convection from overnight looks to persist into the early portion of the Day 1 period. There has been some training of cells overnight and high-resolution guidance has shown that threat to linger beyond 12/12Z. As previously mentioned...very dry soil conditions remain prevalent across much of the Marginal Risk area will mitigate the flash flooding threat...as well as increased cell motion later today. Maintained the Marginal Risk along the corridor of highest expected rainfall totals. Substantive rainfall should be done in most of the Marginal Risk area by 12/18Z Friday in the wake of the cold front which ushers in very cold air. ....Pacific Northwest... The Marginal remains largely the same, but in following with the latest guidance shifting a bit southward in response to the latest guidance. Locally enhanced rainfall rates are expected from later today into early Saturday morning with several bouts of heavy rainfall within the coastal plain extending from northwest CA up through central OR given steepening low- and mid-level lapse rates. There could be some localized flooding with the urbanized corridors along the coast, but nothing too extraordinary in the grand scheme. Bann Day 2 Valid 12Z Sat Jan 13 2024 - 12Z Sun Jan 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL NORTHWEST CALIFORNIA AND A VERY SMALL PORTION OF SOUTHWEST OREGON... ....Northwest Corner of California and Southwest Oregon... On-shore flow of deep moisture is expected to continue from the Day 1 period into the weekend with an additional 2-4" of rainfall...and isolated maximum amounts around 5 inches in the higher terrain along the coast of coastal northwest California and clipping the extreme southwest corner of Oregon. Latest guidance shows enough elevated instability where the combined effects of the moisture plume interacting with the terrain could result in isolated amounts approaching 5 inches. NBM probabilities for at least 3" are remained in the 50-60% range within the King Range for the Day 2 time frame with 2" having a large 50+% area encompassing northwest CA. As a result...an upgrade to a Slight Risk was introduced given the compounding effect of multiple days worth of rainfall. The Marginal Risk surrounding the Slight Risk areas was mostly maintained elsewhere with few changes needed from yesterday's Day 3 ERO. ....New England... Remnant moisture on the eastern flank of the broad cyclonic circulation over the Great Lakes will continue through coastal New England before exiting into Canada the latter half of Saturday. Consistent timing in the models of the area of low pressure resulted in similar QPF amounts over New England compared with previous forecast cycles...resulting in little need for more than a few cosmetic changes to the outlook. All the rainfall and snowmelt within the past series of events will culminate in some localized flood potential. Bann Day 3 Valid 12Z Sun Jan 14 2024 - 12Z Mon Jan 15 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9ugLcGaZjD8eleoV3r1XO6HYdjGj-bIMIVznIOmgxpnY= NKjZkwVKHPH7Dq7-rH4zy81KazxUM8m5SZUHk8Yf6xmlalk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9ugLcGaZjD8eleoV3r1XO6HYdjGj-bIMIVznIOmgxpnY= NKjZkwVKHPH7Dq7-rH4zy81KazxUM8m5SZUHk8Yf31yJOC0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9ugLcGaZjD8eleoV3r1XO6HYdjGj-bIMIVznIOmgxpnY= NKjZkwVKHPH7Dq7-rH4zy81KazxUM8m5SZUHk8YfVqy5gC8$=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) .