Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 01 2023 00:52:48 FOUS30 KWBC 010052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Sun Apr 30 2023 Day 1 Valid 01Z Mon May 01 2023 - 12Z Mon May 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF EASTERN NEW ENGLAND AND FROM SOUTHERN NEW YORK INTO EASTERN PENNSYLVANIA... ....01Z Update... Nudged the periphery of each Slight Risk area a bit based on trends in late afternoon/early evening satellite and radar imagery as well as guidance from the latest runs of the HRRR and 18Z HREF probability guidance. The best moisture transport has shifted eastward since the 16Z ERO as shortwave energy continues to rotate around the anomalously deep system over the Great Lakes. The primary threat remains from northern New Jersey into southern New York and southwestern New England...where 3 hr pressure falls were maximized between 4 mb and 6 mb. Also maintained the Slight Risk in New Hampshire and Maine where the HRRR continues to show 1+ inch per hour rates and A powerful upper low over the Great Lakes working in tandem with a vigorous 500mb vort max racing north along the East Coast will lay the ground work for a robust Warm Conveyor Belt (WCB) of highly anomalous moisture being directed into the Northeast. So much about this setup is associated with statistically impressive anomalies...from near record low 500-850mb heights for late April-early May stretching from the Great Lakes and Ohio Valley that expands into the Northeast later tonight to record low MSLP values in southern Ontario, northern New York, and southern New England...and 850mb winds and the associated Integrated Vapor Transport (IVT). This setup is multifaceted as not only is there the primary upper low over the Great Lakes, but the vigorous negatively-tilted 500mb vort max...with a second system on its heels moving towards New Jersey from the Mid-Atlantic region later this evening.=20 The Slight Risk remains in place from central New Jersey/northeast Pennsylvania into the Poconos and upstate NY. As mean 925-850mb southeasterly flow intersects the approaching cold front making its way through eastern Pennsylvania this evening, thunderstorms with heavy rainfall will be focused along and ahead of the cold front, north of a warm front, and at the nose of a 500mb jet streak over the Delmarva positioning itself over eastern PA and NJ. With soils already saturated, the approaching cluster of storms along the cold front and on the northern periphery of the strengthening surface low will be capable of producing 1"/hr rainfall rates. Latest FFGs are already as low as 1" over parts of northern NJ and south and east of the NY Finger Lakes into the Delmarva. Farther north, New England continues to be ideally placed where the strongest IVT transpires. NAEFS shows IVT values as high as 750 kg/m/s (97.5-99th percentile in central New England) which increases by 06Z to values that are at or above the CFSR climatology for the time of year. IVT values around Portland, ME reach an impressive 1,000 kg/m/s, coinciding within a 60-70 knot 850mb jet that is near record strength for the time of year. 00Z sounding from GYX showed PW values of 1.17 inches with 4000 ft wind speed of 60 kts. PWs along the New England coast are forecast to exceed 1.25" with PWs above 1.0 well inland. With so much moisture present, it is no surprise mean cloud layer RH values (LCL-EL) are above 90% tonight. Warm cloud layers are also quite deep with latest RAP guidance suggesting by 06Z tonight that southern ME has warm cloud layers as deep as 8,000 feet. This will undoubtedly be an efficient rainfall producer, but rates are still capped at generally below 1"/hr due to the lack of sufficient instability. Along the coast, MUCAPE could approach 250 J/kg, and that could be enough for areas north of Boston on north along the southern ME coast for localized rainfall rates up to 1"/hr. Farther inland, hourly rates would be more likely confided to the 0.5-0.75" range, which is quite heavy, but leads to a more gradual increase in the flood threat than a bigger flash flood threat. The 18Z HREF does show just how wet the area is likely to be, highlighting 6-hr QPF > 10 year ARIs between 06-12Z Monday that are 40-70% in western ME. With this being the case, have maintained the Slight Risk from the White Mountains to the southern and central ME coast. The White Mountains are likely to see the heaviest rainfall totals due to the prolonged stretch of enhanced upslope flow that could result in localized amounts surpassing 5". River and stream flooding is likely to occur along with a flash flooding threat in poor drainage pots, heavily urbanized areas where a greater concentration of hydrophobic surfaces exist, and along complex terrain. The Marginal Risk spans from the Adirondacks and southern New England to Downeast ME. Bann Day 2 Valid 12Z Mon May 01 2023 - 12Z Tue May 02 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Mullinax Day 3 Valid 12Z Tue May 02 2023 - 12Z Wed May 03 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!47tEtdEeSfrWUt3YizGlp4_P6wd4t9NzaMH-oJiBZrt4= Pu_Ax1QWch6pNqOuvpVbFjbb6mV2ale9h55ykQBtpv50LY8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!47tEtdEeSfrWUt3YizGlp4_P6wd4t9NzaMH-oJiBZrt4= Pu_Ax1QWch6pNqOuvpVbFjbb6mV2ale9h55ykQBtmcwgpLU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!47tEtdEeSfrWUt3YizGlp4_P6wd4t9NzaMH-oJiBZrt4= Pu_Ax1QWch6pNqOuvpVbFjbb6mV2ale9h55ykQBtQuo3o0w$=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) .