Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 24 2024 01:02:44 FOUS30 KWBC 240102 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 802 PM EST Mon Dec 23 2024 Day 1 Valid 01Z Tue Dec 24 2024 - 12Z Tue Dec 24 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA AND SOUTHWEST OREGON... An atmospheric river will move into northern CA and southwest OR tonight into early Tuesday. This is a fairly progressive system, which will limit the magnitude of rainfall totals and flood risk. However this is a dynamic system with a strong upper jet and deep mid level trough moving inland. These features will help enhance=20 ascent and also allow for some weak instability along the front=20 supporting low topped convective elements locally enhancing=20 rainfall rates. The 18z HREF neighborhood probabilities of=20 exceeding 0.5" in an hour increase and expand in coverage ~05z=20 across southwest OR and northwest CA and peak in the 60-90% range=20 from 06z-12z, with the higher probabilities shifting southward=20 with time. There are even some 1" an hour probabilities showing up=20 in the 18z HREF, peaking ~40% in the King Ranges between 06z and=20 08z. The 22z and 23z run of the HRRR also shows peak hourly rain=20 ~0.75-1" along a narrow band from the King Range into far=20 southwest OR between ~05z and 08z. These rates decrease some as the low topped convective line moves inland away from the slightly=20 higher instability just offshore, but both HREF probabilities and=20 recent HRRR runs still support localized rainfall exceeding 0.5"=20 in an hour inland. Despite the progressive nature of this system,=20 the potential for these aforementioned higher rates does introduce=20 some risk of rock and land slides long with minor flooding of=20 urban and other flood prone areas. Antecedent rainfall has resulted in above average soil saturation and near to above average=20 streamflows, making the flood risk a bit higher than it otherwise=20 would be for such an event. Guidance also suggests we may see anther uptick in rainfall rates=20 around 12z over the northern Sacramento Valley and adjacent=20 foothills. A combination of enhanced low level convergence driven=20 by stronger southerly flow up the valley, and slightly higher=20 instability values over the valley by this time, supports this=20 uptick in rainfall rates potentially towards 1" in an hour. These=20 higher rates will likely be fast moving by this time, and so the=20 heaviest rain at any one location should not last more than an=20 hour or so. This will help limit impacts, although rates could be=20 high enough to still result in some flood impacts. There is a=20 chance rainfall rates between 0.5" and 1" in an hour could move=20 over the Park burn scar around 12z. This will need to be monitored=20 closely, as rainfall rates of this magnitude could result in debris flows.=20 Chenard Day 2 Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN AND CENTRAL SIERRA NEVADA FOOTHILLS AND FROM EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY... ....2030Z Update... Some modest adjustments were made to the Marginal Risk area across the Sierra Nevada foothills based on the new HREF FFG exceedance probabilities which suggest a tad more potential a little farther south into the central Sierra Nevada foothills for some runoff concerns. The 12Z CAMs do suggest some potential for broken convection with 0.50"+/hour rainfall rates impacting the eastern portions of the Central Valley and into the adjacent terrain. Otherwise, essentially no changes were made to the Marginal Risk area over portions of eastern TX and into the Lower MS Valley given good continuity with the QPF in the 12Z guidance. Orrison Previous discussion... ...Sierra Nevada Foothills.. Atmospheric river from previous period will bleed into the D2 time frame with the primary shortwave trough quickly propagating inland with increased forcing upon arrival into the Northern and Central Sierra, less so for areas back into the coast due to negative vorticity advection. Locally enhanced rainfall will be plausible the initial portion of the period with the heaviest likely aligned along those Foothill regions of the Northern Sierra, mainly within elevations below 8000ft, although snow levels will fall below 7000ft towards the end of the more significant precipitation time frame late Tuesday morning. Additional totals of 1-2" are possible within a 6-10 hour window prior to the precip ending leading to a low-end potential for flash flood concerns just outside the Valley that extends from Redding down to Sacramento. Higher runoff capabilities due to terrain orientation and soil moisture anomalies running closer to normal will present some potential for flood concerns despite this being an event that doesn't maintain a more prolonged precipitation signature. The previously inherited MRGL risk was generally maintained, however some of the risk area was cut out due to provide a gradient between the Sierra locations that will trend to more winter precipitation and the areas that will remain liquid through much of, if not the entire duration of the event. ...Arklatex and Lower Mississippi Valley.. Surface ridge over the eastern CONUS will slowly drift further to the northeast allowing for a more broad return flow regime to affect areas across the Western Gulf into the Lower Mississippi Valley. A steady flux of higher theta_E's will begin moving northward out of the Western Gulf, carrying as far north as the Red River before stabilizing as we reach into OK. Aloft, a digging mid- level shortwave will exit the Central Rockies with sights on the Southern Plains leading to enhanced left exit region dynamics and surface cyclogenesis in-of the Red River Valley with a cold front extending from the base into TX and a small warm front lifting northward around the Arklatex. The classic mid-latitude cyclogenesis will create a sector of modest destabilization with the primary axis aligned from southwest to northeast across east TX up through the Arklatex, eventually extending northward into AR and southern MO. The current indications are the best organized convective schemes will be situated around the small warm sector along and ahead of the approaching cold front with relative buoyancy and increasing upstream shear allowing for scattered convection to develop late Tuesday afternoon through the evening. As of this time, the deep layer moisture pattern is still relatively meager compared to some of the more impactful events that occur in this area of the country, however there is enough instability and PWAT anomalies creeping between 1-1.5 deviations above normal to constitute some isolated flash flood concerns, mainly within those stronger cores. SPC D2 risk includes a targeted Marginal Risk for severe weather, overlapping the inherited MRGL risk ERO for the period. Areal rainfall averages will be between 0.5-1" across east TX and 0.75-1.5" across AR, but there is a growing consensus among the CAMs to have scattered instances of a quick 2-3" of rainfall within the best convective environment during the pattern evolution with a 5-10% risk of >3" within the tail end of the 00z HREF neighborhood probs. This threat is likely still within the lower end of the MRGL risk threshold, but the convective premise was enough to maintain general continuity from the previous issuance. Kleebauer Day 3 Valid 12Z Wed Dec 25 2024 - 12Z Thu Dec 26 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL WASHINGTON AND OREGON... ....2030Z... Some minor expansion of the Marginal Risk was accommodated across the Olympic Peninsula to account for heavier rains impacting western WA and relative to where the snow levels are forecast to be in the Olympics. There is very little change to the larger scale synoptic setup as there will be arrival of a new atmospheric river across the region for this period. Orrison Previous discussion... Another atmospheric river surging off the Pacific will make an appearance into the Pacific Northwest by Wednesday afternoon through the end of the D3 cycle. Model guidance is keen on a surge of moisture represented by fairly elevated IVT signatures on the order of 600-800 kg/ms within the global ensemble blend. There is some discrepancy between one of the main deterministic (GFS) and the other global members carrying more of a 25th percentile outcome in total precip due to a less robust IVT pulse comparatively. When assessing the ensemble means from the GEFS compared to the deterministic, the ensemble sways more in favor the scenario of greater magnitudes into the IVT channel leading to a more pronounced atmospheric river regime. This trend leaned more into maintaining continuity in the inherited MRGL risk across the PAC NW, although a few changes were made in the proposal. The first change was to scale back on the eastern extension of the MRGL risk due to considerably less deep layer moisture advecting inland at this juncture for the D3 time frame. FFG indices are still pretty high all the way towards the windward side of the Cascades, so the prospects for FFG exceedance were pretty low and generally fall below the 5% threshold criteria. The second change was to cut out the higher elevations in the Olympic Peninsula due to the primary ptype trending towards snow with more emphasis on rainfall closer to the coast and below 4000ft MSL. The risk area still encompasses the lower elevations surrounding the Olympics and channels into the Olympic National Forest on the southern flank of the Peninsula. Anticipate totals of 2-4" with locally as high as 5" in the risk area extending from coastal southwest OR up through the Olympic coast of WA. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5BBGX8nvo5IX2VjmvdZSTbRvyapcyx-VJ9sUluzr24pg= vR3n23-Sb2fmxrKM8YTU_tod9iWhULkoNIy_wa-PCE0CvA0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5BBGX8nvo5IX2VjmvdZSTbRvyapcyx-VJ9sUluzr24pg= vR3n23-Sb2fmxrKM8YTU_tod9iWhULkoNIy_wa-PApdidFw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5BBGX8nvo5IX2VjmvdZSTbRvyapcyx-VJ9sUluzr24pg= vR3n23-Sb2fmxrKM8YTU_tod9iWhULkoNIy_wa-P58MKS08$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .