Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 11 2023 01:06:18 FOUS30 KWBC 110106 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 805 PM EST Fri Mar 10 2023 Day 1 Valid 01Z Sat Mar 11 2023 - 12Z Sat Mar 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SAN JOAQUIN VALLEY AND FOOTHILLS OF THE CENTRAL CALIFORNIA SIERRAS ALONG WITH PARTS OF SOUTHERN CALIFORNIA... 0100 UTC Update -- Modifications to the Day 1 ERO were mainly the result of observational trends. The southern shift of the AR has resulted in clearing parts of the northern Sierra into NV out of the Day 1 ERO. Meanwhile, the reduction of low-mid layer moisture transport as per ARDT and CW3E IVT forecasts is resulting in a reduction in short term (hourly and sub-hourly) rainfall rates, to a level where the High Risk is no longer needed. Within the Moderate Risk areas, additional pre-frontal rainfall (eastern San Joaquin Valley-foothills-western slopes of the central-southern Sierra) may still bring up to 0.50"/hr rainfall rates per the 18Z HREF probabilities. Hurley ....Previous Discussion... Widespread heavy to excessive rainfall will continue through the day on Friday over an area already soaked by a prolonged wet period and by recent rainfall from a passage of several waves.=20 With a fetch of moisture originating south of 30 degrees North latitude...precipitable water values of 1.5 inches were about 50 nmi off the California coast between the Bay area and the Transverse Ranges and this airmass will continue to be drawn inland. The latest numerical guidance continues to show the focus of heavies rainfall working its way southward along the Coastal Ranges as the channel of best on-shore flow gets redirected with time, This keeps one QPF bullseye focused near the coast and another in the upslope regions of the Sierra Nevada foothills with WPC deterministic pretty close in terms of placement and amounts in the 4 to 9 inch range which leaves room for isolated higher amounts. Highest amounts are where the flow aligns with the terrain to focus a prolonged period of QPF as well as areas that have enhanced rainfall amounts due to instability that the HRRR redevelops during the afternoon with MUCAPE values at or slightly higher than 250 J per kg north of the Transverse Ranges. By early evening, however, precipitation rates begin to wave as the moisture transport of deep moisture becomes weaker and directed farther south. Antecedent conditions remain incredibly favorable with a well-above-normal snowpack in the Sierras and soils remain near saturated area-wide via NASA Sport imagery so the combination of rain and snowmelt (especially below 5500 ft) pose the potential for widespread flooding is considerable, especially within the High Risk areas although the risk of flooding/rapid inundation remains a concern in areas of lesser QPF given the antecedent conditions which would result in much of the water turning immediately to runoff in the Slight and Moderate Risk areas. The strong atmospheric river should push IVT values greater than 400 kg/m*s past the Sierra Nevada and into the central Great Basin. While rainfall rates should remain meager and under 0.5" per hour, the combination of rainfall amounts up to an inch and snowmelt could lead to ponding of water and rises to creeks, streams, and rivers. The latest HREF 24-hour QPF probabilities for exceeding 100 year ARI are also elevated for this area. Bann Day 2 Valid 12Z Sat Mar 11 2023 - 12Z Sun Mar 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE CENTRAL/NORTHERN CALIFORNIA SIERRA FOOTHILLS AND FOR PORTIONS OF THE MID-SOUTH... ....2030Z Update... ....California... While the main atmospheric river event will have ended across the foothills of the Sierra Nevada mountains in California, ongoing and very persistent unidirectional westerly flow of Pacific moisture upsloping into the terrain, along with nearly ideal antecedent conditions consisting of fully saturated soils, a deep snowpack at the higher elevations that will continue melting below 6,000 ft, and ongoing flooding in the area were the factors that led to an upgrade in the flooding threat Saturday to a Slight Risk. Again, conditions Saturday won't be nearly as wet as the ongoing atmospheric river impacting areas further south currently, but the additional rainfall certainly won't help. With up to 3 inches of additional rain into the mountains, ongoing flooding will at least continue through Saturday (and beyond), and may get a bit worse for those river systems that drain the areas that see those highest rainfall totals. Snow levels will hover in the 5,500-7,000 ft range, which for the northern Sierras will be all but the highest peaks, while those higher values will persist further south, where there's still expected to be a melting snowpack to further contribute to the flooding situation on the southern end of the Slight Risk area. ....Mid-South... The upgrade to a Slight Risk across much of Arkansas, northern MS, and the Memphis area of TN is similar to CA in the sense that the very wet antecedent conditions will be the dominant factor driving the concern for flash flooding Saturday night. A rather fast-moving squall line is forecast by much of the CAMS guidance to initiate over AR and persist into TN and MS starting late Saturday afternoon and persisting into Saturday night. For the most part, any one portion of the Slight Risk area will only see 1-3 hours of rain with this line of storms, but the storms will have 1,000-1,500 J/kg of CAPE and PWATs around 1.25 inches of juice to work with, thus the potential for heavy rain is considerable with these storms. If the storms were moving over drier soils this would be a Marginal risk...as such this is considered a low-end Slight risk and upgrades at this point are highly unlikely. Wegman ....Previous Discussion... ....California... Lingering shower activity will continue in the Sierra mountains of central and Northern California especially, but to nowhere near the degree as from Friday into the early morning hours on Saturday. The challenge will be what lingering impacts there will be as the area gets mostly light rain on Saturday in an area that will have very little tolerance for any additional rainfall. Given the very real potential for even the 1-2.5 inches of new rainfall Saturday...there is concern that any on-going flooding concerns could be made worse in addition to possibly resulting in additional flooding concerns. While there remains some potential for a Slight Risk upgrade in the central and northern Sierras as snow levels rise a little bit through the 6-7k feet range during this period...that was not done at this point. Note that the primary driver of flooding will be antecedent conditions in this area, and not so much additional rainfall. ....Mid-South... There remains a lot of spread in the guidance as to where the heaviest rainfall will be seen in this area on Saturday. For the most part, the significant rainfall will hold off until late Saturday night. The GEFS keeps its greatest moisture profile back across Arkansas and northern Mississippi into the overnight hours Saturday/early morning hours Sunday...while the areal coverage from the ECENS extended from southern Arkansas into the Tennessee Valley (with the 10/00Z GFS lending more support to the ECENS placement than it doe to its on ensembles). Regardless...recent heavy rainfall has suppressed flash flood guidance in the area.=20 As a result, a broad Marginal still is valid to depict the uncertainty in placement. As mentioned previously...the Marginal Risk area may need to be shifted from its present location but the chances of needing an upgrade are low. Bann Day 3 Valid 12Z Sun Mar 12 2023 - 12Z Mon Mar 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA... ....2030Z Update... ....California... The Marginal risk across northern California was upgraded to a Slight Risk with this afternoon's update. Rainfall rates will be gradually increasing through the day Sunday into Sunday night as the next atmospheric river begins to take shape further west across the Pacific. Expect 1-3 inches of additional rain into the coastal mountains of far southwestern OR and northern CA. While these areas haven't been hit as hard as areas further south, the lack of a true dry break will keep soils saturated, so antecedent conditions will remain favorable. Further east across the foothills of the northern Sierras, we are on rinse and repeat mode. Very similar rainfall totals (1-3", locally higher in the highest elevations) are expected again on Sunday as on Saturday, and in the same areas. Thus, the rationale for the Slight upgrade remains the same as on Day 2/Saturday. We are expected to simply continue to see Pacific moisture being driven into northern California on a more southwesterly flow (a noteworthy difference from the atmospheric river presently ongoing). Nonetheless, PWATs will be increasing through the overnight period in anticipation of the next AR which begins on Monday. Any ongoing flooding will likely be maintained again on Sunday as any dry periods remain short enough to remain essentially inconsequential towards improving the flooding situation. Speaking of broken records, the snow levels will remain largely the same, only very slowly rising through the 5,500 to 6,500 ft elevation range through this period. Thus, the highest elevations will continue adding to their snowpack, while the lowest elevations continue melting it off. ....Southeast... Model spread remains high as to the timing and evolution of the heaviest rains across AL and GA on Sunday. What looks most likely is the ongoing squall line from Day 2/Saturday night will continue across the northern parts of the Marginal Risk area Sunday morning, then dissipate or largely dissipate, then afternoon convection refires along the line Sunday afternoon, which may continue into Sunday night. The bigger potential threat will be the second round of convection, which will develop either over southern AL/GA or the FL Panhandle. This is because the front driving this convection looks to slow down or stall somewhere in that area. As alluded to, there is significant uncertainty as to where that front stalls, and consequently how much convection develops along it. A stalled front will certainly favor training thunderstorms progressing eastward. Fortunately, other than the ongoing storms over south GA presently, the soils in this area are VERY dry. Thus, the Marginal risk is maintained for this issuance pending greater confidence on both timing and location of any training storms. Wegman ....Previous Discussion... ....California... One...or more...shortwaves embedded within the broader scale trough will help the mid-level flow become increasingly zonal from the eastern North Pacific Ocean into the northern portion of California on Sunday. This should result in an increasing areal coverage of generally light to possibly moderate rainfall and renew flooding concerns in an already water-logged area. Spaghetti plots of SREF QPF area show potential more than 2 inches in the complex terrain in the along the coast and into the northern Sierra...while the GEFS is not as wet. With antecedent conditions being so wet and some uncertainty in when the next upstream system approaches the area (tending to think Monday)...will have an area similar to the Day 2 outlook area but increase it to a Slight Risk. ....Southeast... The spread in model solutions on Day 2 persists into Day 3 as the system makes its way eastward...with the ensembles tending to show a broad north to south spread with NMM members of the SREF being farther north extending from the Tennessee Valley eastward into the Carolinas while the ARW members were largely confined to the southern end of the Appalachians into the Southeast U.S.. Only part of the area has experienced above normal rainfall lately...and a Marginal was placed to cover the overlap between the lower flash flood guidance and where the NMM/ARW members and the GEFS/ECMWF showed some common ground. Future adjustments are likely assuming the timing and placement get in better agreement. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7AgRQV0jNTFv_RPTzLaZb3tAz1T7epTKeAxQBs1IKpYM= IZfgf9Uo_efgGSeDX0TCgid4zdvo_Yy9BES4fpzS-kR9WaQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7AgRQV0jNTFv_RPTzLaZb3tAz1T7epTKeAxQBs1IKpYM= IZfgf9Uo_efgGSeDX0TCgid4zdvo_Yy9BES4fpzSy1SIvs8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7AgRQV0jNTFv_RPTzLaZb3tAz1T7epTKeAxQBs1IKpYM= IZfgf9Uo_efgGSeDX0TCgid4zdvo_Yy9BES4fpzS30hkGw0$=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) .