WeatherWheeler35 Posted April 30 Share Posted April 30 Weather alerts from the WX weather app Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Sun Apr 30 2023 Day 1 Valid 16Z Sun Apr 30 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 TO THE MID ATLANTIC REGION... ...Northeast & Mid-Atlantic... 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 today's 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 today and Northeast tonight, to record low MSLP values in southern Ontario, northern New York, and southern New England tonight, and perhaps most notable for the basis of this forecast discussion, 850mb winds and 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 in the Southeast will be responsible for additional rounds of strong-to-severe thunderstorms in the southern Mid-Atlantic. Meanwhile, a second low pressure area will develop over southeast Virginia this afternoon and head for NJ and Long Island later this evening. The double-barrel low structure (low in southern Ontario, low along the Mid-Atlantic coast) will work with high pressure wedged in place over northern Quebec to produce an intense southeasterly LLJ that delivers copious amounts of moisture to the northern Mid-Atlantic and all the way to northern New England. The Slight Risk remains in place from Baltimore on north and east through northeast MD, the northern DelMarVa, eastern PA, all of NJ, the NYC metro area, and into the Poconos and upstate NY. As mean 925-850mb southeasterly flow intersects the approaching cold front making its way through western PA this afternoon, additional heavy thunderstorms will form along the cold front crossing the central Appalachians, north of a warm front lifting north through the Chesapeake Bay, 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" from the DC/Baltimore metro area to northern NJ and south and east of the NY Finger Lakes. The Marginal Risk extends as far south as Norfolk, VA and northeast NC where the possible strong-to-severe storms may result in localized flash flooding this afternoon. Farther north, New England will 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) for 00Z Monday, and they increase by 06Z to values that are above not only the observed 06Z values in the CFSR climatology for the time of year, but near the Portland area, above all hours in the CFSR record. 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. 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 12Z 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. ...Florida... With the line of storms rapidly moving south and east across the Florida Peninsula, the flash flood threat is quickly winding down in these areas. Have dropped the Marginal Risk inherited from the overnight shift. Mullinax Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Quote Link to comment Share on other sites More sharing options...
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