Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 25 2023 00:58:37 FOUS30 KWBC 250058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 757 PM EST Sun Dec 24 2023 Day 1 Valid 01Z Mon Dec 25 2023 - 12Z Mon Dec 25 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS INTO THE MIDDLE AND LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST... ....01z Update... Both Slight risk areas were removed for this update, given the combination of low instability (MU CAPE indicated to be less than 500 J/kg across the entirety of the risk area) and dry antecedent conditions (NASA SPoRT 0-1m soil moisture averaging near the 10th percentile, and locally can be much lower). The hi-res guidance suggests that rates will only occasionally (and briefly) reach 1"/hr across the Central/Southern Plains and Middle MS Valley, perhaps reaching as high as 2"/hr at times farther south across the Lower MS Valley and Central Gulf Coast. These rather marginal rates should be well tolerated by the dry soils, while short-term localized totals of 1-2" and 2-3" (for north and south respectively) are generally near or below corresponding (3-hr) flash flood guidance (FFG). While the rainfall should largely remain beneficial for the drought-stricken region, the broader Marginal risk was left in place (and confined to where additional rainfall was ongoing or likely to occur) due to the potential for a localized instance or two of low-end flash flooding. Churchill ....16Z Update... Very few changes were needed with today's midday update. The weather pattern remains on track, with potential for training convection into the Gulf Coast and slow moving storms, potentially with an embedded line expected around the KS/MO border in the northern Slight. Wegman ....Previous Discussion... Mid-upper trough is expected to close off at 500 mb by 12Z Christmas Day across the Central Plains. Strong southern stream enhancement (130-150 kt upper jet streak moving across South TX) will boost the deep-layer ascent and south-north ageostrophic low-level moisture/theta-e transport into eastern portions of the Lower MO Valley-Central and Southern Plans as well as the Lower MS Valley and central Gulf Coast region. PWs >1.5" will expand along the Upper TX Coast and across the central-eastern Gulf Coast areas Sun and Sun night, while PWs climbing to 1-1.25" across the Lower MO Valley into the western Ozarks average some 3 to 4 standard deviations above normal across these areas for late December. Two separate Slight Risk areas were maintained from yesterday afternoons Day 2 ERO, with minor adjustments based on the 00Z guidance trends. The first one is across the Gulf Coast from eastern LA and far southern MS-AL to the western FL Panhandle. Over these areas, robust low-level moisture/theta-e transport, aligned low-mid level flow with an anomalous LLJ (favoring increased upwind propagation/backbuilding and thus risk for cell training), and at least modest deep-layer instability (at least 500-1000 J/Kg), will lead to a favorable thermodynamic and kinematic profile for heavy rainfall, especially given the active, vigorous southern stream. 850 mb wind and moisture flux anomalies over these areas peak around 3 standard deviations above normal per both the SREF and GEFS, indicative of a favorably rich thermodynamic profile. Furthermore as noted above, the eastward progression of any pre-frontal quasi-linear convective segments may retard a bit overnight given the vertical shear profiles, i.e. becoming more conducive to training. The second Slight Risk area across eastern KS-western MO into northeast OK and the western Ozarks is also supported by ensemble consensus for robust moisture transport, with 850 mb wind and moisture flux anomalies quite high for late December (between 4 and 5 standard deviations above normal). Deep-layer, elevated instability over this area will be more limited (i.e. MUCAPEs generally less than 500 J/Kg), however at the same time current FFG values are lower over these areas despite the relatively low soil moisture percentiles per the NASA SPoRT analysis. FFG values of 1.5 to 2" in 3 hours is noted across much of the Slight Risk area, while the 00Z CAMs do show isolated stripes of 3+ inches, the bulk of which falling in less than 6 hours. Hurley Day 2 Valid 12Z Mon Dec 25 2023 - 12Z Tue Dec 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN APPALACHIANS... ....2030Z Update.... Some fairly significant changes were made to the Day 2 forecast. A Slight Risk was introduced in coordination with GSP/Greer, SC forecast office for portions of the southern Blue Ridge and Great Smokies. Increasing forecast rainfall, with some MUCAPE values to 500 J/kg should allow for upslope to enhance any convection that will remain heaviest on the southeast facing slopes of the mountains. 24 hour totals may approach 5 inches on the tallest peaks, with a widespread 3 inches throughout the Slight Risk area expected on Christmas/Monday into Monday night. A Marginal Risk area was introduced in coordination with SEW/Seattle, WA forecast office for the Olympic Mountains. Rainfall totals of 3-5 inches are expected, with snow levels remaining high enough to only impact the highest peaks of the eastern Olympics. While the event is only expected to be 24 hours long, the forecast rainfall amounts are high enough (and have been increasing) such that flash flooding and riverine flooding of the rivers that drain the Olympics are possible. The Marginal Risk for coastal FL, GA, and SC was downgraded with this update. Decreasing rainfall totals along with meager instability and a progressive movement to the rainfall should all work in tandem to produce rainfall totals that largely remain under 1.5 inches. Certainly some convective elements remain likely, but they should move over any given area fast enough to be a minimal flash flooding threat. Wegman ....Previous Discussion... Closed mid-upper low will move very little during day 3 (12Z Christmas-12Z Tue), as upper level difluence increases over the southeastern U.S. The aforementioned subtropical jet will remain quite robust, with 250mb winds between 150-170kts moving into the western and central Gulf Coast regions Monday night. A rather quick uptick in deep-layer ascent and low-level theta-e/moisture transport from the GOMEX is expected, although per the latest GEFS and NAEFS, moisture flux and IVT anomalies start off near +2 early in the period across the FL Panhandle, then decrease thereafter. Also limiting the potential for a Slight or better risk over much of the Southeast region will be the faster progression of the leading cold front, thus increasingly transient quasi-linear convective segments over most areas. One area that does bear monitoring for a possible upgrade to a Slight Risk is across the eastern slope of the Southern Appalachians (northern GA to Upstate SC and western NC). This as the strengthening SE-SSE low-level flow provides some upslope enhancement, as most of the models show a heavier rainfall footprint across this region with areal-average totals between 1.5 and 3.0+ inches. However, any elevated instability will be relatively weak/shallow, with MUCAPEs generally under 500 J/Kg. This will limit the 1 and 3 hourly rainfall rates somewhat, which along with the current low 0-100 cm soil moisture percentiles (less than 20% per NASA SPoRT), will likely lead to a more marginal excessive rainfall threat with the expectation of at most localized short-term runoff issues. Hurley Day 3 Valid 12Z Tue Dec 26 2023 - 12Z Wed Dec 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE BLUE RIDGE MOUNTAINS OF VIRGINIA THROUGH SOUTH CAROLINA... ....2030Z Update... In coordination with RAH/Raleigh, NC and RNK/Blacksburg, VA forecast offices, the Marginal Risk was downgraded for the coastal Carolinas. Once again instability will be meager, along with minimal instability. Expect the 1-2 inches of rain will fall lightly over much of the day, which should be handled well by the area rivers and streams, which are at or below normal levels. The Marginal Risk was maintained for the Blue Ridge as easterly flow off the Atlantic combined with upslope flow should allow for a local maximum of rainfall along the mountains, where due to topography is a bit more prone to localized flash flooding. The Marginal Risk was extended southwestward along the Blue Ridge to correlate better with the Day 2 Slight risk, as the rain, while ending, should continue past 12Z Tue and into the Day 3/Tue period. Any ongoing flash flooding from Day 2 in the Blue Ridge of the Carolinas may continue into Tuesday morning. Wegman ....Previous Discussion... Deep, cutoff low over the central U.S. will slowly pivot across the Lower MO Valley by the end of day 3 (12Z Wed). Meanwhile, along the eastern seaboard, another surface low will develop along the tightening low-level baroclinic boundary near the Gulf Stream off the Carolina coast. Broadening upper level difluence across the Mid Atlantic, along with increased left-exit region forcing within the right exit region of the strong subtropical jet streak, will favor a fair quick uptick in deep-layer moist isentropic ascent. SE-SSE 925-850 mb flow anomalies are not overly impressive (at best ~1 standard deviation above the norm), and as such, neither are the low-mid layer IVT and moisture flux/transport anomalies. However, much of the guidance does show multiple footprints of 1.5 to 3 inches of rain on Day 3 within the outlook area, along with some, albeit meager, elevated instability per the 00Z ECMWF (MUAPEs 250-500 J/Kg). Given the recent heavy rain events the past 2 Sundays, which is still reflected in the modestly-high relative soil moisture analysis per the latest NASA SPoRT (50-65 percent in the top 0-100cm layer), along with the areas of lower FFGs, cannot rule out localized areas of excessive short-term runoff. As such, will maintain the Marginal Risk area inherited from yesterday's Day 4 ERO, though based on the slower trends in the guidance, have trimmed the outlook area to leave out MD along with eastern and far northern portions of VA. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8D6-YaETGkewHtDS4dPPW3bqK_JBsVEn7DXh5J-IsO_u= Kc0tdHfkMNo_rMmnYMqgvPdvli6jmtDtHxTfvIVykN4TXGA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8D6-YaETGkewHtDS4dPPW3bqK_JBsVEn7DXh5J-IsO_u= Kc0tdHfkMNo_rMmnYMqgvPdvli6jmtDtHxTfvIVyUpUoqg4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8D6-YaETGkewHtDS4dPPW3bqK_JBsVEn7DXh5J-IsO_u= Kc0tdHfkMNo_rMmnYMqgvPdvli6jmtDtHxTfvIVygkxT1Qo$=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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