Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jul 21 2023 00:46:15 FOUS30 KWBC 210046 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 845 PM EDT Thu Jul 20 2023 Day 1 Valid 01Z Fri Jul 21 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... Maintained the Moderate Risk of excessive rainfall over portions of eastern Colorado and western Kansas as scattered to numerous afternoon showers and thunderstorms propagate off the higher terrain and encounter deeper moisture and a strengthening low-level jet. The convection has been aided by an impressive shortwave trough digging through the area in northwest flow aloft....with deep layer ascent being aided by the right rear quad of an upper level jet streak. Models still point to bulk shear on the order of 40 to 50 kts...suggesting that organization into an MCS appears to be on track for later in the evening and overnight hours. 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. From an ingredients perspective...mid/upper level moisture advecting in from the Great Basin should increase precipitable water values 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...but the rainfall rates still have the potential to overwhelm the infiltration rates of the ground on small enough time scales. As a result...saw little reason to make many changes especially with the latest runs of the HRRR still supporting localized 3 to 5 inch amounts. ....Central Plains Eastward Into the Southern Tennessee Valley... As convection moves out of the Western High Plains immediately adjacent to the Rockies...storms will continue moving into an area where precipitable values have increased to 1.75 inches or greater north of a wavy quasi-stationary front draped from west to east across the area. Nearly zonal flow around the base of a longwave trough remains entrenched over the area with weak and difficult to time shortwave impulses embedded with the flow. Precipitable water value around 2 inches would be above the 90th percentile for the date according to the SPC sounding climatology...which models show being reachable. The deep moisture will be overlapped with 3000-4000 J/kg MUCAPE resulting in extreme thermodynamics to support intense rainfall rates and heavy to potentially excessive rainfall totals. Mean 0-6km winds of 20-30 kts suggest storms will generally propagate to the east from Kansas into southern Missouri but this flow aligned to the stationary front and in the presence of more westerly inflow could result in backbuilding storms at times...though expectations are that storms later tonight should tend to be more progressive than convection earlier in the day. In the very near term...enough dry air has advected in to minimize rain chances in areas of saturated soils in the Mid-Mississippi Valley and southern Tennessee Valley which are particularly vulnerable to flooding from any additional rain. Maintained a Marginal to cover the chance of rain moving back as moisture advection kicks-in.=20 ....Carolinas to Eastern Great Lakes/Mid-Atlantic and Northeast... The main focus for heavy to potentially excessive rainfall continues to be associated with a potent shortwave rotating through the eastern CONUS mean longwave trough. The center of the trough will be crossing the international border soon but associated height falls...PVA and difluence aloft east of the center will still be driving force in showers and thunderstorms tonight. Concern for flooding is focused where the a line of thunderstorms has become parallel to cell motion from is for training of cells on the south end of a line of convection. Deep layer ascent will advect east impinging into favorable thermodynamics characterized by precipitable water values approaching 1.5 inches according to 21/00Z sounding that keep building into the 1.5-1.75 inch range overnight...about +1 sigma, on 20 kts of 850mb SW inflow, and MUCAPE of 1000-2000 J/kg. As previously mentioned...antecedent soil conditions across the region are generally hydrophobic 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. Note that the only change of note made to the Marginal Risk area was in portions of the Carolinas/southern Mid-Atlantic region where guidance has consistently kept dry. Bann 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!4ZjqlmEW1BdLsaZIUT4JEACia_jJV18dwP-t33WY42vD= 9uJWCCqLPnUkZolFgb-WJBCcymJ9LeCJGngiy-qLhggE69s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4ZjqlmEW1BdLsaZIUT4JEACia_jJV18dwP-t33WY42vD= 9uJWCCqLPnUkZolFgb-WJBCcymJ9LeCJGngiy-qLVI0o32c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4ZjqlmEW1BdLsaZIUT4JEACia_jJV18dwP-t33WY42vD= 9uJWCCqLPnUkZolFgb-WJBCcymJ9LeCJGngiy-qLgHTf2zk$=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) .