Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 06 2023 08:31:09 FOUS30 KWBC 060831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Sun Aug 06 2023 Day 1 Valid 12Z Sun Aug 06 2023 - 12Z Mon Aug 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST/NORTHERN PLAINS... ....Northern Plains... A Slight Risk remains in place across parts of the Upper Midwest/Northern Plains as a vigorous vertically-stacked low makes its way eastward ever so slowly across the region. An axis of 1.75-2" precipitable moisture plume will continue to wrap into the system...providing a sounding profile favorable for heavy rainfall and rainfall rates. Latest guidance shows an axis of 1 to 3 inches in a northwest to southeast axis from Iowa into Illinois...with some of the models with higher resolutions showing embedded higher maximum amounts. HREF 40 km neighborhood probabilities for 1 inch per hour amounts are at or above 30 percent along this axis by afternoon and maximize over 40 percent in parts of southeast Iowa by late afternoon/early evening and then begin to taper off by late evening. There is a better signal from the HREF that 3 hour QPF may exceed 3 hour flash flood guidance than 1 hour QPF exceeding 1 hour QPF...mainly across parts of Iowa that received heavy to excessive rainfall earlier on Saturday.=20 ....Northeast IL/Northwest IN Eastward to Parts of New York and the Mid Atlantic... A compact mesolow has occluded while tracking east into northern Indiana by early this morning with generally light to moderate rainfall amounts. With the models still pointing to the possibility of an uptick in rainfall rates later today...re-aligned the Marginal risk area a bit in lower Michigan. Still not expecting much in the way of backbuilding, cell merger activity, or training. As the system continues to push eastward...precipitable water values of 1.8 to 2,2 inches get drawn into the system by this evening with growing coverage of showers and thunderstorms. Some of the convection should be able to produce some downpours given that environment. Opted to introduce a Slight Risk area over parts of the Mid-Atlantic/I-95 corridor where convection from the late afternoon into the late evening or or early Monday morning will be force-fed the moisture by an increasing low level jet...at least enhancing the risk of excessive rainfall totals. A greater concern this far east occurs after the end of the Day 1 period. ....Pacific Northwest... Highly anomalous moisture also persists across portions of the interior Pacific Northwest, with PWATs to 1.25 inches running more than 3 sigma above normal for this time of year. The concern is with daytime heating Sunday afternoon that the storms that form are likely to train westward across portions of WA that are not equipped to handle much heavy rain all at one time. Since the forcing is weak on Sunday, the area remains in a Marginal Risk. Bann Day 2 Valid 12Z Mon Aug 07 2023 - 12Z Tue Aug 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE NORTHEAST AS WELL AS NORTHEASTERN WASHINGTON AND NORTHERN IDAHO... ....Much of the Northeast... Guidance continues to struggle with the placement of heaviest rainfall across this region on Monday. A developing surface low starts the day near Chicago and will track to near Lake Ontario by Tuesday morning. Along this track, the areas likely to be hardest hit with the rain are from the I-95 corridor from Baltimore to NYC northward into northern NY and New England. Isolated rainfall totals to 4 inches are possible. This encompasses areas which are still hydrologically sensitive following heavy rainfall and flooding over the past few weeks...particularly northern NY into northern New England. Some expansion was done to the Slight risk area in eastern Pennsylvania given an increase in QPF with little need for changes in areas where orographics influence rainfall totals compared with the previous outlook. Expect more widely scattered convection from the southern Appalachians north through the Midwest. ....Northeastern WA through Northern ID... A strong, but slow-moving and shearing shortwave in the upper levels will track across central WA on Monday. With highly anomalous atmospheric moisture already in place, it's likely this upper level forcing will enhance local rainfall totals across this region. This area has been hit with rainfall yesterday, today, and is expected tomorrow, so the likely wettest day of the group being the last day of the rain will mean FFGs will be at their lowest in the area when the heaviest rain falls. T Bann Day 3 Valid 12Z Tue Aug 08 2023 - 12Z Wed Aug 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NEW ENGLAND AND NEW YORK ON TUESDAY... ....Northeast... Increasing dynamics associated with low pressure that will have made its way eastward from the Upper Midwest and Great Lakes region will aid increasing coverage of showers and thunderstorms capable of producing heavy rainfall rates and excessive rainfall.=20 A plume of precipitable water values in excess of 1.75 inches should already by in place from the Mid-Atlantic region north/northeastward that can be tapped by the vertically stacked system. The threat of excessive is expect to be driven by intense rainfall rates...especially more in the south and in the east...while concern for portions of upstate New York into Vermont will be a combination of heavy rates with the deep moisture followed by the upper system over Canada that begins to change course and possibly bring renewed convection later in the period.=20 This area in particular is still recovering from recent flooding.=20 There was too much spread in the guidance to place a Moderate with any confidence. The 06/00Z NAM and UKMET depict 2 to 4 inch amounts just north of the international border while the ECMWF is consistent in 4 or 5 inches along coastal Maine. If the NAM solution verifies a bit farther east...the concern for flash flooding will be considerably higher given their hydrologic sensitivity. Surrounding the Slight is a broader apron of Marginal Risk given uncertainty in how quickly rainfall will be moving away and how much will have already fallen. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-won9lzLAafMZfnfnQOrK1-YaVbILX7XISjjSwhEmHZ8= Rzw-FB1rRq2qdJF0lD7u5M_Z1ZW9W03qdxwiYXJ8N7XtDoU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-won9lzLAafMZfnfnQOrK1-YaVbILX7XISjjSwhEmHZ8= Rzw-FB1rRq2qdJF0lD7u5M_Z1ZW9W03qdxwiYXJ8AbrIJqo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-won9lzLAafMZfnfnQOrK1-YaVbILX7XISjjSwhEmHZ8= Rzw-FB1rRq2qdJF0lD7u5M_Z1ZW9W03qdxwiYXJ8PLexq9M$=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) .