Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 07 2023 08:28:30 FOUS30 KWBC 070828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 AM EDT Mon Aug 07 2023 Day 1 Valid 12Z Mon Aug 07 2023 - 12Z Tue Aug 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE NORTHEAST INTO THE MID-ATLANTIC REGION AS WELL AS NORTHEASTERN WASHINGTON AND NORTHERN IDAHO... ....Much of the Northeast U.S.... A potent mid/upper level shortwave trough will make its way eastward from Illinois to portions of the central Appalachians during the day while an anomalously strong jet for the season makes its way from the Ohio Valley into parts of the Mid-Atlantic...helping to keep a feed of deep moisture over the eastern U.S.. Exactly how this interacts with a shortwave trough initially over Michigan before heading north of the border...and any associated height falls over the eastern Great Lakes and Northeast U.S. continues to result in poor run to run consistency...model QPF spread...and associated excessive rainfall potential. With the event moving into the Day 1 timeframe...an increasing amount of CAM guidance is available. The HREF paired-matched mean showed one wave of QPF moving quickly into New England early today before an additional round of active convection capable of intense downpours develops in the afternoon focused farther south which eventually translates/spread north and east. 40 km neighborhood probabilities from the 07/00Z HREF show the highest probabilities for 1-hour QPF to exceed one/two inch(es) developing late afternoon/early evening over parts of the Mid-Atlantic region in proximity to the potent mid/upper level shortwave...although the signal for 1 hour QPF to exceed flash flood guidance/3 hour QPF to exceed flash flood guidance is greater in portions of New York state where Flash Flood Guidance is lowest. For this reason...opted not to make too many adjustments to the northern portion of the Slight Risk area but pulled the southern portion of the Slight Risk in response to the potential for downpours with 2+ inch per hour rates. With so much interaction at the synoptic scale still to be sorted out...additional adjustments one way or the other are still possible. ....Portions of the Northwest U.S.... Maintained the Slight Risk over portions of Washington/Idaho/Montana and Oregon with a fairly subtle southward expansion towards the Blue Mountains. Low pressure prior to the start of Day 1 has drawn moisture into the region from the east.=20 This moisture combined with the slow moving surface low will help fuel another round of afternoon and evening convection. Given the anomalous amount of moisture...with precipitable water values some 150 to 200 percent of normal in northern Idaho...locally heavy downpours that result in flooding are certainly possible. The most vulnerable areas being over and near recent burn scars...although instances of flooding can also occur elsewhere.=20 Thinking is that the NAM is too aggressive with its QPF...a bias with this type of flow pattern... ....Western High Plains... Scattered showers and thunderstorms will develop over parts of Colorado and Wyoming before spreading eastward. Storms will be moving into an airmass with deeper moisture which should allow for an uptick in rainfall rates with time. Locally heavy rainfall rates and isolated heavy rainfall amounts may lead to isolated instances of flooding or at least run-off problems in regions of poor drainage. Bann Day 2 Valid 12Z Tue Aug 08 2023 - 12Z Wed Aug 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NORTHERN NEW ENGLAND AND NORTHERN NEW YORK AND THE CENTRAL PLAINS... ....Northeast... Convection capable of producing heavy rainfall rates and excessive rainfall amounts should be on-going mainly across northern New York and northern New England as Monday's system continues on its northward track. One area of surface low pressure is forecast to be located just north of New York while a second area of low pressure begins to organize south of Maine. Instability appears to be on the weak side but precipitable water values across the region should still be in excess of 1.5 inches...and on the order of 1.75 inches closer to the coast. That should still be sufficient to produce locally heavy rain rates early in the period...with the HREF neighborhood probabilities showing the potential for highest rainfall rates getting shunted eastward during the afternoon. The UKMET and the CMC both attempt to develop a decent band of deformation rainfall north of the border which stays north of the border...while the GFS was tended to rotate a second round of precipitation across northern New England later in the day. Kept a Slight Risk area in the area...trimming some area in deference to a model signal for a somewhat faster departure while keeping the risk across the north until it becomes more clear how energy north of the border will evolve. ....Southeast U.S.... Maintained the Marginal Risk area along and immediately south of an unusually well-defined front for this time of year where precipitate water values at or above 2 inches are expected to be in place. Exactly where a morning MCV will be located over Mississippi or Alabama...and how much interaction between any mesoscale boundaries...such as sea breezes or outflow from nearby convection...makes it difficult to pinpoint the most likely area to receive excessive rainfall.=20 ....Western Plains... Another day of late day and evening convection...with at least some threat of excessive rainfall...is possible as return flow draws moisture farther into the western Plains on the west side of a quasi-stationary front. Thinking is that the convection will initiate over the Western High Plains with the concern for excessive rainfall increasing as the convection moves east/southeastward towards deeper moisture as shown by precipitable water values of 1.5 inches. There was a fair amount of spread in the QPF placement over the region...tended to favor the more southern solutions like the ECMWF/CMC/UKMET (ignoring its 8+ inch convective-feedback bulls eye over northeast Nebraska) due to the better instability. Bann Day 3 Valid 12Z Wed Aug 09 2023 - 12Z Thu Aug 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO VALLEY INTO THE SOUTHEAST U.S.... ....Illinois into the Tennessee Valley and portions of the Southeast... Convection is expected to develop during the late afternoon/evening and grow up-scale as shortwave energy initially over Kansas/Nebraska heads eastward...leading to height falls aloft overspreading the Mid-Mississippi Valley by 10/00Z. Gulf moisture gets pulled northward during the day with both the GFS and NAM showing precipitable water values exceeding 2 inches from the southeast corner of Missouri into parts of Arkansas, Mississippi and Alabama by 09/18Z while the flow aloft ahead of the strengthening shortwave becomes increasingly difluent. Models tend to develop convection during the first part of the Day 3 period...but the expectation is for the risk of highest rainfall rates and accumulations will not occur until the later part of the period due to the better thermodynamics and the arrival of 70+ kt speed maximum aloft. Given the time of year, the precipitable water values are above normal but not terribly so...only 2 to 2.5 standardized anomalies above climatology in Missouri/western Tennessee by Wednesday evening. However...the moisture flux is 3.5 to 4+ standard anomalies above climatology. The NBM and Ensemble Bias Corrected QPF focused 2 or 3 inch amounts along an axis from far southern Illinois into northern Tennessee...and the 07/00Z NAM and 06/12Z UKMET QPF axis was farther north while the GFS was farther south (supporting the GEFS idea of 3 inch contours into northern Georgia. Despite the spread...felt confident enough to begin realigning the axis a bit but left it fairly broad to deal with the spreads of QPF solutions and related excessive rainfall threat. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_yC4yTI0JSl21D97Ot9oQqQp-5UtAYck7rPQIDRbUn_X= 1rbIBOgSqTl0tmyNGqGvSPBrx7aCdhw0xE_aWLK-7ocFps0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_yC4yTI0JSl21D97Ot9oQqQp-5UtAYck7rPQIDRbUn_X= 1rbIBOgSqTl0tmyNGqGvSPBrx7aCdhw0xE_aWLK-qpjuNqA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_yC4yTI0JSl21D97Ot9oQqQp-5UtAYck7rPQIDRbUn_X= 1rbIBOgSqTl0tmyNGqGvSPBrx7aCdhw0xE_aWLK-O1318o8$=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) .