Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 537 PM EDT Sun Apr 01 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 02/0000 UTC thru Apr 05/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Update No significant changes were made to the preliminary Day 1 package. Pereira Day 1 ...OH Valley into the Mid Atlantic and southern New England... A shortwave will continue tracking quickly eastward across the MS Valley and into the Mid Atlantic tonight. WAA ahead of this feature, coupled with a favorable upper jet structure...supports an area of rain/snow with this wave. Note an impressive area of omega in the snow growth zone north of the low...with negative EPV above the frontogenetical zone. Thus appears like a good setup for some pretty strong banding and efficient snowfall where temperatures are cold enough. See the QPFHSD for more information on the snowfall potential with this system. From a QPF perspective, this supports staying on the higher side of guidance with the magnitude from KY northeast across WV into southern PA and into NJ and far southern New England. Locally heavy rains exceeding 1" are possible this evening across KY...although the lack of instability should prevent amounts from getting much higher. Models are in better agreement now with the axis as well, although some differences remain. In general, the 12z NAM and HREF are a bit further north than the global guidance. WPC went with a consensus of the 12z HREF mean and the 12z GFS/0z ECMWF...with a bit more weighting given to the further north HREF. Will have some dry air in place to the north, but given the strong omega noted above, think this should be overcome to a degree, as depicted in the HREF. This, combined with the seemingly realistic extrapolation of the current activity ongoing over the MS Valley in the high res guidance...led us to trend in their direction. Overall this keeps good continuity from our previous forecast. ...Pacific Northwest into the Northern Rockies and Northern Plains... A consolidated area of strong mid level energy will drop southeast into the Pacific Northwest through tonight. As this energy interacts with the terrain and the arctic boundary, anticipate widespread rain/snow. From the Pacific Northwest into the Northern Rockies models were in good agreement...and thus a multi model blend should work well. However, differences emerge across the Northern Plains from eastern MT into the Dakotas. The 12z NAM is stronger/more closed off with the mid level energy...resulting in a further north boundary and thus further north QPF axis across MT into the Dakotas on Monday. The NAM and the 12z HREF members are outliers depicting this further north solution. The GFS/ECMWF/UKMET/GEM have all been very consistent depicting a further south solution. Given this consistency and agreement, we think the NAM represents an unlikely solution...and thus WPC stayed closer to a non NAM and non HREF blend for this area during the second half of the day 1 period. Days 2 & 3 ...Pacific Northwest... Day 2 looks to have the largest break in notable precip in the next week. Low pressure along the northern BC coast will direct a tropically sourced plume of moisture into the PAC NW coast beginning Tuesday night. Rates are expected to increase to moderate Wednesday in preferred western slopes of the Olympics and WA/northern OR Cascades as a plume of 0.75 inch PW comes ashore. Expect up to an inch QPF for these preferred spots in Day 3 (00Z Wed-00Z Thu) with an active and rather wet period increasing thereafter per 12Z guidance consensus. ...Northern and Central Rockies Eastward Across the Plains to the Great Lakes... Developing low pressure will shift east from WY Monday evening to across the Great Lakes Tuesday night. Precipitation will be broad across the Dakotas Monday night, but will become increasingly focused and intense in the deformation zone north of the surface low over the Upper Midwest late Monday night through Tuesday. The 12Z GFS had the majority of the weighting in the QPF blend with similar solutions the 00Z ECMWF and 12Z UKMET receiving the rest of the blend. Confidence has increased in the track and intensity of the precip with heavy rates likely in narrow deformation zone bands from near NE/SD late Monday night across southern MN and central WI earlier Tuesday to lower MI later Tuesday. 1.25 inch PW ahead of the surface low will lift across IL/IN/OH (Two standard deviations above normal) on 50+kt southwesterly 850mb Tuesday morning. Progressive heavy showers and thunderstorms in this area are not expected to cause any more than potential localized flooding issues. ...Northeast... Low pressure will continue to develop as it moves northeast from the eastern Great Lakes Wednesday. 50 to 70kt southwesterly flow ahead of the low will advect one inch PW up the northeastern CONUS all the way across Maine through Day 3 (00Z Wed-00Z Thu) which is two to three standard deviations above normal north of the Mason-Dixon line. Both the leading warm frontal rain and pre-cold frontal rain will feature heavy elements, but will be progressive. The QPF 1.0 to 1.5 inch across the northeastern CONUS with enhancements of up to 2 inches over northern Maine. Wintry precip is expected at the onset here before changing to rain. Flooding concerns are possible over northern New England where the melting snow pack will contribute to the runoff. ...TN and OH Valleys to Gulf Coast Eastward... A cold front associated with the developing surface low pressure system over the Great Lakes region will be the focus for convection from the Gulf Coast northward into portions of the Tennessee and Ohio Valleys on Tuesday. The 12Z GFS was preferred for this region as well with lesser weight given to the 00Z ECMWF and 12Z UKMET. Confluent southerly flow at H85 coming northward from the Gulf ahead of the front will result in precipitable water values to increase to roughly 1.5 inches along the Gulf and into the range from 1.25 inches to 1.50 inches into the Tennessee Valley by late Tuesday afternoon which is 1.5 to 2 standard deviations above normal. Heavy rainfall rates are likely in an area that has experienced multiple rounds of heavy rainfall over the past couple of months. Continued the marginal excessive rainfall risk for now given the progressive nature of the cold front. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml