Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 08 2023 00:49:47 FOUS30 KWBC 080049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 748 PM EST Tue Mar 07 2023 Day 1 Valid 01Z Wed Mar 08 2023 - 12Z Wed Mar 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND EASTERN OKLAHOMA... The forecast remains on track for the overnight period as the threat of heavy rainfall increases across portions of the Southern Plains this evening. An upper-level pattern featuring a longwave trough over the West and a ridge axis spanning from the central Gulf Coast to the Ohio Valley combined with a sharp frontal boundary and increasing moisture will lead to the threat of flash flooding. Shortwave energy is rotating through the base of the western trough, amplifying the downstream mid-level flow as backing flow aloft over the Southern Plains and Lower MS Valley encourages the return of low-level southerly flow (with 850 mb winds of 20-25 kts currently analyzed). Moisture is becoming increasingly focused north along and ahead of a well-defined front and weak low pressure center draped across the Red River Valley of the South (and stretching east-southeast across the Lower MS Valley). Convection has begun to initiate over central OK along an inverted trough, which will support increased convergence and stationary/training cells as the low-level jet continues to pump moisture northward. Aside from increasing isentropic ascent, there is a modest axis of elevated instability with MUCAPE values of 500-1000 J/Kg advancing northward across south-central OK. Precipitable water values have also risen to 1.2" (near the daily max moving average for OUN, per SPC sounding climatology), and may increase further to approach 1.5". This is reflective of not only the low-level Gulf of Mexico moisture transport, but also the infusion of mid and high-level subtropical moisture from the eastern Pacific Ocean. The convection that that is quickly developing will likely be capable of producing rainfall rates of 1.0-1.5"/hr, and the guidance continues to support the potential for multiple rounds of convection that may repeat/train over the same area late into tonight. The 18z HREF suite suggests the potential for maximum rainfall amounts upwards of 2-3" during this time frame, with the greatest chances for these higher amounts along and just north of the Red River (per HREF 2" neighborhood exceedance probs of 30-60%, and 3" exceedance probs of 10-20%). Relative to the prior update, a south-southwestward trend in the guidance continues, which resulted in a small southwestward shift of the SLGT risk area. The intense rainfall rates may fall over some relatively sensitive soil conditions as a result of the most recent rainfall event (primarily across the southeastern portion of the SLGT risk area). The latest NASA SPoRT soil moisture anomalies in the 0-40 cm layer are running high across far southeastern OK (locally as high as the 70-80th percentile range), while the anomalies are running low across central OK (as low as the 10-20th percentile). While the rainfall will be largely beneficial for the long-term across drought-stricken portions of central OK, the dry soils may actually help to exacerbate any excessive runoff in the short-term. Churchill/Snell Day 2 Valid 12Z Wed Mar 08 2023 - 12Z Thu Mar 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF ARKANSAS, SOUTHEASTERN OKLAHOMA, AND PORTIONS OF NORTH TEXAS... ....2030Z Update... Some fairly significant changes were made with this afternoon's update in coordination with the TSA/Tulsa, OK and LZK/Little Rock, AR forecast offices. The model trends continue to decrease the forecast of storm total rainfall across the Slight Risk area. While there remains some overlap from the Day 1 Slight Risk area, most of the expected rainfall is largely offset to the southeast from the Day 1 Slight Risk area. Despite anomalous atmospheric moisture expected over the area, the CAMs and longer range models for this time frame remain in disagreement with each other as regards the potential for training convection and the tracks of the convective activity. Regardless, the continued southward and drier trend, with widespread rainfall totals closer to 1 inch and lowering chances for localized amounts much above that, the flooding potential has decreased enough that a Moderate Risk for flash flooding was no longer needed. Despite the highly saturated soils, the addition of only about an inch of rain, that may be separated into multiple rounds of fractions of an inch of rain should allow those soils to handle this little bit of extra rainfall. FFGs are over 2"/1 hr and close to 3"/3-hrs in the Slight Risk area, which is now considered unlikely to occur outside of an isolated instance from a stronger thunderstorm. Wegman ....Previous Discussion... The heavy rainfall threat from day 1 extends into day 2 and potentially overlaps with regions that contain sensitive antecedent soil conditions, which may exacerbate the possibility of numerous instances of flash flooding. Relatively organized convection is expected to be ongoing across AR and southeast OK early Wednesday morning as the shortwave energy responsible advances to the east across the lower/mid Mississippi Valley. This convection should weaken and become less widespread by midday as instability lacks downstream. This should yield a lull in activity until the low-level jet reloads by Wednesday night. Anomalous PWs (above the 90th climatological percentile) will be readily available as convection develops in response to sufficient 850 mb inflow and MUCAPE values upwards of 1000 J/kg. A lingering stationary front will also remain in the area and drape from the lower Mississippi Valley to northern TX, likely sinking southward some during the day. However, a stronger shortwave crossing the Four Corners region should eject into the High Plains by Thursday and aid in increasing ascent over the Southern Plains, which will support convective development to the north of the front and spanning from southeast OK to central AR. The Moderate Risk area incorporates a region where guidance highlights both the greatest chances for additional rainfall amounts in the 1-3" range and the potential for 1-1.5" per hour rainfall rates, while also overlapping with saturated ground conditions. This rainfall, combined with the previous day convection, could yield areal averages of 3"+ in the 48-hr period. Given the antecedent conditions (AHPS 14 day precip departures are 200-300 percent of normal and NASA SPoRT soil moisture percentiles in the 10-40 cm layer are above 80 percent in areas) and likely overlap of heavy rainfall areas, the spatial coverage of flash flooding impacts should be enough to support the maintained Moderate Risk. High-end impacts may be highly dependent upon the convection overlapping with the susceptible soils and CAM trends (as well as the outcome of Tuesday nights rainfall) will be closely monitored to determine if the outlook area remains viable. The broader Slight and Marginal Risk areas remain in decent shape and incorporate locations where more isolated-to-scattered flash floods are likely along the modest forcing stretching just to the north of the frontal boundary. Eastern regions remain most susceptible due to wet antecedent conditions, while the potential for higher rainfall rates may be closer tied to the better instability over northern TX and near the Red River Valley of the South. Snell Day 3 Valid 12Z Thu Mar 09 2023 - 12Z Fri Mar 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA... ....2030Z Update... Some fairly significant changes to the inherited forecast were coordinated in both of the ERO risk areas in CA and also AR/MS/AL. Please see below: ....Central and Northern California... Starting with the area where concerns/forecast impacts have increased...the atmospheric river expected to impact much of the West Coast remains on track to impact the area this afternoon. Guidance trends continue to increase the forecast precipitation in this area. Unlike several of the previous events into the West Coast, this atmospheric river will have a tropical connection that will advect prodigious amounts of both Pacific moisture and also sub-tropical warmth into California. Thus, snow levels are expected to rise well above the levels seen in previous events. =46rom about the latitude of San Francisco south, snow levels may reach as high as 10,000 feet starting Thursday night, and those snow levels will rise rapidly as the onset of warmer air will move in quickly. Further north into OR & WA, the tropical warmth will be brief in southern OR and the northernmost counties in CA, only lasting a few hours before a trailing cold front moves in and returns snow levels closer to the 3,000 ft "normal". Meanwhile from San Francisco south, those higher snow levels will persist for several days. Thus, a first obvious area of uncertainty is much of northern CA. How far south the aforementioned trailing cold front gets will heavily influence the flooding potential in this area. For the Day 3 ERO, a larger spread in the risk areas from the MDT to the south, SLGT in the middle, and MRGL to the north towards the OR/CA border was intentional to account for this uncertainty. For the mountainous areas of the Moderate Risk area, the biggest point of uncertainty isn't meteorological as much as hydrological. It's no secret that the Sierras and coastal ranges have gotten prodigious amounts of snow already this winter, and the unusually high snow levels (by this winter's standards) advecting into this area will result in rain falling onto snow and snowmelt at elevations that have an extremely deep snowpack. Thus, the uncertainty lies in how much rainfall will be absorbed by the snowpack before there is significant release of that water into the rivers. It's likely some of the QPF will simply be absorbed into the many feet of snow at the highest elevations, but lower elevations, generally below 5,000 ft appear most likely to not have the snowpack necessary to absorb the multiple inches of rainfall expected. Therefore, there are likely to be rapid increases in river levels once the heaviest rain gets going. The addition of mountain snowmelt to the already heavy rain was a big determining factor in the upgrade to a MDT risk, along with the coordination with the affected WFOs to issue Flood Watches. It's also worth mentioning that while not flooding related, the heavy rain falling into the deep snowpack will result in rapid increases in the weight of the snow on the roofs of buildings, increasing the threat of roof collapses. Affected communities are urged to remove the existing snow from their roofs to mitigate this. Lower elevation snow is likely to melt completely or in significant portion with this event, which will increase the flooding threat even if the higher elevation snowpack absorbs much of the rain. ....Portions of the Mid-Mississippi Valley... In contrast to California, and following the theme from Day 2, the Moderate Risk inherited was removed with this afternoon's update, in coordination with the TSA/Tulsa, OK, LZK/Little Rock, AR, JAN/Jackson, MS, HUN/Huntsville, AL, and BMX/Birmingham, AL forecast offices. In fact, portions of eastern OK and western AR underwent a rare double downgrade from a Moderate Risk directly to a Marginal. This was largely decided based on guidance trends moving southeast in favor of the current Slight Risk area into southeastern AR, northern MS, and northern AL. Even despite this shift in the axis of heaviest rainfall, the total forecast rainfall has also been decreasing, in some areas being cut by 50 percent. The lack of overlap with where the flooding rains are expected on previous days and the decreasing rainfall amounts in general even where the heavier rain is expected were the primary drivers for the lowering of risk levels across this area. Otherwise, many of the meteorological factors described in the previous discussion remain valid, but where the storms form and how much instability those storms will convert to heavy rain has decreased. Wegman ....Previous Discussion... ....Southern Plains, Lower Mississippi Valley, and Mid-South... The prolonged heavy rainfall threat impacting the Lower Mississippi Valley and nearby surrounding regions will conclude on Thursday as a potent shortwave trough swings eastward towards the Ohio Valley. Initially, ongoing convection in response to a sufficient 40 kt low-level jet near the Red River of the South intersecting a draped stationary front is expected to be situated across parts of eastern OK to central AR. This may also potentially occur over areas forecast to receive widespread total rainfall amounts of 1-3" on Tuesday and Wednesday. This morning convection is likely to slide eastward to the north of a warm front stretching from FL to the central Gulf Coast in response to the eastward progressing shortwave. PWs greater than 1.25", MUCAPE up to 1000 J/kg, and a modest 30-40 kt 850 southwest inflow should help this convection maintain strength through the afternoon into northern MS and AL. Another round of showers and storms are set to form upstream along and ahead of an advancing cold front, with PWs still greater than +2 standard deviations above climatology, supporting the potential for intense rainfall rates. This last round of rain will also occur throughout regions of the Lower Mississippi Valley, where several inches of rain is already expected by Thursday. The 24-hour rainfall forecast during this time frame depicts the potential for 1-3" rainfall amounts, which is relatively similar in magnitude to the prior two days. However, the Moderate Risk was maintained for a region that remains highly susceptible for flash flooding, as seen by NASA SPoRT 0-40 cm soil moisture percentiles of 80-95 percent. For these locations, areal coverage of flash flooding concerns could be widespread, but the 95th percentile NBM depicts the current ceiling for rainfall amounts remains rather underwhelming and generally under 3". So while extreme amounts of rainfall are unlikely, flooding concerns remain due to modest convection overlapping with highly sensitive soils. Guidance does show the potential for a weaker shortwave and thus a more suppressed look in the precipitation pattern, adding more uncertainty to the forecast. If this were to occur and the heavy rainfall threat shifts even more south (out of the MDT risk), the excessive rainfall threat may greatly diminish as it is highly dependent on overlap with sensitive terrain. The broader Slight Risk was shifted during this update to match with the southern trend and the potential for intense rainfall along the warm front into northern MS and AL. ....West Coast... A strong atmospheric river is set to surge into the West Coast by the second half of the Day 3 period (Thursday night) and steadily increase in intensity through 12z Friday. This surge of moisture has trended faster with recent guidance, thus the main reason for the upgrade to a Slight Risk for much of central and into northern California. The upper-level setup for this event is quite impressive as an upper-level low north of Hawaii and associated shortwaves rounding the parent low act to "scoop" subtropical and tropical moisture from this region northeastward towards the West Coast, while a separate upper-level low dropping south from off the coast of British Columbia to the Northwest coastline reinforces the moisture surge and focuses the push into California. IVT values of 800-1000 kg/m*s per the ECMWF and GFS are forecast to impact a broad region and not remain confined to a singular intense axis. The resulting PW values are forecast to reach above 1.00" (a very impressive +4 standard deviations above climatology). This creates the potential for widespread 1-3" QPF amounts through 12z Friday across much of central/northern California and into coastal Oregon. Upslope enhancement along the California coastal ranges and foothills of the Sierra will support higher rainfall totals up to 4-5" and a greater threat for flash flooding. Snow levels will be much higher than recent storms given the moisture source in play, quickly rising to about 8000 feet. These high snow levels are also likely to lead to rain east of the Sierra crest and near the Lake Tahoe region. This means all coastal ranges and much of the Sierra foothills with receive heavy rain on top of snow, increasing the threat of rapid runoff due to snowmelt. Along with the snowmelt threat, soils remain overly saturated per the NASA SPoRT 0-40 cm moisture percentiles. USGS streamflow gauges also show numerous rivers running above to well above normal throughout central California. The rather broad nature of incoming moisture surge may actually limit the magnitude of flooding impacts as showers with embedded heavy rates appear most likely, versus extreme rainfall rates along a confined axis. However, the upslope and snowmelt component (2"+ of SWE per NOHRSC) is widespread and of enough concern to highlight much of California in a Slight Risk of Excessive Rainfall, with the threat also lingering after this time period. Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-VVcFSTWdVuxmAKfLrkHjwG1H4chOVrNhYWm3JEr3aE3= PkBWA65AgcyQz6vKCGNfjxRQsVlIaIyfXPNvbdAkhlDdHYA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-VVcFSTWdVuxmAKfLrkHjwG1H4chOVrNhYWm3JEr3aE3= PkBWA65AgcyQz6vKCGNfjxRQsVlIaIyfXPNvbdAktmYFIFU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-VVcFSTWdVuxmAKfLrkHjwG1H4chOVrNhYWm3JEr3aE3= PkBWA65AgcyQz6vKCGNfjxRQsVlIaIyfXPNvbdAkx4FQGEY$=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) .