Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon Jun 26 2023 16:26:00 FOUS30 KWBC 262022 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 PM EDT Mon Jun 26 2023 Day 1 Valid 16Z Mon Jun 26 2023 - 12Z Tue Jun 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS MID-ATLANTIC... ....Eastern and Southern U.S.... An anomalously deep upper low will continue to develop and move across the upper Great Lakes today, with an associated surface front pushing across the Ohio Valley into the central Appalachians today. A moist airmass ahead of the front (PWs at or above 1.5 inches), interacting with strong upper level divergence and increasing destabilization during the day is expected to set the stage for widespread convection developing across portions of the Northeast southward through the Mid-Atlantic into the Southeast. While the transient nature of the system and developing cold pools are likely to support progressive storms, the environment is expected to generate intense rainfall rates, which may produce isolated to scattered flash flooding concerns. Wet antecedent conditions supported by above-average rainfall over the past seven days, may leave portions of Virginia into the Southeast more prone to flash flooding. The 00Z HREF Ensemble Agreement Scale shows two areas where heavy amounts are more likely -- one over eastern Pennsylvania into northwestern New Jersey and the Catskill Mountain region of Update New York; and the other centered over south-central North Carolina. 12Z HREF Neighborhood probabilities still indicate amounts of 2-3 inches are likely across the southern area, with 3 inches or more probable in the northern area. Extended the Marginal Risk area northwestward from the Gulf of Mexico across Louisiana and far eastern Texas where convection has fired each of the past couple of days along an old boundary. RAP runs from this morning show frontogenesis in the area this morning should be tapering off by late afternoon or early evening...but cell motion looks to be aligned with where convection occurred/tracked earlier this morning. Also introduced a Marginal Risk area over parts of southern Florida where ingredients for locally heavy to excessive rainfall remain in place...with greatest concern being over the east coast metro areas even though the HRRR sub-hourly QPF maximum (in excess of 1.5 inches) has greatest areal extent west of the most urbanized areas. WPC Mesoscale Precipitation Discussion 588 has more focused mesoscale details. ....Southern Montana into Northern Wyoming and Western South Dakota... Increasing ascent ahead of an upper trough moving through the Intermountain West and increasing moisture supported in part by southeasterly low level inflow are expected to support showers and storms developing across the region later today. Once convection from earlier this morning dissipate...additional storms are expected to develop across the high terrain in southwestern Montana and northwestern Wyoming this afternoon/evening before propagating east along an axis of deeper moisture centered across southeastern Montana and northeastern Wyoming into the Black Hills region. Although widespread heavy amounts are not expected, locally heavy amounts may produce isolated flash flooding especially given the anomalously wet antecedent conditions. ....Northern Cascades... Easterly low level winds, weak flow aloft, and PWs approaching an inch are expected to support slow-moving showers and storms. These may create isolated flash flooding concerns, especially over areas recently impacted by wildfire. Pereira/Bann Day 2 Valid 12Z Tue Jun 27 2023 - 12Z Wed Jun 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST AND THE MID-ATLANTIC, AND THE NORTHERN ROCKIES AND PLAINS... ....Northeast/Northern Mid-Atlantic... The upper low moving across the upper Great Lakes on Day 1 is forecast to transition to an open wave and slow as it moves across the lower Great Lakes on Day 2. Deep southerly flow ahead of the trough will continue to support ample moisture, with precipitable water values remaining around 1.5 inches along much of the Mid-Atlantic and Northeast seaboards. Models show ongoing convection, potentially producing locally heavy amounts and isolated flash flooding concerns early in the period across portions of New England. Then later in the day, destabilization and mid-level energy lifting along the leading edge of the trough are likely to support the redevelopment of showers and storms back to the west across the Mid Atlantic into Upstate New York, which may also produce some additional isolated flash flooding concerns. Still enough uncertainty about how much rain falls on Day 1...and where...at this point to preclude a Slight Risk. ....Northern Rockies and Plains... A slow-moving trough in the west and sufficient moisture will support another day of showers and storms developing over the mountains of western Wyoming and Montana. Farther to the east, leading energy moving out of the trough, interacting with deepening moisture along a strengthening low level jet is expected to support showers and storms developing along and ahead of a front dropping southeast through the High Plains. Although widespread heavy amounts are not expected, locally heavy amounts may produce an isolated flash flooding threat, especially given the wet antecedent soils. Pereira/Bann Day 3 Valid 12Z Wed Jun 28 2023 - 12Z Thu Jun 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST, UPPER MIDWEST, AND THE NORTHERN ROCKIES... Overall...the 12Z suite of numerical guidance maintained decent run to run consistency so nothing more than a few costmetic adjustments were made. Bann Previous Excessive Rainfall Discussion... ....Northeast... An upper trough will linger for yet another day over the Northeast, channeling both deep moisture and mid-level energy across portions of the region. Daytime heating is likely to help support additional showers and thunderstorms, with locally heavy amounts possible, especially across portions of eastern Update New York into New England. ....Upper Midwest... Mid-level energy emanating from the trough in the West, along with an amplifying trough approaching from the northwest over central Canada is expected to support an increasing chance for rain across portions of the region on Wednesday into early Thursday. Deepening moisture (PWs increasing to around 1.5 inches), along with some potential for southwest-northeast training, may produce at least some locally heavy amounts from northern Minnesota to the western U.P. of Michigan. ....Northern Rockies... With an upper trough remaining positioned to the west and a relatively moist airmass still in place, isolated to scattered showers and storms are expected to develop once again across the region. These may produce locally heavy amounts and generate isolated flash flooding concerns, especially given the wet antecedent conditions. Pereira $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .