Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 08 2022 00:46:10 FOUS30 KWBC 080046 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 745 PM EST Mon Nov 07 2022 Day 1 Valid 16Z Mon Nov 07 2022 - 12Z Tue Nov 08 2022 ....A MARGINAL RISK FOR EXCESSIVE RAINFALL EXISTS IN AND NEAR THE MOUNTAINS OF THE CENTRAL & SOUTHERN CALIFORNIA COAST... Across the West Coast, a minor expansion of the Marginal Risk area was made across the coastal ranges of central CA where a period of locally heavy rain is expected late Monday night into early Tuesday morning as a renewed surge of low level moisture transport begins to impact the region south of a deep layer cyclone. Farther south, the Marginal Risk remains for an isolated occurrence of flash flooding which could occur with upslope flow and anomalous moisture into the Transverse Ranges but the bulk of the heavy rain threat is expected to occur beyond 12Z Tuesday. Roth/Otto Day 2 Valid 12Z Tue Nov 08 2022 - 12Z Wed Nov 09 2022 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS THE TRANSVERSE RANGES OF SOUTHERN CALIFORNIA... ....Southern California... On Tuesday, the vertically-stacked low starting the day centered off the coast of northwest CA will begin driving southeastward into central CA by early Wednesday morning. A strong upper jet with shortwaves rounding the base of the low will drive into SoCal on Tuesday. Overall this will be a very dynamic system, with plenty of large scale forcing for ascent. A moisture plume with PWs around 1" will move into SoCal, with values around or above the 90th percentile for early November. The combination of anomalous moisture and plenty of upper level support will result in heavy rainfall rates in the mountains especially, but also into the L.A. Basin down to San Diego. Upslope low level flow will be maximized in the Transverse Ranges, which should result in enhanced rainfall totals of 3-6", which will be falling on top of an additional 1-2" prior to 12Z Tuesday. Amounts closer to 1-1.5" are expected in the lower elevations of SoCal. Given the strong dynamics and height falls over the area, not surprising that some weak instability is also showing up in the guidance with MUCAPE values up to 500 J/kg depicted in the 12Z model guidance. Not much instability, but enough to support some embedded convective elements within the rain shield Tuesday and when combined with peak 850 mb winds of 40-50 kt directly orthogonal to the terrain, locally high end rainfall totals appear likely. The 12z HREF members support 50% probabilities of 0.5" in an hour rainfall in these heavier convective showers/storms starting around 15Z but ramping up to over 90% with a frontal band of heavy rain containing near 30% probabilities for localized rainfall rates around 1" in an hour mainly in the 18-00Z time frame...centered from the western San Gabriel into the San Bernardino Mountains..as plausible as well...mainly in the favored terrain, but even a lower risk along coastal areas. After coordination with LOX/SGX, a targeted Moderate Risk of Excessive Rainfall was introduced for the Transverse Ranges from Los Angeles into southwestern San Bernardino counties. The Moderate includes the sensitive El Dorado and Apple burn areas which had significant debris flows in September of this year. The Slight Risk was expanded westward a bit across Orange County to include the Bond burn scar in the Santa Ana Mountains. Isolated flash flooding, rapid runoff, and urban flooding are also possible for areas along the coast as well. ....Foothills of the Sierra Nevada... A Marginal Risk was introduced for this update into portions of central California where the potential will exist for 0.5+ in/hr rainfall rates below snow levels which are forecast to be as high as 6000-7000 ft at times. The main time frame of concern will be from 15Z to 00Z, in advance of the advancing cold front. Areas at most risk will be burn scars from the past couple of years which dot the region highlighted with the Marginal Risk. ....Portions of the High Desert and Great Basin... Moisture hitting the West Coast will have a much easier than normal time getting past the mountains into interior sections of southern NV, northwest AZ, and southwest UT on Tuesday given the strong dynamics and inland advancement of the IVT plume. Looking like rainfall amounts of 1-2" will fall across this area, with localized amounts around 3" in the favored south southwest facing slopes. Not expecting much in the way of instability this far inland through 12z Wed, which should limit peak rainfall rates below 1 in/hr, but the strong IVT and orographic effects should still push hourly rainfall totals above 0.5" at times. Overall these rainfall totals and rates should not result in widespread or significant flooding impacts. However minor flooding and runoff issues are still probable with rainfall of this magnitude. ....South Florida... The center of the tropical/subtropical system should be well offshore Tuesday and Tuesday night...however the mid level low associated with this system is very broad. Embedded shortwave energy on the periphery of this broader mid level low may track across FL this period, and guidance indicates PWs potentially increasing to around 2". There remains at least some risk of convective bands moving into southeast FL during the day or overnight period, which could potentially pose a localized urban flash flood risk with tall/skinny CAPE profiles in forecast soundings and low level onshore flow of 35-45 kt / coastal convergence axis setting up. Farther north, 12Z members of the 12Z HREF showed the potential for a narrow axis of heavy rain to set up near/just north of Cape Canaveral. Potential for 3-5 inches of rain exists where onshore flow of 30-40 kt at 850 mb is forecast by the latest model consensus. However, the area potentially impacted looks fairly localized and CAPE appears as if it will focus in the lower levels due to mid-level dry air in forecast soundings. Current thinking is that a Marginal Risk need not be extended northward to include the northeastern peninsula as 14-30 day rainfall has been mostly below average...although portions of the St. Johns river valley remain in flood stage. Chenard/Otto Day 3 Valid 12Z Wed Nov 09 2022 - 12Z Thu Nov 10 2022 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN FLORIDA... ....South Florida... Heavy rainfall from Nicole is likely to impact Florida Wednesday into Wednesday night. PWs should get up over 2.25" as the tropical system moves across the state. Still some spread on the exact evolution and structure of the system, but model guidance does seem to be coming into better agreement with the 15Z NHC track for Nicole. Through 12z Thursday the current model consensus remains for 2 to 4 inches of rainfall along portions of the east coast of FL, with localized amounts around 6" seeming probable. Of course some of this will be dependent on the strength and organization of the system as it moves across but it seems an increased pressure gradient on the north side of the system will help to focus the heaviest rain right of the tropical system's track. Although the expected forward speed of the system should put a cap on how high rainfall totals can get. At this time a Slight risk seems warranted along the east coast of FL, with scattered flash flooding probable as the system approaches the coast and moves across the state. Only minor changes were made for the Day 3 which was to expand the Slight Risk westward in the vicinity of the Orlando metro region. As the event draws nearer, reductions to the southern extent of the Slight/Marginal Risk across southern Florida may be needed but enough uncertainty remains at this point to leave these areas untouched for now. ....Upper Midwest... With a strong/deep upper level trough over the west, and impressive ridging over the Southeast, a strong gradient will exist across the Plains and MS Valley. Increasing southerly flow will result in increasing moisture, with PWs by 12z Wed (forecasts near 1.2") already near early November daily record values over portions of SD/MN/WI. Convection is likely to be ongoing at 12z Wed across portions of SD/MN/WI along a frontal boundary and low level moisture convergence axis. Some locally heavy rain is possible with this activity, but probably not much of a flash flood risk given weaker deep layer forcing and only marginal instability. However the front is forecast to stall out during the day Wednesday into Wednesday night, with the stronger deep layer forcing moving into the region ahead of the western upper trough. This in turn will also ramp up the low level moisture transport with flow at 850mb just above 50 kt forecast toward 06Z Thursday across the Missouri River Valley, directed into the stalled front. Thus the setup appears favorable for convective development and some potential training/backbuilding near the front Wednesday night from southeast SD into southern/central MN, northern WI to the U.P of MI where forecast Corfidi vectors are near zero or oriented opposite of the mean flow. A main limiting factor will be instability, with values expected to peak around 500-1000 j/kg and there is not much in the way of an upstream pool of instability. So activity may struggle to maintain intensity as instability gets eroded, especially for locations on the northern and eastern side of the Marginal Risk area. Localized rainfall totals of 2-3" are expected, but with higher potential for these values across southeastern SD into southwestern MN. It should be noted that the event does continue into Thursday (day 4), so event total rainfall will be even higher and we could eventually see a bit greater flood risk evolve. Chenard/Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_k36hkYjA2-2oiKzZxFEINvZyP9FWHmnz1joIvWGtKln= NJ6Kna_AkX5n2vxYX7e1Sc45tEy05uLivTw_T29d4oKdVtc$=20=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_k36hkYjA2-2oiKzZxFEINvZyP9FWHmnz1joIvWGtKln= NJ6Kna_AkX5n2vxYX7e1Sc45tEy05uLivTw_T29dGMRIz2k$=20=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_k36hkYjA2-2oiKzZxFEINvZyP9FWHmnz1joIvWGtKln= NJ6Kna_AkX5n2vxYX7e1Sc45tEy05uLivTw_T29dja5EPHs$=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) .