Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 20 2023 16:23:35 FOUS30 KWBC 201623 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1222 PM EDT Thu Jul 20 2023 Day 1 Valid 16Z Thu Jul 20 2023 - 12Z Fri Jul 21 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN COLORADO INTO WESTERN KANSAS... ....Lee of the Central Rockies into the Central to Southern High Plains... 1630Z Update: The incoming 12Z high-res suite of guidance continued the trend of showing a congealing MCS developing across the High Plains of CO/WY this evening and then diving southeast into the Central Plains. This will be driven by an impressive shortwave digging through the persistent NW flow, with ascent aided by the RRQ of an upper jet streak. There is some uncertainty into how quickly this will evolve, but current radar trends are already a bit ahead of guidance, so there may be some priming that occurs before more significant initiation develops later. With bulk shear 35-50 kts, organization into an MCS appears likely, and while this may feature progressive motions, rainfall rates from the HREF are progged to exceed 2"/hr at times. Some training along the flanks is possible, which could result in locally 3-5" of rain, with the highest HREF probabilities now focused across western KS. The inherited MDT risk was expanded slightly SE to match where the highest rainfall rates may exceed 1-hr FFG, but despite some lower signal now appearing in the CO High Plains maintained the MDT risk there for more sensitive soils and still moderate SREF/GEFS probabilities for 3" of rain. Previous excellent discussion from overnight still highlights the details of today's flash flood threat: A favorable setup for heavy rainfall is expected today/tonight across portions of eastern CO into western KS. In the mid levels, shortwave energy will rotate east southeastward on the northeast side of the Southwest upper high, pushing east southeast from the lee of the Central Rockies into the Central to Southern Plains. A jet streak in the upper levels will be favorably positioned for enhanced upper level divergence. In the lower levels a surface front will drop southward, with post frontal upslope flow and strong 850mb convergence helping initiate convection. This activity will likely enhance into organized convection that will push southeastward through the evening and into the overnight hours. Deep layer moisture will increase across the region as well, with mid/upper level moisture advecting in from the Great Basin, resulting in PWs over the climatological 90th percentile. Not looking at extreme instability, but HREF mean MUCAPE does get into the 1000-2000 j/kg range...plenty to support heavy rainfall rates given the forcing and moisture in place. The main question from a flash flood perspective will be the duration of these heavy rainfall rates. Most indications are that this activity will be pretty progressive off to the southeast, likely putting a cap on the upper rainfall potential. This is evident in the HREF neighborhood QPF probabilities, with a 40-70% chance of exceeding 3", but only a 5-15% of exceeding 5"...thus swaths of 3-5" seem most likely with this event. However given the degree of deep layer forcing present, tend to think this will initially be a messy convective evolution...with several convective clusters developing. Some merging of these clusters is likely, which should locally extend the duration of heavy rainfall rates and enhance the flash flood potential. And even though extreme rainfall totals are less likely, HREF probability of exceeding FFG is quite high, up to 50-80%...so seems pretty likely that we will end up with enough coverage of FFG exceedance to warrant a MDT risk upgrade. So even though the threat of 6"+ amounts is low, the coverage of 2-5" amounts should be enough to result in numerous instances of flash flooding later today into tonight, especially given the high rainfall rates expected. Portions of this area saw heavy rainfall and flash flooding Wednesday evening, further increasing confidence in the need for a MDT risk for today into tonight. The eastern extent of the MDT risk was bound close to the edge of the higher HREF probabilities. We did extend the MDT a bit further southwest than the HREF would support...to account for antecedent rainfall, a further southwest QPF footprint in some of the global models, and a common bias of models under-doing QPF over these further west areas. Scattered to numerous instances of flash flooding are anticipated within the MDT risk area, with localized significant impacts a possibility. With time convection should tend to become more linear and progressive in nature, so did not not extend the MDT into central KS...but some flash flood risk does likely still exist as this organized convection pushes east across KS and northern OK later tonight...with a Slight risk in effect for these areas. ....MO/AR into KY and TN... Another active day of convection is likely across the Mid-Mississippi Valley eastward into the Tennessee Valley. A wavering front will meander across the area again today draped from west to east as nearly zonal flow around the base of a longwave trough remains entrenched over the area. Modest shortwave impulses embedded within this flow will pivot west to east through the day, combining with the low-level baroclinic gradient to produce waves of enhanced ascent. Some of this is materializing as morning MCS activity over Missouri and Tennessee, and it is likely residual outflow boundaries and differential heating boundaries will drive additional thunderstorm development today. PWs across the area will remain elevated, reaching as high as 2 inches, above the 90th percentile for the date according to the SPC sounding climatology, overlapped with 3000-4000 J/kg MUCAPE resulting in extreme thermodynamics to support heavy rain. During this time as well, moisture confluence will be maximized as transport vectors converge into the boundary, providing additional moist ascent for convection. Mean 0-6km winds of 20-30 kts suggest storms will generally propagate to the east, but this flow aligned to the stationary front and in the presence of more westerly inflow could result in backbuilding storms at times, especially the first half of today. With rain rates progged to be 1-2"/hr, any training could produce more than 3 inches of rain in some areas. This heavy rain through the first half of today could also occur directly on top of soils that are extremely saturated and vulnerable from widespread 2+" of rain the last 24 hours, with locally 6-11" measured. The high-res unfortunately features a lot of spread as to where the heaviest rain axis will occur, with the experimental RRFS_a and TL RRFS ensemble being the most aggressive overlapping heavy rain atop the saturated soils while other high res features axes that could be north or south of the heavy rain. Based on current radar, the threat for an overlap has increased, but a lack of ensemble support precluded an upgrade to a MDT risk. Still this appears to be a higher-end SLGT risk with training likely at least through the aftn producing 3-5" of rain in some areas. A second round of convection is likely this evening/tonight as instability climbs along the wavering boundary and the LLJ ramps up again to enhance moisture advection into the region. This could also be impacted by the leading edge of an MCS emerging out of Oklahoma late, which will ride along the greatest baroclinic/instability gradient. This rain should occur south of the earlier footprint, but training of rainfall rates nearing 2"/hr could still produce an increased flash flood risk. Some uncertainty as to how this will evolve, but the SLGT risk was adjusted southward just slightly from inherited to account for the most recent guidance. ....Northern Mid-Atlantic and Northeast... A potent shortwave rotating through the eastern CONUS mean longwave trough will traverse from the Great Lakes into Ontario, Canada today, driving a wave of low pressure and associated warm front/cold front eastward. Height falls, PVA, and convergence along the front will combine with increasing LFQ jet-level diffluence to drive pronounced ascent across the area through tonight, resulting in widespread coverage of thunderstorms, with a solid line possibly developing ahead of the front. This deep layer ascent will advect east through the day, impinging into favorable thermodynamics characterized by PWs surging this evening to 1.5-1.75 inches, about +1 sigma, on 20 kts of 850mb SW inflow, and MUCAPE of 1000-2000 J/kg. While overall the moisture is more typical of late July than anomalous, some training potential noted by Corfidi vectors aligned to the lifting warm front, with rain rates progged by the HREF reaching 1-2"/hr, could result in rainfall amounts reaching 3 inches in some areas, especially where training can occur along the advecting boundary. Antecedent soil conditions across the region are generally hydrophilic as 7-day rainfall has been only 25-50% of normal leading to below normal USGS streamflow anomalies and FFG that is generally 2"/3hrs. The HREF exceedance probabilities for 3-hr FFG are also modest. The exception to this is across eastern PA in the vicinity of the Poconos and into parts of Upstate NY near the Catskills. Here, exceedance probabilities are higher, there is longer duration of forcing/impressive moisture overlap, and antecedent soils are more vulnerable noted by high streamflow anomalies. After coordination with the WFOs, the SLGT risk was pulled eastward to reflect the highest exceedance probabilities and greatest risk for heavy rain over more sensitive soils. Weiss Day 2 Valid 12Z Fri Jul 21 2023 - 12Z Sat Jul 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR THE MID-SOUTH, NORTHEAST NEW MEXICO AND THE NORTHEAST... ....Mid-South/Southeast... An axis of above average PW values will stretch west to east from the Southern Plains into the Lower MS Valley/TN Valley and Southeast in the vicinity of the west to east oriented front forecast to be moving slowly south and southeast on Friday. An area of organized convection should be ongoing at 12z Friday in the vicinity of OK/AR. This activity may very well survive as it tracks east southeast along/near the aforementioned boundary. Exact convective mode/evolution becomes more uncertain as Friday progresses...but the overall environment will remain conducive for heavy rainfall with low level convergence and deep layered west northwesterly flow supporting some training/backbuilding potential. Some chance activity will have enough of a southward progression that the eventual flash flood threat ends up more muted, but given the ingredients and potential in place, think a broad Slight risk is warranted at this time and we can fine tune as we get closer. Models continue to nudge southward with the favored convective axis, so the Slight risk was expanded in this direction. ....NM... Low-level easterly upslope flow will strengthen Friday and Friday night over northeastern NM as the cold front continues to sink southward through the Southern High Plains. Meanwhile this area is clipped by the right entrance region of an upper jet to the north, resulting in a notable uptick in upper level divergence. This combo of low level upslope and convergent flow and divergent flow aloft should support scattered to numerous convective development Friday afternoon. Model guidance still supports the Slight risk across this region, with pockets of 2-4" of rain appearing likely. ....Northeast Mid-Atlantic into New England... Convection should be ongoing at 12z Friday across portions of the eastern Mid-Atlantic into eastern NY, but will probably be in a gradual dissipating state. The synoptic pattern becomes more stagnant Friday, with the trough slowing as it lifts north, and the upper jet remaining in a favorable position for enhanced upper level divergence. This pattern will result in a slowing of the eastward moving cold front as well. Thus as morning convection dissipates it is becoming increasingly likely that we see additional convective development by afternoon from eastern PA into the Northeast. With the slow movement of the front, these cells may tend to briefly anchor in place and/or only gradually move eastward with time. The 00z ECMWF and most of the HREF guidance is pretty aggressive with this convective development and rainfall totals. Through 00z Saturday the HREF is indicating the potential for swaths of 2-3" rainfall where these slower cells materialize. Given the wet pattern of late over this area, soil conditions and streamflows are more susceptible than normal for flash flooding. Thus it seems probable that at least an isolated to scattered flash flood risk will evolve across portions of eastern PA into eastern NY and into VT. There remains some question regarding the exact position of the front by Friday afternoon, and exactly how the transition from dissipating morning convection to developing afternoon storms evolves. Do think this event has higher end Slight risk potential, and can not rule out an embedded smaller MDT risk as the event nears. The lack of anomalous PWs is one limiting factor, but this should be overcome to some extent by the strong forcing and slow movement of cells. ....Southeast FL... Added a small Marginal risk across southeast Fl with this update. There is a notable uptick in HREF QPF output Friday afternoon as an approaching low level front helps enhance convergence and potentially help organize convection a bit more than a typical day. Plenty of instability and moisture to support heavy rainfall rates, so think at least a localized flash flood risk could evolve if convective clusters happen to merge over an urban area. Chenard Day 3 Valid 12Z Sat Jul 22 2023 - 12Z Sun Jul 23 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST AND SOUTHEAST... ....Southeast... As the front drops southward on Saturday an axis of plentiful instability and moisture will remain in place near the Gulf Coast from LA to the FL Panhandle. The front will become nearly stationary by this time, with persistent sfc-850mb convergence in place. The deep layered flow is pretty unidirectional out of the west, which does raise some concerns for west to east training and/or backbuilding of cells. The upper jet to the north keeps this region within the favorable right entrance region as well. This combo of persistent low level convergence, unidirectional flow parallel to the front, and persistent upper level divergence does suggest the potential for an increased flash flood threat. With that said, opted against any Slight risk upgrade at this time in favor of just keeping a broad Marginal risk. FFG is very high over this corridor, and thus a large Slight risk seems unlikely to verify. A more focused Slight risk may eventually be needed, but not confident on where to place that at this time. Convective mode/evolution will be a big factor in where any more organized threat does evolve...as well as whether convection can train inland, or if tends to propagate southward into the Gulf with time. Will continue to closely monitor, and as we get closer confidence in the details should gradually increase. ....Southwest... A broad Marginal risk was generally left unchanged from what was inherited across portions of eastern AZ into NM. The setup over NM does not look as good as it does Friday, but enough moisture and instability persists to support isolated to scattered convective development. Any cells that do form will be capable of dropping a quick 1" or so of rain, and thus localized flash flooding of any more susceptible areas is possible. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_B96VYFtYensKnEt5xvJE1_l1o7TNSHuzgOanci5J-Dr= lVFMb2rFyjelBbc_-HNqihs_bPRF6kVTE0b6NiuwbyeVT9g$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_B96VYFtYensKnEt5xvJE1_l1o7TNSHuzgOanci5J-Dr= lVFMb2rFyjelBbc_-HNqihs_bPRF6kVTE0b6NiuwYN0Xr_s$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_B96VYFtYensKnEt5xvJE1_l1o7TNSHuzgOanci5J-Dr= lVFMb2rFyjelBbc_-HNqihs_bPRF6kVTE0b6NiuwcrE7XV8$=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) .