Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 23 2023 01:00:17 FOUS30 KWBC 230100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 759 PM EST Fri Dec 22 2023 Day 1 Valid 01Z Sat Dec 23 2023 - 12Z Sat Dec 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF SOUTHERN ARIZONA AND PORTIONS OF FAR SOUTHEASTERN CALIFORNIA... ....01z Update... The forecast remains largely on track since the last update, as higher rainfall rates (localized 1-2"/hr) have been realized over the past several hours in the vicinity of the southern AZ/CA border. An impressive comma head structure is now evident on radar mosaic imagery in association with the deep closed low, but convection along the eastward leading edge has diminished in organization and intensity as of late. Even still, some localized 1-2" totals (per MRMS estimates) have already contributed to possible flash flooding in southwest AZ. The risk of additional flash flooding will spread into south-central and much of southeastern AZ, as instability remains (or increases to) 250-500 J/kg. Precipitable water values are also near 1.0" (about the max moving average, per sounding climatology) with deep layer shear of 40-60 kts. Some regeneration of convection is also possible along the backside of the low (per some hi-res model solutions), which may allow for additional localized totals of up to an 1" (and some new or reaggravated flooding) across far southeastern CA and southwest AZ. As a result, the main change for this cycle was to trim southwestern portions of southern CA from the Marginal and Slight Risk areas. The bulk of the more intense rainfall should conclude by around 06z, with some lingering impacts through 10-12z. Churchill ....16Z Update... No major changes to the outlined risk areas from previous forecast issuance as the current synoptic evolution of our closed low remains on track. Newest probabilistic guidance from the 12Z HREF signals a pretty decent convective pattern later this afternoon across the Colorado River Basin starting in Imperial and eastern Riverside counties in CA, moving east into Yuma/La Paz in AZ with greater than 90% potential for neighborhood reflectivity of at least 40 dbz with 50 dbz coming in at 60-70% over the aforementioned area. This correlates well with the 2-year ARI exceedance probability of >90% over much of Yuma/La Paz counties extending northeast into the I-10 corridor in AZ, and the 10-year ARI running up to 60% as of the latest guidance. This is likely due to the increased ascent pattern of the low traversing overhead within low-level moisture advection on the lead side of the low. PWAT indices on the latest mesoanalysis indicate a tongue of 1+" values extending from western Sonora up over to the CA Baja and points north. This places the Colorado River Basin as the focal for deeper moisture presence, thus better chance for higher rain rates once convection ensues. Steep lapse rates off the KSGX sounding this morning peg a picture of the expected instability pattern that will transpire for points east. Neighborhood 1"/hr rain rates have bumped to near 70% probability across the above area, so expectation is for locally heavy rainfall with totals between 1-2" and a max of up to 2.5" as a possibility today which signals flood concerns rising. Further east, the steady plume of low-mid level moisture will continue to advect northeast into south-central AZ with moderate to locally heavy rainfall anticipated for several hrs before the pattern shifts to the east as the low migrates towards the Four Corners area. Tongue of instability can be found within the corridor of higher IVT's lifting northward within the axis of difluence on the lead side of the broad circulation. This will maintain the possibility of localized flooding due to the onslaught of rain with weak convection embedded in the precip field. Best chance for flooding within that zone will be within the urbanized areas and low-water crossings over the low desert within Pima county up to the north of the Tuscon metro. The SLGT risk will suffice given the potential.=20 Kleebauer ....Previous Discussion... The deep, closed low centered just offshore southern California/northern Baja early Friday morning will begin to pick up speed as it moves eastward toward central Arizona by early Saturday morning, opening up as it does so. This will put much of southern California, far southern Nevada, and Arizona in a favored region of deep-layer forcing/lift while anomalous S-SSW low-level flow positions a plume of higher moisture initially into southern California then across Arizona during the afternoon/evening hours. Moisture anomalies with this system are expected to remain well above normal, 3 to 4 standard deviations above normal for this time of year for both PW and 850-700 mb moisture flux. PWs in fact are forecast to reach or exceed 1" at times, especially across southwest to south-central Arizona. The GFS and ECMWF IVT forecasts from CW3E show an impressive surge approaching 500+ kg/m/s into central Arizona. During the afternoon, some limited instability may develop across the Desert regions of CA and eastward into southern Arizona, which may allow for some localized higher rain rates approaching 0.50-1.0"/hr at times per the 00Z CAM guidance. Overall, a broad area of 0.75-1.5" totals are expected for the 24-hr period ending 12Z Saturday, with some localized/embedded higher amounts up to 2.5" possible in any thunderstorms. The latest (00Z 12/22) HREF neighborhood probabilities of 24hr QPF exceeding the 2 year ARI are aoa 50 percent for the bulk of the Slight area, with probs of exceeding the 10 year ARI up to 40-60% along the southern CA-AZ border. Hurley Day 2 Valid 12Z Sat Dec 23 2023 - 12Z Sun Dec 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTH TEXAS AND SOUTHERN OKLAHOMA... Longwave pattern will shift eastward with sights on the plains by D2 as large scale ascent increases exponentially by late tomorrow afternoon into early Sunday morning. Increasing signal of a conglomeration of thunderstorms in two areas are forecast; the TX Gulf coast, as well as through portions of west TX into the I-35 corridor. Rainfall totals on the order of 1-2" are becoming much more likely within the above corridor, including some localized 2.5-3" totals showing up on multiple sets of deterministic. A lot of this stems from a favorable ascent pattern maximized through a jet coupling between the sub-tropical jet extending from the Pacific into adjacent Mexico, and a budding jet on the lee side of a the upper trough traversing the Rockies. This pegs much of the southern plains within a core of deep layer ascent over a steady theta-E advection regime caused by meridional flow ahead of the mean trough. Difluent signature is textbook for precipitation to break out over the southern plains and trek eastward as the steering pattern dictates. Deep layer shear and backed flow across central and east TX will maintain a streamline of Gulf moisture to interact with the approaching trough causing a clustering of storms to migrate from the western rolling plains into central and north TX by the second half of the period, heading straight into areas that have some appreciable rain within the last 24 hrs. NASA SPoRT soil moisture indicator pinpoints a swath of moist soils within the north-central area of TX through the DFW metroplex. Given the forecast rainfall and convective regime anticipated over the area, this aligns with favorable heavy rainfall potential and the possibility for flooding. The DFW area will be the primary target in this case due to the impervious surfaces within the metroplex, so wanted to make sure they were the center of focus. The secondary area of note is across the Gulf coast of TX where the Houston metro would be the next favorable candidate for possible upgrade. The ECMWF and ECENS are insistent on a heavy rain threat near the coast, which would make sense from an instability standpoint as the best theta-E values and highest moisture concentration are within the coastal plain. The problem is the lack of agreement on the potential from other guidance as the camps in places are; storms pertrude inland and lead to enhanced rainfall along the immediate coast, or storms remain over the Gulf and fail to make advancement inland and the setup dwindles to just some showers with scattered storms. Rainfall totals are nothing to balk at (1-2"), but with the FFG guidance above that for 1/3/6 hr rain rates and the total only 1-2" for the event, leaned away from an upgrade, but wanted to make mention as if the trends are more favorable, would not be surprised to see a SLGT extension to include the immediate TX coast around the Houston metro over into perhaps Beaumont. Kleebauer Day 3 Valid 12Z Sun Dec 24 2023 - 12Z Mon Dec 25 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE OZARKS AND ARKANSAS RIVER VALLEY SOUTH THROUGH THE ARKLATEX AND CENTRAL GULF COAST REGION... ....20Z Update... Only change to the previous forecast was an expansion of the SLGT further north and west to include more of the I-44 corridor as recent trends in guidance pinpoint a better potential for heavy rainfall for areas near and east of Tulsa. In coordination with WFO Tulsa, was sure to extend the risk area to encompass the areas in question. The longwave pattern is developing more continuity within the deterministic with the ensemble means shedding light on the two areas of interest over the Mid-Mississippi Valley and the central Gulf coast where clusters of storms will propagate inland over southern and southeast LA into the coastal areas of MS/AL. Rates are not anticipated to be higher than 2-3"/hr and with FFG guidance remaining very high for the coastal areas, will likely not see an upgrade to a higher category unless guidance shifts significantly in the overall magnitude. Kleebauer ....Previous Discussion... Aforementioned mid/upper trough is expected to close off at 500 mb by 12Z Christmas Day across the Plains. Strong southern stream enhancement (130-150 kt upper jet streak moving across South TX) will boost the deep-layer ascent and south-north ageos low-level moisture/theta-e transport into eastern OK-TX as well as much of AR-LA and the MS-AL Gulf Coast regions. PWs >1.5" begin to creep into the Upper TX Coast region early in the period, then across southern LA and coastal MS later Sun-Sun night. The anomalous low-level 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, robust southern stream which is indicative of the current strong warm-phase ENSO pattern. As such, for the most part we have maintained both the Slight and Marginal Risk areas with minimal changes from yesterday's Day 4 ERO. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7J-pwCDMzqcQvlgtcRAzheWgkQ0QVIJeBjQug-ZseF6T= 9ZpM8Q-KyowXPZqT1Mh_Fme2aIMhgcIL8ERyA6Zb-pM6v_k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7J-pwCDMzqcQvlgtcRAzheWgkQ0QVIJeBjQug-ZseF6T= 9ZpM8Q-KyowXPZqT1Mh_Fme2aIMhgcIL8ERyA6ZbUI7Zf_I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7J-pwCDMzqcQvlgtcRAzheWgkQ0QVIJeBjQug-ZseF6T= 9ZpM8Q-KyowXPZqT1Mh_Fme2aIMhgcIL8ERyA6Zb0ViBQLA$=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) .