Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 07 2023 08:29:48 FOUS30 KWBC 070829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 AM EST Tue Mar 07 2023 Day 1 Valid 12Z Tue Mar 07 2023 - 12Z Wed Mar 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL/EASTERN OKLAHOMA AND NORTHWEST ARKANSAS... The forecast remains on track for the new Day 1 period as the threat of heavy rain increases across portions of the Southern Plains after 00z tonight. 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 rotating through the base of the western trough will amplify the downstream mid-level flow as backing flow aloft over the southern Plains and lower MS Valley will encourage the return of low-level southerly flow. Moisture will become increasingly focused north along a well-defined front draped across the Red River Valley of the South and stretching east-southeast across the lower MS Valley. Initially, the forcing along the front will be rather modest, but by the evening and overnight hours, the arrival of low-amplitude shortwave energy in the deep layer southwest flow aloft along with some uptick in the low-level jet should encourage the development of showers and thunderstorms along and north of the front for areas of central/eastern OK and north-central TX, before gradually shifting into adjacent areas of northwest AR and southwest MO in vicinity of the Ozark Plateau. The initial convection extending to the north of the front is also likely to be co-located along an inverted trough, which will support convergence and stationary/training cells as the low-level jet continues to pump moisture northward. Aside from increasing isentropic ascent, there will also be the nose of at least a modest axis of elevated instability with MUCAPE values of 500+ J/Kg advancing up across south-central OK that will facilitate convective development, with 1000+ J/kg not that far away in northern TX. The PW anomalies increase with time and reach upwards of 3 standard deviations above normal in the 00Z-12Z/Wednesday time frame, with actual PW values close to 1.50" and near the daily max moving average for FWD and OKC according to SPC's sounding climatology page. This will be 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 which is showing up in a well-defined manner in the global models. The convection that does develop will likely be capable of producing rainfall rates as high as 1 to 1.5 inches/hour given the relatively efficient moisture transport regime and available instability, and the guidance supports the potential for multiple rounds of convection that may repeat/train over the same area. CAMs suggest the potential for maximum rainfall amounts upwards of 3-4" during this timeframe, with the greatest chances for these higher amounts across parts of central and eastern OK. 00z HREF has 30-40% neighborhood probabilities of exceeding 3" here. The only minor trend with recent guidance is to initiate convection farther to the southwest over parts of northern TX and limit the heavy rainfall potential over southwest MO. Additionally, these intense rainfall rates may fall over some relatively sensitive soil conditions as a result of the most recent rainfall event. The latest NASA SPoRT soil moisture anomalies in the 0-40 cm layer are running high (locally in the 80 to 95 percentile range). USGS data also shows streamflows running high along the OK/AR border and over into the Ozark Plateau. Thus, given the sensitive antecedent conditions, and heavier rainfall potential for the latter part of this period, a Slight Risk has been maintained with only minor adjustments to the Marginal Risk area. Scattered areas of flash flooding can be expected within the highlighted area. Snell Day 2 Valid 12Z Wed Mar 08 2023 - 12Z Thu Mar 09 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN OKLAHOMA THROUGH CENTRAL ARKANSAS... 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 FROM EASTERN OKLAHOMA THROUGH CENTRAL ARKANSAS... ....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 will have probably had several inches of rain 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, 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. Upslope enhancement along the 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 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!-yxD8nuobY8ctgDk2WlLps-Ds5oqO_2tETBx1rMltxaG= o60vxHMKjRf5krT68tLShmU1Ldw0WP2N9C-T07ePfcfCoB4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-yxD8nuobY8ctgDk2WlLps-Ds5oqO_2tETBx1rMltxaG= o60vxHMKjRf5krT68tLShmU1Ldw0WP2N9C-T07ePeHzxKq8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-yxD8nuobY8ctgDk2WlLps-Ds5oqO_2tETBx1rMltxaG= o60vxHMKjRf5krT68tLShmU1Ldw0WP2N9C-T07ePFTBztFY$=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. 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