Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 01 2024 08:01:17 FOUS30 KWBC 010800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Mon Apr 1 2024 Day 1 Valid 12Z Mon Apr 01 2024 - 12Z Tue Apr 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL EXTENDING FROM THE OZARK REGION NORTHEASTWARD ACROSS THE MID MISSISSIPPI VALLEY INTO THE OHIO VALLEY... Highly amplified longwave pattern across the west will begin to push eastward with increased meridional flow ahead of the mean trough ushering warm, moist air poleward. To the north over the Mid-Mississippi Valley through the Ohio Valley, a stationary front will drape over the aforementioned regions with some oscillation expected through Monday as strong southerly winds act upon the frontal positioning. A sharp convergence pattern will setup in-of the Midwest extending eastward through the Ohio Valley and western Mid-Atlantic. Theta-E forecast depicts a well-defined boundary where the low-mid level convergence regime will be able to take=20 place. Modest buoyancy and sufficient shear will generate a period of moderate to heavy rainfall from convective development as the=20 large scale ascent increases in favor thanks to the longwave=20 progression and attendant shortwaves ejecting northeast out of the=20 southern plains. The boundary itself will provide the necessary focal point for heavier rains to develop and ride west to east along the zone of greatest low-level convergence. The best axis of heavy rain looks to occur from eastern MO through central IL/IN, through southern OH into central WV with totals of 1.5-2.5"=20 increasingly likely during the late-afternoon and evening with some local totals exceeding 3" if settled within spots of favorable=20 training. 00z HREF EAS is depicting a fairly stout swath of at=20 least 1" of total precip (70-90%) and a probabilistically=20 significant range of 20-40% of at least 2" across the area of=20 interest. This is plenty to maintain continuity for the SLGT risk=20 that was in place earlier with only some minor adjustments made to=20 extend the northern fringe of the risk area a bit further north and bring up the southern edge to align with model consensus.=20 Further to the southwest over the Ozarks and southern plains, convective development ahead of an advancing cold front will add to the opportunity for localized flash flooding concerns, especially considering the favorable LLJ pattern developing by evening providing a better sheared profile capable of discrete supercells within a moisture laden environment (PWAT anomalies approaching 2-3 deviations above normal). Any direct impacts from organized convection, whether it be multi-cellular or=20 supercellular will have the opportunity to put down locally=20 enhanced rainfall capable of flash flooding, especially within any=20 urbanized corridors or terrain. The best risk will lie over the=20 Ozarks of northern AR and southern MO which is currently placed=20 within the southern envelope of the SLGT risk forecast. MRGL risk=20 expands across northeast OK, through the central Midwest over into=20 the Ohio Valley with the eastern fringes banked against the=20 Allegheny Front in WV.=20 Kleebauer Day 2 Valid 12Z Tue Apr 02 2024 - 12Z Wed Apr 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE EASTERN CONTINENTAL DIVIDE FROM SOUTHWEST PENNSYLVANIA DOWN THROUGH EASTERN TENNESSEE... Our highly amplified upper trough will continue to press east with a potent shortwave exiting to the northeast ahead of the mean trough axis. A strong, digging 5H vort max will dive out of Canada, inducing a phased mid and upper level pattern which will enhance downstream diffluence in wake of the complex evolution. A strong SLP will move into the Midwest and intensify rapidly before occluding into MI. A sharp cold front will press east out of the Mississippi Valley with increasing convergence ahead of the boundary allowing for a period of thunderstorms to develop downstream across portions of MS/AL/TN up into WV. Instability pattern is most favorable to the west of the Continental Divide located from the WV High-Country down through northern MS/AL which also aligns within the left-exit region of a strengthening 150-160kt upper jet max pivoting out of the base of the mean trough. With deep, moist advection ongoing ahead of the trough, area convection will be able to tap into the favorable dynamics and present a period of locally heavy rainfall anywhere within the axis of instability, especially ahead of the cold front where there will be focus mechanism for development and maintenance. A SLGT risk=20 was added to the terrain within the Appalachians and adjacent areas west of the mountains due to favorable convective modes and=20 complex terrain allowing a heightened risk for flash flooding in=20 those areas more prone in these setups.=20 Across northern WV into MD/PA, a multi-round setup of moderate=20 rainfall will allow for priming of the local soils within the=20 complex terrain west of the Blue Ridge which will enhance potential for flash flooding as the second wave of rain enters the picture Tuesday evening. CIPS analog package from this evening had a decent signature for mean rainfall within the aforementioned zone with a few events that had a history of producing some localized flooding within the terrain. The prospects were not as analogous further east of the Continental Divide, however some rainfall totals approaching 1-1.5" could offer flood concerns within those urban corridors east of the Blue Ridge, as well as lower elevation towns within the major river basins thanks to multiple rounds of rainfall. The MRGL risk was maintained for the Mid-Atlantic due to the potential with a possible upgrade pending short term trends and observations from the initial pattern evolution today and early Tuesday.=20=20 Kleebauer Day 3 Valid 12Z Wed Apr 03 2024 - 12Z Thu Apr 04 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS NORTHEASTERN NEW JERSEY, NEW YORK CITY, LONG ISLAND, AND PORTIONS OF SOUTHERN NEW ENGLAND... A deep, closed upper low over the Midwest will move east with an occluded SLP over MI transferring energy to another SLP developing over the Mid-Atlantic and intensifying as it moves northeast. A strong easterly component will advect deep moisture off the Atlantic into the Northeast US as cyclone intensifies off the Northeast coast leading to heavy rainfall focused over northern NJ up through coastal Southern New England. Latest trends in guidance are signaling a locally heavy rain event for the coastal plain extending from just south of NYC up through coastal Southern New England with the heaviest rainfall along the I-95 corridor from southern CT through eastern MA. Rates will be borderline within the 0.3-0.5"/hr territory at peak intensity, but much of this will fall within a span of 12-18 hrs until you are north of CT where the=20 occlusion process will maintain a lingering precip field through=20 late Wednesday into Thursday morning. The question initially was=20 the ratio of rain to winter precip and when/where that will take=20 place. As of now, the prospects for most, or all rain will be along the coastal plain with only a slight chance of the storm ending as a mix as the heights crash with the upper low moving overhead the=20 end of the forecast period. A combination of strong onshore flow=20 and heavy rain will enhance local flood prospects within those=20 coastal towns as modest surge coupled with the rain will present a=20 better risk of flooding over the areas near the Atlantic.=20 Heavy rains within the urban corridor spanning from northern NJ,=20 through NYC up to Boston will also have their potential with the=20 best chances occurring Wednesday afternoon into the first half of=20 the evening. Totals of 1-2" will be common within the above areas=20 with a max of 2.5" plausible across eastern MA where heavier precip will focus along the coastal front. Once you are north of the MA=20 Turnpike, the prospects for winter will be higher with an=20 expectation of a mix or all snow once you set sights away from the=20 coast and places further inland at elevation.=20 Considering the expected evolution of the synoptic pattern and the forecast for significant precip totals over the course of=20 Wednesday into Thursday, there was plenty of credence to maintain=20 the previous MRGL with an expansion to the southwest into the NYC=20 metro and surrounds. This was also agreed upon by the Upton WFO in=20 collaboration for the expansion prospects.=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-rJBjz0bbW24Ycgwq8FiBcLUjum2Ylevj7KayA69YfHf= dEW5gO26ucDuulFG69gmTG1LnIUk64MI_dDQ5LrC5FwzG14$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-rJBjz0bbW24Ycgwq8FiBcLUjum2Ylevj7KayA69YfHf= dEW5gO26ucDuulFG69gmTG1LnIUk64MI_dDQ5LrCbcs2r0E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-rJBjz0bbW24Ycgwq8FiBcLUjum2Ylevj7KayA69YfHf= dEW5gO26ucDuulFG69gmTG1LnIUk64MI_dDQ5LrC1zcynV4$=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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