Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 25 2023 19:46:39 FOUS30 KWBC 251946 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 345 PM EDT Fri Aug 25 2023 Day 1 Valid 1915Z Fri Aug 25 2023 - 12Z Sat Aug 26 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS THE CENTRAL ROCKIES INTO THE CENTRAL PLAINS AND PORTIONS OF SOUTH-CENTRAL APPALACHIA... 1930 UTC Update... Added a Slight Risk to eastern Kentucky into Ohio, far western Virginia, and eastern Tennessee as activity over eastern KY looks to repeat as it progresses south down the instability gradient with sufficient westerly low level inflow. Further info can be found in MPD 977 and any subsequent MPDs. Jackson 1600 UTC Update -- The latest satellite IR loops show two primary areas of interest today -- one currently across the Central Rockies-High Plains associated with the remnant moisture from TC Harold, the other with the MCS remnants and renewed convection currently across southern WV into eastern KY and far SW VA. There isn't much of a reflection aloft (i.e. well-defined MCV) from the overnight convection tracking S-SSW across the central-southern Appalachians, however with the surface front nearby, modest low-level moisture/moisture flux anomalies per the 09Z SREF, and continued positive dCAPE/dt downstream, have maintained the Marginal Risk area through central-eastern KY, eastern TN, and western NC. Otherwise, per the 12Z CAMs, the trend for more spotty convection and a more limited (<5%) flash flood risk was noted elsewhere across the Mid Atlantic Region closer to the coast, so have removed the Marginal in that area. Elsewhere, based on the latest (12Z) deterministic and probabilistic guidance, including the HREF exceedance probabilities, have extended the Slight Risk a little farther east along the northern KS-southern NE border, while extending the Marginal Risk through southern IA-northern MO, along the periphery of the upper high and along/near the surface stationary boundary, to also account for the latest observational and model trends. Hurley Previous discussion below.. ....Southwest to the central Rockies and Adjacent Central Plains... Energy that had once been Harold will continue to make its way around the periphery of building mid-level heights over the Plains states...with a surface reflection being a low center to the east of the Central Rockies and the presence of anomalously deep moisture setting the stage for a potentially impactful event.=20 Precipitable water values of 1.75 inches over eastern and central Kansas/Nebraska gets pulled westward by low level easterly flow feeding into the area of low pressure by this evening...which helps push precipitable water values over northeast Colorado and nearby southeast Wyoming to 3 standardized anomalies greater than climatology. The 25/00Z HREF's highest probabilities of the 24hr QPF exceeding 2 inches in 24 hrs peaks over 50 pct where flow over a northwest-southeast front aids lift as well as an second area farther south given proximity to better instability (1000-1500 J/Kg surface-based CAPE per the HREF). By later in the period...the surface low begins to move east and a line of precipitation to slowly press southward. Local precip maxima from deterministic models and signals from the 25/00Z HREF EAS probabilities show a potential for locally heavy rainfall with enhanced rates of up to 1"/hr and totals near 1-2". This area was already covered in Thursday's Day 2 ERO and needed little change. Afternoon and early evening convection is expected in and near the higher terrain around the 4-corners region which poses a low-end risk of excessive rainfall given favorable moisture profiles from the area soundings. ....Northeast and Mid Atlantic and into parts of the Ohio Valley/Southern Lakes.., Area low pressure will track to the northeast from the Great Lakes area on Friday with increased mid-level difluence out ahead of a stronger shortwave trough digging eastward out of Ontario. First round of rainfall will be on-going at the start of the Day 1 period at 12Z today...which eventually exited off Southern New England. A second round of convection will develop near a warm front pushing northward along the coast with a trailing cold front pressing east along the base of the surface low. General instability axis will preside within the warm sector from the Mid Atlantic up into New England with PW anomalies a solid 1-2 standard deviations above normal, especially across New England where the heavy precipitation signature is most prominent. Any thunderstorm will be capable of producing locally heavy rainfall with rates up to 1"/hr. A secondary area of focus back into eastern KY extending back into parts of Indiana with diurnally driven convective initiation in a region of moist confluent flow. Precipitable water values will climb during the afternoon to values between 1.75 and 2 inches along this corridor which should support local downpours...especially where terrain can help provide focus for the flow.=20 Bann Day 2 Valid 12Z Sat Aug 26 2023 - 12Z Sun Aug 27 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS THE SOUTHERN-CENTRAL ROCKIES AND CENTRAL PLAINS EASTWARD INTO PARTS OF THE LOWER MISSOURI AND MID MISSISSIPPI VALLEYS AS WELL THE TENNESSEE AND LOWER OHIO VALLEYS... 2000 UTC Update -- Based on the latest deterministic and probabilistic guidance and trends (esp with CAMs), have decided to combine Marginal Risk areas to include parts of the Lower-Mid MS Valley. Partly to maintain continuity from the D1 Outlook area, while also aligning with the expectation that the excessive rainfall risk will be elevated along/near the stationary with weak upper level height/thickness diffluence between the contracting deep-layer ridge (SW) and digging upper trough (NE). Moreover, the models show some enhanced forcing (upper divergence...low level FGEN) within the right-entrance region of a 70-90kt upper level jet streak traversing the Great Lakes region. At this point, the guidance, including the CSU UFVS-trained first guess fields, maintain an isolated or marginal threat for flash flooding, given the lack of more upscale organization. Obviously that could change, at least over a targeted area, depending on any organized convection (MCS) overnight into early Sat. Right now the guidance QPF spread, especially among the CAMs, is too high for a Slight Risk area. Hurley Previous discussion below... Locally heavy rainfall is expected across parts of the southern and central Rockies as shortwave energy ejects eastward and out over the adjacent Plains into the Mid-Mississippi Valley later today/tonight. This channel is where the numerical guidance places highest precipitable water values and it is where model QPF is highest. There was a slight southward shift in the placement of the area based on the latest model runs but overall the difference reflected little shift in forecast reasoning. Deterministic and probabilistic QPFs have came up across parts of New Mexico and eastern Arizona on Thursday. Those increased amounts persisted in the 25/00Z model runs...so few changes needed there. Also introduced a Marginal Risk area over portions of the eastern Tennessee Valley as well, and as such have expanded the Marginal Risk over that region. The models show weak shortwave energy settling in from the north accompanied by precipitable waters exceeding 2 inches late in the period. Given the diffuse nature of the shortwave energy...overall confidence is below average. On the other hand...this appears to be the beginning of an expanding area of rain over parts of the area farther east on Day 3.=20 Finally...with models showing the back edge of rainfall lingering over the far eastern bit of downeast Maine...and a couple of the ensemble members showing potential for amounts approaching in inch above what is forecast on Day 1...introduced a Marginal risk there. Bann Day 3 Valid 12Z Sun Aug 27 2023 - 12Z Mon Aug 28 2023 ....THERE ARE MARGINAL RISK AREAS EXTENDING FROM PARTS OF THE ROCKIES-HIGH PLAINS EASTWARD TO THE SOUTHEAST-LOWER MID ATLANTIC REGION... 2000 UTC Update -- Few changes made to the Day 3 ERO. Did expand the Marginal Risk farther NW into WY and far south-central portions of MT, given a bump in deterministic and probabilistic QPF from the 12Z guidance, responding to an uptick in upper level divergence Sun afternoon-evening coinciding with the anomalous deep-layer moisture (PWs 2-2.5 standard deviations above normal per the latest GEFS and SREF). Instability should be marginal (MUCAPEs averaging 250-500 J/kg), but considering the deep-layer lift and moisture will likely be sufficient for localized hourly rainfall rates of 0.5-1.0". Hurley Previous discussion below... ....Rockies and High Plains... Weak northerly flow with a broader 500 mb ridge will allow minor shortwave troughs to slowly propagate southward from Colorado into New Mexico. Those shortwaves will be able to tap precipitable water values of 0.8 to 1.2 inches along their path...with lower values being in Arizona. Thinking is that at least some of the resulting late day and evening convection will be able to produce local downpours that results in run off or local flooding...especially over recent burn scars..normally dry washes and arroyos. ....Eastern parts of the Central-Southern Plains into much of Arkansas and northern Louisiana... A shortwave dropping southeast from the western high plains will help focus storms along a cold front that produce local downpours as it moves into a region with precipitable water values at or slightly above 2 inches. Timing is uncertain...but think the GFS idea of development during the late afternoon and persisting into the evening is reasonable.=20 ....Parts of the Southeast into the Lower Mid Atlantic Region... Models agree on an expanding area of rainfall over parts of the Carolinas Sunday into early Monday...in part due to an amplifying mid- and upper trough over the eastern United States. As it advances southward...it helps develop low pressure along a cold front over the Southeast U.S.. An airmass with precipitable water values generally above 2 inches will be in place...leading to forecast soundings favorable for heavy rainfall rates. Given antecedent conditions...think most places should be able to handle the forecast rainfall...although rainfall intensity may still result in local run off or flooding problems so maintained a Marginal for the time being...and may be a primer for bigger rainfall concerns beyond the end of Day 3.=20 Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8RuP5yqjYonMRsBhAdtCwDJSN3QLC7sCgWQKiz6EoxGc= J1DwkD127HcgU-YIKmWhJKl-349ZODfIIVj6RmGQF24D6oE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8RuP5yqjYonMRsBhAdtCwDJSN3QLC7sCgWQKiz6EoxGc= J1DwkD127HcgU-YIKmWhJKl-349ZODfIIVj6RmGQFJ0V5Uw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8RuP5yqjYonMRsBhAdtCwDJSN3QLC7sCgWQKiz6EoxGc= J1DwkD127HcgU-YIKmWhJKl-349ZODfIIVj6RmGQDIJt1vM$=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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