Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 20 2023 19:51:41 FOUS30 KWBC 201951 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 351 PM EDT Thu Jul 20 2023 Day 1 Valid 1744Z 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 AND PARTS OF TENNESSEE... ....MO/AR into KY and TN... Special 1730Z Update: Added a targeted MDT risk to parts of central and eastern TN, including Nashville, to account for backbuilding convection noted on radar. The model guidance is still struggling with the evolution, so the upgrade is driven primarily by radar trends in a region of still impressive moisture confluence and thermodynamic advection. Mesoscale Precipitation Discussion more fully highlights this threat. Previous Discussion from 1600Z: 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. ....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. ....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 PARTS OF THE SOUTHERN HIGH PLAINS, SOUTHEAST/MID-SOUTH, AND NORTHEAST/MID-ATLANTIC... ....Northeast and Mid-Atlantic... A potent shortwave and vorticity max rotating through the broad longwave trough will eject across northern New England Friday, lifting into Quebec by Saturday morning. This feature will drive height falls across the region, pushing an area of low pressure eastward through the St. Lawrence Valley and dragging the associated cold front into New England and the Mid-Atlantic. Forcing for ascent will be enhanced by diffluence within the LFQ of a zonally oriented jet streak pivoting through the OH VLY, and it is likely this will result in a broad area of impressive synoptic lift on Friday. This will act upon favorable thermodynamics characterized by PWs surging northward in the pre-frontal regime to 1.25-1.5 inches, slightly above climo according to the NAEFS tables, and MUCAPE reaching above 1000 J/kg. This will produce widespread convection with rainfall rates progged by the HREF to be 1-2"/hr in clusters of storms organized through 25-35 kts of effective bulk shear. 0-6km mean winds are progged to be 20-30 kts, so storm motions are likely to be progressive overall. However, there is at least some temporal overlap of aligned mean winds with the approaching front, suggesting a short-duration training threat. This reflected by simulated reflectivity showing lines of convection moving across the area during the aftn/eve, as the atmosphere re-destabilizes post some morning convection. This area has been extremely saturated recently noted by AHPS rainfall that is 300-400% of normal the past 7 days resulting in compromised FFG and USGS streamflow anomalies that are almost uniformly above the 90th percentile. If storms can organize and train just a bit more impressively than current progs suggest, this could require a targeted MDT risk. However, after coordination with BTV/BOX/OKX, opted to keep a SLGT risk for locally 3+" of rain with this update. ....Mid-South/Southeast... A mid-level shortwave and associated potent vorticity maxima will race WNW to ESE from KS to SC Friday, helping to amplify the longwave trough all the way to the Gulf Coast. Flow across the region in the base of this trough will be weakly cyclonic (almost zonal), within which a cold front will just gradually sag southward through the day. During this evolution, a 300mb jet streak will amplify across the OH VLY/Mid-Atlantic and then extend a tail southward to place the favorable RRQ for ascent at least peripherally overtop the front. This will result in broad large scale ascent favorable for convective development, and it is likely scattered to widespread thunderstorms will occur during peak heating and into the evening, with nocturnal MCS development also possible in some areas. One of these MCS may persist after 12Z Friday as it emerges from the Southern Plains, and while confidence in the evolution and track of this feature is limited, it could produce heavy rainfall rates as it moves eastward into the TN VLY through the morning. This may weaken, as is typical, during the aftn, but result in enhanced moisture convergence along its track, especially where interaction with the cold front occurs. Additional development is likely along the front itself as it impinges into favorable thermodynamics noted by PW anomalies according to NAEFS of +1 sigma overlapped with MUCAPE of 2000-3000 J/kg. Any MCS development will likely track along the greatest axis of moisture convergence and the strongest CAPE gradient, which could also experience thunderstorms training along the front through boundary-parallel mean flow. The inherited SLGT risk was expanded slightly to the south/east to best reflect the greatest exceedance probabilities from the HREF, but was kept out of FL after coordination with JAX/TAE. ....New Mexico... Weak shortwave impulses embedded within an amplifying ridge over the Desert Southwest will produce ascent within an environment of increasing heat and instability. Progs suggest MUCAPE will rise above 1000 J/kg during peak heating, which will correlate well with the timing of a cold front sagging into the Four Corners from the northeast before weakening during the day. This will lead to an overlap of ascent from modest PVA and low-level convergence along the front, with additional lift being provided through increasing NE/upslope flow. The NAEFS ensemble tables indicate that a narrow corridor of +1 sigma PW will advect into New Mexico Friday as well, providing sufficient moisture for the intensifying ascent to produce scattered showers and thunderstorms with rainfall rates exceeding 1"/hr according to the HREF neighborhood probabilities. These storms are expected to be slow noted by 0-6km mean winds of just 5-10 kts, and may be tied to terrain features initially before organizing into clusters within 25-35 kts of effective bulk shear. Where the most intense rates can persist the longest, generally near terrain, 2-3" of rainfall is possible with locally higher totals, and the SLGT risk was adjusted to match the highest probabilities for 3" extending into the Sandia Manzano Mountains. ....Southeast Florida... The inherited MRGL risk was adjusted slightly for cosmetic purposes to best match the recent guidance trends. Another day of scattered afternoon air-mass type thunderstorms is likely across the FL peninsula, but some slightly greater organization and coverage is expected than the past few days. PWs around 2" resupplied through onshore flow from the Gulf of Mexico will combine with MUCAPE reaching 3000 J/kg to provide extreme thermodynamics to support rainfall rates of 2-3"/hr within convection. Although most of the development will occur along the typical sea breeze/gulf breeze with additional storms occurring along outflows, slow height falls and the peripherally distant RRQ of a jet streak over the Southeast will drive stronger deep layer ascent to result in more coverage across the peninsula. With the mean synoptic flow out of the west, this could produce storms that organize and stall along the eastern pinned sea breeze. The greatest risk for this is across the urban treasure and gold coasts where slowing of storms could produce more than 3" of rain and isolated runoff/flash flood issues. Weiss 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, SOUTHEAST, AND SOUTH FLORIDA... ....Southeast... A continued amplification of the eastern CONUS longwave trough will drive subtle height falls southward, pushing a cold front towards the Gulf Coast. This front will stall in a generally west to east fashion Saturday as it becomes entrenched within more zonal flow at the base of this broad cyclonic flow. Height falls, low-level convergence along the front, and weak embedded impulses will be the primary drivers for ascent, but a poleward arcing jet streak pivoting into New England will also drive some enhanced lift through its diffluent RRQ. The overlap of this ascent should produce widespread convection Saturday, with impressive moisture confluence impinging into the front working to sustain convection with rain rates of 1-2"/hr. The flattening of the trough atop the front will result in nearly unidirectional low and mid-level flow, becoming more parallel to the front, which could result in more significant training of convection. Where this occurs, both the GEFS and ECENS probabilities suggest a low-end threat for more than 3 inches of rain. FFG across this area is quite high, so exceedance to drive flash flooding is uncertain. For this reason the MRGL risk was maintained despite a modest uptick in QPF. If the signal becomes more robust for more efficient training, a targeted SLGT risk may be needed, but at this time confidence is not high enough for an upgrade. ....Southwest... The inherited MRGL risk was adjusted only cosmetically to account for new guidance, but remains generally unchanged for portions of AZ and NM. The mid-level ridge will gradually expand and intensify Saturday, but lobes of vorticity rotating around the periphery will continue to produce waves of enhanced ascent. This lift will occur within still favorable thermodynamics as PWs are near normal and MUCAPE reaches 1000 J/kg. This will result in scattered, primarily aftn/eve, convection with rainfall rates possibly exceeding 1"/hr. Although organization and intensity may be somewhat muted compared to Friday, storm motions will remain quite weak and variable, so any heavy rain producing cell could yield more than 1" of rainfall as noted by low probabilities in the GEFS and SREF guidance. This could result in isolated instances of flash flooding, especially if these cells move across more vulnerable areas. ....South Florida... Anther day of afternoon thunderstorms is expected Saturday as an overlap of impressive instability of 2000-3000 J/kg and PWs around 2" support diurnal convection. Forcing for ascent will increase once again as the longwave trough across the eastern CONUS amplifies further to produce at least modest height falls into Florida, while the associated upper jet streak shifts off the Southeast coast to leave modest upper diffluence over the peninsula. The overlap of this ascent and favorable thermodynamics will produce scattered to widespread thunderstorms with rainfall rates of at least 2"/hr at times. Although organization is not expected to be quite as impressive as on D2, the heaviest rainfall may again occur along the pinned east coast sea breeze, over areas that could receive significant rain on D2 to prime the region. After coordination with WFO MFL, a small MRGL risk was added. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9YYpzjdUYKvwsf-1HRMW6tQDn05XY-O2wpIJTJsADgzE= J7p3rkAusCiHXrgdQCC0I_pFaYUR-js0SjPfGXACM2jOp4M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9YYpzjdUYKvwsf-1HRMW6tQDn05XY-O2wpIJTJsADgzE= J7p3rkAusCiHXrgdQCC0I_pFaYUR-js0SjPfGXACh7_ztNY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9YYpzjdUYKvwsf-1HRMW6tQDn05XY-O2wpIJTJsADgzE= J7p3rkAusCiHXrgdQCC0I_pFaYUR-js0SjPfGXACc-ft1is$=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) .