Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 07 2022 16:00:35 FOUS30 KWBC 071600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Mon Nov 07 2022 Day 1 Valid 16Z Mon Nov 07 2022 - 12Z Tue Nov 08 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE TRANSVERSE RANGES OF SOUTHERN CALIFORNIA... ....16Z update... No significant changes were made for this update. Across the West Coast, a minor expansion of the Marginal Risk area was made across the Santa Cruz Mountains where there will be a longer time period for locally heavy rain to occur as a renewed surge of low level moisture transport begins to impact the region late Monday night. Farther south, an isolated occurrence of flash flooding could occur with upslope flow and anomalous moisture into the Transverse Ranges but the bulk of the heavy rain threat will occur beyond 12Z Tuesday. Forecast reasoning below remains on track for California. For Arkansas, a frontal boundary extending westward across the lower Mississippi Valley into central Texas will become more ill-defined through Monday evening into the overnight hours as surface ridging continues to build south across the southeastern quadrant of the nation. The threat for additional heavy rain across the Mississippi River Valley appears to be fading as cloud tops are warming overall (as of 1545Z) across the region as short term forecasts indicate expanding of mid-level ridging across the region and the 12Z hires model suite does not support further heavy rainfall for the remainder of the outlook period. For Texas, there could be a localized spot or two of heavy rain as moisture continues to increase but the coverage appears fairly limited to introduce any risk area for this update. Otto ....previous discussion follows... ....Central and Southern California... The deepening trough and vertically stacked low will continue propagating south down the coast from Washington State to just west of northern California on Monday. This will cause the longwave trough to amplify in a positively-tilted direction. The result will effectively be a shifting on the main onshore moisture flow southward down the California coast into central and southern California. The jet streak rounding the base of the trough will provide the best forcing as a lobe of vorticity remains well offshore. The deeper moisture plume will continue to slowly drop in latitude and focus toward the Transverse Ranges into the L.A. Basin. Integrated Vapor Transport values will be weak to modest a best with 300-400 kg/m/s values initially directed a bit south of due west, but with the deepening trof further north it is expected to back and maintain a longer duration through the afternoon into the morning overnight period on the 8th. Rates are not expected to exceed .5"/hr until this redirection/secondary surge occurs later in the overnight period (after 00z), but still remain low in the 10-20% range for Southern California. Still the shear duration of .25"/hr rates in the Transverse Ranges will support localized of 1.5-2.5", worthy enough to maintain the Marginal Risk for the region. Further north, the timing of the sharpening/approaching embedded shortwave off the NW CA coast, will support strengthening southwesterly confluent flow after 08.06z with 850mb winds increasing to 40-50kts by 12z and bring nose of .7-.8" TPW values to the Santa Cruz Mountains. IVT values increase to 400-500 kg/m/s values and Hi-Res CAMs ramp up to 1-1.75" totals between 06-12z. While this is on the edge of a large enough area for a risk area, it does dove-tail nicely with the thinking within the Day 2 ERO period and allows for some remaining uncertainty for timing given the slight trend faster over the last few cycles.=20=20=20=20 ....Arkansas through Northeast Texas... There remains a non-zero chance for thunderstorms capable of intense rainfall rates extending from a weak shortwave/low level inflection across the central AR/MS Mississippi Valley, slowly expanding westward into Northeast Texas from the start of the forecast period through the morning into early afternoon. This is response to weak FGEN forcing/convergence along the morning return warm front from the higher theta-E air streaming off the western Gulf of Mexico. Moisture flux convergence and enhanced deeper layer moisture will be consolidated near the inflection where PWat values of 1.75-1.9" are well above climatological averages close to 95th to 97th percentiles. Modest instability of 500-750 J/kg of MUCAPE will be available for these likely slower moving cells (given proximity to the weak low to mid-level wave) and as such Hi-Res CAMs support localized rates of 1.5"-1.75"/hr. Slow westward propagation as increasingly unstable air may allow for additional scattered development upstream into TX through mid/late morning with better instability given 1000-2000 J/kg of CAPE maybe available, yet this is also a factor of slightly enhanced capping relative to further east and less focused moisture convergence to support even more isolated nature to these stronger thunderstorms. HREF probabilities are not suggestive of rates over 2", though 2"/3hr totals do maximize around 12-15z in SE AR with 50% probabilities. The fact there is already ongoing convection that may have exhausted some localized pockets of enhanced instability may suggest Hi-Res CAMs supporting the HREF may be a bit too slow. While FFG values for the region have rebounded, they may have too quickly given NASA SPoRT LIS relative soil saturation values are in the 40-50s ranges across much of E AR, N LA and parts of NE TX and best heavy rainfall signal lies on the fringes of these areas. As such, a small Marginal Risk was considered across southeast AR, but probabilities are likely too low extending back toward the west into TX. Gallina Day 2 Valid 12Z Tue Nov 08 2022 - 12Z Wed Nov 09 2022 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS 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". 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. Not much, but enough to support some embedded convective elements within the rain shield Tuesday. The 00z HREF members support 0.5" in an hour rainfall in these heavier convective showers/storms, with localized amounts around 1" in an hour plausible as well...mainly in the favored terrain, but even a lower risk along coastal areas. A Slight Risk of Excessive Rainfall was maintained for this forecast cycle. The Slight is focused mostly in the mountains, but isolated flash flooding, rapid runoff, and urban flooding is also possible for areas along the coast as well. As always, areas near recent burn scars will be most at risk to debris flows and other impacts associated with heavy rain. Only minor adjustments were made to the outlook with this update, with the main adjustments just a northward expansion of the Marginal Risk into the Santa Cruz mountains, where an upward QPF trend is noted in the favored upslope areas. ....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, but the strong IVT and orographic effects should still push hourly rainfall totals into the 0.25"-0.5" range. 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 is quite a bit of model spread this time frame, but the 00z EC and UKMET remain elevated with QPF along coastal areas. This will be pretty dependent on how the broader mid level low and associated shortwaves rounding it evolve..so the flood risk this day is more conditional in nature. But there is 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. Chenard 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 of the system, but model guidance does seem to be coming into better agreement. Through 12z Thursday the current consensus is 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. Nonetheless the above mentioned totals seem like a pretty safe floor for totals with this event given recent trends...with the potential there for higher amounts. 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. ....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 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, with the stronger deep layer forcing moving into the region as well from the west. This in turn will also ramp up the low level flow and 850mb moisture transport 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. With PWs near climatological max values, strong forcing, and a stationary front, the ingredients for heavy rainfall are there. The 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. Do think that there is enough going for the event to support a Marginal risk and isolated flash flood risk, with localized rainfall totals of 3" a possibility. Still some chance this event over performs given the setup, so will need to monitor trends. However, at this point, forecast instability fields suggest this is only a lower end flood risk through 12z Thursday. 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 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_3MYSZSEKQI6pXLlTCbQ-VIwWgtIauPL-TB_UCBJT32d= TzbK4utS7xpsynHQkngftHL6rOH9jaGfTQtKw2FRNaveSR4$=20=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_3MYSZSEKQI6pXLlTCbQ-VIwWgtIauPL-TB_UCBJT32d= TzbK4utS7xpsynHQkngftHL6rOH9jaGfTQtKw2FRAL1qQ_A$=20=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_3MYSZSEKQI6pXLlTCbQ-VIwWgtIauPL-TB_UCBJT32d= TzbK4utS7xpsynHQkngftHL6rOH9jaGfTQtKw2FRE7YTTpY$=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) .