Quantitative Precipitation Forecast
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000 FXUS04 KWBC 250701 QPFPFD Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri May 25 2018 Prelim Day 1 QPF Discussion Valid May 25/1200 UTC thru May 26/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Overview... The flow pattern is not too exciting except for a closed low entering the west coast and a broad area of anomalously low heights in the Gulf of Mexico / northern Caribbean where the National Hurricane Center places high probabilities of subtropical or tropical depression development in the near future. A shortwave digging into the back side of the Gulf trough early this morning will help induce inverted troughing and enhanced southerly flow of very moist air downstream over south Florida today. Elsewhere northern stream waves will promote a few mesoscale thunderstorm clusters over the upper Midweset / Great Lakes and New England. Diurnal convection should again become fairly widespread in coverage over the southern tier, and the low along the west coast will yield organized rainfall, especially northern California and southern to eastern Oregon. WPC QPF was derived from a near 50/50 blend of our in-house ensemble and the HREF blended mean. Six-hour QPF probabilities from the GEFS were also helpful. ...Western U.S... Models are very generous with rainfall in the difluent region north to northeast of the closed low center. Although the low is forecast to open up late in the Day 1 period there should be a lengthy duration of favorable ascent profiles and southeasterly low level inflow from northern California into Oregon, and catching part of northwest Nevada. The event may begin with some convective enhancement and then transition to longer duration stratiform rainfall behind the surface boundary. An environment more supportive of deep convection and/or more repetitive convection should exist farther east, from southeast Oregon into parts of northern Nevada and much of Idaho. Instability drops off with eastward extent through NV/UT given drier air in that region, but thunderstorms should be able to make some eastward progress before drying out. WPC favored the NAM CONUS NEST for its more expansive coverage, but perhaps the HREF mean for more toned down amounts. Still, areal average rainfall greater than an inch is forecast in parts of CA/OR. The odds of training cells appears lower today, but hourly rain rates will likely exceed a half an inch locally during the stronger individual thunderstorms. ...Florida... With a shortwave trough dropping in to strengthen the broad upper trough over this region, the models develop an inverted trough axis over south Florida along with upwind convergence in the stengthening and very moist southeasterly flow coming off the Caribbean. There is an increased model signal for widespread 1 to 2 inch areal average rainfall, and the potential for locally more intense amounts as the pattern remains locked in, allowing repeated tropical convection into the Keys and parts of south Florida. Burke $$

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