Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 08 2022 15:58:51 FOUS30 KWBC 081558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1058 AM EST Tue Nov 08 2022 Day 1 Valid 16Z Tue Nov 08 2022 - 12Z Wed Nov 09 2022 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS THE TRANSVERSE RANGES OF SOUTHERN CALIFORNIA... 16Z Excessive Rainfall Outlook Discussion... Early morning water vapor satellite imagery showed multiple vort centers embedded within a broader upper-level cyclone...with the southern vorticity center becoming the dominate player in terms of supporting a growing area of rain that will continue to move south and east today and tonight. The latest high resolution guidance still shows the potential for heavier rain associated with the leading edge of a vigorous upper level low will move down the coast, but directing internal convection straight into the coastal mountains. Thus, much of the expected rainfall today will likely fall in the few hours the heavier rain band/front is over any particular part of the coast. A broad 2 to 3 inches of precipitation is expected in the coastal mountains from Malibu northward...with locally higher amounts. In areas farther south...rainfall won't be as heavy nor have the rainfall intensity at lower elevations and urbanized areas of So-Cal, the lower FFGs and still steady rainfall should result in better-than-isolated flash flooding. Saw little reason to change the Moderate Risk area or the surrounding Slight Risk area at this point...with the best flood threat will be around the lower elevation burn scars. Also maintained the Marginal Risk area farther inland...due to the decidedly lower instability. Even though amounts in excess of an inch are forecast, rainfall is expected to be more stratiform in nature with lower rates. In Florida, the inherited Marginal Risk area was also largely left unchanged. Rain from the westernmost rainbands of Nicole will just start impacting the Atlantic coast of south Florida overnight tonight. As with all tropical cyclones, there will likely be embedded stronger cells, which when moving into the highly urbanized I-95 corridor could result in isolated flash flooding prior to 12Z Wednesday. Bann ....Previous Discussion... ....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 So-Cal 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 So-Cal, 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 So-Cal. 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 2 Valid 12Z Wed Nov 09 2022 - 12Z Thu Nov 10 2022 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF THE FLORIDA PENINSULA... ....Florida Peninsula... Rainfall associated with what is forecast to be hurricane Nicole will overspread much of the Florida Peninsula on Wednesday. Changes in the guidance include a bit faster movement and maybe a slight northward adjustment. As such the inherited Slight Risk was expanded north and west to include much of the Florida Peninsula except the Gulf Coast. The main consideration to note in addition to those changes is the fact a few of the larger rivers, like the St. John's, remain in Flood stage from Ian a few weeks back. Thus, the prospect of a half-foot and locally more of heavy rain will reopen the wound so to speak. In collaboration with the MLB/Melbourne, FL and JAX/Jacksonville, FL offices, a potential upgrade to a Moderate Risk for all or portions of the First, Space, and Treasure Coasts is probable with future forecast updates. Such an upgrade may also include some of the Orlando Metro as well. ....Upper Midwest... In coordination with the MPX/Minneapolis, MN and DLH/Duluth, MN forecast offices, the inherited Marginal Risk over this area was trimmed significantly with this forecast update. There has been a notable northward trend in the forecast models on the axis of heaviest rainfall. A plume of Gulf moisture will extend all the way up to the upper Great Lakes on Wednesday. Meanwhile a strong front will be in place over this same area. The boundary between a truly wintry air mass to the north of the front and the warm moisture plume over much of the country to the south of the front will result in a rather small area (latitudinally) of heavier rain right along the front where the 2 air masses collide. This front will be essentially stationary, so wherever the front sets up, those areas may see multiple round of training convection. Given the potential for multiple inches of rain, it's fortunate that the area as been exceptionally dry the past few months. This greatly negates the overall flooding threat. However, this small area of heavier rain may set up either over the Twin Cities or Duluth. Thus, changes to the current Marginal Risk area are probable. For now, the Marginal Risk will focus on areas that have seen at least a bit of rainfall recently, so antecedent conditions are a bit wetter. Thus, the area is confined to around Lake Superior. Wegman Day 3 Valid 12Z Thu Nov 10 2022 - 12Z Fri Nov 11 2022 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... ....Portions of the Southeast... The circulation associated with Nicole will continue hitting portions of the Southeast on Thursday. The main areas of heavy rain continue to be focused from coastal southern South Carolina, the eastern half of Georgia, and Florida from Apalachicola east and from Tampa north. Expect an additional 3-6 inches of rain on Thursday with higher amounts locally. This area too has the potential to need additional upgrades to a Moderate Risk with future forecast updates, once the track and speed of Nicole come into better agreement in the guidance. Further north and west from northern Georgia to southwest Virginia, there has been a clear and notable wetter trend in the forecast rainfall, with some northward shift due to an approaching upper level trough. Thus, the Appalachians from GA to VA are looking particularly wetter compared with previous forecasts. Strong southeasterly to easterly flow into the mountains will upslope along the mountains, which will result in a local maximum of rainfall over this area. That being said, this area has been very dry lately, so only the Marginal Risk was expanded into this area. The Slight Risk, while expanded from inherited, is also much smaller than it otherwise would've been if the antecedent conditions weren't so dry. Thus, it remains to be seen how widespread any flooding rains are. ....Upper Midwest... A strong low will develop along a front across the Upper Midwest on Thursday. While it produces blizzard conditions across the Dakotas and northern Minnesota, on the warm side of the storm from southern MN, northern WI, and the U.P. of MI, rainfall will be ongoing from Wednesday over much of the same area, since the front focusing the rainfall will be stalled out. Once again since this area has been so dry the flooding threat is significantly diminished. However, the aggregate of multiple days of rain over this same area should result in an isolated flash flooding threat across the Marginal Risk area. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!47W1ja8I20FJ8wVw1Iu0H31Jr1rF3Du133CtB2ldNe7r= mwBfoOgOQlziX0WR4IlTXhrOY4V-lBm6UNuUaDDNObFa4B8$=20=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!47W1ja8I20FJ8wVw1Iu0H31Jr1rF3Du133CtB2ldNe7r= mwBfoOgOQlziX0WR4IlTXhrOY4V-lBm6UNuUaDDNEWjQfA0$=20=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!47W1ja8I20FJ8wVw1Iu0H31Jr1rF3Du133CtB2ldNe7r= mwBfoOgOQlziX0WR4IlTXhrOY4V-lBm6UNuUaDDN6vv3eVc$=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) .