Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Apr 02 2024 07:46:00 FOUS30 KWBC 020800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Tue Apr 2 2024 Day 1 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 WESTERN PENNSYLVANIA DOWN THROUGH NORTHERN ALABAMA... A highly amplified upper trough will continue to press east with a potent shortwave exiting to the northeast ahead of the mean trough axis this morning, impacting the central Mid-Atlantic. 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. Low pressure over the Mid- Mississippi Valley will strengthen as it moves northeast before occluding into MI as a powerful mid-latitude cyclone (sub-985mb). 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 OH/WV/Southwest PA. Instability pattern is most favorable to the west of the Continental Divide located from eastern OH down through northern MS/AL where the new SPC D1 features an expansive Enhanced Risk with a large Moderate encompassing a good chunk of the Ohio Valley. The combination of modest buoyancy aligning with the left-exit region of a strengthening 150-160kt upper jet max pivoting out of the base of the mean trough will allow for widespread convection nestled within a moisture rich environment as PWAT anomalies reach 2-3 deviations above normal through a large chunk of the eastern CONUS. The deep, moist convective pattern will present periods 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, and along the stationary boundary bisecting the I-70 corridor from PA through OH as the aforementioned shortwave provides ample lift and a focal point for convergence within the boundary. The SLGT risk was maintained from the previous forecast, but did expand the southern and northern edges of the risk area to include northern AL and more of western PA, respectively. The highest risk of flash flooding is likely within western PA due to lower FFG indices in all 1/3/6 hr periods, as well as minor flooding occurring from prior rainfall yesterday which will be exacerbated with the incoming precip. Forecast precip totals of 2-3" are now forecast in-of western PA which aligns well with the latest 00z HREF blended mean guidance and correlated with HREF EAS probability of at least 2" up closer to 25-35% for areas along and west of I-99 out to Pittsburgh. Neighborhood probabilities of 50-70% for at least 1"/hr rates with sporadic signals of 2"/hr (10-20%) are the primary drivers within the SLGT risk over the Southeast U.S and adjacent Tennessee Valley. Given the better surface based instability alignment south of I-64, the threat of higher rates will propose the best, true flash flood potential despite higher totals still expected for areas further north. Regardless, the expanse of the SLGT and MRGL risk areas in place is a testament to a potent mid-latitude cyclone and an amplifying 5H pattern. Additional updates for expansion in both the MRGL and SLGT risk areas are possible as the complex setup evolves. Kleebauer Day 2 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 slowly advance east with an occluded SLP over MI transferring energy to another SLP developing over the Mid-Atlantic and intensifying as it moves northeast. Prevailing southwest flow ahead of the mean longwave trough will maintain a persistent moisture advection regime up through the Mid-Atlantic with PWAT anomalies solidly above 2 standard deviations for much of Wednesday. A strong diffluent pattern ahead of the sprawling upper low will enhance large scale ascent across the central Mid-Atlantic, expanding northeast through Southern New England before the pattern finally shifts out into the Atlantic. Recent trends in both deterministic and ensemble guidance have brought a greater emphasis on an elevated convective regime taking place over the central Mid-Atlantic, including NoVA/MD/PA which encompasses a larger population center and more susceptible region for flooding. Wednesday morning will be the initiation point of the developing convection with rainfall rates generally reaching 0.3-0.5"/hr at peak intensity. Considering naturally lower FFG indices due to urbanization footprint, as well as priming from previous day rains within the river basins, this will promote a better risk for localized flood concerns, especially any area that gets impacted by a heavier thunderstorm or storms. Neighborhood probability of at least 1-2" of total precip has crept up from the last succession of ensemble runs with locations within the Lower Susquehanna Valley now running upwards of 50% to see at least 2" within the period from 00z Wednesday through 00z Thursday based on the latest 00z HREF. The period begins at 12z Wednesday, so the effects from one period to the next will be compounding with ongoing rainfall likely until at least 21z before the surface reflection moves off to the northeast over Long Island and eventually off Cape Cod by Thursday AM. As we head northeast into places like Northern NJ, NYC, and Southern New England, the primary focus will turn to a strong onshore component with a developing axis of deformation to the northwest side of the rapidly intensifying cyclone. Temps aloft down to the surface will remain sufficient for keeping the ptype most, if not all rain for areas like Northeast NJ, NYC, LI, up through coastal Southern New England. This is the area of interest for flood prospects as rainfall totals will reach over 1" pretty easily with 1.5-2HREF" likely during the Wednesday afternoon and evening time frame. When this is coupled with the forecast coastal flooding due to the prevailing onshore flow from the deepening low pressure, any additional rainfall will exacerbate flooding concerns in-of the urban corridors along and south of I-95 up into Eastern MA. Models have become a bit colder further north up around Boston's latitude, so there will be sharp northern extent for flooding capabilities as winter weather will become the primary focus for the interior areas of NY state up through interior Southern New England. Considering the nature of these setups and the positioning of the coastal front over Eastern MA, decided to maintain continuity from the previous forecast and kept the MRGL risk in place with the biggest change being further south with the extension down through central NJ, the eastern half of PA, into the DC/Baltimore metroplex encompassing Central MD and Northern VA. Kleebauer Day 3 Valid 12Z Thu Apr 04 2024 - 12Z Fri Apr 05 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt__;!!DZ3fjg!6R_tngtXCSyfI3wbi8JkHRP5sBPQPl1Plbgphw-G3SEeEIv5Yz9STJDjBtftmYFqATkK2DlVrMd0-smIKuHsBlXLXJQ$ Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt__;!!DZ3fjg!6R_tngtXCSyfI3wbi8JkHRP5sBPQPl1Plbgphw-G3SEeEIv5Yz9STJDjBtftmYFqATkK2DlVrMd0-smIKuHsVNvHERY$ Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt__;!!DZ3fjg!6R_tngtXCSyfI3wbi8JkHRP5sBPQPl1Plbgphw-G3SEeEIv5Yz9STJDjBtftmYFqATkK2DlVrMd0-smIKuHs32XnZ5g$ = = = --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .