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  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 


...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.


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.


  Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt 

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