Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 26 2023 00:56:14 FOUS30 KWBC 260056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 855 PM EDT Fri Aug 25 2023 Day 1 Valid 01Z Sat Aug 26 2023 - 12Z Sat Aug 26 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE COLORADO ROCKIES EAST ACROSS THE CENTRAL PLAINS... ....Central Plains... Impressive MCS slowly organizing along a stationary front and ahead of a surface low over northwestern KS will continue shifting east overnight. Ample moisture and instability has led to hourly rainfall estimates exceeding 2.5" in the heaviest parts of this multi-cell activity. Increasing SWly flow into the frontal zone this evening will provide reinforcing moisture and instability ahead of the activity is high - around 4000 J/kg according to recent RAP output. Given low mean layer flow (Wly jet around the sprawling high is up over the Dakotas/MN), slow progression east is expected through the rest of the night as heavy rates are maintained. Therefore, the Slight Risk is extended east across northern Kansas and along the Nebraska border then into NW Missouri including St. Joseph and the KC metro. ....Four Corners States... A vort lobe over northeast NM will continue shifting northeast overnight. Widespread heavy thunderstorms have developed ahead of this lobe over southeast CO where there was ample afternoon instability. A cold front has pushed through into NM and subsidence/drying has reduced instability ahead of the current activity and should lead to weakening within a few hours. Until then, the Slight Risk is maintained and heavy thunderstorms/scattered instances of flash flooding are expected to continue. Farther west is general monsoonal flow west of the central U.S. ridge which should allow some further diurnal activity to lead to isolated flash flooding for a few more hours before dissipating. The Marginal Risk is maintained for portions of UT/AZ/NM/western CO. ....Lower Midwest into Tennessee Valley and Southern Appalachians... Moist confluent flow along a slow moving cold front will continue to allow heavy thunderstorms to have a southerly component to their direction. Earlier activity centered on eastern KY has developed a cold pool and is much more progressive, allowing the Slight Risk to be removed. However, precipitable water values of 1.8 and 2 inches along this frontal zone will continue to support local torrential rain and an isolated flash flood threat, so a Marginal Risk is maintained. ....Northeast and Mid Atlantic and into parts of the Ohio Valley/Southern Lakes.., A lack of activity and decent mean layer flow ahead of an upper trough axis should keep activity over the Mid-Atlantic and Northeast progressive enough overnight to allow the Marginal Risk to be removed. Jackson 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!4WHEztZXAIeXl51cXQOW5W5sTL0Bf-zAECZXeSJha1fl= zUQUcrGbTSovrOh-mOxMP445AG_BZNb8c_-Tq4vyjtGPAkA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4WHEztZXAIeXl51cXQOW5W5sTL0Bf-zAECZXeSJha1fl= zUQUcrGbTSovrOh-mOxMP445AG_BZNb8c_-Tq4vydV2NU9c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4WHEztZXAIeXl51cXQOW5W5sTL0Bf-zAECZXeSJha1fl= zUQUcrGbTSovrOh-mOxMP445AG_BZNb8c_-Tq4vyblAdlJU$=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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