Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 08 2023 15:48:59 FOUS30 KWBC 081548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1047 AM EST Wed Mar 08 2023 Day 1 Valid 16Z Wed Mar 08 2023 - 12Z Thu Mar 09 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF ARKANSAS, SOUTHEASTERN OKLAHOMA, AND PORTIONS OF NORTH TEXAS... ....16Z Update... For today's morning update, the first round of convection that moved through southeastern OK and across southern AR is rapidly advecting east and is moving across the MS River. All of the CAMs guidance suggest that there will be a long break for the rest of the day across southern AR. Meanwhile, isolated to widely scattered convection across north central TX is expected to eventually organize a bit better late this afternoon through tonight, but track more northeasterly across southeastern OK and possibly into northwestern AR. A stronger storm last evening dropped a narrow area of 2 inch rainfall near Broken Arrow, OK, but otherwise most areas got around 1" of rain, which was well handled by the local river system with no flash flooding impacts seen. Going forward into tonight, the guidance again shows scattered convection that may have brief cellular towers that locally produce heavier rain, but the vast majority of folks in the Marginal Risk area will only see light rain. Thus, the flash flooding threat continues to decrease, such that the Slight Risk is no longer needed. The Marginal Risk still implies an isolated flash flooding event or 2 will be possible, which remains the case if any of the convection grows stronger than expected or multiple cells train over a narrow corridor.=20 Wegman ....Previous Discussion... The heavy rainfall threat on Day 1 extends from ongoing convective activity over southeastern OK and AR early this morning that is expected to push southeast before redeveloping showers and storms near the Red River Valley potentially overlap these areas this evening. The excessive rainfall threat to start the period is in response to shortwave energy advancing to the east across the lower/mid Mississippi Valley. Convection should weaken and become less widespread by midday as instability lacks downstream, but may reinforce the stationary boundary draped across the region and aid in forcing later today from northeast TX to southern AR. Overall, a lull in activity is then anticipated until the low-level jet reloads by tonight. 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. The lingering stationary front will also remain in the area and extend from the lower Mississippi Valley to northern TX. Meanwhile, 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. At the same time, a pivot point in the frontal boundary over parts of southern MS could also be a focus for slow-moving localized convection and intense rainfall rates over 1"/hr. The Slight Risk area incorporates a region where guidance highlights both the greatest chances for additional rainfall amounts in the 1-2" range (locally up to 3") and the potential for 1-1.5" per hour rainfall rates, while also overlapping with saturated ground conditions. 00z HREF neighborhood probabilities of 6 hr QPF exceeding 2" are highest (greater than 30 percent) in the ArkLaTex region through midday. This rainfall, combined with the previous rain, could yield scattered areas of flash flooding. 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) over parts of AR and southeast OK, terrain may be more sensitive for ponding of water in low-lying areas and urban flooding. A recent trend in guidance has been to slide the potential for 1-2"/hr rates to the south along with a gradual southward trend in the stationary front. This has led to the heavier rainfall axis forecast to occur the south and not overlapping with even wetter antecedent conditions over central AR, thus a subtle southward shift to the Slight Risk and Marginal Risk. Snell Day 2 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... ....West Coast... A strong atmospheric river is set to surge into the West Coast by the second half of the Day 2 period (Thursday night) and steadily increase in intensity through 12z Friday. The updated ERO only incorporates minor changes from the previous forecast. This surge of moisture has continued to trend somewhat faster, with rain now expected to enter northern California before 00z Fri. 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 this 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, but last for 12 hours during this forecast period. The resulting PW values are forecast to reach above 1.00" into the central valleys (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 SW 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 tropical 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, which prompted the expansion of the Slight Risk into far western Nevada. This means all coastal ranges and much of the Sierra foothills will receive heavy rain on top of snow, increasing the threat of rapid runoff due to snowmelt. The most significant snowmelt is expected to occur below 5000 ft, where snowpack is shallower and temperatures will be warmer. The Moderate Risk was drawn tightly to this elevation as a result. 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 events with extreme rainfall rates along a confined axis. However, the upslope and snowmelt component (2"+ of SWE per NOHRSC), as well as the depth of the IVT values is widespread enough to support the Moderate Risk. ....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 are possible by Thursday. The Slight Risk area over southeast OK and the ArkLaTex was maintained/stretched westward to account for morning convection occurring over areas that should be more sensitive due to prior rainfall. The heaviest 24-hour totals during this time frame are actually forecast outside of the Slight Risk and into northern MS and AL, but these areas have been relatively dry and early green-up should limit the flash flooding potential to more of a localized threat. Given southward trends in the guidance with better overlap of moisture and instabiuility, the Marginal Risk was also shifted to the south and west. Snell Day 3 Valid 12Z Fri Mar 10 2023 - 12Z Sat Mar 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL CALIFORNIA... The strong atmospheric river impacting California is anticipated to continue into Friday, while gradually shifting the focus southward. The greater IVT values(800-1000 kg/m*s) are forecast to occur between 12z and 18z per the ECMWF and GFS. This better moisture surge is also expected to be over central California and into parts of the Transverse Ranges of southern California. Similar to Thursday night, snow levels will remain very high relative to recent events this winter and into the 8000-10000 feet range. Snow levels will fall back to below 7000 feet by Friday night behind the cold front, but this will be as the heavy precipitation threat is waning. 24-hour QPF amounts of 2-4" are forecast along the central California Coast and Sierra foothills, with locally higher totals possible. 1-3" of rain is also likely into the San Rafael mountains of southern California. In total, the entire event ending Friday is forecast to produce upwards of 6-8" of rainfall across the central California coast and Sierra Nevada (although the highest terrain above 8000 ft will see snow). A Moderate Risk areas encompass parts of the central California coast/ranges and the Sierra foothills, where orographic enhancement should support a continuation and several additional hours of 0.5-0.75" per hour rainfall rates. This, combined with any additional snowmelt and severely saturated ground conditions, should support the potential for widespread runoff concerns and the potential for significant flooding impacts. Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5LpPlGLas5xYrmIyOQlclr3tc_znS6yskUS-W67nUUo9= mIZz98ZeiJhjc53uLC3mJtnM4Ztqn8dXP3DPiNtDuyar1Ww$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5LpPlGLas5xYrmIyOQlclr3tc_znS6yskUS-W67nUUo9= mIZz98ZeiJhjc53uLC3mJtnM4Ztqn8dXP3DPiNtDNOgao7Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5LpPlGLas5xYrmIyOQlclr3tc_znS6yskUS-W67nUUo9= mIZz98ZeiJhjc53uLC3mJtnM4Ztqn8dXP3DPiNtDlr0IVUc$=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) .